~ : - 2a?mj?s>> /41 /fi^SWX.0. LIB RAFLY OF THL UN IVER5ITY or ILLINOIS 630.7 Hfeb no. 1-84 **$00M ^c-1/.i -; '^m^'^m "> - C /T aSf^S^- UNIVERSITY OF ILLINOIS Agricultural Experiment Station. URBANA, DECEMBER, 1901. BULLETIN No 67. APPLE SCAB. BY GEORGE P. CLINTON, M.S., ASSISTANT BOTANIST, AGRICULTURAL EXPERIMENT STATION. Investigations by the Department of Horticulture of the Uni- versity of Illinois upon the disease commonly known as Apple Scab were published in Bulletin No. 54. These studies were mostly con- nected with preventive measures and the conclusions were mainly based upon field experiments. The present publication records the results of botanical studies made during- the years 1898, 1899, and 1900. Scab is a disease of the leaves and fruit of apples (Fig. 2) caused by a parasitic fungus botanically named Fusicladium dendriticum. Many fungi possess at least two stages ; namely, a summer or temporary spore stage adapted to spread the disease during the growing season of the host, and a winter or permanent stage adapted by greater protection afforded the spores to carry them through the winter and thus insure the infection of the host 109 i io BULLETIN NO. 67. {December, again the following- year. These stages usually differ greatly in appearance and may occur on different parts of the same host and at different times of the year, or they may occur on entirely different plants, thus causing one to regard them as two distinct species until their life history has been thoroughly worked out. The apple scab constitutes the temporary stage of a fungus and one of the points of investigation was to determine if there were a permanent stage connected with it, or, if not, how the fungus passed the winter. Accordingly careful examinations were made of the twigs and old fallen leaves during the fall and spring with the result that on the latter in the spring there was found a fungus that appeared to be the permanent stage. Germination of the spores of this in Van Tieghem cells in 1898 showed that the myce- lium from them could give rise to spores similar to those of the scab stage. This fungus had been long known to botanists but under an entirely different name, being called Venturia chlorospora. At this time it was not known to the writer that the latter had been suspected by others as the permanent stage of the apple scab fungus. An examination of literature, however, showed that Goethe as early as 1887 had suggested the relationship of the scabs of pears and apples to mature stages on the dead leaves ; that Bre- feld in 1891 figured a temporary spore stage obtained from a Ven- turia on pear that was similar to the leaf scab of that plant ; and that Aderhold, 1894 and later, had made a very comprehensive study of the scabs of apple, pear, and cherry and had connected them with species of Venturias that develop on the dead leaves of these hosts. From the results of these and the writer's investigations there is no doubt that apple scab is merely a parasitic summer stage of a permanent saprophytic fungus occuring on the fallen leaves. It also appears to be this latter form that is largely (in its immature condition) responsible for carrying the fungus through the winter. At least the writer has been unable to find any satisfactory evi- dence that the scab stage persists on the young twigs, as has been stated by some investigators, and by means of a new crop of scab spores in the spring spreads the disease to the young fruit and leaves. Neither was there found any evidence that the old spores lived over the winter on the fallen leaves, or that the mycelium in these gave rise to a new crop for spring infection. INJURY. This is a common disease of the apple wherever grown though the amount of damage inflicted may vary considerably in different IQOI.] APPLE SCAB. Ill localities and especially during- different seasons. It occurs on the leaves usually in distinct, small, circular, olive colored areas and in time is likely to cause more or less puckering- of the tissues or even killing- the plant cells beneath. When very vigorous on young- leaves it may cause them to turn yellowish and drop from the tree, though usually the leaves remain attached until fall when they may be shed somewhat prematurely. By these means the fung-us interferes with their function as food producing 1 agents and thus more or less indirectly affects the g-eneral health of the tree. On the fruit it produces the "scabby" spots familiar to every- one. These are roundish, olive-colored, roug-hish areas -usually about one quarter of an inch in diameter, or by coalescence more or less covering- the surface of the apple. When the very young" fruit is very badly infected it may cause distortion and interfere with the normal development in size, or, if severe enoug-h, cause the young- apples to fall from the tree. The injury of the fung-us is thus direct and easier of determination. By these direct and indirect effects the quantity of fruit pro- duced may be considerably lessened and the quality made inferior. It is hard, hcwever, to give the exact loss caused by such a disease unless one carries on careful experiments to determine quantity and quality of fruit produced from sprayed and unsprayed trees of the same variety grown under otherwise similar conditions during- a series of years. Most estimates given from such losses are mere guesses. There can be no doubt, however, that thousands of dol- lars are lost each year even in this state alone, and that during- certain years the loss is much greater. Professor W. J. Green, of Ohio, who has carried on careful spraying- experiments ag-ainst scab gives the following- as results in an orchard in 1897: "The average per sprayed tree was 6.75 bushels and of the unsprayed 2.42. Fifty sprayed trees produced 4.33 bushels of apples more per tree than the unsprayed, which was secured at a cost of not more than 20 cents per tree. In the case of the Northern Spy and Baldwin the actual profit derived from the treatment was more than $5.00 per tree." This fung-us, on the whole, seems to be more injurious in the states of the Mississippi valley than in some of the eastern coast states. Its injury, as stated before, also varies considerably dur- ing- different years. This is larg-ely due to weather conditions. It does most injury those years which have unusually wet spring's, especially if cold damp weather occurs during- the unfolding- of the leaves. Its severity also seems to bear some relation to the amount of the Venturia stage that has developed on the old leaves. 112 BULLETIN NO. 6j. [December, There appears to be no such thing- as a scab-proof variety. Various writers have recorded observations showing- that certain varieties have been more severly injured than others and there is no doubt that this is the case. It is very likely that such differences are due in large part to the thinner or more easily punctured cuti- cle of the scabbier varieties. The writer has also observed that scab is a lover of neg-lected orchards orchards which have not been properly sprayed, pruned, or cultivated and that it seems to be less abundant on very young- trees, especially where such trees are isolated. This latter is probably due to less favorable conditions for original infection. Scab has been known to botanists as a fung-ous trouble of the apple since the first part of the nineteenth century. Schweinitz, one of the first collectors of fungi in this country, found it at work on Newton Pippins in New York and Pennsylvania in the early thirties of the past century. Curtis records it as common on apples in North Carolina in 1867, and other early writers have listed it from various states. The earliest reference to its appearance in this state that has been found is a note in the Report of the Illinois Horticultural Society for 1863, pag-e 119, where Mr. Murtfeldt, of Og-le County, reported Carolina Red June, and Early Red as small and inclined to scab. Professor Burrill in 1883 writing- of it as an apple scourg-e in the Mississippi Valley, said in part: "Everyone must have noticed last year a peculiar shriveling- and curling- of the apple leaves, beg-inning- in many cases soon after the bursting- of the buds in spring- time and continuing- more or less during- the season. * * * The apples have been, in numerous instances, so scarred and spotted as to be practically worthless, except for stock or cider. * * * In Champaig-n County the Early Harvest apples, among- others, were divided half way to the core, or even deeper, by several irreg-ular fissures, leaving- knobby lobes, more or less covered by a layer of blackened corky tissue." The writer in his collecting- in recent years over the state has always found apple scab more or less com- mon and often doing- serious injury. PREVENTION. It is not the purpose of this bulletin to g-o into detail concern- ing- the prevention of this disease. It is known, however, that the injury can be stopped largely by proper attention to spraying-. Since the experiments started under the direction of Galloway, of the U. S. Department of Agriculture (by Goff in Wisconsin and by Taft in Michig-an) there has been much written by various station APPLE SCAB. 113 workers concerning- the prevention of scab through spraying-. As a result of these activities Bordeaux mixture has been found the most valuable fungicide. The present tendency is to use it made as fol- lows : 41b. copper sulphate, 41b. fresh lime, and 50 gal. water. The copper sulphate may be dissolved in hot water, the lime slaked in a suitable quantity of water, the two then being mixed together and strained into a barrel containing the remaining amount of water. Much of the success in the use of a fungicide depends upon the proper application and the time of spraying. The trees should be thoroughly sprayed. The first application should be made soon after the leaves begin to unfold, followed by a second soon after the petals begin to fall and by a third in less than two weeks if the season has been favorable for the development of scab. Later sprayings are apt to burn the foliage and russet the fruit and so are not very desirable. Some writers advocate two sprayings before the petals fall, the first with the swelling of the buds and the second after the leaves have expanded but before the petals are open. Winter spraying with a strong solution of copper sulphate has also been found to retard the first appearance of scab, but it is questionable if it is worth the extra expense and trouble if the first treatment with Bordeaux mixture is made promptly and thoroughly. In case the codling moth is injurious Paris green, at the rate of 1 Ib. to 200 gal. of the Bordeaux mixture, may be added for the second and third sprayings. Besides spraying proper attention should be given to pruning, as properly pruned trees are more easily and cheaply sprayed and also because unpruned trees afford greater protection against quick evaporation of moisture and thus so far favor infection by scab. The same holds true of proper cultivation, since orchards neglected in this respect offer greater protection to the fallen leaves, which., as we have seen, are the means by which the permanent stage de- velops and produces infection in the spring. No careful experi- ment of raking together and burning all of the fallen leaves from an orchard has been carried on and so nothing definite can be said of the advantage of this procedure. However, in a small plat in an orchard from which the leaves had been so removed the first appearance of scab on those trees seemed to have been retarded and reduced. Such work to be of any value should be done very thor- oughly in the autumn after all of the leaves have fallen. . FUSICLADIUM OR PARASITIC STAGE. TIME OF APPEARANCE, ETC. Scab makes its first appearance early in the spring, usually soon after the leaves begin to unfold, H4 BULLETIN NO. 67. [December, and it is while these and the fruit are in a young- condition that the fungus can best infect them. While it is to be found from the first appearance of the leaves until they drop off in the fall, there are two periods of greatest activity in spreading-, one early in the spring- and of the greater importance and the other in the autumn. This latter infection does little damag-e but is important because it is probably larg-ely responsible for the development of the mature stage. During- the years 1898 to 1900 a careful watch was kept of apple trees to determine first appearance, manner of infection, and relative abundance of scab. In 1898 the first sig-n of scab was seen on May 2d, though from its condition then it probably made its appearance several days earlier. At this time the oldest leaves had not reached their full size. On some trees the scab appeared more abundant on the lower than on the upper branches ; it was much more abundant on those leaves developed from flower buds than on those from the later purely leaf buds ; it was most abun- dant on the lower surface in the vicinity of the midrib. This lat- ter condition showed that the first infection had taken place very early, before the leaves were entirely unfolded. As apple leaves are folded on the midrib as an axis with the upper side within and the edges somewhat involute, the lower surface next the midrib is the first and most exposed part. The fact that the scab was most abundant on the leaves produced from flower buds sug- gests the possibility of insects as carrying agents for the spores, or it might be that such leaves being the first to unfold merely show the scab earlier on this account. On the very young fruit the scab often appeared at one side and toward the blossom end. The spring was cold and damp and as a result scab was much worse this year than either of the other years. There was also found an abundance of the Venturia stage on the old leaves before and dur- ing the first appearance of the scab. In 1899 a very careful examination showed the first scab on May 5th, on a very few leaves. This year it was slow in appear- ing and the oldest leaves escaped more than the later which were developed during a more moist period. The scab also appeared much more commonly on the upper sides of the leaves. The early spring was unusually warm and dry and therefore very unfavor- able for the spreading of the disease. The Venturia stage was also much later in developing- and many perithecia were found which seemed never to have matured their spores. This stage was much scarcer than it was the previous year. In 1900 the first sign of scab was found on a crab apple on 19!-] APPLE SCAB. 115 / May llth. This tree had been badly infested the previous year, and when first examined only two or three scabby leaves were found. The fungus did not make a general appearance in orchards until quite late, the latter part of the month. In one orchard ex- amined on May 21st no scab was found on a certain tree. This was examined again on June 4th and found badly scabbed on most of the lower leaves except the oldest. This tree had branches that reached down to the ground. The orchard was uncultivated and that spring- the Venturia stage had been found fairly common on the old leaves. The following 1 counts of leaves taken as they came show the relative abundance of scab on the lower and upper branches of the tree : Total leaves examined. Scabby. Total scabby spots. Branch in top 118 3 3 Branch on ground 99 53 520 A later examination showed that the scab had spread some- what in the top of the tree. These conditions seem clearly to in- dicate that the original infection had come by means of the lower branches from the Venturia stage on old leaves on the ground. Often the primary infection seems to be slight and in such cases it is from the spores developed from these that the scab be- comes spread over the tree. Sometimes it seems to appear on one tree and spread from this to the others. Very often trees show no difference in the development of scab on different parts. ON THE LEAVES. Scab generally occurs in roundish colonies about one quarter of an inch in diameter (Fig. 4). These may re- main entirely distinct or run together so that they loose their in- dividuality. This is often the case where they are crowded along the prominent veins. It is a common thing for the colonies to start over the veins as if these offered more favorable places for in- fection or for growth. The spots have an olive green color and under a magnifier are seen to consist of distinctly toothed and branched fungous strands (apparently on the surface of the leaf) that radiate from a common center. Sections through scabby spots seen under the microscope show the mycelium as a single thread in thickness (but no doubt several lying side by side) creeping over and closely applied to the top of the epidermal cells but beneath the cuticle and narrowing down at the end to a point by means of which it separates the cuticle from the epidermal cells (Fig. 7). The cuticle on the young leaves is not very easily made out but on the older ones where it has attained some thickness it becomes more evident. By this manner of devel- oping the fungus is protected on the outside by the cuticle while 1 16 BULLETIN NO. 67. [December, by being- directly applied to the cells on the inner side it is able to take nourishment by absorption ; no haustoria were found. The mycelium sometimes pushes down somewhat between the epider- mal cells but very rarely, if ever, goes deeper in green leaves. When this subcuticular mycelium, which averages 4 or 5 ;>. in diameter, is ready to form spores it divides up into more or less distinct, hyaline, roundish cells about the same diameter as the threads (Fig 1 . 8 and 9). Often there is but a single layer of these, but sometimes, especially above veins and woody tissue, this sub- hymenium is -two or more cells deep. From these are developed upright at first bluntly lanceolate to ovate shaped cells, or cotfidio- phores, that penetrate the cuticle (Fig-. 6). They have a thicker, reddish, outer wall and a very thin hyaline inner one, and occur in groups of greater or less extent, generally touching- each other a their broader bases but separated at their apices. The spores are formed by the protrusion from the apex of the conidiophore of its inner wall and contents as a hyaline roundish body, with more or less of a distinct base (Fig-. 7 and 8). This, when fully grown, forms the lanceolate, acute, reddish olive colored spore. When mature the spore is pushed off by the development in a similar manner of a second one. Each time a spore is formed, however, the conidiophore is lengthened by the remaining short basal part. This at first consists of only the hyaline inner wall, but develops with the spore the outer colored wall. By this means the conidiophore is not only lengthened but given a banded appear- ance something like short joints of an extended sliding tube (Fig-. 10). The conidiophores thus in time become cylindrical with short swollen bases, their final length being determined by their age and the character of the season as affecting the number of spores formed. The old conidiophores sometimes develop a septum or two. On an apple leaf gathered the latter part of June there was seen a conidiophore that had developed ten of these spore bands. The scab colony soon assumes its full size and the chief change during the season is the development of some new conidiophores and the growth of the old ones. Toward the end of the season the cells of the subcuticular mycelium may increase somewhat in size and become more or less tinted, but the chief rhauge in this respect comes after the leaves fall and their tissues die. Earlier or later in the season the action of the subcuticular mycelium, possibly in part through pressure, but probably largely through irritation, causes a more or less complete collapsing of the epidermal cells. This no doubt causes the puckering often seen on badly scabbed leaves. Scab spores are chiefly lanceolate, varying from narrowly lyOI.] APPLE SCAB. lanceolate to ovate (Fig. 16) . They are widest near the base which tapers somewhat to the truncate end or former point of at- tachment Toward the usually acute apex they taper gradually, but are sometimes slightly constricted giving a somewhat fiddle shape. Usually they are one-celled, though a septum is somtimes developed near the center. In color they are reddish olive and in size generally vary from 12 22 ,"- in length by 6 9 . in greatest width. ON THE FRUIT. On the apples the fungus produces the so- called "scabby" spots (Fig. 5) which are usually l /% to y$ of an inch in diameter though by coalesence they may more or less cover the surface. These develop vigorously on the very young fruit but after the apples obtain some size their growth is slow and usually there is little danger from further serious infection. This is due to the very thick cuticle developed on the fruit. The scab colonies when fresh have a dark olive green color with, usually, a narrow lighter border. The cuticle of the apple becomes ruptured and flaked off, and in time the olive colored fungous cells are also more or less worn away so that the color may assume a reddish brown shade due to the exposed dead tissues. Sections under the microscope show that the development of the fungus is somewhat different from that on the leaf. This and the microscopic appearance caused botanists originally to consider the forms on the leaf and fruit as two distinct fungi. The difference is not so great as it appears and is due chiefly to the greater de- velopment on the fruit of the subcuticular mycelium which in the more advanced stages often forms a tissue many cells deep (Fig. 15d). This vigorous development seems to be due to the very thick cuticle (Fig. 13c) which evidently makes it much more diffi- cult for the penetration of the fungus to the outside (Fig. 14) and as a result there is formed quite a tissue before the conidiophores are pushed through and developed. While at first this subcuticu- lar mycelium is made up of the small, roundish, hyaline cells these grow in size and especially at the outer surface become more or less colored like the conidiophores. In the apple stored over winter, shown in Fig. 15, these cells had all become thus colored and gave the appearance of a resting stage of the mycelium. The action of the fungus often causes more or less of a collapsing or flattening of the host cells beneath and this results in the forma- tion of a sort of corky tissue (Fig. 12b) which affords protection to the plant against evaporation and the entrance of other fungi. Scabby apples that have remained outdoors all winter, because of the general rotting produced by other fungi, show little evidence i6 BULLETIN NO. 67. [December, of scab and there is no ground for supposing- that they are the means of any spring- infection. Stored scabby apples, having- been protected ag-ainst decay, show some evidence that the mycelium at the margins of the colonies is still alive and capable of producing a few spores. ON THE TWIGS, ETC. It has been claimed by some botanical writers that the apple scab fung-us also developed on the young twig-s and by this means was carried over the winter and by the pro- duction of scab spores in the spring- caused the infection of the leaves and fruit. During- the three seasons the writer has had the fung-us under observation very careful examinations have been made of the twigs at different times of the year to determine if this was the case. In only one instance was the fung-us ever found growing- on the branches and that exception was a badly infected crab-apple which had several twig's of the current year's growth considerably infected. On this tree the scab was also found abun- dantly on the leaves and petioles and in a few cases on the pedicles, calyces and styles. The tree was thoroughly examined ag-ain the next spring- but no sig-n of the fung-us was found on the twig-s. It is not uncommon for the scab to occur on the leaf petioles but it never seems to develop down upon the twig-s in such cases. In the spring-, before the appearance of the fung-us, loose spores have been scraped from the twig-s, but it appears hardly probable, if alive, that these are the means of any considerable infection. GERMINATION OF SPORES. Fresh spores placed in water in Van Tieg-hem cells g-enerally show some beginning- to g-erminate inside of twelve hours and by the end of twenty- four hours these may have g-erm-threads several times the leng-th of the spores (Plate I, Fig-. A). The g-erm-thread may emerge from either end or the sides of the spore but it most frequently comes from the pointed or apical end. Usually the g-ermination does not proceed further than to form a simple, rarely slig-htly branched, hyaline, septate thread somewhat narrower than the spore and often many times its length. In apple leaf broth or apple leaf agar the germination is much more vigorous, the germ-threads often becoming branched and more elongated, forming a slight mycelium. Often the ends of the threads or branches become somewhat swollen, more closely septate, and develop short, more or less knobbed, or irregular branches that when cut off by a septum in some instances have a spore-like ap- pearance. These peculiar tips usually become tiuted like the spores (Plate II, Fig. A). Their nature is not very evident. They may possibly be only a sort of resting condition of the mycelium since IQOI.] APPLE SCAB. IIQ frequently they send out more slender hyaline threads as if in ger- mination (Plate II, Fig 1 . C) and these again may develop similar tips. Occasionally in these cultures the mycelium formed true scab spores. These are usually produced on short hyaline side branches not especially differentiated as conidophores. The first spore formed may be pushed aside by the development of a second and often several are seen detached in the neighborhood. These spores are colored and otherwise similar to those produced in nature (Plate I, Fig. D). ARTIFICIAL CULTURES. The writer usually found it rather difficult to obtain cultures of the scab fungus from Petri dish sep- aration cultures of the spores. This was partly due to frequent development of surface bacteria over the agar, especially on fresh apple leaf agar, but chiefly to the very slow development of the scab colonies compared with those of the molds whose spores had been introduced. It usually took from two to four or five days be- fore the scab colonies could be detected by the naked eye. These then showed as very small lax growths entirely embedded in the medium but with time increasing in size and growing somewhat into the air. These colonies when very young could be traced down to the spores. Figure 17 shows in part the mycelium of one of these colonies still attached to the spore. Such colonies while young were transferred to test tubes of old apple leaf agar, new apple leaf agar, beef broth agar, beef broth gelatin, beef broth, beef broth corn meal, old apple leaves, and young apples. The growth in these media differed somewhat, chiefly as follows : ON OLD APPLE LEAF AGAR. The colonies were about \ to ^ inch in diam- eter and composed of a very prominent dense growth of dirty white threads into the air. Spores were rarely found and these disappeared through germination (Fig. 23). ON NEW APPLE LEAF AGAR. The chief difference between this and the above was the more prolific development of spores and the somewhat slighter growth of threads into the air. ON BEEF BROTH AGAR. This medium being lighter colored than either of the above showed a dark growth in the agar. There was also a slight growth of dirty white threads in the air. Spores were produced somewhat, at least at first. (Fig. 22). ON BEEF BROTH GELATIN. The growth in the medium was very dark and in striking contrast to it, while the growth in the air was very slight. Spores were found but usually disappeared through germination. (Fig. 25). IN BEEF BROTH. The growth was confined to the liquid or sides of the tube at surface and was similar to that in the gelatin. (Fig. 24). ON BEEF BROTH CORN MEAL. On this medium the growth most nearly ap- proached the appearance in nature. A dense, short, olive green felt developed on the surface with no growth of dirty white threads in the air. Spores were pro- duced very abundantly and did not disappear through germination. (Fig. 26). 120 BULLETIN NO. 67. [December- ON YOUNG STERLIZED APPLES. The growth usually was not very vigorous and was chiefly at place where agar was introduced with the mycelium. In some cases the dirty white threads spread out over the apple but caused no scab like appearance. ON OLD STERLIZED APPLE LEAVES. The growth here was also chiefly in the region of the introduced agar forming more or less of a prominent tuft of dirty- white threads which ran out somewhat on surface of leaf (Fig. 21). Growths on old leaves were tried in the hope that the Venturia stage would be developed but were not successful in this respect. While these cultures were unsuccessful, in the case where lately fallen apple leaves containing fine colonies of scab were placed in test tubes on moist cotton plugs and left until the next year the Venturia peritheria were found abundant though immature. The mycelial threads of cultures are first hyaline but in time become tinted especially those in the medium or against the glass of the test tube where they often assume a dark olive green color. They vary in diameter chiefly from 2^ v to 5 ;j- and have at first uniform or vacuolate protoplasmic contents (Fig". 19), but in time are apt to be more or less filled with highly refractive granules of varying sizes (Fig. 18). In the young threads the cells are com- paratively longer than in the older, broader ones. In the latter there may occur circular swellings as if due to vigorous osmotic action (Fig. 20; Plate II, Fig. Be). In old dried out cultures highly refractive spore-like bodies, which may possibly have served as chlamydospores (Plate II, Figs. E, F) were sometimes found. ARTIFICIAL INFECTION. Because of the general presence of scab on apple trees artificial infection of leaves out doors is not safe unless on isolated trees not liable to develop scab. Most of the experiments tried were with one or two year old seedlings planted in crocks and kept in doors at a higher temperature than the out- side. In such cases the spores were usually sprayed in water on the leaves, (especially when they were very young) and the moisture prevented from evaporating by covering the trees for a few days with bell jars lined with moist paper. For some reason it was found rather difficult to infect the leaves of such trees, though the conidial stage of the powdery mildew had no difficulty in producing natural infection. Out of the various trials made at different times less than half of the trees produced leaves with scab and never with any abundance. In one instance where very young leaves were sprayed on three successive days with water containing spores and kept for ten days under a moist bell jar there were eventually found only three leaves with a total of five scab colonies. In those cases where colonies developed they were usually first seen about three weeks after the spores had been put on the leaves. The ex- periment was also tried of spraying spores on the unfolding leaves of different varieties of apples whose twigs had been cut off and IQOI.] APPLE SCAB. 121 placed in water under bell-jars but no results were obtained in these cases since the leaves all dropped off by the end of two weeks. VENTURIA OR SAPROPHVTIC STAGE. ON DEAD LEAVES. After the scab infected leaves fall from the trees in the autumn and lose their green color the subcuticular mycelium slowly changes. The rounded cells become larger (eventually 5 10 <>. in diameter) and gradually tinted to a reddish olive color and also give rise to similarly colored mycelial threads that penetrate into the interior of the leaf. In sections made of fallen leaves gathered in October there were seen signs of the for- mation of perithecial bodies and occasionally threads could be traced connecting these with the subcuticular cells. The perethe- cia generally originate in the loose part of the leaf a short distance above the lower epidermal cells, and while still young show an outer covering of colored cells and a few inner hyaline ones. The Venturia sjage is thus produced from the mycelium of the scab stage, the dead leaves allowing easy penetration into the interior and offering no hindrance to further development. From the fact that the p^rithecia most frequently occur on the lower side of leaves and are not found unusually abundant on or near con- spicuously scabby spots, it seems quite probable that the less con- spicuous scab colonies originating in the fall on the under side of the leaves are largely responsible for the development of this stage. The relationship of these two forms is well shown by the fol- lowing data obtained regarding their development. May. Scab first appears on young apple leaves and fruit and during this month and June obtains its greatest foothold. July to September. The warm, generally dry, weather is not very favora- ble for spreading the disease to the leaves, and fruit usually suffers but little from further infection. September and October. Scab appears to develop somewhat more abundantly especially on the lower surfaces of the leaves, but not necessarily in vigorous fruiting condition. October and later. On the fallen dead leaves there are signs of the forma- tion of the perithecia of the Venturia stage. October to April. Perithecia slowly develop as weather conditions prove favorable. April and May. Perithecia with mature ascospores are now found. June. Venturia stage disappears. The Venturia stage was found most abundant the spring of the year apple scab was the worst. After one severe winter it was apparently injured by the cold since many of the perithecia never came to maturity. It was never found on any other leaves than the apple and succeeded best where these were protected by sod, especially where more or less heaped together in fence corners. 122 BULLETIN NO. 6j. [December, The perithecia show on the leaves as small black pustules gen- erally scattered but sometimes congregated in small groups often on greyish spots which may mark the place of the fall scab colony. They frequently occur in the vicinity of the veins. Sometimes they are found on leaves that show no sign of scab to the naked eye. When mature they are more or less loosely imbedded in the leaf tissues and at the time of their disappearance infected leaves often show numerous small holes where they had been im- bedded. The writer has never found them upon the old fruit and only once on the twigs and in this case the single large perithecium was probably blown there. MICROSCOPICAL APPEARANCE. Prom the examination of va- rious specimens the microscopical characters may be given as fol- lows : Perithecia when mature very dark, light scarcely shining through unless crushed and then showing outer wall of dark olive green color, spherical or subspherical, generally 90 150 p. in diameter; reticulations (Pig. 27) not evident 'until perithecia are crushed, irregularly polygonal, 5 15 ,; ostiolum showing at first as a lighter spot, usually on a broad somewhat beaked projection (Fig. 29 and 30); bristles (Fig. 28) absent or surrounding the pro- jection, six or more in number, easily detached, of same color as perithecium, simple, tapering somewhat from base to apex, usually about 35 fj. though varying at least from 25 75 v in length; asci (Fig. 27; Plate III, Pig. A) numerous, at least when mature with- out paraphyses intermixed, without very evident stipe, oblong to spatulate, frequently slightly curved, 55 75 n x 6 12 /*; spores (Fig. 27; Plate III, Fig. B) eight in an ascus, upper single ranked, lower irregularly two ranked, two celled with ratio of length of cells about 2:3, oblong, with upper cell usually slightly wider than lower, light reddish or olive brown, 11 15 ;j- x 5 7 ,. GERMINATION OF SPORES. Spores when placed in . water in Van Tieghem cells germinate rather abundantly within the first twenty-four hours. They become considerably swollen and then send out a germ-thread from one or both cells, but most frequently only from the lower cell. While these threads may originate from any part of the cell they usually come from the free end. They at- tain a length several times the length of the spore, taper somewhat toward free end and become septate at the base (Plate III, Fig. E). In apple leaf agar the germination was more vigorous with the production of the curiously enlarged and irregularly knobbed or branched tinted tips (Plate IV, Fig. C) that were also produced in cultures from scab spores. In one culture the mycelium also gave rise to true scab spores (Plate IV, Fig. D). APPLE SCAB. 123 ARTIFICIAL CULTURES. Colonies were produced from spores in Petri dish separation cultures like those from scab spores, small, lax, and of slow development. So far as tried the growths on va- rious media were similar to those of the scab stage. The fungus did best on apple corn meal where it produced characteristic threads and spores of scab. The culture work with this stage was not carried on very ex- tensively because of the difficulty of finding- spores in 1899 and be- cause in 1900 much of the time during- the season of the fungus was spent working with another fungus supposed to be probably an immature condition of the Venturia. This was finally seen to be Sphaerella and was determined by Ellis as Sphcerella pirina, E. & E. This fungus also is found on the dead leaves, apparently quite common. It occurs most abundantly upon the upper surface and produces more or less grouped perithecia that are smaller and originally not so deeply imbedded in the leaf as those of the Ven- turia. The mature perithecia also lack the bristles that are usually present with the Venturia and the spores differ in being smaller and hyaline. IDENTITY. The apple leaf Venturia has been commonly called Venturia chlorospora (Ces.) Karst., which name was also made to in- clude various forms on the fallen leaves of other hosts, such as Salix, etc. Aderhold has recently shown that these forms are distinct, even those occurring on the closely related hosts pear and cherry being different species from that on the apple. For this latter form he has adopted the name Venturia inceguatis (Cke.) Ad., using the specific name first applied by Cooke. The latter de- scribed this as a new species, Sphcerella incequalis, in Seem. Journ. Bot. in 1866, Niessl in 1881, Rabenhorst's Fungi europaei no. 2663, placed this species under the genus Didymosphaeria as D. incequalis (Cke.) Nessl. Winter, instead of Aderhold, seems to have been the first to place the species under the genus Venturia as the specimens on apple leaves distributed in 1880 in de Thii- men's Mycotheca universalis no. 1544 are called Venturia incequa- lis, Wint. in litt. NOMENCLATURE. Fries was probably the first botanist to give a name to apple scab. About 1819 in Nov. Fl. Suec. he applied the name Spiloccea Pomi to the fructigenous form. In his Syst. Myc. in 1829 he also described this species and this is the reference for the name usually given by botanists. On the same page of this latter work he de- scribed a fungus on the leaves of apple and pear that to the writer 124 BULLETIN NO. 67. [December, seems to be the leaf form of scab, though Sptloccea epiphylla is never given as one of its synonyms. Persoon in 1822 under the name Fumago Mali also described a fungus on the leaves of apple that by some botanists has been questioningly referred to scab. In 1833 Wallroth unquestioningly found the leaf form and named it Cladosporium dendriticum. Fuckel in 1869 placed this under the genus Fusicladium and since that time scab has been commonly called Fusicladium dendriticum (Wallr. ) Fckl. In 1834 Schweinitz described a new species on Newton Pippin apples from this country under the name of Dothidea pomigena.* This has been considered by some botanists as probably another name for the fructigenous form of this fungus. The writer was inclined to this view and so recently examined Schweinitz's specimen in the Herbarium of the Philadelphia Academy of Science. While the specimen was in poor condition it was seen that it was not scab, being more like the fly speck fungus in its macroscopic appearance. Schweinitz, how- ever, did list apple scab from this country under the names Spil- occea fructigena or S. Pomi. He seems to have originated the name fructigena though from his reference one would suppose that Fries was authority for it. The specimen in the Schweinitz col- lection is labeled Spiloccea Pomi. In 1851 Bonorden described a scab fungus under the name of Fusicladium virescens. There is some doubt whether this is a synonym of the apple or the pear scab. Bonorden gives apple as the host for his fungus but his figures more nearly resemble the scab of the pear, and so some botanists give it as a synonym of this. The last person to give a new name to this fungus was de Thiimen who, in 1875, described the form on the fruit as Napicladium Soraueri. Since it has been clearly shown that scab is merely a stage of a Venturia it must now be named under that genus instead of under Fusicladium. Venturia incequalis (Cke.) Wint., seems an appropriate name, though some botanists would use for the spe- cific name the one first given to any of the stages of the fungus, which in this case is Pomi. Another possible objection against the use of the above name is that Cooke's Sphcerella incequalis also included the form on the pear, which is now considered a distinct species. The different names applied to the scab stage, as found by the writer, are as follows : Spiloccea Pomi, Fr. Nov. Fl. Suec. 5 : 79. 1819. ? Fumago Mali, Pers. Myc. Eur. 1:9. Ib22. *Sturgis (Am. Rep. Conn. Agr. Exp. Stat. 21:171) thinks that this descrip- tion applies to what he calls the Sooty Blotch fungus. IQOI. J APPLE SCAB. 125 ? Spilocaa epiphytta, Fr. Syst. Myc. 3 : 504. 1829. Cladosporium dendriticum, Wallr. Fl. Crypt. Germ. 1 2 :169. 1833. Spilocaa fructigena, Schw. N.A.F. : 297. 1834. ? Fusicladium virescens, Bon. Handb. Allg. Myk. : 80. 1851. Fusicladium dendriticum, Fckl. Symb. Myc. : 357. 1869. Napicladium Soraueri, Thura. Hedw. 1 4 : 3-4. 1875. Fusicladium dendriticum f. Pyri Mali, Thiim. Myc. uni. no. 1174. 1878. Fusicladium dendriticum, var. minor. Sacc. fungi Ital.y. 782. 1881. Fusicladium dendriticum var. Soraureri, Sacc. Syll. Fung 1 . 4 : 346. 1886. Fusicladium dendriticum forma microsperma, Routn. Fungi sel. exs. no. 5592. 1890. EXSICCATI. Fusicladium dendriticum (Wallr.) Fckl. has been distributed in a number of different exsiccati of which the following have been examined by the writer and found to be this fungus : Spiloccea Pomi, Fr. on apple fruit. Scler. Suec. exs. no. 260. Cladosporium dendriticum, Wallr. on apple leaves. Herb. crypt, belg. no. 695. Cladosporium dendriticum, Wallr. on Pyrus Malus. Klotz. herb. viv. myc, no. 1883. Cladosporium dendriticum, Wallr. on leaves Pyrus. Klotz. herb. viv. myc. II. no. 766. Napicladium Soraueri, Thiim. on apple fruit. Thiim. Myc. uni. no. 91. Napicladium Soraueri, Thiim. on apple fruit. Roum. Fung. sel. Gall. exs. no. 154. Fusicladium dendriticum forma microsperma, on leaves of apple. Roum. Fung. sel. exs. no. 5592. Fusicladium dendriticum f . Pyri Mali, on leaves Pyrus Malus. Thiim. Myc. uni. no. 1174. Fasicladium dendriticum (Wallr.) Fckl. on leaves Pyrus Malus. Sey. & Earle Eco. Fungi no. 38. Fusicladium dendriticum (Wallr.) Fckl. on fruit Pyrus Io- wensis. Sey. & Earle Eco. Fungi, no. 420. Fusicladium dendriticum, Wallr. on leaves Pyrus Malus. Ell. N. A. F. no. 372b. Fusicladium dendriticum (Wallr.) on leaves Pyrus Malus. Ell. & Ev. Fungi. Col. no. 464. 126 BULLETIN NO. 67. {December, Fusicladium dendriticum (Wallr.) Fckl. on leaves Pyrus Malus. Briosi & Cavara Fung-, par. no. 140. Fusicladium dendriticum (Wallr.) on leaves Pirns pruni folia. Rab-Wint-Paz. Fungi eur. no. 3997. Besides these there were found a number of specimens distrib- uted on Crataegus and Sorbus under various names that have the appearance of this fungus, but which were not included with the above because of lack of knowledge concerning- their permanent stage. BIBLIOGRAPHY. Apple scab has been the subject for much writing on the part of botanists and horticulturists. This is due to its wide distribu- tion and the considerable injury that it causes. Much of this literature relates to experiments for the prevention of its ravages, especially by the use of fungicides. The following references relate to articles and notes that the writer has indexed from his examination of such literature as was at his disposal. No doubt many more references could be added to the list: Aderhold, R. Die Perithecienform von Fusicladium dendriticum (Venturia chlorospora f. Mali). Ber. Deutsch. Bot. Ges. 12: 338-342. 1894. Shows the relationship between F. dendriticum on living apple leaves and V. chlorospora f. Mali the permanent stage on the dead leaves. Aderhold, R. Litterarische Berichtigung zu dern Aufsatze ueberdie Perithecien- form von Fusicladium dendriticum, Wallr. Ber. Deutsch. Bot. Ges. 13: 54 - 5. i8 Q 5. Speaks of earlier investigations connecting Fusicladium dendriticum with Venturia stage. Aderhold, R. Die Fusicladien unserer Obstbaume. Landw. Jahrb. 25 : 875 - 914. 1896. [Illustr.] Ibid. 29 : 541 - 587. 1900. [Illustr.] Gives detailed accounts of the scabs of apples, pear and cherry and shows their relation to Venturia stages on dead leaves of these hosts. Aderhold, R. Revision der species Venturia chlorospora, incequalis und ditricha autorum. Hedw. 36 : 81. 1897. [Illustr.] Describes the different species of Venturia and gives their hyphomycetous stages, placing Fusicladium dendriticiim under Venturia inceqiialis (Cooke) Ad. Alwood, W. B. Fusicladium dendriticttm. Virg. Agr. Exp. Stat. Bull. 24 124. 1893. Mentions this as cause of one of the serious apple diseases in this state. Andre, Ed. Les Fusicladium et nos Vergers. Rev. Hort. 60 : 246. 1888. Gives short description of apple scab. Anon. Apple and Pear Scab. N. Y. Agr. Exp. Stat. Bull. 35 1606-7. 1891. Describes briefly effect of fungus and recommends preventive treatment by spraying. 1001.] APPLE SCAB. 127 Anon. Treating Apple Orchard for Scab. N. Y. Agr. Exp. Stat. Bull. 84 : 19-20, 31-33. 1895. Recommends time for spraying and notes varieties injured by Bordeaux mix- ture. Anon. Apple Scab. Year Book, U. S. Dept. Agr. 1894 : 577. Ibid. 1 895 : 587. Ibid. 1 896 : 625 . Ibid. 1 897 : 673. Gives treatment for prevention of scab. Arthur, J C. Apple Scab and Leaf Blight. Ann. Rep. N. Y. Agr. Exp. Stat. 3 : 370. 1885. Gives short note on damage caused by this fungus. Bailey, L. H. Scab-proof Apples. Card. & For. 5 : 442. 1892. States that there is no marketable scab-proof variety, Baldwin the least susceptible. Bailey, L. H. The Recent Apple Failures. Corn. Agr. Exp. Stat. Bull. 84 : 58- 63,66-8. 1895. [Ulustr.] Attributes apple scab as chief cause of failures and discusses the fungus and methods of combating it. Beach, S. A. Wood Ashes and Apple Scab. N. Y. Agr. Exp. Stat. Bull. 140 : 665-90. 1897. Reports experiments of applying wood ashes to the soil as not preventing scab. Beach, S. A. and Paddock W. Apple Scab. Ann. Rep. N. Y. Agr. Exp. Stat. 14:345-7- 1896. Discuss briefly apple scab and prevention by Bordeaux mixture. Beach, S. A., Lowe, V. H., and Stewart, F. C. Scab. N. Y. Agr, Exp. Stat. 170:385-7. 1899. Give short account of life history and methods of prevention. Berkeley, M J. Why Do Pears and Apples Crack ? Card. Chron. 1855. 724. 1855. States that this due to Spiloccea Pomi, the fructigenous form of Helmintho- sporium Pirorum, but does not recognize specific difference between the forms on apple and pear. Bizzozero, G. Fusicladium dendriticum var. minor, Sacc. Fl. Ven. Crittog. 1 =510. 1885. Follows Saccardo in giving form on apple as variety of that on pear. Blair, J. C. Spraying Apple Trees with special reference to Apple Scab Fun- gus. 111. Agr. Exp. Stat. Bull. 54 : 181 -204. 1899. [Illustr.] Gives results of spraying, directions for making fungicide, description of ma- chinery, etc. Bonorden, H F. Fusicladiitm virescens. Handb Allg. Myk. : 80. /./,/. 94. 1851. Describes this as a new genus and species and says in part "Kommt in Garten auf veredelten Apfelbaumen vor." Winter gives this as a synonym of pear scab and Bonorden's figures certainly more nearly resemble that species than the one on the apple. Brefeld, O. Venturia ditricha f. Piri. Unter. Gesammt. Mykol heft. 10 : 221. /. 7,f.5Q, 60. 1891. Shows similarity of a stage produced from above fungus to the scab of pear and apple. 128 BULLETIN NO. 67. [December, Briosi and Cavara Fusicladium dendriticum (Wallr.) Fckl. Fung. par. no 140. 1892. Give illustrations and specimens of this fungus together witha short description. Burrill, T. J. Notes on Parasitic Fungi. Agr. Rev: 86. .1882. Gives an account of apple scab and two other fungi. Burriil, T. J. An Orchard Scourge. Trans. Miss. Vail. Hort. Soc. 1:202-7. 1883. Gives an account of apple scab and the damage caused in Illinois. Burrill, T. J. The Apple Scab Fungus. Trans. 111. Hort. Soc. 1900:86-97- 1901. Writes a popular article on our present knowledge of apple scab. Butz, G. C. Apple Scab. Penn. Agr. Exp. Stat. Bull. 43 : 13, 16. 1898. Treats of apple scab and its prevention by spraying. Card, F. W Apple Scab in Nebraska. Card. & For. 8 :28. 1895. Thinks little scab found in .this state is due to dry weather. Chester, F. D The Scab of the Apple and Pear. Del. Agr. Exp. Stat. Bull. 3:6-7. 1888. Gives short note on above including preventive measures. Chester, F. D. Experiment in the Treatment of Apple Scab. Del. Agr. Exp. Stat. Bull. 29:18-24. 1895. Gives favorable results from spraying with Bordeaux mixture. Chester, F. D. Experiment in the Treatment of Apple Scab. Del. Agr. Exp. Stat. Bull.'34:i4-9. 1897. [Illustr.] Records favorable results from spraying with Bordeaux mixture. Chester, F. D. Continuation of the Work on the Treatment of Apple Scab. Ann. Rep. Del. Agr. Exp. Stat. 11 :27-3o. 1900. Describes successful spraying experiments with Bordeaux mixture. Clark, J W. Spraying for the Codling Moth and Apple Scab. Mo. Agr. Exp. Stat. Bull. 13:6. 1891. Reports favorable results from spraying with Bordeaux mixture. Close, C. P. Apple Scab. Utah Agr. Exp. Stat. Bull. 65 : 67 8. 1900. Gives note on appearance of scab and remedy. Cobb, N. A. Apple Scab. Agr. Gaz. N. S. Wales 2 : 216, 492. 1891. Reports this on increase and suggests use of fungicides. Cobb. N. A. Apple Scab, "Tasmanian Black Spot." Agr. Gaz. N. S. Wales 3 : 276-8; 1892. [Illustr.] Gives short botanical account of the fungus and of the best fungicides for prevention. Collier, P. Apple Scab and Pear Scab. Ann. Rep. N. Y. Agr. Exp. Stat. 9 : 337-9. 1891. Describes effect of this on leaves and fruit and suggests preventives. Comes, O. Fusicladium dentritician, Fck. Critt. Agr. : 394. 1891. Gives short account of this fungus. Cooke, M. C. Spharella incequalis, Cke. Seem. Journ. Bot. 4 :248-9, 1866. [Illustr.] Describes this as a new species on various hosts of the Pomaceae. Cooke, M. C. Spha-rellaina:qualis t .\i.^, . - / \LJ y, J _ v r ?._/ ,y ^c. ^ s lt<^ HTci&ft i >