IHV ^^^^^^^^^M SB 732 Iff ^^^^^^^^^M .H4 -i j- 1 ^M ■i ; 1 1 W^V\^^'"^^ °^^^\/'\ ^^K*\^^^% "-%^*' /'\ N^r. '* ^^^ \by rO^ ^^0^ ^0 .^^. V •- ^^•^^. BULLETIN 318-110-15h OF THE UNIVERSITY OF TEXAS No. 135 ISSUED SEMI-MONTHLY SCIENTIFIC SERIES NO. 14 NOVEMBER 15, 1909 Symptoms of Disease in Plants BY FREDERICK D. HEALD Professor of Botany in the University of Texas PUBLISHED BY THE UNIVERSITY OF TEXAS Entered as second-class mail matter at the postoffice at Austin, Texas Cultivated mind is the guardian genius of democracy, . . . It is the only dic- tator that freemen acknowledge and the only security that freemen desire. President Mirabeau B. Lamar. h2 L^ar-' ?'■■'■"! MAV 19 19ie SYMPTOMS OF DISEASE IN PLANTS It is luglily iiuportniit that nil persons cnoa.u'ed in <;cncral farming, truck gardening, fniit raising, or tioiiculture should Ik al»lo to recognize the presence of disease. In their ninte way plants tell ns when they are suffering from constitutional or contagions diseases if we are only able to interiiret their lang-^uige. It is evident that many people do not j'ecog- nize the presence of diseases, for it is not an nncommon thing to receive replies like the following to the letters of inquiry that are sent out over the state: "There are no plant diseases present in our locality"; "All the crops in our region are free from diseases" ; "No diseases in our country." The diseases that atfect plants may l)e divided into three different groups: first, those disturbances of nutrition which are due to unfavor- able conditions in the environment of the plant, or to the conditions in- herent in the ])lant itself; second, trouliles due to the presence of para- sitic plants, generally either bacteria or fungi, but sometimes to flower- ing ])lants which deform, stunt, or kill the affected plant by their pres- ence; third, those "caused by insect pests, which are in many cases very evident, and in other cases not easily detected. Many of these insect troubles are hardly to be classed as plant diseases, while many are as truly diseases as those produced by parasitic fungi. In the nature of the resulting symptoms, there are no sharp and fast lines that can be drawn between many insect and fungus troubles, and often a careful micro- scopic examination is necessary in order to diagnose the disease. The first thing of importance for the farmer is to detect the ]iresence of the disease and then to endeavor to find out its cause, nature, and probable outcome and the treatment which should be employed, by consulting those who have made the subject a special study. Before taking up the main part of this paper, which will deal prin- cipallv with the symptoms of diseases produced by bacteria and fungi, l)rief mention may be made of a number of flowering ])lants which are parasitic upon other flowering plants. The best example in this class is afforded by the various species oP dodder (Cvscuta), twining vines, devoid of chlorophyll or leaf green and possessing only rudimentary leaves. Dodder twines around the stems of the host plants much as a morning-glory twines around its support. The dodder is devoid of chlorophyll and is not attached to the ground by roots, so that it must ol)tain all of its food by robbing the host upon which it is growing. It does this bv sending its sucking organs or ■4 University of Texas liavstoria into the stems around which it grows. One of our cultivated plants seriously infested with dodder is alfalfa. Seeds of four different .species of dodder may be found in alfalfa seed, and several of the species Fig. 1. — Dodder or "Love Vino," a serious parasite upon alfalfa. Original. are reported as doirg eonsidoral)l(' liarm in various parts of the alfalfa- growing territory. The dodder is a rampant grower, and soon kills Symptoms of Disease in Plants 5- alfalfa l)y rcibhinc" it of food and smothering it with its dense growth of intertangling stems. Many of onr forest and shade trees are attacked by another type of parasite, the mistletoe (Phoradendron sp.). This is a parasite which pos- sesses green coloring matter and is consequently able to supply a part of Fig. 2. — Trees badly infested with mistletoe (Phoradendron flavescens) Photograph in School of Botany collection. its own food. The mistletoe does, however, stunt and deform the host plant on which it is living, the amount of injury depending upon the extent of the infection. The distinction between true parasites and epiphijtes, which merely find lodgment on the surface of the plant on which they are growing,. 6 University of Texas should 1)G kept in mind, since several such plants are common in this region. The so-called '"long moss" {Bendropogon usneoides) which grows npon various f6rest and shade trees is a familiar example of such an epiphyte. This plant is not a moss as most people suppose, hut is a flowering plant, wdiich merely finds lodgment upon the branches of trees Fig. 3. — Tree completely covered witli "liall ]\Ioss" (Tillandsia reciirvata). Photogra])li t'ron: School of Botany collection. and does not rol) them of any of tlioir accumulated food material. The "short moss'' or "ball moss" {Tillandsia reciirvata) is equally familiar, and in this section of the state is more conspicuous upon park and shade trees occu])ying the liigher locations, while tbe ""long moss" is ])redomi- nant on trees occui)ying tbe valleys. Botli of these rpipJiytcs probal)ly SynipUjins of Disease in Plants 7 cause injury to tlic trees wliieh liai'hor them, hut this can only be the case when the "moss" becomes especially abundant. By crowding tlie foliage of the tree, checking the development of yonng shoots, and harboring in- sect pests, the "long moss" or "short moss" may be detrimental to the life of the trees npon which tliey are growing, Imt they should never be looked upon as parasites. The following outline will give a survey of tlu' ])rincii)al symptoms of disease in ])lants, placing most empliasis on those diseases due to bacteria and fungi ; 1. Discoloration or change of color from the normal. (a) Pallor. Yellowish or white instead of the normal green. (b) Colored spots or areas on leaves or stems. AVhitish or grey: mildews ; white rusts, etc. Yellow: many leaf spots. Eed or orange: rusts, leaf spots, etc. Brown: many leaf spots. Black: black rust, tar spots, etc. Variegated : leaf spots, etc. 2. Shot-hole: perforation of leaves. 3. Wilting: "damping off." "wilt," etc. 4. jSTecrosis: death of parts, as leaves, twigs, stems, etc. 5. Reduction in size: dwarfing or atrophy. 6. Increase in size : hypertrophy. 7. Eeplacement of organs by a new structure. 8. Mummification. 0. Change of position. 10. Destruction of organs. 11. Excrescences and uuilformations. Galls : pustules, tumors, corky outgrowths, crown galls, etc. Cankers: malformations in the bark generally resulting in an open wound. Punks or conchs and other fruits of fleshy fungi. Witches' l)rooms. Rosettes and hairy root. 12. Exuflations. Slime flux. Gummosls: especially for stone fruits. Resinosis: especially for coniferous trees. 13. Rotting: Dry rot and soft rof : the "gangrene" of plant tissue. Root rots: alfalfa, cotton, beets, cherry, etc., generally woodv or fleshv roots. 8 University of Texas Stem or trunk: dry rot of trees; rot of modified stems like rliizomes, Ijulbs, or tubers. Buds. Fruits: flesliy fruits of various kinds. It is quite common to find the foliage of trees in the prairie regions showing a pronounced yellow cast instead of the normal green color. This yellowing is especially prominent in the regions where the soil is decidedly alkaline or where there is a seepage of ground water from high irrigation ditches to a lower level, or where both of these conditions pre- vail. The conspicuous sickly yellow corn plants that may be found in Fig. 4. — Powdery mildew (Mierosphaera sp.) of tlie lioney suckle. The leaf nearljr covered by the fungus shows groups of minute black bodies, the spore -fruits or perithecia. Original. the early part of the season upon flooded fields or in water-logged soils is another example of this marked symptom of disease. The lack of sufficient iron in the soil will also cause a plant to pass into what is technically termed a chlorotic condition, that is, the leaf green or chlo- rophyll is either not developed at all or only to a slight extent. These chlorotic plants can often be restored to their nornial condition by supply- ing them with iron. The occasional white plants that appear in corn fields are some of our plant "albinos," and are suffering from rathe]' complex disturbances of their nutrition. Anotlier physiological trouble, the "mosaic disease," common in the tomato and the tobacco plants, is Sympiom.s of Disease in Plants 9 characterized by the mottled appearance of tlie leaves, or, in other words, the leaf is somewhat variegated with yellow and green. When the leaf shows colored areas or spots that are more or less definite and circnmscribed, and different from any normal coloration we may be fairly certain that it is suffering from cither an insect or fungus disease, with the ])rcsum]>tion in favor of the latter. Fig. 5. — Crimson rambler rose showing powdery mildew (Sphaerotheca pannosa) on stem and leaves. The leaves are curled and reduced in size. Original. White patches more or less definite or somewhat diffuse are generally due to either some of the "Powdery Mildews" or to the "Downy Mil- dews." We have a good illustration of this symptom in the powdery mildews of the apple, peach, cherry, grape, gooseberry, wheat and other grasses, the elm, catalpa, mesquite, the lilac, honeysuckle, crimson ram- bler and other roses, golden glow, etc. In all of these cases the fungus 10 University of Texas tliat causes the disease Lives on the surface of the host plant, or is what may be termed an external parasite. During the first part of the season the surface of the leaves ot stems of the host plant is covered with an in- terlacing tangle of minute wliite fungus fiUimcnts. and the powdery ap- pearance is duo to the production of large numbers of minute repro- Fig. 6. — Twig from a young cherry tree infected with powdery mihlew. {Podo- sphaera oxycanthae). The mildew is very abundant on the lUider surface of the leaves and causes the leaf blades to roll. Original. ductive bodies or .spores, wliicli Ijccome lioaiied up on tlie wliite patches. Later in the season many of these "Powderv ^fiblews" |)roducc winter fruits, wliicli sbow as small black bodies scattered over the wbite ])atche3 or grouped in special clusters. (Fig. 4.) In some of the ''Powdery Mildews" tbe presence of the fungous para- Symptoms of Disease in Plants 11 site causes more or less discoloration of the atfected leaves togellier with more or less curling or rolling. This is especially marked in tlie powdery mildew of the cherry. (Fig. G.) In the powdery mildew of many roses, the leaves are cui-led and at the same time stunted or dwarfed in si;^e. In the downy mildew of the grape, wliich is an internal parasite, wo often find the under side of tlu' leaf covei'cd with irreuiilar wliitisli naTclies Fig. 7. — Leaf of grape vine showing the coiisjjicuoiis patches of downy miklew (Peronospora viticohi) upon the under surface. Original. which arc formed hy tlio spore-producing parts of the fungus that einerge from the interior of the leaf. It may be mentioned in this connection that the upper side of the leaf, directly over these spots is at first a yellow color, but later changes to a dark brown. When the disease is very jircva- lent, the leaves may turn brown throughout, shrivel somewhat, and fall from the vines, or thev may be cast while thev are still more or less 12 University of Texas mottled with green, yellow, and brown areas. Other downy mildews showing somewhat the same symptoms may be found on cucumbers, let- tuce, spinach, and onion among garden vegetables, and upon some of our forage crops like millet, clover, and alfalfa. In some diseases which are not due to "Powdery" or "Downy" mil- dews, the appearance of white patches upon the under surface of the leaves with yellow areas upon the opposing surface is characteristic. This is especially noticeable in a serious leaf-spot disease of the parsnip. (Fig. 8.) Fig. S. — Poiiion of parsnip leaf affected by fungus (Cercosporella sp.), which produces conspicuous white spots upon the under surface. Original. The "white rusts" generally show more definite wdiitish pustules which eventually rupture and allow the escape of the spores. The white rusts may be observed upon cress, the sweet potato, morning glory, turnips, radish, cabbage, horse-radish and related plants, and also upon the leaves of the oyster plant. The white rust of the oyster plant may be so severe as almost to kill the tops and thus greatly reduce the development of the roots. In some diseases the affected spots are of a grayish color, either uni- form throughout or with margin or center of a different color. In a leaf Symptoms of Disease in Plants 13 disease of the elm, the leaf is covered with irregular grayish patches with dark or black, somewhat papillate centers, while parts of the same leaf may be brown and dead. In the leaf-spot of the common strawberry the Pig. 9._Leaves of oyster plant affected with white rust (Albugo tragopogonis). The tips of some of the leaves have turned brown and have begun to curl. Original. fungus causes numerous circular grayish spots surrounded by quite defi- nite purple borders. A disease of the prickly pear, called anthracnose, is not uncommon in the Southwest. Circular grayish, slightly sunken, 14 Universittj of Texas patches, one-half inch or more in diameter, witli dark or black centers, are qnite characteristic of this disease. In an anthrocnose of the box. the mature spots are grayish, one-fourth to one-half inch or more in diam- eter, and surrounded by a narrow brown border, or show more or less con- centric zonation. In the early stages of the disease the spots are yellow and turn gray with the development of the disease. Many fungi tbat inhal)it leaves have a localized effect and produce more Fig. 10. — Leaves of the cork-winged elm showing abundant scab spots (Gnomonia nlmea ) . x 2. Orioinal. or less circular or slightly irregular spots of a nuirked yellow color. In such cases the disease is generally characterized as a "leaf-spot.'' Symp- toms of this kind may be noted in the leaf-spots of tbe oak and violet, and in the antJiracnose of cucumbers and melons. Sometimes the change in color becomes more general, and the whole leaf turns yellow and falls from the tree as in the early summer stage of apple scab. Another marked symptom of disease is the occurrence of red or orange i:^ijiit [itdiiifi of Disease in /'hi ills 15 sjiots or [lustulc's. The "Ihists" of our cereals and otlier crops received their name from the fact that in the early stage of the attack, reddish or brownish spore-producing- pustules give the stems or leaves a rusty ap- pearance. Sometimes these rust spots may l)e few in miiiihcr, at other times so numerous that almost the entire surface of leaf or stem is cov- ered, and thus the vigor and productiveness of the crop is seriously im- paired. In all of our cereal rusts, black pustules apjiear on leaves and stems as a later growth. I'he red pustules always predominate and give character to the disease in the "orange rust" of wheat and the "crowned rust" of oats, while the black stage or black rust is the conspicuous stage Fig. 11. — A segment of priekly pear showing several anthracnose spots ( Gloeosporivun sp. ) . Photograph (by F. D. Heald and F. A. Wolf. in the "stem rust" of wheat. Some of the other cultivated plants affected by rusts are the clovers and alfalfa, cotton, beans and peas, beets, aspara- gus, plum, peach, cherry, tig, cottonwood and willow, roses, carnations and chrysanthemums. The cluster-cup stage of many of the rusts pro- duce fairly large, well-defined orange-colored spots in many cases. This symptom is well illustrated in the well-known "cedar rust" of apple leaves, and in the cluster-cup stage of the "crowned rust" of oats on the leaves of the buckthorn. Deeper red or even purple colored sjwts or ex- tended areas indicate the presence of a leaf parasite in many cases. This is often noticed in the early stages of the shot-hole diseases of the cherry 16 University of Texas or the peach, and is especially characteristic in the bacterial blight of sorghum, Johnson grass, or Kaffir corn. In the last-mentioned case the dark red or purple patches are generally somewhat elongated and more or less irregular. The leaves of dandelion affected by rust generally show a reddening of the whole leaf, while the under sides of blue-grass lenves affected with the powdery mildew frequently show spots of a deep purple color. Fig. 12. — Violet leaves affected with leaf-spot ( Alternaria violae) . Pliotograph iby F. D. Heald and F. A. Wolf. The pods of beans frequently become conspicuously "rusted." The presence of rust-colored, sunken patches, either scattered or very almn- 'dant, is characteristic of the so-called anthracnose of beans. The disease is esj>ecially noticeable on the wax-podded varieties, and the majority of farmers and gardeners speak of the trouble as "rust," although the fungus which causes the trouble does not belong the the true rust fungi. Definite brown areas, either small or somewhat extended, are quite cliaracteristic of spot diseases of either leaves or fruits. The leaf-spots Symptoms of Disease in Plants 17 of apple, pear, and plum are common diseases in this and adjoining states. As a result of these troiiljles, the leaves beoin to drop and a pre- Fig. 13. — Leaves of Johnson grass affected with hacteriosis (Bacillus sorghi). Photograph by F. A. Wolf. mature defoliation is the result, thus sending the tree into winter rest with an insufficient supply of reserve food for the growth of the follow- 18 University of Texas ^K. y^ M^^ HL^' ' ^ ^d^SSmSM) \ \ \}^^MSBi aSsaBB**^^^^^ *T^fT^ ^^^^^ ^ X^^RilH^^^^^^llBr^ ^ Fig. 14. — Apple leaf from the under surface showing conspicuous patches of rust ( Gymnosporangium juniperi-virginianae) . Original. Fig. 15. — Pods of hlack wax Jieans affected with anthraenose, commonly called "rust" (Colletotrichum lindemuthianuni) . Original. Sympioms of Disease in Plants 19 ing season. As a result of tJie leaf-spot, Ben Davis trees are often de- foliated when other varieties are in full foliage. The leaves of tomato plants often show an abundance of small brown spots. As these spots become more plentiful, the intervening areas turn yellowish, the leaf begins to shrivel and curl and finally drops off. JMany gardeners d(j not recognize this behavior of tomato leaves as a symptom of disease, since it is of such common occurrence. The early blight of potatoes is char- acterized by brownish spots on the leaves that often show more or less of a concentric zonation. As the fungus spreads, the spots may coalesce. Ylg. 10. — Leallets from tomato plants afi'ected witli leaf-spot (Septoria ]yco.persici) . Original. and thus the entire leaf will be killed. Sometimes the brownish color is not confined to definite spots, but is more general as in the difi^use form of apple-scab, which shows upon either the under or up|oer surface of the leaves. The definite scab spots on the fruits or leaves of apples, pears, or peaches may be gray, brown, dark olive green, or even almost black. More or less deep cracks or fissures often appear in later stages of scab. In the Ascochyta blight of the pea, the leaves and pods become covered with 30 University of Texas Fig. 17. — Peaches showing abundant brown spots sometimes called "peach freckles" (Cladosporium carpophiliim) . Original. more or less circular brown areas of dead tissue. The brown patches may coalesce to form extended dead patches on either leaves or pods. A careful examination of the patches will show minute black specks, the fruits of the fungus, scattered over the middle of the spots. Many other Fig. 18. — Pods of peas showing difi'erent degrees of abundance of the Ascochyta (blight (Ascochyta pisi). Original. Symptoms of Disease in Plants 21 Fig. 19. — Leaf-spot of rhubarb, showing shot-hQle effect and concentric zonation. Original. Fig. 20. — Tar-spot (Rhytisma acerina) of willow and silver maple. Original. <5tO University of Texas examples of a somewhat similar character may be mentioned, as the leaf- S2>ots of beets, watermelon, maple, mulberry, chrysanthemum, rose, rhu- barb, etc. Other troubles, such as spray injury, or sun-scald, may be indicated by brownish discolorations, but in the majority of cases this sjmiptom indicates a fungus disease. The appearance of black spots upon stems or foliage is not an uncom- mon symptom of disease. One of the most striking cases of this char- acter is in the so-called "tar-spots" of our maples, willows, and live oaks. Leaves affected by this trouble show large, irregular l)lotches of blackened tissue — in fact, they look almost as if they had been spattered with drops of tar. When these spots become abundant, the intervening tissue of the Fig 21. — Fly-speck fungus (Leptotliyriuin pomi) on an ai>ple. Original. leaf turns yellow, and the leaves are drop]ied quite early. In severe epi- demics considerable injury may result from these "tar-spots.'' In all of our cereal rusts, the winter stage is indicated by either small black cov- ered pustules or by elongated black streaks having a somewhat powderv appearance. The winter spore pustules of the orange leaf rust of wheat are small and covered, and are generally confined to either leaf blade or sheath, while the winter spore pustules in the stem rust of wheat, rye, or oats are in the form of elongated naked streaks, wliich are more numer- ous on the stem than on any other parts of the plant. In a leaf disease of the honey locust, the leaflets often show numerous small l)lack spots on the under side: in many cases tliese black spots become so numerous Syiitpfonis of Disease in Phnils 23 24 University of Texas that they coalesce and cover the entire surface of the leaflet, hence the common name "black leaf" disease. Another fungus trouble that is quite common in orchards is the so-called "fly-speck" fungus of the apple. Fruits attacked by this fungus show small clusters of circular black spots which on account of their characteristic appearance have suggested the bo 5 common name. This fungus is more common and evident on the yellow skinned varieties. The "sooty" fungus produces larger, and more diffuse, smoky brown, or almost black, irregular patches upon the epidermis of apples. In some of the anthracnoses the affected areas are circular in outline Symptoms of Disease in Plants 25 and show concentric circles of minute black spots, the fruiting pustnles of the fungus which causes the trouble. It has been shown that this zonation is flue to the alternation of light and darkness. The anthracnose of the box and of some of the squashes and gourds, and also the an- thracnose of the tomato afford excellent examples of this symptom. Leaves which show circular or sometimes irregular perforations have probably been affected by some fungus parasite. In many cases leaves so affected will show brown spots of dead tissue which have not yet dropped out. Several diseases have been called "shot-hole" diseases on account of this peculiar effect upon the foliage. Notable among these troubles is the shot-hole disease of the cherry and plum. This is some- Fig. 24. — ^Sliot-hole disease ( Cercospora sp.) of tlie Virginia creeper. Original. times called "rust" by orchardists, but the true cherry rust is an entirely different disease. Nearly all of the English Morello cherry trees in bome sections of the conntry have been killed during the past few years by this so-called "shot-hole" disease. In this disease of the cherry the leaf tissue adjacent to the affected spots frequently shows a marked purple colora- tion, while at certain stages in the development of the disease the inter- vening areas may be distinctly yellow in color. Sometimes the leaves fall from the tree before the perforations are complete, while in other cases the leaves still hanging upon the tree will be found to contain numerous perforations. Both Cercospora and Pliyllosticta spots of the Virginia Creeper fre- 26 University of Texas quently sliow a marked shot-hole effect. The perforations are so numer- ous that in some cases they might easily be mistaken for the work of insects. Such a Ccrcospora spot is shown in the accompanying illustra- tion. (Fig. 24:.) A sudden wilting of a plant which can not be explained by dry condi- tions of soil or air is often due to the attack of a parasite. Both seedlings and mature plants exhibit this symptom, and the loss which is occasioned by diseases having this attendant symptom is often enormous. In a Fig. 2.5. — Shot-Iiole disease (Cylindrosporium padi) of the plum. Original. seedling the disease in Avhieh this symptom is prominent is often called "damping off." The young plant drops dead, as it were, for a fungus parasite has entered the stem at about the ground level, and has struck a vital Ijlow, interrupting essential physiological functions. A disease of cotton seedlings of this nature is prevalent througliout the South, and is known to the planters as "sore-shin." A disease of the squash and related forms called "wilt" is not uncommon. In this trouble the para- site enters tlic stem and grows rampantly in the water vessels or ducts Syinpfoiiis of Disease in Plants 27 until they are completely plugged, and the supply of water to the foliage is interrupted. Often the first indication of the presence of this disease is the sudden wilting of the plant, no previous warning symptom having made its appearance. Fig. 26. — '•Damping off" or '-sore-shin" of cotton seedlings (Rliizoctonia sp.), Each seedling shows a dead brown area at the ground level. Photograph by F. A. Wolf. In some diseases, the death of parts of the plant, as leaves, twigs, stems, flowers, etc., is the first symptom that is observed. In the fire- blight of pears and apples the young terminal twigs with their leaves and 28 University of Texas flowers are killed and even large branches are involved and succnmb. The leaves turn brown and dry up while still hanging upon the tree, and the twig will be found to be dead down to a certain point, which marks the advance of the bacterium which is the cause of the disease. In this disease small gummy drops which consist of myriads of these bacteria, often ooze out from the dead bark. Sometimes the blossoms of plums turn brown and die prematurely as if blasted by frost or fire. This Fit -Pear trees killed by "fire bliglit.'' Tree in the foreground covered with dead leaves, brown and curled. Photograph by F. A. Wolf. blighting of the blossoms is due to a different organism, the brown rot fungus, which causes the rotting of peaches, plums, cherries, and some- times apples later in the season. In many diseases a more or less marked reduction in size, a dwarfing or atrophy of the whole plant or some of its organs, is evident. A crippled and deformed individual or a poorly developed organ, with im- poverished nutrition, is the result in many coses. The dwarfing or re- Symptoms of Disease in PlanU 39 m " to ^ 3 30 University of Texas duction in size of apples by attacks of scab is often prominent. Many apples seriously affected do not reach over one-fourth the natural size, while others are gnarly and deformed, owino; to localized atrophy of the tissues of the fruit. The same may be said of apples aifected by "cedar- rust," although perlmps the effect is not as striking as in apple scab. In a disease of the peach, known as "little peach," a marked reduction in size of the fruit is one of the attendant and characteristic symptoms. Every farmer is familiar with the shrunken and shriveled appearance of wheat from badly rusted fields. The energy of the plant is sapped by the rust fungus, and as a result the grains are poorly developed, lessening Syirrptoins of Disease in Plants 31 32 University of Texas the yield and qualit}'- of the crop. The dwarfing of stem, leaves, roots, flowers, or flower parts, or even the complete arrest of seed development, is often the result of the inroads of some parasitic fungus. While a large number of fungi produce more or less extensive atrophy or dwarfing of parts of their host, others cause abnormal enlargements of organs or parts of organs. The so-called "plum pockets" or "fools" is a Tig. 31. — Wheat grass affected witlh ergot ( Claviceips purpurea). Original. •striking illustration of hypertropliy. In this disease the outer layers of the fruit become thick and fleshy, while the pit remains undeveloped, causing the modified fruit to be hollow, hence the other common name of "bladder plums" which is sometimes employed. The flower parts of the radish and related plants are frequently enlarged and deformed as a re- sult of the attacks of white rust, while the catkins of some of our trees Syinplonis of Disease in Plants 33 shoM' striking modilications of size and cdlor due to the stininlatino' effect of their fungns guests. The twigs, petioles, and leaves of our com- mon ash often show orange-colored enlargements due to the growth of the cluster-cup stage of a rust fungus. The vegetative parts of plants, as stems, leaves, and roots, often exhibit peculiar enlargements, but most of these may be considered under the head of excrescences and malforma- tions. It sometimes happens that a parasitic fungus produces a new structure which takes the jilaee of some normal organs of (he plant, as in the so- Yig. 32. — ^Rye grass afl'ected with ergot (Claviceps ,purpurea ) . Original. called "ergots'' of our wild and cultivated grasses and rye. These ergots are horny resting bodies or sderotia of the fungus that occupy the same position as the kernel or seed, and appear only with the destruction of the seed-producing structure. These ergots not only affect the seed pro- duction of the host plant, but they contain poisonous principles which produce serious disease when ergot-infested hay is fed to cattle. The production of "mummies'' is another characteristic symptom of disease in fruit trees. Apples that are l^adly affected with either brown rot or black rot often dry up slowly and remain hanging on the tree over 34 University of Texas winier in a more or less hard, shriveled condition. These mummies at the end of the winter period of rest may produce a crop of spores v/hich Yvill spread the disease. This spore-production may take place while the mummies are still hanging on the tree or on the fallen mummies. The formation of mummies is especially common in plums and peaches that are affected with the brown rot. It must he at once evident that mum- mies should be destroyed in order to prevent production of new crops of spores and thus to lessen the ravages of these rot-producing diseases in an orchard. The practice of allowing rotting plums or peaches to fall to the ground and remain beneath the tree should certainly he discouraged. Change of position is a symptom of disease that is sometimes over- looked. This is well illustrated in some plants which are affected with Fig. 33. — Normal apple and "mmiimy'" produced by the black-rot fungus (Spliaeropsis maloruni). Photograph 'by F. D. Heald and Leva Walker. a rust fungus, in the cluster-cup stage. Some of the spurges which are normally more or less prostrate or creeping become erect when attacked by a fungus of the kind mentioned. The common garden weed known as "pusley" grows normally in a prostrate condition, but when it is at- tacked by the "white rust," many of the seriously affected branches be- come more or less erect or ascending. This same symptom is present in some tree diseases, in which normally horizontal limbs or branches become more or less erect. As a result of the attacks of a fungus parasite a complete destruction of organs may result. This effect is well illustrated in the majority of our cereal smuts. In the loose smut of wheat, for example, the complete inflorescence is destroyed, the glumes and other flower parts being re- duced to a powdery mass of black material, the smut spores, Avhich Symptoms of Disease in Flanis 35 finally drop away, leaving nothing Imt the bare central axis of tlie head. The same effect with bnt little deviation may be noted in the naked and covered smuts of barley, and the loose smut of oats, while in the kernel smut of sorghum, the kernel smut of oats, and the bunt of wheat, it is Fig. 34. — 'Winter barley afl'ected with loose or naked smut (Ustilago nuda). Original. the berry alone M'hicli is destroyed, the surrounding parts remaining in- tact. In the last mentioned cases the berry or "seed" may show an in- crease of size or a modification of form Avith the complete destruction of all its tissue except a surrounding membrane which serves to confine the 36 University of Texas mass of smut spores. When the membrane is rui)tured, the interior brown mass crmnbles to powder, since it is simply a loose aggregate of spores which have been formed at the expense of embryo and endosperm. In ■sorghum affected with the kernel-smut each grain or kernel of the dis- eased head or inflorescence is destroyed and the whole head has a much more compact form than is characteristic for the normal inf!orescence. AVheat fields in which l)unt or stinking smut is present emit a character- istic odor, and an examination of affected heads will show that each Fig. 35. — Winter barley affected Avitli covered smut (Ustilago hordei), smutted head has all of its "berries" destroyed, and further that all heads from a given "stool'' or plant are invaded and destroyed if the fungus is present in any. Wheat from fields infested with bimt is often dark in color due to the immense numbers of spores lodged upon the sur- face of tlie kernels and especially collected in the ■'lirush'' or tuft of liairs at the distal ends of the berries. In many cases wheat is so seriously infected with bunt that the yield is enormously reduced and the market value of the actual vield materiallv decreased. Symptoms of Disease in Plants 37 As a result of Iho stiimilalinti- effect of a fungus yiarasite, parts or organs of the host phmt may exhibit variously formed excrescences or malformations. These malformations may he in tlie form of pustules or small hlister-like eknations upon the surface of the lenf or stem, as in the "white rusts,'"' or the blistered areas may be quite extensive and cause more or less deforming and rolling of the leaf as in peach 'leaf- curl." Sometimes the ahnormal formation is in the form of a smui Fig. 30. — Normal and smutted heads of sorghum; normal kernel and smutted kernel (Sphacelotheca sorghi). Original. mass or tumor which, when mature, is filled with a brown or black powder, the spores of the fungus. In the smutted corn plant these tumors may occur on any part of the plant, not a single aerial organ l>eing exempt, although the majority of people think of corn smut as affecting only the ears or tassels. The head-smut of sorghum simu- lates corn smut somewhat in general external appearance, l)ut in this case the whole inflorescence is destroyed by the time it emerges from the leaf sheath. The so-called "cedar apple" is a good illustrarion of a fungus gall. 38 University of Texas Fig. 37. — Bunt or stinking smut of wheat, (a) whole head affected with smut; (,b) smutted grains; (c) normal grains; (d) smutted grain broken to show smut; (e) normal grain divided in the middle; (f) spores much enlarged; (g) germination of a spore. From a drawing by Venus W. Pool. Symptoms of Disease in Plants 39 These brown cedar apples may be present on tlio cedar trees in largo numbers in case of trees that stand adjacent to an apple orchard, and they may vary in size from about that of a radish seed to nearly two inches in diameter. Badly affected cedar trees may be so covered with these galls that many of the branches droop with tlie added weight ]ike Fig. 38. — Heads of wheat wliieh have heen dwarfed and blighted by scab (Fusarium culmorum). The head overgrown iwith the cottony mycelium was kept in a damp chamiber over night. Original. the branches of a heavily laden fruit tree. With the onset of the first warm rains of spring, the ''cedar apples" produce numerous orange- colored projections which stand out in all directions and thus give rise to the characteristic gelatinous rosette-like structures. I have known peoph:" to admire the "cedar apples" as tlie true fruit of the cedar, or to 40 University of Texas w Si/mptoniM of Disease in Plants 41 look upon the gelatinous rosettes as the flowers of the cedar, little realiz- ing that the structures are not nonual, but due to a parasitic fungus which may often seriously affect the lilc of the cedar and greatly impair the productiveness of adjacent a|)ple orchards. The gelatinous projec- Fig. 40. — Head siimt ( Spliaoelotheea reiliana) of sorghum. Original. tions soon dry up and leave the old dead galls hanging up(m the branches. Here may also be mentioned tlie disease of plums and cherries known as "black-knot." This disease is often not noticed until the conspicu- ous black enlaroemcnts liecomc evident later in tlie season. The enlarge- 42 University of Texas Fig. 41. — ^Cedar apples (Gymnosporangiiim juniperi-virginianse) on the common ceaar, one mature and the other from the previous season. Original. Fig. 4-2. — Cedar apples showing the production of the gelatinous sori. Spring stage. Orioinal. Symptoms of Disease in Plants 43 ments are somewhat irregular, roughened, and generally extend for some distance along the length of the affected twigs. When young or in the early S2>]"ing the newly formed knots are olive green in color. In this Fig. 43. — Black knot (Plowriglitia morbosa) on the Burbank plum. Original. condition they produce a crop of spores that spreads the disease, while later the older knots produce a second kind of spores. The fungus that causes the black^knot lives perennially in the twigs and branches 44 University of Texas Fig. 44. — Potato showing;' ileep sc.ib and surt'aco spots (Oos|)ora sca'bies). Original. and consequently new growths appear each year unless the affected limbs are pruned off'. In several sections of the United States plum orchards are seriously affected, and it is not uncommon to find the wild plums in many regions covered with malformations of the character described. Fig. 45. — Crown-gall on the iieacli. Original. Syiiiiitoiiis of Duoasc in Flanin 45 In some fiuiii'ons diseases the iiiair()nii;!ti]]fvinoia nieoalonia) . Orioinal. the opinion that this last type of root-rot is induced primarily by eitlier unfavorable soil or climatic factors, or both, which so lowers the tone and vigor of the plants that various saprophytic forms come in and complete the work of destruction. Tm'o different root-rots of cotton are known. One of these is very common in Texas and has received the common name of the "Texas root-rot." It is not uncommon to find fields in which 25 to 75 per cent of the crop has been ruined l)y the inroads of these root-rotting parasites. Some of our fruit trees, such as the clierry and apple, and also forest 58 University of Texas Symiitoms of Disease in Plants 59 and shade u'ee?, are attacked Ijy root-rotting fungi. TIio wood of root arid even the crown is slowly disintegrated, and the mechanical structures are so weakened that severe storms may cause the affected tree to be blown over. White strands or fibrils, the vegetative body of the fungus, may sometimes be seen upon the fractured surface. In other cases the aU'ected tree may remain standing until it finally succumbs to the attack. Modified stems, like tubers, rhizomes, bulbs, or corns, which serve as storage organs for reserve food, suffer in many cases from rot-producing fungi or bacteria. These storage organs are affected in much the same Fig. CO. — Aipple rotted by black mold (Rhizopus nigricans). Specimen was placed in a damp chamber to cause the mold to grow over the surface. Original. way as the fleshy roots, but dry rots are not uncommon. The potato is affected by several diseases in whicli a soft rot of tlie tubers is a marked symptom. This is true in the disease known as tlie late blight, a fungus disease, and also in the bacterial blights, while in some cases a soft rot may result when the tops are not atfeeted. In the dry-rot of the potato the fibro-vascular ring often shows a conspicuous darkening when the tubers are cut across, and the badly rotted portions become shrunken and darkened and more or less corroded. Diseases of this kind may be spread by planting affected seed. Iris rhizomes, canna root-stocks, 60 University of Texas Symptoms of Disease in Plants 61 liyacinth or ealla lily bulbs, onions and similar niodified storage oro-ans are often affected by organisms which produce soft rots. An immense amount of loss is caused in standing timljer and in fruit trees from dry-rots of the trunk due to wood-destroying fungi which gain an entrance through wounds or otherwise. It frequently happens that the trunk of a tree is thoroughly permeated by the vegetative hijphae of the fungus l)efore any external evidences of its presence can be detected. These wood-destroying iimgi may give rise to external fruiting bodies, either toadstool-like or bracket sporopliores, when they have reached a sufficient vigor of growth. The affected wood is gradually disintegrated hy the work of the fungus which digests the woody elements. It is thus transformed into a brittle or even punky condition that renders it unfit for the purposes wliieli it must serve as a functional part of the tree, and lessens or destroys the value of the wood for hunber. "Wood affected by ■dry-rot is often discolored, sometimes showing a marked blue, pink, yel- low, or reddish-brown coloration. Black lines may often be noticed cross- ing irregularly through the wood or surrounding definite areas. The vegetative body of the fungus is frequently invisible to the naked eye, hut in some cases it shows as tine strands or ropes, or even lamellae that run between the wood iiljers or along the silver grain. In a few cases bvtds are affected by rot-producing fungi or bacteria. In the tropical regions the cocoanut is seriously affected by a bacterium which causes a rotting of the large terminal bud. In this connection the black rot of the cabbage may be mentioned, for the cabbage is really a large bud. This disease is of bacterial origin, the organism entering at the leaf margins. They make their way down through the veins to the central axis and frequently cause a complete rotting of the fully or par- tially developed heads. This disease is particularly destructive on low, heavy soils. One of the l)est illustrations of a fungus bud-rot is to be observed in a serious disease of certain varieties of greenhouse carnations. In this disease the fungus may gain an entrance into the bud at an early stage of its development, and cause a complete rotting of the petals be- fore the calyx has opened. In other cases the rotting starts later and the flower may be partially opened before its development is arrested. In these affected flowers the claws of the petals will be bro^ra and soft even though the exposed portions are normal in appearance. A minute mite is constantly associated with this disease and the evidence points to the fact that the mite acts as a carrier for the fungus, transferring it from bud to bud. The rotting of fruit is always caused l)y the inroads of bacteria or fungi. In the majority of cases it is some filamentous fungus that causes the trouble. The fruit-rotting fungi may gain an entrance through '^vounds or bruises, or in other cases they may penetrate the uninjured 62 University of Texas Fig. 62.— Normal Belinower apple and coal black "mummy' produced by brown rot fungus (Sclerotinia fructigena). Original. MB Sf Symptoms of Disease in Plants 63 skin. In some eases the rotting may Ijegin wlien the fruit is still imma- ture and hanging on the tree, and the work of destruction may be con- tinued after the crop is harvested. Many fruit-rotting fungi only at- tack the ripe fruit, just at time of nuiturily or after it has been harvested or during storage. Soft fruits, like strawberries, suffer rapid decay from fungi, while fruits like apples with a protective epidermis may be preserved for a long time. The brown rot of peaches, plums and cherries is very destructive in certain regions when conditions become favorable. Affected, fruits show brown patches of soft tissue. The patches spread rapidly in circumfer- ence until the whole fruit is involved, and if the weather is moist these rotted areas will show conspicuous pale brown conidial tufts which pro- duce myriads of spores that spread the rot to other fruits. It is these rotting fruits that produce the "mummies" already mentioned. A fungus similar to that which cau'^es the brown rot of peaches, plums, and cherries also affects the apple, it may work in the same way, or it iuay transform the apple into a blaek mummy, shiny and coal black, with no external evidence of the fungus. This rot of the apple should not be confused with the true black rot which works in a somewhat similar way. In the true black rot the surface of the apple soon becomes covered with minute black pustules and the fruit be- comes more or less shriveled and shrunken. In both the black rot and the brown rot the pulp is colored a dark browai, the black coloration being due to changes in the skin. The bitter rot shows brown circular areas with concentric zones of pink spore-pustules. This is one of the most destructive apple diseases known for the central Mississippi Valley region, especially for southern Illinois and Missouri and adjacent terri- tory. Green mold is one of the most destructive fungi for stored apples. Apples affected by this rot often show only brown patches in wdiich the tissue becomes rapidly softened and the whole apple transformed into a soft rotten mass. In moist conditions the bluish-green spore-tufts of the fungus may be seen upon the surface of the fruit. In addition to the various rots mentioned, a white rot of the apple has been recently ob- served in which the entire pulp may be transformed into a soft mass without any discoloration. By paying attention to proper spraying to prevent insect injuries and other fungus diseases, by careful handling in harvesting, and by storage under conditions that retard the growth of fungi, much of the trouble from rotting of apples can be prevented. O V ■/ ^o.'-^-/ \'^-^\/ -o^^^-,0' A. ^ *y^^- ,0'' A .^'"'-. t^o^- ^^--^ ^ o V °o> .^^ ^^-n *1 C» *! •1 o> LIBRARY OF CONGRESS 002 81 1 462 A #