UNIVERSITY OF CALIFORNIA PUBLICATIONS. 
 
 University of California— College of Agriculture, 
 
 AGRICULTURAL EXPERIMENT STATION. 
 
 E. W. HILGARD, Director. 
 
 ORANGE AND LEMON ROT. 
 
 By C. W. WOODWORTH. 
 
 NAVEL ORANGE, ONE FOURTH SHOWING THE EFFECT OF THE ROT. 
 
 BULLETIN No. 139. 
 
 (Berkeley, February, 1902.) 
 
 SACRAMENTO: 
 a. j. Johnston, : : : : : superintendent state printing. 
 
 1902. 
 
Digitized by the Internet Archive 
 
 in 2012 with funding from 
 
 University of California, Davis Libraries 
 
 http://www.archive.org/details/orangelemonrot139wood 
 
ORANGE AND LEMON ROT. 
 
 By C. W. WOODWORTH. 
 
 The present season has been more than usually favorable to the decay 
 of oranges, especially in early shipments. Every year the loss from the 
 rotting of both oranges and lemons is very considerable. The cause of 
 the rot and the precautions that should be taken to avoid the trouble 
 are very rarely understood by those who handle the fruits; and there 
 can be no doubt that a fuller knowledge of the subject would enable 
 growers and shippers to avoid much of the loss. The present bulletin 
 is prepared in response to requests from numerous growers for informa- 
 tion on this subject. 
 
 The Cause. — The cause of the rot of oranges and lemons is the growth, 
 through their substances, of a mold fungus known scientifically under 
 the name of Penicillium digitatum. The growth of this plant within 
 the fruit causes a softening and breaking down of the tissue, a very 
 characteristic change in the flavor of the juice, and sooner or later a 
 very pronounced discoloration of the affected part. The fungus belongs 
 to a genus consisting of a number of well-known species, all having 
 much the same manner of growth and producing decays on various 
 substances. The name "blue mold" applies to the whole group. The 
 best known species is Penicillium crustaceum, or, as it is more commonly 
 called, Penicillium glaucum. This species is one of the common forms 
 of rot-producing fungi that attack deciduous fruits, but it is probably 
 even better known from its attack upon all manner of substances in the 
 household, such as cooked foods, clothing, etc. While Penicillium crus- 
 taceum is thus found in a great variety of situations, it appears that 
 Penicillium digitatum attacks only citrus fruits, confining itself wholly 
 to these. 
 
 Nature of the Fungus. — The form of the fungus while it is growing 
 through the tissue of an orange is that of innumerable fine white threads, 
 known as "vegetative or growing mycelium," the individual threads 
 being called "hyphae." (Plate 1.) These hyphse penetrate the fruit in 
 much the same way as the root-hairs of higher plants penetrate the soil. 
 Indeed, they are quite similar, except that the hyphse are not attached 
 to a more complicated organ like the rootlet from which the root-hair 
 
— 4 — 
 
 Plate 1. Vegetative mycelium from a culture of the 
 fungus in water. Very much magnified. 
 
 ^ 
 
 v^KPr 
 
 Plate 2. Rotten orange, partly unwrapped, showing mold in white and 
 
 blue condition. 
 
— 5 — 
 
 Plate 3. Fruiting hyphre in the " white mold " con- 
 dition. Surface view as seen under the microscope. 
 The same fruit shown in Plate 2. 
 
 springs. After the vegetative mycelium has somewhat exhausted the 
 substances of the fruit, or if the latter becomes too dry for rapid 
 growth, the fungus prepares to develop a form of fruit of its own. 
 Fruiting hyphse grow outward 
 to the surface of the fruit, or, 
 as is sometimes the case if 
 there is a cavity within the 
 fruit, they grow to the surface 
 of the cavity and there form 
 the fruiting mycelium. This 
 is a dense mass of threads, 
 white in color and soft and 
 •downy in texture. (Plate 3.) 
 It is this stage of the growth 
 that the growers call " white 
 tmold." It is merely a stage 
 in the development of the 
 fungus following the growth 
 of the vegetative mycelium 
 within the fruit, and the fruit- 
 ing hyphse are, as it were, 
 the fruit spurs upon which the crop of the fruit of the fungus will be 
 borne. The fruit of the fungus consists of minute, oval bodies, called 
 .spores, of dull, greenish-blue color. When they become numerous enough 
 
 'J *5 "*"* ****? #»~ ^° ^ide the white mycelium 
 
 upon which they are borne, 
 
 the whole surface changes to 
 
 blue, and it is then called 
 
 "blue mold." (Plate 2.) The 
 
 V ?Jm|l fruiting hyphse producing the 
 
 I spores first divide into a num- 
 
 |J§ ber of branches, suggesting 
 
 ■ 
 
 §f the name of the species (" digi- 
 gf tatum," or fingered), and each 
 W of these branches as they in- 
 crease in length become con- 
 stricted, and finally divide 
 into a large number of oval 
 fruits, the conidia-spores } which 
 for a long time hang together 
 like a string of beads. (Plate 
 -4.) They are easily broken apart, however, and produce then a fine 
 powder that can be carried by the wind, and so are distributed far 
 and wide. 
 
 fJW 
 
 
 * ; - 
 
 4 •*■'* 
 
 Plate 4. Conidia-spores— still more enlarged. 
 
— 6 — 
 
 There is another form of fruit produced by plants of this genus 
 Penicillium, known as the asco-spores, being formed in little sacks called 
 asci; but they occur only rarely, and, as far as we know, have not 
 been recognized in this species. They doubtless occur, however, and 
 enable the fungus to pass seasons that would kill most of the conidia- 
 spores. The conidia-spore is the means of rapid reproduction, and the- 
 only one that is highly significant from the practical standpoint, 
 because it is responsible for practically all of the infection of our fruit 
 with the rot disease. 
 
 When and Where It Enters the Fruit. — The rot of citrus fruit is not 
 
 Plate 5. Navel orange, showing rotten spot. 
 
 usually a disease of the orchard. In lemons the infection occurs almost 
 entirely in the curing-house; and in oranges, as a rule, after they are 
 packed and on their way to the East. Navel oranges, however, very 
 often come into the packing-house badly infected by the disease. The 
 trouble begins at the navel end, and may be scarcely visible from without; 
 though commonly a slight split, or perhaps a little gum, will indicate 
 the point of entrance of the fungus. (See Frontispiece and Plate 5.) In 
 this case the trouble clearly arose in the field, and even began before the 
 fruit was ripe. It is usually confined to a limited part of the fruit, 
 perhaps the upper end of one or two sections, and very often produces; 
 
— 7 — 
 
 spores within the cavity caused by the shrinkage of the affected tissue, 
 so that the affected part may be badly discolored. In any citrus fruit a 
 bad wound of the surface is apt to be followed by the development of 
 the disease in the tissue just beneath, with the general characteristics 
 specified above as occurring in Navels. 
 
 In the packing-house or in transit the point of attack may be the 
 navel end, but it is more commonly where two fruits are pressed 
 together. Usually only one of the fruits so touching is affected at firsts 
 though after it becomes thoroughly rotted the disease usually communi- 
 cates to the other. If the conditions are favorable to the growth of the 
 fungus, it may spread from a single affected fruit to all those adjacent, 
 and in time to the whole box. 
 
 How It Enters the Fruit. — This disease, being entirely a matter of 
 the fruit and belonging particularly to ripe fruit, evidently always 
 gains its entrance to the fruit from the outside and never from the tree. 
 The conditions necessary to accomplish this are: 
 
 First — That the spore of the fungus should rest upon or near the sur- 
 face of the fruit. It may be carried there by the wind, or by touching 
 decayed fruit upon which the spores are being produced. 
 
 Second — Sufficient water upon the surface of the fruit to cause the 
 germination of the fungus. 
 
 Third — The right condition of temperature. The fungus will grow in 
 such a range of temperatures, however, that this condition may be con- 
 sidered to be practically always present. 
 
 The germination of the spores of the fungus is thus seen to be much 
 the same as the germination of the seed of a higher plant. 
 
 The reason that the navel end is particularly liable to the attack of 
 the rot fungus is that in case a drop of moisture finds its way within 
 this structure, it is less liable to rapid evaporation, and so favors the 
 germination of any spores that may also find their way there. The 
 same explanation accounts for the common abundance of this fungus in 
 fruit with broken skin.' The point at which moisture will accumulate 
 and remain longest when fruit is sweating after packing, or while it is 
 stored in the packing- or curing-house, is the point where the fruit 
 touches an adjacent fruit; and at this point, therefore, the germination 
 of the fungus most commonly occurs. 
 
 PREVENTIVE MEASURES. 
 
 From the above review of the manner of growth of the fungus pro- 
 ducing the rot of these fruits, it is evident that our preventive measures 
 must lie wholly along one of two lines: either by preventing the spores 
 from gaining access to the surface of the fruit, or by the prevention of 
 the germination of the spores after they are there. For the latter pur- 
 
— 8 — 
 
 pose several methods have been employed, usually without knowledge 
 of the reason of their efficiency, and therefore often in a manner to 
 defeat the very object sought. Those that have proven valuable are 
 Refrigeration, Ventilation, and Wrapping. 
 
 Effects of Refrigeration. — One of the most available means for abso- 
 lutely preventing the growth of the fungus, even though abundant upon 
 the surface of the fruit, is to so lower the temperature where it is stored 
 that the fungus will not grow. This method is followed very largely by 
 shippers of green deciduous fruits, who have to contend against the 
 closely related Penicillium crustaceum. If sufficient refrigeration is 
 maintained, the fruit will be entirely safe from the attack of the fungus. 
 In the use of refrigeration it should be clearly understood that the cold 
 temperature is likely to condense a large amount of water upon the 
 fruit, and as soon as the temperature is allowed to rise to the point 
 where the fungus can grow, the conditions are extremely favorable for 
 its rapid germination. This has given rise to the belief of many that 
 fruit taken out of refrigeration is much more subject to rot than it was 
 before. The skin may have really become softened by the water during 
 refrigeration, so that this may in some measure be true. The common 
 experience that washed fruit rots worse than that which is brushed dry, 
 may be true for a similar reason. But in any case, the presence of 
 water upon the fruit is always essential for the entrance of the fungus; 
 and if fruit taken from refrigeration is immediately thoroughly dried by 
 arranging for sufficient ventilation, there would be no greater suscepti- 
 bility on account of the cooling. The use of refrigeration for citrus 
 fruits will probably never be resorted to as extensively as for deciduous 
 fruits, for the reason that the control of rot is not so difficult a matter. 
 
 Effects of Ventilation. — The use of ventilated cars, or the ventilation 
 of the curing-house, is chiefly calculated to prevent the rot by carrying 
 off the moisture that may accumulate on the fruit in the sweating 
 process, or on account of the rapid lowering of the temperature. The 
 cooling that is accomplished by the evaporation of this moisture is 
 sometimes thought to act like refrigeration, and may, it is true, slightly 
 decrease the rate of growth of the fungus after it germinates, but can 
 not produce a temperature low enough to prevent its germination and 
 growth. Certainly, the important matter in ventilation is the rapid 
 removal of any condensed moisture that may gather on the fruit. If 
 this moisture is removed promptly enough, so as not to give time for the 
 germination of this mold fungus, the fruit will not decay, but will stay 
 sound until it would ultimately dry up and mummify. 
 
 The problem in ventilation, then, is to maintain the mean, and avoid 
 on the one hand too much drying, and on the other the presence of water 
 
— 9 — 
 
 for any considerable period upon the surface of the fruit. In small lots 
 this is not particularly difficult, but in larger rooms there is some diffi- 
 culty in securing a uniform ventilation; and therefore it is desirable, 
 especially in curing lemons, that the temperature be kept as uniform as 
 possible; particularly avoiding rapid lowering of temperature that might 
 result in the deposition of water on the fruit. 
 
 Effects of Wi^apping. — The practice which is almost uniformly adopted 
 for citrus fruits, and which is rapidly extending also to deciduous fruits, 
 viz., wrapping them in tissue paper, is an extremely efficient means 
 of decreasing danger from rotting. The reason for this is that the 
 paper absorbs water very freely and will take up a very considerable 
 quantity. The paper will have to become very wet before it will give 
 up enough to the spores of the fungus to permit them to germinate, 
 and so it regulates the matter to a large extent; because when the tem- 
 perature falls the paper merely becomes moist, and when the tempera- 
 ture rises this moisture is evaporated, and if there is fair ventilation it 
 will be carried off and the fruit never become really wet. If the fruit 
 sweats too much, however, owing to poor ventilation or rapid lowering 
 of temperature, the paper will not be sufficient to prevent the accumu- 
 lation of the water on the fruit and the germination of any spores that 
 may be there. Only within certain limits, therefore, is the wrapping of 
 fruit a preventive of infection by the fungus. 
 
 DESTRUCTION OF THE SPORES. 
 
 Thus far we have considered measures that have been employed to 
 prevent the germination of the spores. Quite as important a matter 
 is that of decreasing their abundance. This is a subject that has not 
 been given the attention that it should have. I doubt if there is a 
 packing-house in the State in which, in the midst of the season, the 
 odor which is so characteristic of the spores of this fungus is not at 
 once evident to every visitor. It would appear almost surprising if a 
 single fruit could escape the presence of the spores of the mold upon its 
 surface if it passes through some of the packing-houses in which the 
 dust is so filled with them. The means that may be employed to 
 diminish the number of spores in the packing- or curing-house are of 
 three kinds: disposal of infected fruit, disinfection of the house in 
 summer, and the use of sulfur. 
 
 Disposition of Decayed Fruit. — It is a common practice to throw 
 decayed fruit in a pile in the immediate vicinity of the packing- or 
 curing-house; and here it continues to decay and produce countless 
 millions of spores, which are freely carried by the wind, and to this is 
 due the thorough infection of the atmosphere, referred to above. There 
 
— 10 — 
 
 is no means better calculated to disseminate the disease than this prac- 
 tice. Fruit should never be allowed to become " blue." By the time 
 it reaches the white-mold stage it should either be destroyed by fire, or, 
 what is probably more feasible, be buried. If it is buried deep enough, 
 go that it will not be turned up by plowing, or in any other manner? 
 the decay of the fruit will be completed, and in time the spores of the 
 fungus will disintegrate and the disease will not spread. When the 
 fruit is stored in bins for some days, so that infected fruit will have 
 time to become moldy, these moldy fruits should be removed with great 
 care, so as not to scatter the spores. Attention to this matter can but 
 have a beneficial effect upon the prevalence of the disease. 
 
 Summer Disinfection. — In practically all parts of the State the long, 
 dry summer period affords a very available time for the thorough disin- 
 fection of the packing-house. The mold spores can be killed by pro- 
 tracted drying, and it should be the practice to so thoroughly air all 
 packing-houses during the hottest and driest part of the summer, that 
 they will be entirely free from the fungus for the beginning of the next 
 year's campaign. In most places this is a very easy operation, and 
 should never be neglected where feasible. 
 
 Sulfuring. — When it is not possible to disinfect the house in the 
 summer by drying, as suggested above, or by whitewashing, it may be 
 possible to destroy the spores by the free use of sulfur. This is also 
 possible during the actual packing season. The sulfur should be burned 
 so that the fumes will come in contact with every part of the inside of 
 the packing- or curing-house. Usually these buildings are not tight 
 enough to permit of very thorough work, so that no prescription of the 
 amount to use can be safely made. The material is not expensive, so 
 that the best policy is to use it very liberally whenever the presence of 
 the fungus in quantity is known or suspected to exist in the house. 
 
 ORCHARD PRACTICE. 
 
 A great deal of fruit undoubtedly goes into the packing-house already 
 covered with the spores of this rot-fungus. As much carelessness in the 
 disposal of rotten fruit is seen in the orchard as has been pointed out 
 above as occurring about the packing-house. Rotten fruit is very com- 
 monly allowed to fall upon the ground, and is turned under with the 
 plow, to be turned up again in subsequent cultivations. In almost any 
 orchard in which the disease is prevalent, one would not have any diffi- 
 culty in finding oranges or lemons, covered with quantities of mold- 
 spores, lying upon or just beneath the surface of the ground, from which 
 they may be blown over the fruit with the dust. 
 
 It is not only the Navels that are thus endangered; but these fruits 
 
— 11 — 
 
 are probably the only ones that commonly become rotten before they 
 are picked. All the fruit in the orchard is liable to become affected 
 after it is packed, as soon as the spores existing on its surface have an 
 opportunity to germinate. It is particularly desirable, therefore, that 
 in the orchard all decayed fruit be gathered and buried so deeply that 
 it will not be turned up by the plow; or that it should be otherwise 
 disposed of. This may be done by digging a deep hole every little dis- 
 tance in the orchard, and piling in the rotten fruit from adjacent trees. 
 Another method would be to gather and compost decayed fruit, thus 
 destroying the spores; in this way finally using the fruit for fertiliza- 
 tion, as mentioned by Professor Hilgard in the paragraph below. 
 Either method provides a safe and rapid means of disposing of the 
 refuse fruit, which will certainly very materially decrease the amount 
 of spores, and therefore the liability to rot. 
 
 THE FERTILIZING VALUE OF CITRUS CULLS. 
 
 In connection with the foregoing discussion of the cause and preven- 
 tion of the rot of citrus fruits, it seems desirable to discuss a point very 
 frequently mooted, viz., the fertilizing value of such culls. Since, 
 from Professor Woodworth's investigations, it is clear that the culls are 
 not merely useless, but a positive detriment if allowed to rot under the 
 trees, as is commonly done, the question of their disposal raises that of 
 their value in restoring the plant-food they have derived from the soil. 
 It is clear that each fruit represents the ingredients which will be 
 required to produce another in the future; and in accordance with the 
 golden rule of returns to the soil, they should be given back. The 
 question arises, however, as to the exact value represented, and the 
 expense the grower can afford to go to save it, as against purchasing 
 fertilizers in the market. 
 
 The ash-percentage of fresh citrus fruit is (in California) .43 per cent 
 on the average. Analysis shows that of this amount about one-half is 
 potash, while .053 per cent is phosphoric acid. Or, stated for 1,000 
 pounds of oranges, we have 2.11 pounds of potash, 1.25 pounds of 
 phosphoric acid, and, besides, 1.83 pounds of nitrogen in the fresh fruit. 
 Assigning to these ingredients their accepted commercial values for 
 such conditions, we find the manurial value of the 1,000 pounds, aside 
 from the humus, to be-about 30 cents; it will not exceed 40 cents. 
 
 As an alternative of the deep burying recommended by Professor 
 Woodworth in order to render the culls innocuous to the crop, I have 
 heretofore suggested composting with lime and earth on special piles* 
 the compost to be hauled out to the orchard after the complete decom- 
 position of both fruit and mold-spores. Considering that the culls are 
 
— 12 — 
 
 usually gathered into piles anyway, this would involve little outlay 
 beyond that for lime; and for this purpose the spent lime from the beet- 
 sugar factories would be available, provided it were sold at its true 
 manurial value only, which is about $2.30 per ton in the moist condi- 
 tion, or $3 in the dry. When fresh lime is used it will be best to 
 employ it in the unslaked condition, so that it will help to take up the 
 moisture from the fruit. The compost thus produced would be a sub- 
 stantial addition to the productiveness of the grove. But each one must 
 judge according to his location, etc., whether this or the purchase of 
 fertilizers would pay best. The tree roots would eventually find the 
 pits in which the culls were buried; but to plow them in deep enough 
 to prevent their being brought up by the plow afterward, would need 
 the "master's eye." [E. W. H.]