SYMPTOMATOLOGY OF DEFICIENCIES AND TOXICITIES OF CITRUS

Transcription

1 FLORIDA STATE HORTICULTURAL SOCIETY 145 SYMPTOMATOLOGY OF DEFICIENCIES AND TOXICITIES OF CITRUS The ability to recognize the symptoms of de ficiencies and toxicities as they present themselves in citrus trees has become of prime importance in the fertilization and handling of groves. Our soils work has not yet reached the point where we can definitely correlate soils analyses with tree condition even when such soil analyses are made in the laboratory under closely supervised con ditions. Even after such correlations are estab lished it will be necessary to develop accurate quick methods adapted to our soils before such tests can be put into actual commercial use. On the other hand, the citrus tree itself has been found a very accurate indicator of nutritional con ditions and the symptoms indicative of certain de ficiencies have been found very specific as well as extremely sensitive. The symptoms of four de ficiencies in citrus have now been worked out in a fairly complete way and will be presented in detail in this paper together with the symptoms of one toxicity which has frequently been mistaken for a deficiency of some sort. The four de ficiencies discussed will be those of zinc, copper, manganese and magnesium and the toxicity will be that of boron. It should be pointed out perhaps that no attempt will be made to go into the realm of conjecture concerning other deficiencies that may exist but for which the exact symptoms have not been de termined. Various types of plants have widely different mineral requirements. Crotalaria, for instance, will grow luxuriantly on a soil so de ficient in zinc that tung trees will hardly grow at all; thus, a soil that might be deficient in some element in terms of a certain crop may contain adequate for another. Moreover, symptoms of certain specific deficiencies vary widely in dif ferent crops; thus, typical "frenching" is due to zinc deficiency in citrus, but equally typical french ing in tung trees is caused by manganese de ficiency; so that it may not be concluded that be DR. A. F. CAMP Citrus Experiment Station Lake Alfred cause citrus shows a certain symptom which re sembles the symptom of a deficiency in cotton, that this same deficiency actually exists. The sub sequent descriptions will, therefore, be limited to those symptoms which have been carefully worked out on citrus. Any discussion as to the reasons for their occurrence will have to be omitted for the present owing to lack of time. In this paper the term deficiency will be used in connection with such things as Frenching and Dieback, i. e., sine deficiency and copper deficiency. While it may be argued that this may not prove to be technically correct, such terminology has the distinct advantage of tying up the name with the element used as a corrective. Moreover, the fact that there is a particular derangement that responds only to the application of a certain ele ment and that this application can be made to the tree instead of the soil, would tend to substantiate the idea that it is a deficiency. The fact that the process involved is complicated and indirect would not vitiate the correctness of the term. ZINC DEFICIENCY OR FRENCHING As far as possible this description will be limited to the symptoms per se and the manifestations present when other deficiencies modify the symp toms will be omitted. Foliage Symptoms: Frenching as observed in Florida has characteristic leaf symptoms which can be easily recognized. It is characterized by green areas along the midrib and the main lateral veins, the remaining areas being light yellow to almost white. The relative amounts of green and yellow tissue vary from a condition of mild frenching in which there are only small yellow splotches between the green veins to a condition in which only a portion of the midrib toward the base of the leaf is green and the remainder of the leaf light yellow to almost white. In immature leaves some of the smaller veins may be green 10 Horti

2 146 FLORIDA STATE HORTICULTURAL SOCIETY and present a lacework of green on a light yellow background. As the leaves mature the small veins may lose their green color, but small green spots 2 or 3 mm. in diameter may persist. In the milder cases the leaves are almost normal in size, while in severe cases the leaves are small and abnor mally narrow, having not over a tenth of the area of normal leaves. The tendency to produce un usually narrow pointed leaves is one of the very distinct symptoms of frenching. Growth Symptoms: In mild cases, frenching may appear on occasional weak twigs, the rest of the tree remaining green, or it may be general over the tree with little reduction in leaf size. In the latter case the growth is likely to tend to an unusual erectness in contrast to the usual spread ing growth. When more severe, frenching occurs over the entire periphery of the tree. The growth is shortened and this combined with the short narrow leaves gives the tree a peculiar brushy ap pearance and this is accentuated by the develop ment of numerous adventitious shoots. All the twigs are weak and die off quickly so that there is always an excess of dead wood in the tree. In contrast to the persistent dying back over the periphery of the tree there is a vigorous develop ment of watersprouts on the large limbs and trunk. These watersprouts are commonly free of frenching and give the tree a dense center of growth in contrast to the weak, dying appearance of the outer portions of the tree. Such trees may decrease in size very rapidly due to the dying back of the twigs. Fruit Symptoms: The fruit borne on weak twigs carrying small frenched leaves is unusually small, the rind very smooth and the color, when ripe, is unusually light. The pulp of such fruits is woody with a very low juice content. In con trast to the fruit borne on the frenched twigs is the large coarse fruit on the watersprouts. In severe cases the production is mostly culls which are either very large or very small, and very little good fruic is produced. Distribution: Frenching is more common on oranges than on grapefruit and least common on tangerines, though alt may be affected. The seedy varieties of oranges, such as Pineapples, are usu ally more severely affected than Valencias, while it is still less common on Parson Browns and other early oranges, and thus may be related to crop strain. Seedling trees are seldom as severely affected as budded trees, but on occasion may suffer seriously. Generally speaking, frenching is commonest in Pineapple oranges, but can be found in any varitey of citrus. Modifications of Frenching: The above descrip tion applies primarily to frenching uncomplicated by other deficiencies. In such cases a zinc spray applied during the spring will cause the frenched leaves to green up rapidly and this will be fol lowed by a flush of growth with normal leaves and characteristics. The small frenched leaves do not grow appreciably after they have become green, but continue on the tree, giving a peculiar mixture of foliage. In contrast to the above there are many cases complicated by other factors such as magnesium or copper deficiency. Unfortunately such combinations of deficiencies are more common than a straight zinc deficiency and such cases do not respond satisfactorily unless all the needed elements are supplied. Frenching was long con sidered a symptom of dieback, copper deficiency, because it is almost always associated with dieback. This sort of situation is not strange since factors that tend to produce a deficiency of zinc are likely to also eliminate or tie up such ele ments as manganese, copper and magnesium. COPPER DEFICIENCY OR DIEBACK AND AMMONIATION (Exanthema of Fawcett) Dieback and its associated ammoniation is con fused with frenching only through persistent as sociation of the two troubles. When separated from frenching the symptoms are distinct as will be noted from the following description. B. F. Floyd has described dieback in detail so only the main points will be mentioned here. Foliage: The foliage in the earlier stages is unusually dark green, the leaves are large and thick and unusually vigorous, which is the first symptom looked for. On the severely affected I wigs which will be mentioned later, a few small weak leaves may develop and shed, but these are only on twigs in the process of dying. Growth: One of the first growth symptoms looked for is the appearance of "S"-shaped, soft, over-vigorous twigs. These combined with the dark green foliage mentioned above are taken as

3 FLORIDA STATE HORTICULTURAL SOCIETY 147 definite first indication of dieback. As the trouble progresses gum pockets appear between the bark and the wood, and an unusual excretion of gum will take place at any break or cut. Severely af fected twigs will show excrescences of gum near the terminal end and such twigs usually lose their leaves and die back for some distance. As the case progresses the tree will show a body of dark green, heavy growth with a great many dead ter minal twigs. Multiple buds are numerous so that a great many twigs develop and many of these die. The bushy growth of small twigs frequently con sidered symptomatic of dieback are primarily as sociated with zinc deficiency rather than copper deficiency. Fruit: The manifestations of copper deficiency on the fruit have long been termed "ammoniation." The young fruit is light green in color disfigured by light brown markings, or excrescences which darken as time progresses and may become black by the time the fruit is half grown. Severely af fected fruits are likely to drop while less than \JA to 2 inches in diameter, splitting is common and a considerable percentage of the splitting will be across the axis. Gum pockets are common in the rind and around the seeds of ammoniated fruit. Less severely affected fruits will be large and coarse, with large dark markings and frequently gum around the seeds. In severe cases, no fruit of first or second grades will be produced, but a scattering of ammoniated fruits is commonly the forerunner of severe dieback and a truer indi cator of copper deficiency than twig or leaf symp toms for zinc and magnesium deficiencies, and may be so severe as to restrain the vigorous growth of twigs and leaves so that the commonly recognized foliage and growth symptoms never appear. Distribution: The use of copper sulfate for the correction of this trouble has been known for many years and this combined with the general use of bordeaux has kept copper deficiency at a minimum, but there is evidence to indicate that it would be general throughout the citrus growing, area if it were not for the use of copper. It is generally worse on oranges than on grapefruit or tangerines. MAGNESIUM DEFICIENCY OR BRONZING The term bronzing or copper leaf is rather more loosely applied than the terms frenching and dieback and is used to designate a type of chlorosis which is very common throughout the citrus belt. It has been particularly severe during the last sev eral years on the lighter sands and has caused a very considerable reduction in the beanng ca pacity of trees. The symptoms described here are limited to those apparently caused by a de ficiency of magnesium, although it is common to find the symptoms of severe manganese deficiency included. The narrower limitation seems more desirable since the broader use of the term is likely to cause misunderstanding. Foliage: Bronzed leaves do not show the pat tern of green veination which is characteristic of frenching, but do show in the later stages an over all cadmium yellow to bronze cast. This may start as two elongated patches, one on each side of the midrib, and spread outward to the pe riphery of the leaf until only a green midrib is left and the green along the midrib may gradually fade until it is only a wedge-shaped area at the base of the leaf which remains green. In some cases the entire leaf takes on an over-all yellow to bronze cast and no green at all is apparent. Leaves when affected with this trouble never show the narrowing that is characteristic of frenched leaves and normally are all full size, having de veloped first as normally-appearing green leaves and later becoming bronzed in contrast to leaves on zinc deficient trees which are frenched from the beginning. In severe cases the leaves may be slightly reduced in size and this is particularly characteristic of the type in which the leaf is en tirely yellow. Bronzing usually appears in the late summer or early fall on heavily fruited trees and particularly on limbs carrying a cluster of fruit, whereas light fruited trees or limbs that are not carrying fruit will remain green.. When the trouble is severe and a large proportion of the leaves is affected, there will be a heavy leaf fall and a consequent heavy reduction in the effective leaf surface of the trees. Such trees do not respond to fertilizer treatment satisfactorily and while the leaf char acteristic may be and probably is associated with

4 148 FLORIDA STATE HORTICULTURAL SOCIETY crop strain, it cannot be restrained through ap plication of nitrogeous fertilizer. Growth: Trees affected primarily with bronz ing do not take on the bushy appearance of trees affected either with dieback or frenching. They retain their naturally spreading shape, but con siderable dead wood appears and frequently large limbs will die out and have to be removed so that the trees in severe cases become progressively thinner both as to foliage and growth so as to become almost skeletonized. The profuse devel opment of watersprouts characteristic of both dieback and frenching does not take place and the tree has a hard, weak appearance. Fruit: In the early stages of bronzing, trees usually bear heavily, but as the trouble progresses the production is reduced due to the reduction in bearing surface and also due to the production of large quantities of small sizes. Other than that the fruit seems to have a normal quality and fair texture until the advanced stages are reached and the tree is almost dead, at which time the production may drop to almost nothing. In mild cases of bronzing production may be maintained at a high level for several years and the fruit have a reasonably good quality. In such groves the trees seem to recover during the spring and early summer and bronzing appears in the fall and the trees become progressively weaker until the crop is removed, at which time the trees start to green up again. This may be considered a marginal case which may suddenly develop into a very severe form with a heavy loss of wood. Distribution: Bronzing is generally much worse on grapefruit than on oranges or tangerines, and is particularly associated with seediness in grape fruit. Marsh Seedless very seldom shows a great amount of bronzing and the same is true of the Thompson Pink, which is a seedless pink sport of Marsh. It appears to a considerable extent on oranges, however, and seems to be worse on Pine apple and probably next on Valencias, thus being associated with both seediness and late maturity and the consequent crop strain that results. Early varieties such as Parson Brown and Hamlin do not show as much of this, but on occasion may show severe cases. It is not very pronounced on tangerines, but occasional cases occur. It is very common to find it associated with frenching and where copper sprays have not been used for disease control or for corrective purposes, dieback may also appear, but not in its character istic form due to the restrained growth. MANGANESE DEFICIENCY Manganese has been used extensively on citrus on calcareous soils for what is known as marl chlorosis or marl frenching. Its use on. acid soils as well as marl soils has been recommended by Bahrt and considerable work has been done by Bahrt and his associates along these lines with excellent results. The symptoms have not been thoroughly described, however, probably due to the fact that they commonly occur associated with frenching and it is only since the work on zinc has reached an extended scale that manganese de ficiency symptoms have been accurately separated from frenching. The symptoms as given below will be incomplete because of the fact that the work on this phase of symptomatology has not been completed at the present time. Foliage: Leaves showing manganese deficiency take on much the same pattern as is present in frenching with a green midrib and green lateral veins. The tissue between the veins, however, does not take on a yellow color, but remains a light green so that there is a dark green pattern of midrib and veins with a background of lighter green. In mild cases it may resolve itself into a scries of light green patches along the periphery of the leaf between the lateral veins. On the immature leaf this pattern will be fairly bright and quite easily observed, but as the leaves mature they take on a peculiar dull slate cast and unless the investigator is looking closely, the pattern may be overlooked. This dull slate cast seems to be rather definitely associated with manganese de ficiency not only in citrus but in other plants and makes the leaves extremely difficult to photo graph. On the marl soils the pattern is usually more pronounced and the leaves as they mature take on a grayish to slate cast. This peculiar symptom is probably due to a combination of zinc and manganese deficiency with the latter being predominant, but with the more pronounced zinc deficiency covering it up to a certain extent. The use of manganese for more characteristic types of frenching was common a number of years ago,

5 FLORIDA STATE HORTICULTURAL SOCIETY 149 but experiments in the last few years by the author have indicated that the characteristics of frenching occurring on these soils do not respond to man ganese treatment but to zinc. Both on the marl soils and on the acid sands, frenching if present in abundance will almost completely mask man ganese deficiency and it will only be observed after zinc has been applied. The leaves do not take on the characteristic shape that is associated with frenching and will only have a pronounced pattern when they are quite young. The trouble seems to be associated primarily with growth con ditions and appears to the greatest extent on trees that are in vigorous growing condition. Growth: No characteristic growth symptoms have been observed to date, but Bahrt and the author have observed an improvement in tree con dition and a general improvement in growth fol lowing manganese treatments on deficient soils. Fruit: No definite fruit symptoms have been associated by the author with manganese deficiency, but Bahrt reports increased intensity of color fol lowing applications of manganese and has re ported this to be particularly noticeable in tan gerines. He has also reported the fruit is firmer and better able to withstand shipping. Some chemical relationships have also been established and the readers are referred to Bahrt's paper on this subject. Distribution: The symptoms of manganese de ficiency have been found general throughout the citrus belt during the last year both on acid sands and on the alkaline soils along the coast. They have been much more pronounced on grapefruit, tangerines and Temple oranges than they have on round oranges. They have been found almost universally on tangerines except in those groves where manganese has been used either alone or in the fertilizer. Where frenching or dieback or both are present in profusion, or where bronzing is severe, only an occasional typical leaf may be found, but when such groves are treated so as to eliminate the other deficiencies, manganese de ficiency will show up in abundance. The observa tions in connection with this work have indicated that frenching probably occurs under practically the same soil conditions as manganese deficiency, but due to its more pronounced pattern and also to its effect on growth, masks the symptoms of manganese deficiency. Where frenching is se vere, characteristic manganese deficiency can sel dom be found, but commonly appears as soon as zinc is applied. Particular attention is called at this time to manganese deficiency because of the fact that many growers have noticed it and have merely considered it partially corrected frenching instead of a distinct deficiency. BORON TOXICITY Coincidental with the increased use of borax and other boron compounds in packing houses, there has been occurring in groves a peculiar chlorotic condition which has been found to be due to excessive applications of borax or other boron compounds to the soil. This has been brought about mainly through two practices: first, the dumping of culls in the grove and the careless inclusion of fruit that has been dipped in borax and not thoroughly washed afterward, and seconally, the leaving of boxes which have been treated with borax in the field for extended periods so that the rain may wash the borax off the boxes onto the ground around the trees. It has also occurred to a considerable extent around packing houses where borax has been extensively used and the borax tanks are allowed to drain near adjacent citrus trees. In some cases where the entire box of fruit including the box is dipped in borax solution and the boxes after removal of the fruit have been stacked on a platform, the rain has often washed enough borax from the boxes to kill all the citrus trees and other vege tables near the box platform. The grower who finds this trouble in his grove and who is unfa miliar with it commonly associates it with french ing because there is a certain amount of reremblance between the two troubles. For this reason the symptoms of boron toxicity will be given below. Foliage: Following application of an excessive amount of a boron compound to a tree, the leaves will begin to show a yellowing near the tip fol lowed by a splotchy yellow coloration along the peripheral portions of the leaf. The yellow color will increase in intensity to become a dark bronze color. Semicircular areas of dead tissue will oc cur around the edges of the leaf or the entire tip of the leaf may be killed. These leaves shed at

6 150 FLORIDA STATE HORTICULTURAL SOCIETY the junction between the petiole and the leaf, and in severe cases the trees may become quickly de foliated. Almost immediately such trees will put out a succeeding flush of growth in which the leaves will be almost white with a narrow green midrib. This stage and the succeeding stages are quite commonly mistaken for frenching, but the characteristic narrowing of the leaf is not present and in fact as the flushes progress in severe cases, tlie leaves become smaller and rounder than nor mal. The severely affected leaves mentioned above will shed quite early and successive flushes will follow and where the rains are plentiful or where irrigation water has been used to wash the borax out of the soil, they will gradually begin to recover with the leaves showing less of the yellow condition and more green. The yellow color occurs nearer the tip of the leaf and does not have the characteristic light yellow to white of frenching, but soon takes on a deep yellow which is very distinct. In the early stages the leaves may show a large number of small yellow dots scattered over the surface of the leaf and these are particularly noticeable on the under side of the leaf and near the tip. The leaves also have a peculiar characteristic leathery texture which is distinct from the ordinary smooth, hard texture of citrus leaves and this symptom combined with the coloration makes it very easy to identify the trouble and to separate it from frenching. Trees that have received an unusually large amount of borax will continue to put out flushes of growth for some time with the leaves becom ing gradually smaller and rounder and showing a great many dead spots around the edges and the trees may become almost defoliated. In milder cases a gradual recovery is made as the borax is leached from the soil and the leaves become very large and thick and the pattern disappears. Growth: In severe cases of boron damage there will be a considerable loss of wood throughout the tree and in extremely severe cases the tree may be entirely killed. Usually, however, a cer tain amount of dead wood develops and the tree starts on a process of recovery with the loss of wood never being very pronounced. Fruit: Severely affected trees usually lose their fruit quickly and do not put on succeeding crops of any size until recovery is practically complete. Distribution: Grapefruit trees have been found to be unusually susceptible to boron damage and most of the cases observed have been oh grape fruit, although a few severe cases have been found on other varieties as well. In experiments it has been found that it takes considerably smaller quan tities of borax to produce severe symptoms on grapefruit than is the case with other varieties. The trouble is usually limited to small areas in the groves where boraxed culls1 have been dumped or where loads of boxes carrying borax on them have been left through a rainy period. It is sel dom found evenly distributed over a grove and may only affect two or three trees here and there where either truckloads of culls or piles of boxes have.been left. The only treatment that has been found of value is an application of irrigation water if available or to wait for the rains to leach it out of the soil. If the trouble is not too severe this will usually occur within twelve months. The commonest place to find this trouble is around packing houses where borax is used extensively for fruit treatment. Treatment with lime is sometimes helpful, but the damage has usually been done before the trouble is noted so that ap plications are too late to be of great value. Other deficiencies and toxicities will undoubt edly be worked out from time to time, but specu lation concerning these is idle except as a guide to research. It is almost certainly true that as the deficiencies above are supplied, the chances of identifying others become greater as is evidenced by the fact that the use of zinc has made the identification of manganese deficiency easier. It is possible, too, that as the work develops, soil tests may become increasingly useful, but at the present time the tree itself is the best indicator of its needs.