COPPER SULFATE AS A CORRECTIVE FOR DIEBACK, A NEW DISEASE OF THE AVOCADO

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Loreen Holland
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1 152 FLORIDA STATE HORTICULTURAL SOCIETY Trapp, and Waldin blocks have developed a severe little-leaf condition during the past two years, and many other trees of these varieties have been showing symptoms sus pected of being mild cases of zinc deficiency. The most pronounced symptoms of zinc defi ciency occurred in the plots receiving a zero percent organic nitrogen formula as the fer tilizer treatment. Six trees of each fertilizer plot in the Trapp block, were sprayed on Sep tember 6 with the zinc sulfate-lime formula, leaving three trees in each fertilizer plot as checks. Little new growth appeared in these trees following the treatment, probably be cause of serious infestation of Dictyospermum scale which occurred generally in the block. Some improvement in color of the foli age was evident in the sprayed trees by Jan uary 1, The January freeze severely damaged many of the trees in the block so that further observations have been suspended for the present. Discussion It would appear from the experimental work to date that little-leaf of the avocado is a severe manifestation of zinc deficiency, and that it may be corrected readily by ap plications of zinc sulfate combined with lime as a spray. Experimental work has not been continued long enough to answer definitely the question of optimum times and amounts, of zinc to apply. It is probable, from the results ob tained in experimental tests on other crops by other workers, that the most effective time to apply the spray is just before a ma jor growth cycle. For the present, the formula of zinc sulfate-lime is tentatively recommended for the correction of severe cases of zinc deficiency, and one-half that amount is recommended for the correction of mild cases of the trouble. Experiments to be conducted during 1940 should determine, to a large extent, the quantity of zinc neces sary for maintenance of correction. Observational evidence is strongly indica tive that the use of synthetic forms of nitro gen to the exclusion of organic sources of this element is conducive to the development of zinc deficiency symptoms. It is entirely possible that the synthetic forms of nitro gen may be used without abnormal growth response, provided the zinc requirement of the trees is satisfied by application of sprays or by other means. It is also evident that drouthy periods are favorable to the develop ment of zinc deficiency symptoms, provided adequate water is not supplied by irrigation. COPPER SULFATE AS A CORRECTIVE FOR DIEBACK, A NEW DISEASE OF THE AVOCADO GEO. D. RUEHLE and S. J. LYNCH Sub-Tropical Experiment Station, Homestead A serious disease, causing dieback of young avocado trees and their death in many in stances, was observed in Highlands County this past season. It was brought to our at tention in September 1939, when a specimen tree was sent by Mr. W. F. Ward to the Sub- Tropical Experiment Station for diagnosis. Examination of the specimen indicated that the trouble was physiological in nature, since no pathogenic organism could be found in the affected tissues. Reference to a disease of the avocado with similar symptoms could not be found in the available literature. Groves of avocados Under two years of age in Highlands County were visited early in October. The disease was evident in all groves except one, in which the trees were growing in soil that had previously supported a citrus grove. The condition was severe in groves planted on virgin soil. In Mr. Ward's young avocado grove, af fected trees had been given various soil treat-

2 Ruehle-Zinc Deficiency of The Avocado Plate 1. Little-leaf of Lula avocada on left; normal leaf on right Plate 2. Branch of Lula avocado showing severe little-leaf

3 Ruehle-Zinc Deficiency of The Avocado Plate 3. Correction of little-leaf condition on Lula avocado following zinc sulfate spray. Photo taken September 6, 1939; spray applied in May, 1939.

Ruehle-Zinc Deficiency of The Avocado Plate 4. Severe zinc deficiency symptoms on Lula avocado tree. Photo taken Septem ber 6, 1939.

4 Ruehle-Zinc Deficiency of The Avocado Plate 4. Severe zinc deficiency symptoms on Lula avocado tree. Photo taken Septem ber 6, Plate 5. The Lula tree shown in Plate 4 after spraying with zinc sulfate. Photo tak en March 18, 1940; spray applied Sep tember 6, 1939.

RuehleSpraying Experiments for Control of Avocado Anthracnose Plate 1. Stage of development of bloom spikes when first anthracnose spray should be applied.

5 RuehleSpraying Experiments for Control of Avocado Anthracnose Plate 1. Stage of development of bloom spikes when first anthracnose spray should be applied. Ruehle and Lynch-Copper Sulphate as a Corrective Disease of Avocados for Dieback-a New r Plate 1. Old leaves from severely affected tree. Note prominence and discoloration of veins. Plate 2. Multiple bud formation and dieback of young avocado tree. Photo taken November 22, 1939.

$Ruehle and Lynch-Copper Sulphate as a Corrective for Dieback a New Disease\of\bvocados Plate 3. Untreated tree, March 6, 1940, showing severe multiple bud formation and dieback.$

6 Ruehle and Lynch-Copper Sulphate as a Corrective for Dieback a New Disease\of\bvocados Plate 3. Untreated tree, March 6, 1940, showing severe multiple bud formation and dieback. This tree was just beginning to form multiple buds on November 22, Plate 4. Correction of a severe case of multiple bud and dieback of young avocado tree. Copper sulfate applied to soil November 22, 1939; photo taken March 6, 1940.

FLORIDA STATE HORTICULTURAL SOCIETY 153 ments, including a mixture of sulfates of cop per, iron, manganese, and magnesium in a well balanced fertilizer high in organic sourc es of nitrogen.

7 FLORIDA STATE HORTICULTURAL SOCIETY 153 ments, including a mixture of sulfates of cop per, iron, manganese, and magnesium in a well balanced fertilizer high in organic sourc es of nitrogen. Some had been given applica tions of poultry or stable manure, and others had received fertilizer treatments of differ ent analyses. The only treatment which ap peared to be beneficial was the mixed fer tilizer containing copper, manganese, and magnesium. Symptoms Affected trees appear somewhat starved in the incipient stages of the trouble. Older leaves have a dull appearance, with the veins becoming prominent at first and then assum ing a reddish-brown color which may gradu ally spread into the leaf blades (Plate 1). The premature shedding of such leaves may or may not be accompanied by a dying back of the tips. The symptoms in more advanced stages have certain characteristics in common with diebacfc of citrus. Multiple buds form at the tips of the twigs, sometimes forming a clus ter three-quarters of an inch in diameter (Plate 2X These buds attempt to put forth new leaves, but almost immediately begin dry ing up and dying back until the entire twig showing tie multiple bud growth is dead (Plate 3). Occasionally an affected tree will start new branches lower down on the trunk, which may in turn develop the multiple bud condition and die back. This condition may extend over a period of six months or more before the final death of the tree oc curs in severe cases. The disease appears to be confined to trees growing in the light sandy soils of the Ridge section, although a careful survey of the situ ation in other sections of the State is in complete to date. Characteristic symptoms have been observed in a few instances on avo cado seedlings, but the trouble is more pro nounced on budded trees. Dieback without multiple bud formation was observed on a lone seedling tree of bearing age, which was growing in a poultry yard near Lake Placid. This tree had been showing more or less dieback for several years, according to the own er of the property. The disease was not ob served in bearing trees growing in commer cial groves. Experiments The favorable response by affected trees to a fertilizer containing copper, manganese, and magnesium, in Mr. Ward's grove, indi cated that the trouble is probably a manifes tation of deficiency of one or more of these elements. Absence of the disease in bearing trees which have been sprayed annually with copper fungicides for control of fungous di seases, indicated that copper might be a cor rective. Moreover, the absence of character istic symptoms on young avocado trees grow ing in soil which formerly supported old cit rus trees that had been sprayed in past years with bordeaux mixture, was also indicative that copper might be beneficial. Soil applications of copper sulfate and of magnesium sulfate were made to several se verely affected trees in two groves on October 12. The materials, applied in each case at the rate of two ounces per tree, were spread even ly and worked lightly into the soil within a radius of about 18 inches from the trunks. The five trees treated with copper sulfate were showing decided improvement by No vember 22. New foliage which had developed since the treatment was normal in appear ance, and where the multiple bud growth had died back new branches were appearing be low the dead area. Untreated trees and those treated with magnesium were showing more pronounced symptoms of the disease than they had shown six weeks earlier. Additional trees were treated with various materials in the same manner on November 22 in a grove near Lake Placid. Copper sul fate, manganese sulfate, magnesium sulfate, and a combination of copper and manganese were included in this series of treatments. Two trees were also given soil applications of borax at the rate of one-half ounce per tree. The trees were protected through the freeze in January 1940 by banking them with soil, and little damage from cold occurred.

8 154 FLORIDA STATE HORTICULTURAL SOCIETY They were examined early in March for re sponse to the different treatments. The dieback condition had become more pronounced on untreated check trees (Plate 3) during the winter months. On the other hand, five of the six trees receiving the cop per sulfate treatment in November were showing good recovery in March, with nor mal twig and foliage development (Fig. 4). The sixth tree receiving copper sulfate had died before the freeze, probably because the disease had progressed too far before the treat, ment was applied. Of four trees receiving both copper and manganese sulfates, one had been severely injured by the freeze, two had made good recovery, and the fourth was growing feebly in March from basal shoots which had appeared after the top was killed back either by cold or by disease. The results from treating three severely affected trees with manganese alone were conflicting. One had made good recovery, but the others had apparently received no benefit from the treatment. The magnesium treatment had again failed to correct the dieback condition, and borax apparently served only to hasten the death of the trees. The freeze in January severely killed back the trees in the grove where the first treat ments were made, so that further observa tions on their response could not be made. A number of trees in a young avocado planting near Sebring had died from the same disease during the summer of 1939, and scat tered trees throughout the grove were show ing symptoms of the trouble in November. It was suggested to the owner that a copper spray be tried for correction of the condi tion. The suggestion was followed, but un fortunately checks were not left in the grove. All of the trees were sprayed in November with a mixture containing copper sulfate, manganese sulfate, and lime. The grove sur vived the freeze with only slight damage. The trees were generally showing normal growth in March 1940 without evidence of further development of the disease. Discussion The disease of young avocado trees, here in described for the first time in Florida, may be called "dieback" appropriately. Although it has been observed thus far only in groyes planted on the light sandy soils of Highlands County, a complete survey of young avocado plantings may show that it occurs elsewhere. Dieback has been under observation for a relatively short time and hence the informa tion is not complete regarding its etiology and control The experimental work here reported has been exploratory in nature. The work to date would indicate that dieback of young avocado trees is probably a copper deficiency disease, and that it can be corrected by applications of copper sulfate to the soil if the trouble has not pro gressed too far. Detailed recommendations for treatment cannot be given at the present time since the tests have not been extensive enough to answer definitely the question of optimum time, amounts, or even the form of application of copper. The disease has cer tain characteristics in common with "dieback" or "copper deficiency" of citrus. It is entirely possible that copper sulfate applied with lime as a spray will correct dieback of the avocado as it does dieback of citrus, but this point has not been definitely determined by experimental tests.

A Guide to Citrus Nutritional Deficiency and Toxicity Identification 1

HS-797 A Guide to Citrus Nutritional Deficiency and Toxicity Identification 1 Stephen H. Futch and David P. H. Tucker 2 Citrus trees in commercial and dooryard plantings can exhibit a host of symptoms