"Another fungus, a species of Colletotrichum, is often observed in diseased spots on leaves and fruits. This fungus is closely related to Gloeosporium and the injuries with which it is associated resemble those caused by the latter fungus. It is probably the cause of some of the injuries that are classed as anthracnose.
"In the control of these leaf and fruit spots, Bordeaux mixture has given satisfactory results where applied in time. As soon as the injuries begin to appear, spraying should be made and continued until the disease is checked. Two or three applications may be necessary, made at intervals of two or three weeks. If the fruit is near maturity, it is advisable to substitute ammoniacal solution of copper carbonate for the Bordeaux mixture, to prevent any disagreeable stain that may result from the use of the latter. Aside from spraying, all dead wood should be kept out of the trees, as this is likely to harbor these fungi from one season to the next.
"Avocado scab is of more than ordinary interest, owing to its close connection with citrus scab, and the fact that it has come into existence within the past three or four years. It is in all respects a new disease that has had its beginning in Florida.
"Scab is chiefly a disease of the tender growth, and at present it is found more abundantly in the nurseries, where it is particularly severe on seedling plants. It also attacks budded varieties in the nursery. The disease has been found on young and old bearing trees in the groves, affecting the leaves, and in a few cases the injury was observed on fruits. At present it is more common in the nurseries, but it may soon prove a serious pest in the groves.
"Scab forms definite spots or patches on the young, tender leaves and shoots, and severe attacks may cause the foliage to curl or become distorted. The more mature leaf tissue is not affected, but old leaves will be found bearing spots that were formed when the tissue was young. The spots are usually small, raised, circular to irregular, purplish brown to dark in color, and may vary from a sixteenth to an eighth of an inch in diameter. They may appear scattered over the surface, or several may grow together, forming irregular patches. The spots penetrate the leaf tissue, and they are visible on both sides. They are usually more prominent on the upper surface of the leaf, in which case the under surface of the spot will be slightly bulged and marked by a discolored area. The centers of the spots are composed of dead cells, more or less spongy in character and brownish in color.
In the earlier stages the surfaces may show a fuzzy, whitish growth -the fruiting parts of the fungus. The surfaces of older spots are darker in color and frequently covered with a dark webby fungous growth. On young shoots and twigs the spots appear more elevated, small, oval, dark purplish brown to black, and have comparatively smooth surfaces. This same type of spot is observed on the fruits.
"It is plainly evident that the avocado scab fungus is none other than Cladosporium citri, which causes citrus scab. The two fungi agree in structure and growth habits, and both are parasitic on citrus.
"Only tentative control measures for avocado scab can be suggested at the present time. Spraying with Bordeaux mixture for the disease in the nursery has given good results in some cases, in others less satisfactory. If the new growth can be protected while it is putting out, the disease may largely be avoided. The sprayings should be made when the foliage begins to put out, and continued until the leaves are nearly developed. The 1 4 50 Bordeaux mixture may be applied at intervals of ten days or two weeks, or often enough to keep the young foliage well protected. The fungus develops more rapidly during cool weather where moist conditions are provided. Shade and a crowded condition of the trees also seem to favor the development of the scab."
Many growers in southern Florida who have planted the Trapp avocado have been troubled by the trees dying back following the production of a heavy crop of fruit. Krome of Homestead has given this subject much study, and writes as follows regarding it in the 1916 Report of the California Avocado Association:
"Avocado trees of the West Indian race, when in good condition of growth, are prone to put on a tremendous bloom from which a setting of fruit is apt to result so heavy as to be entirely beyond the carrying capacity of the tree. Following this abnormal effort there is often a period of apparent exhaustion during which the tree seems to realize that it has 'bitten off more than it can chew,' and to be seeking the best method to recoup from its over exertion. This is a critical time in the life history of the tree and calls for intelligent handling on the part of the grower. If left to its own devices the tree will endeavor to carry the over crop, draining upon its reserves until its vitality has been seriously impaired. Evidences of this condition are usually very apparent. The tree drops a large portion of its leaves, the younger branches change in color from a dark green to a saffron yellow and no new growth is put on. Lack of sufficient foliage to provide proper shade often results in serious sunburning of the more tender branches, and the low state of vitality lays the tree particularly liable to the inroads of disease, especially of the anthracnose fungus which seldom loses such an opportunity for making an attack. Finally the tree is compelled to drop practically its entire crop of fruit and is left in a condition which means, at the very best, a set-back of two seasons in its development and not infrequently results in its actual death.
"To obviate overblooming, particularly in the case of young trees, is very difficult, for the better the cultural condition of the tree, the more likely this is to occur. The usual procedure has been to thin the over crop of fruit and this method of handling works quite satisfactorily provided the set-back to the tree has not already been brought about through the excessive bloom. However, the avocado requires a longer period than most fruits between the first appearance of the bloom and the setting of the fruit and it often happens that the damage to the tree has made considerable advance before relief by stripping can be obtained. In this event removal of the entire crop and further careful attention is necessary.
" In an effort to overcome this difficulty, I have during the past two seasons resorted to frequent applications of fertilizer, in order to offset the heavy drain upon the vitality of the trees during the blooming period. In the spring of 1916, following a season favorable to growth, the avocado trees at Medora Grove began to bloom about the middle of March. Immediately afterward a light application of fertilizer, carrying ammoniates from readily available sources was made. The bloom was the heaviest known in a number of years and persisted until about the middle of April. Between April 15th and 20th, another light application of the same fertilizer was made and this was followed by a third application the latter part of May, when a fertilizer somewhat higher in phosphoric acid, largely derived from low grade tankage, was used. As a result of this treatment a full crop of fruit was set and in most cases carried through to maturity without damage to the trees. When an over crop was set at first, as a rule dropping took place without a reduction in vitality, until the proper carrying capacity had been reached, and the remainder of the crop was matured. In a few cases stripping was necessary, but among nearly two thousand trees of varying ages, not more than eight or ten showed any appreciable damage."
In both California and Florida, avocados sometimes crack open while hanging on the tree. This has occurred in varieties of the Guatemalan and Mexican races, but is most common in the latter. The cracks are usually situated towards the apex of the fruit, and are often very extensive. W. R. Home, H. S. Faweett, and others have noted the presence of several fungi in the cracks and the flesh beneath them, but up to the present it is believed that these fungi are secondary, and not the cause of cracking.