Typical symptoms are a yellow streaking of the green stems and branches and a yellow-to-red streak on the fruit. On fruit that remains green at maturity, the streak is yellow. On fruit that turns black or purple at maturity, the streak is usually red. A slight deformation and vein chlorosis may occur in the leaves, but the stem and fruit symptoms are the most common. Affected trees tend to have a recumbent, willowy type of growth, and some may be stunted. A checking of bark on mature branches and trunks is often associated with the disease, but it has not been definitely established that this is a symptom of the disease.

Sun blotch is readily transmitted through budwood or graft wood. It is possible that the disease may also be transmitted through the seed, although this has not been proved under controlled, insect-proof conditions. The disease has been observed occasionally on young seedling avocado trees growing under conditions in which it is unlikely that insect transmission could be involved.

Control measures include careful selection of disease-free scion and seed sources and the removal of all seedlings showing sun blotch symptoms, as well as of all off-type seedlings, from nursery plantings.

CANKERS of roots, trunks, or branches of avocado trees may be caused by several organisms. Phytophthora cactorum and P. cinnamomi . are the two fungi most commonly involved in California in cankers of the lower trunk and rootstock. Artificial inoculations show that P. citrophthora, the fungus that causes brown rot of lemon fruits and brown rot gummosis of citrus trees, is capable of invading avocado trunks. A canker of minor importance on branches and trunks is caused by the fungus Botryosphaeria ribis.

Symptoms in the top of the tree vary from gradual loss of vigor and chlorosis of leaves to sudden death of the entire top. Examination of the trunk usually reveals a darkening of the bark and an exudation of powdery white mate-vial from the affected bark. When the bark in the cankered area is cut into, it is found to be brown in color and to have a noticeably sour odor. The brown discoloration often extends into the wood as well. On many trees the trunk is not completely girdled, but the canker may extend up the trunk several feet from the ground. A similar trouble known as collar rot has been reported from Florida.

If detected in a sufficiently early stage, cankers can be controlled by cutting out infected tissue and painting the treated area with a fungicidal paint such as bordeaux paste. In California the Guatemalan varieties of avocado appear to be more susceptible than other varieties to these cankers. It is therefore desirable to bud these varieties high and to avoid covering the bud union with soil in low-budded trees.

DOTHIORELLA ROT, the most important rot of avocado fruit in California, is caused by the fungus Botryosphaeria ribis (imperfect stage: Dothiorella gregaria). The disease is a serious problem on the Fuerte variety in plantings near the coast. In inland areas it is of relatively little importance. The fungus is commonly present on dead wood, dead leaf tips, and debris. It enters the fruit sometime before harvesting. After entering the fruit, it lies dormant, and rot does not develop until the fruit begins to soften, by which time it has reached the consumer. The fact that there is no method of detecting fruit that will develop this rot and culling it out in the packing house creates a difficult marketing problem.

This rot commonly appears first as small, brown, or purplish-brown spots on the green fruit surface. The spots gradually enlarge until much of the surface may be involved. In early stages there is little involvement of the flesh. As the disease progresses, however, the fungus invades the flesh and causes a brown discoloration and an offensive odor. Occasionally the fungus induces a stem-end rot. It may also invade fruit pedicels, causing the fruit to drop. In Florida the fungi Diplodia and Phomopsis are also involved in stem-end rot of fruit.

The following control measures are effective in reducing or eliminating this fruit rot: Removal of dead wood and dead leaf tissue from trees to reduce sources of fungus inoculum; use of all possible measures to reduce tip-burn of leaves; use of low rather than overhead sprinklers; picking of fruit before it reaches the peak of maturity, as it is not so severely affected in early season; and spraying trees.

Research in the 1930's indicated that 8-8 100 bordeaux mixture plus 6 pounds of wettable sulfur gave good control. It has been shown since then that Crag Fungicide 658 (1.5 pounds to 100 gallons), bordeaux 6-6 100, Cuprocide (2 pounds to 100 gallons), and zineb (2 pounds to 100 gallons) are effective in controlling the fruit rot. If rainfall is relatively light, as during the period from 1948 to 1951 in California, two sprays give good control, the first in mid-September and the second in early November.

CERCOSPORA SPOT OR BLOTCH, caused by Cercospora purpurea, is the most important disease of avocado in Florida. Lesions on fruit appear as small, scattered, brown, slightly sunken spots that have a definite outline but irregular shape. Grayish spore-bearing structures of the fungus appear on the spots in humid weather. These fruit spots, which are one-eighth to one-fourth inch in diameter, later develop cracks or fissures, which permit the entry of other fungi that cause fruit decay. The Cercospora fungus also causes small angular spots on leaves.

Research in Florida has demonstrated that the disease can be controlled by two or three copper sprays, the first between May 1 and May 15, the second not more than a month later, and the third a month after the second. The third is usually necessary only for varieties that mature in winter or early spring. G. D. Ruehle has shown that 6-6 100 bordeaux or 4-4-100 bordeaux (the latter where annual spraying is practiced), or wettable cuprous oxide (1.5 pounds to 100 gallons), or copper A (4 pounds to 100 gallons), or basic copper sulfate (3 pounds to 100 gallons) are equally satisfactory.

SCAB, the next most important disease of avocado in Florida, is caused by the fungus Sphaceloma perseae, which attacks both foliage and fruit. This fungus causes corky, raised, brownish, oval-shaped spots on the fruit. As the spots become older they may coalesce and give the fruit a russetted appearance. They may develop cracks that permit entry of other fruit-rotting organisms. Scabby, deforming lesions are also formed on leaves, leaf petioles, and twigs. The disease can be controlled by spraying with 6-6 100 bordeaux or 1-5-100 wettable cuprous oxide, using the same schedule as the one for blotch; very susceptible varieties need additional early applications.

There is considerable variation in susceptibility of the different varieties of avocado to this disease. Lula is listed as very susceptible; Hall, Taylor, and Booth 7 and 8 moderately susceptible; and Fuchsia, Pollock, Booth 1, Waldin, Itzamna, Linda, and Collinson quite resistant.

A THIRD COMMON DISEASE of avocado fruit in Florida, but one causing less damage than the blotch or scab, is anthracnose, or black spot, caused by Colletotrichum gloeosporioides. This disease is characterized by sunken black spots on the fruit, the spots being nearly circular in outline and one-fourth to one-half inch in diameter. As the fruit ripens, the fungus invades the flesh to a greater degree until most of the fruit is rotted. The fungus is unable to enter unwounded fruit. It usually becomes established in lesions caused by Cercospora or Sphaceloma. Where spray applications are made to control Cercospora blotch or scab, no additional sprays are necessary to control anthracnose.

POWDERY MILDEW (Oidium species; perfect stage unknown) occasionally is found in Florida on foliage in nurseries or on. young trees growing in shaded, damp locations. When young trees are affected, tender tips of shoots may be killed back. Dark-green discolorations may appear on the leaves, which show the characteristic white, powdery mildew growth on the lower side. Control may be obtained with lime-sulfur or sulfur dusts if the problem becomes acute.

NONPARASITIC DISEASES brought about either by deficiencies or excesses of certain elements occasionally are found on the avocado.

One of the most common is zinc deficiency, which results in a little-leaf condition, mottling of the leaves, and often a deforming of the fruit. If the deficiency is prolonged, the branches may die back. Best results for control in both California and Florida have been obtained with sprays. Either zinc sulfate and hydrated lime (5 pounds of ZnSO₄ and 2.5 pounds of lime to 100 gallons of water) or zinc oxide (2 pounds to 100 gallons) may be used. For severe cases in Florida, twice the indicated dosage of zinc sulfate and lime is recommended. Sprays should be applied soon after new growth appears.

Iron deficiency occasionally occurs in California, primarily in trees on calcareous soils. It is characterized by yellowing of the major portion of the leaf, with the veins remaining green. Little change is caused in fruit size or shape or in leaf size. Soil applications of sulfur and similar acidifying materials have generally been only moderately successful. Reduction in water applications to trees in the problem soils has usually resulted in a lessening of the chlorosis.

Copper deficiency has been reported in: Florida on young avocado trees. Symptoms include development of S-shaped shoots and lateral branches, premature defoliation, a multiple-bud condition, and dieback. This trouble is usually corrected by soil or spray applications of copper sulfate.

The presence of excess chlorides in soil or irrigation water is the primary cause of tipburn of leaves in California. Tipburn causes a considerable reduction of green-leaf area, with consequent weakening of the tree. The dead areas are also commonly invaded by fungi such as Botryosphaeria ribis, which then spread to the fruit. Control of tipburn is difficult unless water with a lower chloride content is available to use in leaching the soil.

GEORGE A. ZENTMYER is associate plant pathologist in the University of California Agricultural Experiment Station at Riverside, where he is engaged in research on diseases of the avocado and other subtropical plants and on fungicides. He has also worked on diseases of forest trees on the west coast, and on chemotherapy for control of the Dutch elm disease and other vascular diseases at the Connecticut Agricultural Experiment Station.