Diseases of the Avocado

George A. Zentmyer.

The fruit of various species of Persea, a genus native to Mexico, Guatemala, and other Central American and South American countries, has been used for food by the inhabitants of those countries for centuries. Selections from those wild trees, of varied fruiting habits, have resulted in the development of the cultivated avocado trees of California, Florida, and Texas.

Avocado trees were introduced into California in the latter half of the nineteenth century, but the principal development of the industry in California and Florida has taken place since 1930. The industry is of even more recent origin in Texas.

A number of diseases of varying severity affect the roots, trunk, branches, leaves, and fruit of the avocado tree. The diseases are caused primarily by fungi, although one virus disease is known. Several troubles are the result of deficiencies or excesses of certain elements used by the trees.

PHYTOPHTHORA ROOT ROT is the most serious problem in avocado culture in California, where it used to be known as "decline." Root rot of avocado also occurs in Costa Rica, Florida, Peru, South Africa, Honduras, Puerto Rico, Mexico, and Texas. The disease has been of little consequence in Florida, although it seems to be on the increase. Root rot was first noted in 1950 in Texas.

Two factors are involved the soil fungus Phytophthora cinnamomi and wet soil conditions brought about by poor drainage.

The causal fungus was first identified by R. D. Rands in 1922 on cinnamon trees in Sumatra and was first reported on avocado by C. M. Tucker in Puerto Rico in 1928. P. cinnamomi since has been found on an increasingly large number of plants, including pine, pineapple, heather, camellia, rhododendron, cinchona, chestnut, peach, yew, a number of deciduous and coniferous nursery trees, and Lawson cypress. V. A. Wager reported the isolation of P. cinnamomi from avocado roots in California in 1942.

Root rot increased greatly in California between 1940 and 1953. By 1953 it was estimated that 2,500 acres had been affected, or approximately 175,000 trees, including trees that either bad died or were in various stages of disease.

Root rot occurs in California on two main soil types heavy soils and soils having a permeable surface horizon of relatively shallow depth underlain by an impervious layer of clay or rock. In either case drainage is impeded and moisture conditions favorable for development of Phytophthora cinnamomi may occur. In other areas where root rot is found, and on other crops, it is also a disease of heavy or poorly drained soils. On well-drained soils the fungus rarely causes damage.

A gradual deterioration sets in of the above-ground parts of the tree affected with root rot. The general appearance indicates root destruction. Early symptoms include a lighter green color than is normal for leaves, a tendency for leaves to wilt in the presence of an amply moist soil, and a lack of new growth. As the disease progresses, branches die back, many leaves are shed, newly formed leaves are generally small and yellowish green, and fruit does not reach normal size. Frequently an abnormally heavy set of fruit happens soon after symptoms of the disease first appear evidently a reflection of the fact that much of the root system is rotted and food material has accumulated in the top.

The gradual decline in vigor and productivity of the tree usually goes on for several years, although occasionally the deterioration is more rapid. Many of the small feeder roots on affected trees are blackened, brittle, and dead; healthy roots are difficult to find.

The causal fungus is one of the "water molds." It thus needs wet soil conditions for its best development and for production of its two spore stages, swimming spores (zoospores) and resistant spores (oospores). The fungus thrives at moderate temperatures, making no growth below 50 F. nor above 90 F.

No adequate control measures have yet been found for the disease. At the University of California Citrus Experiment Station, investigations have been started on several aspects of control, including soil fumigation, soil fungicides, soil amendments, resistant rootstocks, crop rotations, and irrigation.

In years when rainfall is not excessive, careful irrigation can do much to retard the progress of the disease and prolong the life of affected trees. It has been observed that water use is markedly reduced in trees affected with root rot, evidently because of the destruction of the small feeder roots. Hence, if all trees in a grove, healthy and diseased alike, are given the same amount of water in one irrigation, water will accumulate in the soil around the diseased trees and accentuate the disease situation. To correct this, the soil-moisture conditions around each diseased tree must be determined at the time of irrigation, and individual trees should be irrigated on the basis of water use. This can be done effectively only where groves are irrigated with low sprinklers. Any condition contributing to the development of excessively wet soil conditions is favorable to progress of the root rot disease; hence particular care should be taken to avoid the occurrence of excess water in the grove.

Soil fumigation has various possible applications to the problem. Research in California has shown that several fumigants, such as chloropicrin at 30 to 40 gallons to the acre and D D mixture at 100 to 150 gallons an acre, will kill Phytophthora cinnamomi in soil. Field fumigation, however, has usually resulted in good growth of replants for a period of only 2 to 4 years, followed by recurrence of the disease. These plots were in areas of widespread infection; recurrence of the disease probably resulted from re-invasion of the treated locations by the fungus. The response of the resistant oospores to fumigants is not known, and there is a possibility that some of the reinfection came from germination of those spores. Fumigation may be useful under certain conditions, as in treating nursery soil or in eliminating the fungus from small isolated spots of infection in the grove.

In tests with soil fungicides, the organic fungicide nabam (disodium ethylene bisdithiocarbamate) has been found to be effective in killing Phytophthora cinnamomi in the soil. Research on this treatment is in its preliminary phases, but materials of this type have possibilities for application to various hosts of the fungus, as they are less injurious to higher plants than the common soil fumigants. Soil applications of alfalfa meal (ground stems and leaves) have also been shown in many cases to have some beneficial effect on diseased trees in California; this is attributed to the great increase in saprophytic fungi and bacteria in the soil resulting from these applications, and the possible antibiotic action of some of these micro-organisms against P. cinnamomi.

Phytophthora cinnamomi is very sensitive to drying of the soil. Air-drying a light loam to 1 percent moisture has resulted in kill of the fungus and excellent growth of replants in greenhouse experiments. In irrigated groves the spread of the fungus through the soil may be retarded by maintaining a dry, nonirrigated zone at the edge of an area where the disease is present. It may also be possible to make use of this principle in eliminating the fungus from nursery soil where trees are grown in large tar-paper pots.

The method of control that has the most possibility of success in the long run is the development of a rootstock that is resistant to root rot. Tests of 22 varieties of avocado seedlings, including Guatemalan, West Indian, and Mexican types, in California have shown all are similar in susceptibility. Other species of Persea from Mexico and Central America are being tested.

Inoculations with Phytophthora cinnamomi and limited field observations indicate that persimmon, cherimoya, macadamia nut, and citrus trees are either resistant or immune to root rot. These crops are possibilities for use in replanting areas where avocado trees have been removed because of root rot, at least in California. Other types of subtropical plants may be more adaptable in other regions. The principle of crop rotation is an old and valuable one in control of root rots of annual crops, and may have similar value in controlling avocado root rot.

It should be emphasized that little or no avocado root rot occurs on well-drained soils. A preventive measure, with respect to future plantings, is to plant only clean nursery stock on well-drained soils. Obviously, this is not a remedy for the problem in the case of the thousands of trees already planted on poorly drained soil. Nor does it take into account the fact that in some areas, because of the frost hazard, the best land for avocado plantings is hillside land, where the soil is often heavy or shallow.

Development of a rootstock that is resistant to the disease, that is compatible with present scion material, and that possesses no obvious disadvantages with respect to production or other diseases may ultimately solve the problem. Meanwhile, present measures to permit living with the disease include careful irrigation practices, drying the soil, replanting with resistant crops, and possibly use of temporarily alleviating measures, such as application of alfalfa meal or soil fungicides.

VERTICILLIUM WILT of avocado trees has been known as such since 1948, when I isolated the soil fungus Verticillium albo-atrum from affected trees and demonstrated the pathogenicity of the fungus to the avocado. Occasional reports during the previous 15 or 20 years in California and Florida had described sudden wilting and collapse of isolated trees in well-drained soils. In California the trouble was termed collapse, asphyxiation, or apoplexy and was thought to be the result of exclusion of oxygen from the soil, brought about by sudden saturation Of the soil with water. It is possible that lack of oxygen could cause such symptoms, but since 1948 V. alboatrum has been invariably isolated from trees with these symptoms in California.

Avocado trees affected with verticillium wilt show symptoms similar to those that develop on other woody hosts. The symptoms include a sudden wilting of all the leaves on a part of a tree, or on the entire tree, and the rapid death of the leaves. The leaves turn brown and remain attached to the branches for a long time. Typical brown streaks may be seen in the wood, when the bark is peeled from branches or roots of affected trees. Within a few months after the initial collapse of the tree, vigorous new shoots may appear, and within 1 to 2 years the trees may recover completely. Occasionally trees die from the disease, and occasionally the disease may recur in a given tree. Most of the affected trees observed in California have recovered completely and have shown no further symptoms. Similar observations have been reported in Florida.

Several suggestions as to control can be made. Use of land that has been planted to a susceptible crop should be avoided, and susceptible crops should not be interplanted after the land has been planted to avocados. Other plants affected by the fungus include tomato, pepper, eggplant, berries, apricot, potato, and a number of flower crops.

Severe pruning of diseased trees immediately after first symptoms of the disease appear may speed recovery. Avocado trees that are or have been affected with verticilliurn wilt should not be used as sources of budwood. It is possible that the disease may be transmitted in bud-wood, as with verticillium wilt of rose.