Foot Rot of Citrus Trees

L. J. Klotz, J. F. L. Childs.

Foot rot, mal di gomma, gummosis, and brown rot gummosis are some of the names given a disease that attacks citrus trees the world over.

Foot rot is as good a name as any it has priority of usage. Gummosis is a general term for several troubles in which gumming is a symptom. Brown rot gummosis was intended to indicate the causal relationship between the bark disease and brown rot of citrus fruits caused by species of Phytophthora. Mal di gomma simply means the gumming sickness, whether caused by Phytophthora species or other biological and physiological troubles.

First reported in the Azores in 1834, foot rot quickly spread to Portugal (1845) and other Mediterranean countries. By 1863 it was destroying thousands of citrus trees in Italy and Sicily and had appeared as far away as Australia. It appeared in Florida in 1876 or so and in California about the same time. Between 1834 and 1914 it had spread to virtually every citrus-producing region in the world.

The study of plant diseases was still young but it had made considerable progress by the time foot rot was ravaging the citrus orchards of the Mediterranean region. For example, Charles Moore, who went from Australia to study the foot rot problem in Spain, recognized in 1867 that the disease probably was caused by a fungus parasite. Several fungi and bacteria were falsely accused before it was proved in1913 that Phytophthora citrophthora is the real cause of foot rot. Two other species of phylophthora, P. parasitica and P. palmivora, were later isolated from foot-rot-like infections of citrus trees. P. citrophthora and P. parasitica are isolated with about equal frequency from foot-rot lesions in California.

The most widely recognized symptom of foot rot is the presence of diseased bark and the exudation of gum, usually near the soil line. Affected bark first looks dark and water-soaked, and underground in wet soil it may smell fishy or sour. Gum often exudes from diseased bark in sufficient quantity to flow down the trunk. The gum is most noticeable in dry weather because rain dissolves and washes it away.

Bark lesions usually are more extensive at or below the soil surface, but they may extend 18 inches or more upward. Infected bark is killed through to the wood, which is usually stained brown. In dry weather the dead bark dries, shrinks, and the margins split.

Foot rot may cause severe damage to the below-ground parts of the tree by periodically damaging the smaller permanent roots and the fibrous feeder roots. Under favorable conditions, the small roots are quickly replaced, but damage to the root crown is more serious and permanent. Through the root crown all the sugars and other manufactured food materials must pass on their way from the leaves to the roots. Serious damage to the crown region therefore usually means the death of the tree.

The work of R. E. Smith, L. J. Klotz, and H. S. Fawcett in California, Lillian Fraser in Australia, and Victoria Rosetti in Brazil has given us a fairly complete understanding of the growth relations of the foot rot fungus.

We know, for instance, that the fungus thrives best in damp locations, that free water is necessary for the formation of the swarmspores, and that those spores can swim about like Minute tadpoles until they come to rest on some suitable food material (Possibly a citrus root) or they die.

We also know that swarmspores are formed in spore sacs, or sporangia, which in turn are formed most readily at temperatures near 75 F., and that a drop in temperature, such as usually follows a thundershower, causes the swarmspores to be released into the water. The fungi are sensitive to heat, P. citrophthora being killed, for example, by exposure to temperatures of 112 for 1 minute, 111 for 2 minutes, 110 for 5 minutes, 105 for 90 minutes, 100 for 210 minutes, 98 for 25 hours and 91 for 75 hours.

The acidity of the soil has an important bearing on the survival and virulence of the fungus. Using Rough lemon seedlings grown in pots, Miss Fraser found that the greatest amount of root infection took place in soils between pH 5.4 and 7.5; moderate infection occurred at pH 4.8 to 5.0; and only very slight infection occurred between pH 4.3 and 4.5. The results obtained in the laboratory, confirmed her observations on the incidence of root decay under field conditions.

Another factor that has an important bearing on the incidence of foot rot is the susceptibility of the host. Even before the cause of the disease was known, growers recognized that foot rot occurred more often and caused more damage on some varieties of citrus trees than on others. That was true in the Mediterranean countries, where sour oranges were grown long before sweet oranges, lemons, or limes were introduced and where effects of the disease on the different varieties of citrus could easily be compared. Charles Moore, an Australian, visited Spain in 1867 and found sour orange used widely as a rootstock because it was known to be resistant to foot rot disease. Because of the general use of the basin method of irrigation, which promotes Phytophthora infections of the tree crowns, sour orange is the only rootstock that is acceptable and used in Italy and Sicily.

In 1896 the foot rot problem was studied in Florida and the use of sour orange rootstock was recommended for its control. R. E. Smith in California suggested that considerable immunity could be obtained through using Florida sour orange as a rootstock. At Professor Smith's suggestion, H. S. Fawcett started research on the cause of the disease and in 1913 announced that Pythiacystis citrophthora, later named Phytophthora citrophthora, was the causal organism.

Upper left: Green mold, a common storage decay that affects oranges. Upper right: Apple scald, an important storage trouble that causes much loss of fruit. Lower left: Stem-end rot of grapefruit, a fungus disease affecting the fruit on the trees and in transit. Lower right Phoma spot of tomato, a storage and transportation problem.

The discovery of the true cause of foot rot made it possible to study the susceptibility of the many varieties of citrus. L. J. Klotz and H. S. Fawcett in 1930 published the results of a study of the susceptibility of about 100 species and hybrids of citrus. They found that the various species of citrus and citrus relatives could be arranged in descending order of their susceptibility to foot rot as follows: Lemons (Citrus Limon), limes (C. aurantifolia,) pummelos (C. grandis), grapefruits (C. paradisi), sweet oranges (C. sinensis), mandarin oranges (C. reticulate), citrons (C. medica), sour oranges (C. aurantium), kumquats (Fortunella species), and trifoliate orange (Poncirus trifoliata).

Considerable variation in susceptibility was found among the species in each genus. For example, Imperial grapefruit was found to be no more susceptible than standard sour orange but Pernambuco grapefruit was more susceptible than some of the limes.