Phony Peach and Peach Mosaic

A. E. Cavanagh, C. H. Rothe.

Phony peach and peach mosaic are virus diseases of peach. The first recorded observation of phony peach was made at Marshallville, Ga., more than 50 years ago. By 1915, the disease had increased to the extent that commercial peach growers in central Georgia were alarmed and J. H. Hale, a grower, asked aid from the Department of Agriculture. He said the disorder seemed to be the most serious menace to the industry ever encountered in that locality.

After preliminary investigations were made, the United States Peach Disease Field Laboratory was established at Fort Valley, Ga., in 1921. The phony peach control project was organized in 1929 shortly after the virus nature and communicability of the disease had been demonstrated by Lee M. Hutchins, of the Department of Agriculture.

The disease that later was known as peach mosaic was brought to the attention of the Department in 1931 by inspectors who were making a survey for the presence of the phony peach disease near Bangs and Clyde, Tex. It later developed that a new peach disease, which proved to be peach mosaic, had attracted the attention of Peach growers and pest control officials in Colorado that year. Growers in southern California detected a new peach disease in 1933. It also proved to be mosaic.

Both phony and mosaic produce a symptom complex in peach. Phony-infected trees are dwarfed, have a deep-green color, and present a rather compact appearance in comparison with healthy trees. The internodes are shortened and foliage is flattened. Viewed from a distance, the phony tree has a more or less even outline; a normal tree with long terminal growth presents an irregular outline. Phony trees bloom earlier and come into leaf earlier than normal trees and retain their foliage longer in the fall. Fruit on infected trees is much smaller in size and quantity. Phony weakens but does not kill the tree as do some other virus diseases such as yellows, rosette, little peach, and X-disease.

A chemical test helps to identify phony. Sections of roots or twigs from phony-infected trees show numerous, well distributed, purple spots after 3 to 5 minutes in a solution of methyl alcohol and hydrochloric acid. For general use the solution should be about 15 percent acid. There is no known chemical test as a confirmatory aid in identifying peach mosaic.

Mosaic-infected trees present symptoms that vary with the season, the variety, and the part of the tree affected. The more important symptoms may be classified in five general groups: Color breaking in the blossom petals of the varieties that have large pink blossoms; retardation of foliage development; mottling and deformity of leaves; deformity of fruit; and abnormal twig growth.

Mosaic is transmitted in nature, presumably by insects. It may be transmitted artificially by budding or grafting from any part of the affected tree. Mosaic, like phony, does not kill the tree.

Inspections made in 1929 1952 demonstrated a general distribution of the phony disease in Alabama, southern Arkansas, northern Florida, Georgia, Louisiana, Mississippi, south central South Carolina, and eastern Texas. Local areas of infection were found in Illinois, Kentucky, Missouri, North Carolina, and Tennessee. Isolated cases occurred in Indiana, Maryland, Oklahoma, and Pennsylvania.

Similar surveys in 1935-1952 for mosaic revealed a general infection in Arizona and New Mexico and in parts of central and northeastern Texas; Riverside and San Bernardino Counties, Calif.; Mesa County in Colorado; Bryan County, Okla.; and Grand and Washington Counties in Utah. Localized areas of infection were found in other counties of those States and in southwestern Arkansas.

Both phony and mosaic occurred in 1952 in two States, Arkansas and Texas.

Peach mosaic has been observed in Baja California, Coahuila, and Chihuahua in Mexico.

Nearly 2 million peach trees were made unprofitable by the ravages of phony between 1929 and 1952 and were destroyed. In Georgia more than a million infected trees were destroyed before 1929. In the Southwestern States approximately 300,000 trees are known to have been destroyed because of mosaic infection evidence that, if phony and mosaic were allowed to go unchecked, the peach industry could not prosper in those areas. The several States in which phony or mosaic are most prevalent and in which control programs are centered produce about two-thirds of the peaches in the United States and a yearly average of 10 million peach nursery trees.

THE ONLY CONTROL thus far devised for both phony and mosaic is the destruction, after inspection, of the infected trees. An obstacle is the incubation period, the time between the initial infection and the development of visible symptoms. The period varies from 14 days to 1 year for mosaic and from 1 to 3 years for phony. Obviously, therefore, when all visibly infected trees are eliminated, there are others which are actually infected but in which the disease is in the incubation period and cannot be detected.

In carrying out this process of elimination, one encounters other complicating factors. Almond, apricot, plum, prune, and certain varieties of peach, for example, are tolerant to the mosaic virus. Some of those hosts, in fact, are completely symptomless. Elimination of those species of Prunes, along with the visibly infected peach trees, is therefore necessary in some areas to obtain adequate control. It has also been established that wild plum is a host of the phony virus. Its elimination in orchard areas therefore must be included in an effective control program.

Department investigators in 1948 reported four species of leafhoppers to be vectors of phony. Studies of insecticidal sprays for leafhopper control were started in the hope that they might be another useful weapon against the disease.

Annual inspections have been made of peach trees in the control areas. Infected trees are removed immediately. Mosaic symptoms are more pronounced during the first part of the growing season. Phony manifestations are more evident during the last part. Inspections for mosaic therefore are made from April to August and for phony from June to October, depending on the locality. As an additional aid to control of phony, growers are urged to establish new orchard plantings as far away as possible from old orchards and thickets of wild plum.