Virus Diseases of Stone Fruits

L. C. Cochran, E. L. Reeves.

Stone fruits have been grown in North America for some 300 years, but up to 1930 only five virus diseases were known to affect them.

Peach yellows, the first virus disease known to affect peach, may have been present around Philadelphia as early as 1750. Seven epidemics, the latest in 1920, wiped out orchards in different sections of the country. Little peach, thought to be caused by a strain of the peach yellows virus, became serious in the same geographic area as yellows in the late 1800's and now causes greater losses than yellows. Red suture, believed to be caused by another strain of the peach yellows virus, was first seen about 1910. Rosette and phony disease appeared in southeastern United States at about the same time that little peach appeared farther north. The phony disease has become the most serious virus disease of peach. More than 1,500,000 phony-affected peach trees have been removed from orchards in Georgia alone.

Since 1930, more than 40 new virus diseases have been found in North America on peach, nectarine, plum, sweet cherry, sour cherry, apricot, almond, and many ornamental and wild species of the genus Prunus.

It is indeed strange that so many troubles should have come upon us so suddenly. Growers and research men alike are inclined to ask why and from whence they came. They cannot be attributed to any foreign importation,because apparently only a few of those known in North America have been found in foreign countries and probably these were carried abroad from America. It appears then that most of our stone-fruit viruses are indigenous to North America and have gained access to stocks during the processes of fruit growing.

Peach yellows was recognized soon after peach culture was started in Massachusetts in 1630 or so indicating that the causal virus was present in other hosts, from which it probably spread to peach. As peach culture spread, peach yellows went along until it reached the limit of the range of the peach yellows vector. Peach yellows may have been carried to areas outside of the vector range in nursery stock, but since yellows eventually kills peach trees and there was no vector to spread the causal virus to other trees, the disease never became established.

Little peach and red suture of peach show relationship to yellows and are generally credited to strains that have arisen from the yellows virus. Rosette, on the contrary, has a different geographic distribution than yellows and has few characters in common with yellows. Rosette appeared on peaches soon after peach culture extended into Georgia, pointing to an indigenous occurrence, possibly on wild plums. The phony disease also appears to have started in Georgia, from where it spread north and west.

Peach mosaic was already widely distributed when it was discovered in 1931. Surveys during the next few years showed it present more than 600 miles south of the border in Mexico and in various localities in the Rio Grande and Colorado River Valleys of the United States. The high incidence in several Prunus species in those areas indicates that it may have been present for a long time before it was discovered. Not until it escaped into the areas of intensive culture, where it produced striking effects on peach, was it seen. The distribution indicates that peach mosaic may have been centered in the vicinity of El Paso and was locally spread among the Indian villages in infected plums, which were commonly propagated by transplanting sucker shoots. It was probably introduced into the peach areas of Colorado, Utah, and southern California and Texas in infected nursery stock, which had been grown near infected plums. Sour cherry yellows appears to have been present in orchards for a number of years but was not recognized because of the presence of other disorders which caused leaf casting.

The distribution of X-disease and western X-disease is more difficult to explain. Reports of growers and the extent of occurrence indicate that both had been present in some areas for a long time before they were recognized. The variation in symptom expression in different areas indicates the presence of fairly widely varying forms. When X-disease was first seen it was limited to one locality in Connecticut, but during the next few years it spread rapidly westward in chokecherry and within 15 years was well established in the north central Midwest.

Cherry buckskin was first seen several years before X-disease was recognized on peach, and at that time its occurrence was limited to a small area in the Bay district of central California. It is now recognized that the virus causing cherry buckskin infects peach, with symptoms nearly identical with those of western X-disease. Western X-disease is rapidly becoming widely distributed in the major peach areas of central California. Western X-disease is also widely dispersed in the central valleys of Washington and Oregon east of the Cascade Range, in southern Idaho, and central Utah. The similarity of symptoms on various hosts produced by the viruses causing X-disease and western X-disease of Peach, buckskin of cherry, and possibly others points to the possibility that they may have stemmed from a common source.

The origin of many of the diseases, particularly on cherries in the western United States, is something of a mystery. Stone-fruit culture in that area, particularly the nursery business, is still regarded as a relatively young industry. It seems logical that some of the viruses may have been present on native hosts and as fruit crops moved in they spread to fruit trees. The present occurrence of certain diseases with a relatively slow rate of spread would fit well into such a theory. There is good evidence to show that the cherry mottle leaf disease of sweet cherries was present in cherry trees near Wenatchee more than 20 years before it was recognized. It has been shown to be indigenous in native stands of bitter cherry near infected orchards, but spread into commercial orchards was relatively slow before 1930. Diseases with a rapid rate of natural spread, such as albino cherry in southern Oregon and little cherry of the Kootenay Lake area of British Columbia, point to either a new introduction or introduction into a host favorable to an efficient vector.

To postulate the origin of viruses is like trying to account for other kinds of life. There is no evidence to support an origin other than that they have evolved in the same way that other forms have. When they become prevalent in an area it is merely evidence that they have either been introduced into an environment favorable to them or that they were present and the environment has been changed so that it is favorable.

THE ECONOMIC IMPORTANCE of individual virus diseases affecting stone fruits is strikingly variable.

Peach yellows became so extensive in northeastern United States and the Great Lakes States in epidemic eras that it caused peach culture to disappear from certain localities for a time. Phony peach continues to be a serious threat to peach culture, particularly in Georgia and Alabama. Peach mosaic has caused the loss of more than 200,000 peach trees in southwestern United States and remains uncontrolled in many areas. Western X-disease has caused extensive losses in peach. orchards in Northwestern States and is responsible for disappearance of peach culture in some localities. Surveys in several States indicate that upwards of one-third of all the sour cherries in the United States are affected with sour cherry yellows. Fruit production on thoroughly diseased sour cherry trees is reduced to 50 percent or less, thus indicating a total crop reduction of at least 15 percent. Interpreted in terms of a crop worth 45 million dollars, sour cherry yellows causes an estimated loss of more than 5 million dollars annually.

Some other virus diseases offer potential threats to stone fruits because of their ability to destroy trees or crops, but are limited in distribution and have caused only locally serious losses. Albino cherry is chiefly responsible for disappearance of cherries from the Rogue River Valley in western Oregon, where it is has spread rapidly through orchards. The disease is limited to that area, which is isolated from the more important cherry-producing areas by mountains. Sour cherry pink fruit, cherry rasp leaf, peach wart, peach yellow bud mosaic, and apricot ring pox are diseases that ruin the commercial value of their hosts and have caused serious local losses, but are still limited in occurrence to local areas.

Cherry twisted leaf, cherry mottle leaf, and cherry necrotic rusty mottle, also of only local occurrence, ruin the value of some horticultural varieties of cherries but cause only partial crop losses to others. In contrast to them is little cherry, a disease affecting sweet cherries in the Kootenay Lake area of British Columbia. Its occurrence was limited to a single orchard when first observed in 1933, but during the next 15 years it spread over an area 100 miles in diameter despite mountainous barriers, large lakes, and the great distances between orchards.

Little cherry reduces the fruit size to one-half normal size and ruins the flavor, yet causes no tree or leaf symptoms. The rapid rate of spread through the infected area and the serious effects it has on fruit make it a serious threat to sweet cherries in other districts.

A sizable number of virus diseases affecting stone fruits cause only small losses. Some have produced striking effects on a few trees but are limited in occurrence. Others are of wide distribution, but cause only minor effects. Ring spot is nearly universal in some Prunus species, particularly cherries, in western United States; after the initial stages of infection, however, damage appears to be limited to a possible reduction in tree vigor. While these viruses are relatively unimportant economically, their presence complicates research.

THE EXPRESSION of virus diseases in plants is usually indicated by development of some consistent off-type or abnormal change in appearance as compared to normal or healthy plants; such abnormal characters are referred to as symptoms. Demonstration that a given symptom is caused by a virus is generally accepted if that symptom is reproducible on a comparable previously healthy plant following some mode of transmission in the absence of any visible pathogen. Since plant viruses are obligate parasites which cannot be cultured outside of their hosts and are generally too small for their presence to be determined with ordinary equipment, it is necessary to depend on the symptoms they produce to indicate their presence and determine their identity.