MOST OF THE viruses affecting stone fruits appear to exist in nature in few to many forms or strains. No variants are known for the viruses causing phony peach or peach rosette. Peach rosette kills trees so rapidly that it is entirely possible that variants have not had sufficient time to become segregated. If rosette should infect hosts which it does not kill, variants might become expressed. Peach yellows also kills peach trees, but less rapidly than peach rosette. Little peach and red suture are believed to be caused by strains of the peach yellows virus and may have arisen because of the quality that neither of them kill peach trees as rapidly as yellows. Yellows does not kill plum trees and the little peach and red suture strains may have arisen in plums and spread to peach. Both little peach and red suture appear to be stable strains and can be obtained and identified from orchards at will. Peach mosaic and ring spot, on the other hand, exist in innumerable variants, forming a gradient from one extreme of symptoms so mild that diagnosis is difficult to effects that severely damage the trees. Variations of other features, besides severity, also occur, such as type and amount of mottle, amount of dwarfing, rate of movement of virus through trees, and others. The variations are so numerous that new cultures obtained from naturally infected orchard trees, while they can be fitted into the gradient, cannot be definitely identified with previous cultures.

A number of other stone-fruit virus diseases, such as western X-disease sweet cherry necrotic rusty mottle: and mottle leaf, are known to have variable symptoms, caused by the existence of many virus forms. As evidence accumulates it seems likely that some of the diseases that have been described separately are actually merely expressions of different forms of one virus. Certainly X-disease, western X-disease, and buckskin produce similar symptoms on peach although they produce somewhat different symptoms on sweet cherries in different areas. Lambert mottle and necrotic rusty mottle produce similar symptoms on Lambert cherry but differ on other varieties. Ring spot, as originally described on peach, appears to be very similar to necrotic ring spot described on sour cherry.

THE CONTROL of virus diseases of stone fruits is complex because of the many variable features of the diseases. Measures applicable and sufficient for one disease may not suffice for another or even for the same disease in another area. Before satisfactory control measures can be recommended, the nature of the disease, its distribution, host range, rate and manner of spread, effect on yield, and other facts should be known. In general, control procedure can be divided into two categories: Prevention by exclusion and (if already present) reduction by removal of diseased trees or application of other procedures that reduce their detrimental effects.

All evidence indicates that viruses, like other forms of life, do not arise out of nowhere but are direct descendants of preexisting forms; therefore procedures that exclude them are the most efficient means of control and should be used wherever practical.

Federal quarantines have been promulgated to prevent entrance of viruses and other pests from foreign sources. State quarantines have been established to prevent movement from infected areas in one State to another part of the same State or into other States. In general, quarantines are efficient only where areas are protected by natural barriers, such as bodies of water, deserts, and mountain ranges. Quarantines have also been used on new diseases or for outbreaks of disease in new areas while studies were being made to determine practical means of control or for slowing down spread while control or eradication was being attempted.

RECOMMENDATIONS for control of established virus diseases of fruit trees have generally been to remove infected trees from orchards. For certain diseases and in certain districts that procedure may still be generally recommended. The question as to whether removal of diseased trees, or roguing, is practical depends on the rate of spread, whether spread is originating in the orchard or from outside sources, how much damage the disease causes, and whether there are resistant host varieties or procedures to protect against infection.

Roguing coupled with isolation, use of disease-free nursery trees, and wild-host-removal programs have been the only practical means of control for some diseases, especially those that spread rapidly and severely damage all varieties of a given host. Those procedures have controlled peach yellows and reduced it from a dire threat to the peach-growing industry to the rank of a minor disease. Isolation in intensively cultivated fruit areas may be hard to achieve and may not be necessary for diseases that spread slowly.

For diseases which are generally distributed, have a rapid rate of spread, and have symptomless hosts or other features that make it impractical to remove diseased trees, other procedures of control have to be developed.

THE USE of resistant or tolerant varieties (varieties which, although infected, are not materially damaged), tolerant or resistant top and rootstock Combinations, mild symptom-producing virus forms to protect against infection by more damaging ones, chemotherapy, and control of vectors are approaches which offer promise as control measures. Heat has been used to kill certain viruses in infected bud-wood and nursery trees, and certain chemicals have reduced infection by others in experimentally inoculated trees. No treatment with chemicals, spray materials, fertilizers, or other materials has resulted in the cure of virus-diseased fruit trees in the orchard.

There is ample evidence that stone-fruit virus diseases have been spread in infected nursery stock. Control procedures in general are ineffective unless such spread is stopped. Nursery-improvement programs are under way in several States, but because they are concerned with different diseases and different conditions, procedures and specifications which have been developed are variable. There are some general considerations that are regional in scope and there is need for consideration on a regional basis. Nurserymen need the assistance of research, regulatory, and extension men and growers. Growers need the advantages of starting orchards with better stock and can assist much in the problems by working closely in cooperation with those trying to help them.

L. C. COCHRAN is in charge of investigations of virus diseases of deciduous fruits for the Bureau of Plant Industry, Soils, and Agricultural Engineering. He is a native of Indiana and holds degrees from Purdue University and Michigan State College. Before joining the Department of Agriculture in 1941, he was engaged in investigations of citrus and peach virus diseases at the Citrus Experiment Station of the University of California at Riverside.

E. L. REEVES is stationed at the United States Horticultural Laboratory at Wenatchee, Wash., and is a pioneer in investigations of stone fruit virus diseases occurring in northwestern United States. He was born in Idaho and holds degrees from the University of California and Washington State College.