How Nurseries Get Virus-free Fruit Stock

L. C. Cochran, Earle C. Blodgett, J. Duain Moore, K. G. Parker.

More than 40 virus diseases affect stone fruits. Others attack pome, citrus, avocado, fig, and other fruits. No major fruit crop is free from virus diseases. Some virus diseases have caused the destruction of orchards throughout communities and others have ruined orchards in large areas. Some, more insidious, do not kill trees but take an annual toll by reducing the yield and quality of the crops. Still others produce only mild effects and are important chiefly because they complicate the question of control.

Part of the geographic occurrence of most of the virus diseases affecting stone fruit is traceable to distribution in infected nursery stock. Most fruit trees now are grown from nursery stock produced by budding or grafting the desired variety onto seedlings or rootstocks propagated vegetatively from cuttings. If the variety or the rootstock is infected with a virus, the resulting nursery trees usually will be infected. If the nursery is located near infected trees, viruses may spread naturally into nursery stock during the growing season. When diseased nursery stock is planted in a district where the disease was not previously present, spread may take place to other trees and the disease soon becomes established.

The question of nursery improvement is complex. The many factors that are involved vary among areas because different diseases are present, different varieties of fruit are grown, and different conditions exist in each. It seems impossible to devise a program with provisions that would be entirely applicable to all areas, although some general procedures can be formulated.

Because some virus diseases of stone fruits are known to have been carried in the nursery stock, a logical starting point for improvement is the use of procedures to eliminate them from scion and budwood sources and from rootstock seed sources. Growers of such sources then could be issued certificates indicating the standards that have been met. Certification has value only when it refers to specified definite standards.

Any plan for the production of certified fruit tree nursery stock could well make use of the same principles and procedures developed for certified seed potatoes: Establishing- disease-free foundation stocks true to variety; increasing the stocks in the field under rigid inspection and roguing; passing the stocks for certification if the number of diseased or off-type plants is maintained at or below a standard, which has been determined by practice to be necessary to insure high yields and good quality; and supervising sales to maintain the identity of the certified stock.

Considerable progress has been made toward nursery improvement in the more important fruit-growing States. The program has been mostly voluntary. The approach has been from different angles. Unknown and variable factors have prevented the formulation of any uniform procedure usable in all States.

In most of the States, the first step has been to inspect the orchard trees desired as a source of budwood and the trees adjacent to them. If no symptoms of virus or viruslike diseases are found, the nurseryman who gets buds from them may obtain a certificate that his trees were propagated from sources that had been inspected and found visibly free of virus diseases. The procedure has helped reduce the prevalence of such virus diseases as the peach yellows group, peach wart, and y certain cherry diseases, which are generally expressed on all the horticultural varieties of the affected host. It has also helped in the elimination of cherry diseases like mottle leaf, twisted leaf, rusty mottle, the necrotic rusty mottle; apricot ring pox; the psorosis of citrus; and other diseases that damage some fruit varieties but are only meagerly expressed on others. Orchard inspection has also materially helped in the elimination of certain viruslike non-transmissible but bud-perpetuated disorders, such as sweet cherry crinkle leaf, sweet cherry deep suture, almond bud failure, and Italian Prune leaf spot and sparse leaf, and has assisted in the selection of fruitful types true to variety.

The usual procedure has been for the nurseryman to apply for the service by a given date to the State department of agriculture in his State. The trees then are inspected in the proper season and given some sort of identifying designation. Standards have to be set, such as minimum age of the trees to be used and the distance from the nearest diseased trees. Because most of the diseases spread in orchards, any certificate based on orchard inspection is good only for the year in which the inspection is made and new inspections must be made each year.

Orchard inspection alone is not enough to determine the presence of all the viruses that affect fruit trees. Some viruses that are destructive to one variety may exist in another without symptoms. Buds from such infected but symptomless trees produce similar infected nursery trees, which carry the virus to orchard locations where the trees are planted. The mottle leaf virus ruins the Napoleon (Royal Ann) and Bing varieties of cherries, but may cause few symptoms or none on Lambert.

Environment, such as high or low temperatures, influences the expression Of symptoms of some diseases. Symptoms of leaf yellowing of the sour cherry yellows disease are expressed on sour cherry trees and damage is accentuated in areas where temperatures following petal fall are relatively low, but no leaf symptoms occur in areas where the temperatures are higher. Nursery stock propagated in warmer climates from vigorous-appearing trees can very innocently carry the sour cherry yellows virus and result in serious losses if planted in such areas as those near the Great Lakes, where summer growing temperatures are low.

The western X-disease virus, conversely, may not produce symptoms, especially on sweet cherries growing on mazzard rootstock in areas of high elevation where temperatures are low. Some virus diseases have long incubation periods; hence, if buds are cut from orchard trees in the early stages of infection before symptoms appear, they may carry the virus to nursery stock. The ring spot virus produces symptoms only during the acute or initial stages of infection on many fruit tree hosts, yet buds taken from trees in the chronic stages and showing no symptoms carry the virus.

The presence of viruses in symptomless trees is determined by indexing them on varieties that express symptoms. That commonly is done by budding healthy nursery trees of a susceptible symptom-expressing variety with buds from the suspect trees. Certain varieties and species are known to produce consistent and characteristic symptoms when infected with particular viruses. By use of a combination of such hosts in index procedures, a fruit tree can be tested for the presence of any of the known viruses. In order to keep the number of necessary hosts to a minimum, hosts that will express and differentiate a large number of the viruses may be used. Here is a list of index hosts and the diseases which each may serve to diagnose:

Peach. Elberta: Peach yellows, little peach, red suture, peach rosette, rosette mosaic, phony, peach mosaic, X-disease, western X-disease, yellow bud mosaic, wart, peach mottle, peach necrotic leaf spot, asteroid spot, golden-net, peach calico, peach blotch.

Peach. J. H. Hale: Ring spot, willow twig.

Peach. Muir: Muir peach dwarf.

Peach. Seedlings (open-pollinated seedlings of Lovell and Halehaven have been used): Necrotic ring spot, sour cherry yellows.

Sour cherry. Montmorency: Sour cherry yellows, green ring mottle, necrotic ring spot, pink fruit, peach mottle.

Sour cherry. On mahaleb: Western X-disease wilt and decline.

Sweet cherry. Bing: Buckskin, albino, mottle leaf, rusty mottle, mild rusty mottle, rasp leaf, twisted leaf, tatter leaf, small bitter cherry, western X little cherry, peach mottle.

Sweet cherry. Royal Ann: Black canker, cherry rugose mosaic, pinto leaf.

Sweet cherry. Lambert: Necrotic rusty mottle, little cherry, small bitter cherry, Lambert mottle, Utah Dixie rusty mottle.

Prunus serrulata vars. Shirofugen: Ring spot.

Prunus serrulata vars. Kwanzan: Other latents, rough bark.

Plum. Italian Prune: Prune dwarf. Plum. Shiro: Line pattern.

Plum. French Prune: Prune diamond canker.

Plum. Standard prune: Standard prune constricting mosaic.

Plum. Santa Rosa: Plum white spot. Apricot. Tilton: Ring pox.

Almond. Nonpareil: Drake almond bud failure.

It may not be necessary to index budwood sources in all areas on all of those hosts. For example, our evidence indicates that some of the cherry viruses are not present in peaches in some sections where only peaches are grown. Also, most peach virus diseases (or at least most of those that seriously damage peaches) affect all varieties of peaches similarly, and their presence generally is determined easily by orchard inspection. But cherries appear to be more commonly affected by viruses than peaches. Some viruses are ruinous on one variety of cherry but may infect another with only meager or no symptoms; index procedure to find virus-free trees therefore is needed for cherries more than for peaches.

The question of how much indexing should be done depends on what diseases are present in the area, the destination of the nursery stock, and the variety of the host.

Stone-fruit clones completely free of all viruses are difficult to achieve for a number of reasons. The complete host range of many of the stone-fruit viruses has not yet been determined. Completely satisfactory index hosts for all of the stone fruits have not been determined, particularly because of the variability of reaction caused by different forms of certain viruses. Some stone fruits are nearly universally infected, particularly with some of the latent viruses. Certain viruses (sour cherry yellows and ring spot) are transmitted through seeds. As insect vectors are known for only a few of the diseases, it is not known what measures are necessary to protect healthy stock from outside infection.

The problem of obtaining virus-free stocks by indexing procedure is complex. Index hosts have to be found that will serve with certainty to indicate the presence of a given virus in all its forms. The search for such hosts has been complicated by the fact that stocks of certain of those used were already infected. Orchard trees being indexed often are infected by more than one virus and therefore give confusing results. The long incubation period of some virus diseases, such as diamond canker of French Prune and willow twig of peach, makes the procedure slow and costly.