The climate also favors development of the sour cherry yellows disease, the cause of serious losses. Infected trees or nursery stocks grown in warmer climates may not be seriously affected and may not show leaf symptoms. A simple procedure for testing the sour cherry orchard trees for the presence of the sour cherry yellows virus or freedom from it would be to grow progenies located in an area where, if the virus were present in the progenies, symptoms would be sure to develop on them. Trees of other species could be correspondingly tested by placing buds from them into healthy sour cherry nursery trees. The chief objection to this procedure is that the nursery trees propagated from diseased trees, or healthy nursery trees infected by inoculation from diseased trees, sometimes take 2 years to develop symptoms; thus it takes a long time to get results. Some difficulty has been experienced in obtaining and maintaining disease-free indicator trees. Also, sour cherry does not express symptoms of many of the other viruses which may be present, thus necessitating further indexing on other hosts.

A high percentage of sour cherry trees is infected with the ring spot virus. Build-up of this virus has taken place over the years by indiscriminate propagation from infected trees, in which the virus had become latent; by propagation on infected seedling rootstocks, infected by passage of the virus through seeds; and by orchard spread.

Ring spot is much more prevalent than sour cherry yellows; in fact, all cultures of sour cherry yellows appear to contain the ring spot virus.

The universal occurrence of ring spot with yellows might indicate that yellows is the expression of the combined effect of two or more viruses, of which ring spot is one. Ring spot is known to exist without yellows and may be due to a single virus that is generally a contaminant of yellows. Any index procedure for sour cherry yellows would necessarily have to take ring spot into account.

In Michigan, index procedure for sour cherries has been developed with peach seedlings as indicator plants. Halehaven peach seedlings grown during the current season are budded with cherry buds in late August. If ring spot alone is present in the sour cherry trees from which the peach seedlings were budded, growth of the seedlings the following spring is retarded, buds die on many of the branches, and sometimes the branches die. Subsequent new peach growth from surviving buds assumes a normal appearance.

If sour cherry yellows is present in the cherry trees, the inoculated peach trees will show the retarding and die-back characteristic of and caused by the ring spot virus but in addition subsequent peach growth from surviving buds produces shoots with short inter-nodes and abnormally green leaves crowded into loose rosettes. If neither ring spot nor sour cherry yellows was present in the cherries, the peach seedlings grow normally and should compare with uninoculated checks.

The peach seedling technique is quick and inexpensive. It can be done on a large scale in many areas in the open field. Its shortcomings are that peach does not react well in the greenhouse and cannot be used out-of-doors in regions where peaches are subject to winter injury. In some instances, possibly because of virus forms or individual seedling differences, peach does not give clear-cut reactions. The possibility exists that the dwarfing reaction attributed to yellows is due to a third virus, which is a contaminant commonly associated with sour cherry yellows.

In Wisconsin, indexing procedure has been developed by making use of the fact that ring spot is usually associated with sour cherry yellows. Indexing is done in one of two ways. Scions from the trees are grafted on potted disease-free Montmorency cherry trees in the greenhouse and held at 70 F. for 3 to 4 weeks; if the orchard tree has ring spot, ring spot symptoms will develop on the leaves of the potted tree. The second way is to cut scions from the orchard tree and hold them in cold storage. The tree is inoculated with a known culture of ring spot and observed for symptoms. If no symptoms develop, the tree is assumed to have had ring spot before the inoculation and the scions in storage are discarded. If the tree develops ring spot symptoms, it is assumed that it was not previously infected and the scion wood in storage is used for propagation. By eliminating ring spot, it is reasoned that sour cherry yellows also is eliminated. Our experience thus far has supported this conclusion.

Use has been made in Oregon of two varieties of Prunus serrulata, Kwanzan and Shirofugen, for indexing for the presence of ring spot and possibly other latent viruses. When buds carrying ring spot are inserted into arms of Shirofugen, the buds die without uniting, and gumming lesions are formed around the bud insertion points. The virus apparently moves very slowly, because if the branch is severed below the gumming lesion the virus is removed. By spacing index buds at intervals of 6 inches or less along a branch, a single Shirofugen tree can be used to index a large number of orchard trees. Trees that test negative on Shirofugen are then tested on Kwanzan, because in a few instances a virus has been found that will not affect Shirofugen but will cause a reaction on Kwanzan. Viruses usually spread rapidly through Kwanzan. A Kwanzan tree therefore can be used only for one test.

Prunus tomentosa, Manchu cherry, has been used in Iowa and is reported to be a more sensitive host for ring spot than Lovell peach seedlings. Manchu cherry was found unsatisfactory in California because of variability among seedlings and its failure to give a reaction with forms of the ring spot virus which reacted on Hale peach. Results of tests in Washington indicate that Shirofugen is a much more sensitive test plant than Manchu cherry.

More information is needed before the different index hosts can be evaluated. Where sour cherries are maintained as clones and do not show any symptoms of sour cherry yellows under growing conditions favorable for yellows, it can be presumed with reasonable certainty that they are free of yellows. Peach, sour cherry, and the Shirofugen and Kwanzan varieties of oriental flowering cherry all appear to be of value in indexing for ring spot. Shirofugen can be grown in climates with insufficient chilling requirements for sour cherries and appears to be as sensitive to the ring spot virus as sour cherry or peach.

Programs have been started in some States to develop certified foundation stocks. In some instances nurserymen were furnished budwood direct from orchard trees that had been determined by index methods to be free of virus. Tree performance was determined by observations directly on the orchard tree. In other instances progenies have been grown from indexed orchard trees and bud-wood has been supplied to the nurserymen from the progenies. In a few instances enough budwood for direct propagation of nursery stock has been supplied from progeny trees, but mostly nurserymen have increased their own foundation stocks to supply budwood sufficient for their needs.

Peach budwood heated at 122 F. for 5 minutes was used in Michigan on a sufficiently large scale to show that such treatment was practical for nursery procedure. Experiments earlier had shown that the treatment would eliminate viruses of the peach yellows group and X-disease. Certification on the basis of inspection of orchard trees has been used satisfactorily in Michigan for avoiding the yellows group of diseases in nursery stock; the heat treatment adds assurance against any of these diseases getting through.