The foundation stock-mother block procedure has several advantages over use of approved trees in orchards. Mother-block trees properly isolated can be maintained with less risk of becoming infected by natural spread. Pedigreed stocks, centrally located, can be given proper care and can more easily be checked for off-types or diseases. Centralization also makes for simplification of record keeping, especially as regards progeny performance and tracing troubles which may arise.

It also has limitations. Standards may be hard to determine. It has not been established that certain viruses, especially certain of the latent ones, are sufficiently harmful to warrant exclusion. New diseases are continually being found, and it may be difficult to prevent some of these from getting into stocks. There will be a continuing demand for the new varieties and new strains of varieties, which will pose a continuous problem of what should be stocked.

If the isolation station procedure is generally adopted, much expense can be saved by putting it on a regional basis. In the same way, procedure for interstate shipment of nursery stock could be simplified by interstate agreement on the requirements for certification.

The most serious virus diseases of citrus and pome fruits in most instances can be detected and avoided by orchard inspection. The psorosis disease of citrus occurs in all of the citrus-growing areas of the world. It was distributed in infected budwood before people knew it was caused by a virus. Infected trees do not commonly develop the spectacular scaling lesions and decline symptoms until they are 12 to 16 years old, yet the buds taken from them before the symptoms appear carry the virus. Exocortis, a disease that affects trifoliate orange rootstock, resulting in dwarfing the trees growing on it, can be carried by the top variety without symptoms if it is grown on other rootstocks. Stubborn disease, a third virus disease that reduces the vigor and fruitfulness of sweet orange, also requires several years for recognizable symptoms to develop in infected nursery stock.

All these diseases can be avoided by selecting budwood from vigorous, healthy-appearing orchard trees old enough to show symptoms. For freedom from the exocortis virus, bud-wood must be taken from trees growing on trifoliate orange rootstock. The discovery that psorosis could be diagnosed on the basis of symptoms in young leaves has greatly simplified certification procedure. Sweet orange seedlings can be used to index any species of citrus on which the presence of psorosis cannot be determined by orchard inspection.

None of the three citrus viruses we mentioned appears to have any means of natural spread in North America except by occasional natural root grafts between trees. Production of virus-free nursery stock is therefore an important and efficient way to control them.

The contagious nature of the quick decline disease of citrus, the lack of distinctive symptoms, and the wide occurrence of the causal virus in sweet orange on rootstocks other than sour orange make production of nursery stock free of the quick decline virus impractical as a control procedure within infected areas.

Several virus diseases affect pome fruits, but only one, stony pit of pear, has caused sufficient damage to merit selection of disease-free scion wood. Fruits of the Bose variety on diseased trees are variously misshapen and pitted. Tissue at the base of the pits and around the core becomes hard and stony, making the fruit worthless. The disease can readily be recognized in the orchard just before harvest on fruits of the Bosc variety. Other varieties can be indexed by grafting healthy Bosc on one arm.

One rather serious virus disease of avocado, sun blotch, is the cause of unfruitfulness and misshapen fruits. The expression of the disease is erratic and no good indicator variety is known. The best method of avoiding the disease seems to be by the use of propagation material from trees shown by progeny performance to be free of sun blotch. The disease appears to have been spread chiefly in diseased nursery stock and is the cause of enough damage to warrant efforts to avoid it.

Elimination of viruses and viruslike disorders from nursery stock must precede measures applied in the orchard for effective control. To do this, several general steps are necessary. Viruses and viruslike disorders, which cause obvious symptoms, can be avoided by use of budwood from orchard trees that show no symptoms, are fruitful, and true to type. Such trees preferably should not be in plantings where contagious virus diseases are present and should not in any case be adjacent to virus-infected trees. Screening by index procedure is necessary to avoid viruses that may be latent in orchard trees. When desirable trees are once determined free of virus, they should be propagated on virus-free rootstocks and grown under isolation where they can be maintained under observation and periodic testing to assure virus freedom and desirability of type. Such trees can serve as foundation material from which propagating materials can be supplied to nurserymen for establishing mother tree blocks, which in turn supply budwood for nursery propagation.

It is equally important that orchards producing seeds for growing rootstocks be virus-free and of desirable type.

Specifications for indexing and isolation would necessarily vary with districts, depending on the diseases present and the fruits grown, but effort should be made to devise provisions with enough uniformity to allow for interstate shipment. Growers should demand virus-free trees. Nurserymen need the cooperation and assistance of research, regulatory, and extension men, and growers. Nursery improvement programs are under development in several States and there is reason for optimism.

L. C. COCHRAN is in charge of investigations of virus diseases of deciduous fruits in the Bureau of Plant Industry, Soils, and Agricultural Engineering.

EARLE C. BLODGETT is located at the Irrigation Experiment Station, Prosser, Wash., and holds a joint position of Plant pathologist with the Washington State Department of Agriculture and the Washington Agricultural Experiment Station. He is responsible for developing fruit tree foundation stocks and nursery improvement procedures.

J. DUAIN MOORE is associate professor of plant pathology in the University of Wisconsin and is engaged in investigations of diseases of tree fruits, with particular interest in virus diseases of sour cherries. He is a native of Lancaster County, Pa., and holds degrees from Pennsylvania State College and the University of Wisconsin.

K. G. PARKER is professor of plant pathology at Cornell University. He has been investigating diseases of trees since 1928, and has been in charge of investigations of virus diseases of tree fruits since 1946. He is a native of Indiana and holds degrees from DePauw and Cornell.