Among the varieties resistant to red stele in the United States, with their parentage, the year introduced, and the estimated commercial acreage Planted to them in 1952, are:

Aberdeen, a chance seedling, introduced in 1919, not grown commercially.

Fairland, a cross of Aberdeen and Fairfax, introduced in 1947, grown on 500 acres.

Pathfinder, a cross of Howard 17 and Aberdeen, 1938, on 25 acres. Redcrop, a cross of Aberdeen and Fairfax, 1949, 300 acres.

Sparkle, a cross of Fairfax and Aberdeen, 1943, 1,500 acres.

Temple, a cross of Aberdeen and Fairfax, 1943, 2,500 acres.

Vermilion, a cross of Redstar and Pathfinder, 1950, 50 acres.

The development of resistant varieties has eliminated an important hazard to strawberries grown in infested soils. Of much significance here is the fact that the best soils for strawberries are often well suited for development of the red stele disease. In some States many hundreds of acres of land especially suitable for this crop were made useless for profitable production.

Breeding for resistant varieties is far from completed. Earlier and later maturing varieties than those now available are needed, and larger, firmer-fruited sorts for all seasons would be welcome additions. Varieties adapted to different strawberry regions are still to be obtained.

The plant breeders must now take into account in their work the newly discovered fact that races of Phytophthora fragariae exist. All of the present named varieties of strawberries in the United States are susceptible to a new race race S of the fungus. In Great Britain five and possibly more races exist.

Tests have indicated that a few unnamed selections are resistant to both known races of the causal fungus. They have been obtained from crosses between U.S.-3374 or Md.-683 as one parent, and Fairland or Temple as the other parent. Both U.S.-3374 and Md.-683 have Scottish selections as one parent (CC-18 and BK-46, respectively) and it is presumed that their resistance to race S was inherited from those ancestors, which were introduced because of their resistance to red stele in Scotland.

Because red stele only rarely affects varieties such as Fairland, Temple, and Sparkle, it is assumed that race S is not widespread and may not become the serious problem that the common race (race A) has been. It is expected that new varieties resistant to both known races of the red stele fungus will be available soon. Since red stele is caused by a soil-borne organism, it cannot spread as rapidly as wheat rusts and other diseases caused by wind-borne fungi. Strawberry breeders and growers have little reason to fear a sudden widespread destruction of their resistant varieties by new races of Phytophthora fragariae.

Inheritance studies for resistance to race S have been conducted under conditions highly favorable to severe infection of plants by the pathogen.

Md.-683 or U.S.-3374 when selfed or crossed with Aberdeen or any other varieties susceptible to race S produced seedlings of which 20 to 25 percent were highly resistant to race S. Progeny of the cross between Md.-683 and U.S.-3374 (both parents resistant) yielded more resistant seedlings than crosses involving one susceptible parent. Resistance is partly dominant and is governed by a number of genes. Field studies on inheritance of resistance to race A have indicated that resistance as obtained from Aberdeen is also partly dominant.

During the first few years that seedlings and selections were tested for resistance to red stele, elimination of susceptible material was done in fields naturally infested with the pathogen. Results were often inconsistent because of variable soil moisture conditions and irregular distribution of the disease. Soil infested with the red stele organism has been used more recently in greenhouse benches and has given very satisfactory results. Raised greenhouse benches, about 6 inches deep, are lined with a heavy, watertight material with an overlap in the bottom of the bed for restricted drainage. The infested soil, taken from fields where plants have shown serious infection, is put in the benches to within 2 inches of the top. Enough steamed and co,,. posted soil is added nearly to the top, Some space is left to make watering easier. Between late August and October, seedlings are transplanted from seed flats directly into test benches. They are spaced about 2 inches apart. Good growing conditions are maintained until about the end of November.

Usually by that time the roots have extended to the bottom of the benches and the crowns are about one-fourth to one-half inch in diameter. Heavy, frequent watering is then begun and a temperature of about 45 to 55 F. is maintained. Conducting the test during the winter provides the cool temperature necessary for uniform infection of plants by the organism. Infection occurs quickly under favorable conditions and the major part of the root systems of susceptible plants is destroyed within 2 months after exposure to cool temperatures is begun. Frequent watering of the beds is an important factor in raising the degree and uniformity of infection.

Breeding for red stele resistance was being conducted in 1953 by the agricultural experiment stations of Illinois, Michigan, Maryland, and Oregon. The only program known to be active abroad was at the West of Scotland Agricultural College at Auchincruive, Scotland. Testing of strawberry varieties and selections for resistance to new races was under way in England by C. J. Hickman at the University of Birmingham and in this country at the University of Maryland and the nearby Plant Industry Station at Beltsville.

In the past few years the presence of the virus in strawberries has added another aspect to the work that of maintaining stocks resistant to red stele in a virus-free condition. That has been accomplished with some success through isolation of plantings and a thorough aphid-control program by the use of parathion dust. Most, if not all, of the selections to be exchanged for testing in the future will be virus free. Unless a source of immunity to virus can be found in strawberries, however, virus infection probably always will be a hazard in the breeding work.

Reports of differences in varietal susceptibility show definitely that races of P. fragariae exist in several States and it is especially urgent that breeders, regulatory officials, and others concerned with movement of strawberry plants over long distances be careful to prevent shipment of infected plants.

STRAWBERRY growers who do not have the red stele fungus in their land should be especially careful to prevent its introduction through the use of infected planting stock. Growers in all but the southernmost States should learn to recognize the red stele disease and be certain that they use only disease-free stock. If red stele is in the immediate area, care should be taken to prevent the contamination of uninfected land.

Once land is infested with the red stele fungus the main control measure consists of resistant varieties. There are a number of good varieties which will make perfect growth where susceptible varieties are a total loss. They may not meet all of the many specific requirements of various areas, however.

W. F. JEFFERS is professor of plant Pathology at the University of Maryland. Since 1939 he has been active in research and extension work dealing with the nature and control of red stele disease of straw- berries.

D. H. SCOTT is senior geneticist in the division of fruits and nuts, Bureau of Plant Industry, Soils, and Agricultural Engineering at Beltsville, Md. He obtained his doctor's degree at the University of Maryland in 1949. His special field of work is breeding strawberries, blueberries, and other small fruit.