Red Stele Disease of Strawberry

W. F. Jeffers, D. H. Scott.

Considerable interest always attends the first occurrence of a serious plant disease. Sometimes it can be traced to plants introduced from a foreign country. Sometimes there is some other ready explanation. Often, however, the origin of a new disease remains a mystery.

One such is the red stele disease of strawberry. It is caused by Phytophthora fragariae. It was reported officially in Illinois in 1935 and in Maryland and California in 1936, but it had been seen in all three States a few years earlier.

How such a serious new disease could appear almost simultaneously in locations so far apart is truly perplexing. There was no evidence of introduction of diseased strawberries from other countries, and it seems unlikely that red stele was spread from one location to the others.

The first report of red stele in the world is similarly perplexing. It suddenly appeared in Scotland about 1920. Its origin is unknown. Perhaps the fungus causing it had been present in some other part of the world for centuries without causing noticeable damage and when it reached Scotland and the United States it found very susceptible varieties and favorable conditions.

Another possibility is that the pathogen developed suddenly through the hybridization of other fungi or by mutation.

Its name is descriptive. Its main diagnostic symptom is a red discoloration of the central vascular cylinder the stele of strawberry roots. The redness, most evident in early stages of infection, can be found when other evidence is lacking that plants are diseased.

Soon after primary infection, the stele turns reddish while the rest of the root tissue looks normal. The red stele fungus is closely followed by secondary organisms. The roots die from the tip upwards and become brown. That color masks the reddened stele, but usually there is some extension of the red color upward from the dead tissue. In late stages of infection, diseased roots may be so badly rotted that there is little evidence of red stele. Lateral roots and minute rootlets are also invaded and are killed back to the primary root. The so-called rat-tail symptom results when the upper section of primary roots is still white while the lower portion is blackened. Sometimes there may be several separate infections on a root.

Foliage symptoms of red stele appear after active growth begins in the spring. If the season is dry, infected strawberry plants may die before blossoming. If rainfall is heavy, however, little above-ground evidence of the disease may be apparent until fruit begins to develop. Plants growing in Poorly drained soil usually show foliage effects later than those in well-drained soil.

The first above-ground symptom is a slight off-color in the leaves usually a dull bluish green instead of the normal light green of the young leaves. Soon the leaves of badly affected plants begin to turn red and are noticeably wilted. The entire plant may die within a few days a promising planting of strawberries might suddenly become a total loss.

Red stele commonly causes plants to be severely stunted and discolored Without killing them. Such plants produce only a few small, worthless fruit. Poorly drained places in a large field may contain stunted or dead plants while the rest of the planting is healthy. Many growers therefore have thought poor drainage is the direct cause of red stele. Unless the causal fungus is present, however, the disease cannot develop. The fact is that strawberries are very resistant to direct. damage from poor drainage they prefer soils with a good supply of moisture.

Affected plants that do not die in spring often recover in summer. Rotted roots disintegrate and new ones are produced, so an examination of the roots then will disclose little or no evidence of red stele. The disease usually becomes active the following spring, however, and the plants will again become infected.

As the red stele organism is active only during cool spring weather and the reddened stele becomes obliterated later in the season, positive diagnosis can best be made in March and April a fact of great moment to growers, nurserymen, and regulatory officials who want to get disease-free planting stock. A careful inspection of strawberry fields in spring makes it possible to determine whether the red stele disease is present and to get a clue to the potential fruit production of a planting.

In the few years since its discovery, red stele has become in many localities the most serious disease affecting strawberries. It is especially serious in Maryland, Michigan, Illinois, New Jersey, Oregon, and Washington.

At first it was hard to isolate the fungus that causes red stele, but it was accomplished by scientists in the Department of Agriculture and in England. Since strawberries belong to the genus Fragaria, the fungus was named Phytophthora fragariae.

The fungus can live for many years in fertile soil even if strawberries are not present. Once a field becomes infested, therefore, it is usually useless for further production of susceptible varieties of strawberries.

The fungus spreads through the soil by microscopic swimming cells known as zoospores. These spores require high soil moisture and therefore the fungus is of serious importance primarily in moist or poorly drained areas. The swimming bodies infect roots of strawberry plants and kill the water-conducting tissue (stele), causing it to turn red. Such interference with the water supply naturally causes the plant to wilt and to become stunted or to die. The degree of injury depends primarily on the number of roots affected on a plant. To a lesser extent the amount of soil moisture may affect the degree of injury resulting from red stele.

Flood water and drainage water can carry the zoospores over fairly long distances and thus infect other fields of strawberries. The disease can also be spread from field to field by transportation of soil on machinery, cattle, and such. However, most long-distance spread of the red stele disease is through use of infected planting stock. It is not unusual for a strawberry grower to spread red stele unwittingly through his neighborhood by giving infected plants to his friends. So it is essential that nurserymen and all others who distribute plants obtain special inspection service to be certain that the plants are free of red stele.

The fungus causing red stele is most active during cool, wet weather. Most infection occurs at soil temperatures of 32 to 55 F. Therefore, in most strawberry-growing areas this is the late winter or early spring period. During the summer the fungus becomes inactive and withstands hot summer weather by formation of another type of spore, the oospore. It also is microscopic and has a thickened wall, which enables it to withstand adverse conditions. These spores enable the fungus to remain in soil for many years.

Several strains of Phytophthora fragariae exist. They differ in their ability to infect different varieties of strawberry a variety resistant to red stele in one area may be susceptible in another area or the reverse may be true. Consequently, confusion has arisen concerning resistance of strawberries to the red stele disease. Fortunately new varieties having resistance to several races are being developed. It is advisable to test small plantings of various resistant varieties before making extensive plantings.

SOON AFTER RED STELE was discovered in the United States, it was learned that extensive acreages devoted to strawberries were contaminated. No economically feasible field-control methods were available, and research workers began immediately to test varieties for resistance.

Aberdeen, an American variety, was the only one out of the many tested that was completely resistant to the disease. Aberdeen is too soft, too variable in quality, and too subject to leaf scorch to be an acceptable commercial variety, however, so breeding work was begun in 1937 by the Department of Agriculture to originate commercial varieties of strawberries resistant to red stele.

The principal objective in the breeding work has been to develop resistant commercial varieties for different seasons and different regions of the country. More recently the objectives have been expanded to include resistance to at least two races of the red stele fungus and for varieties adapted to both frozen-pack and fresh-market use. Studies on inheritance of resistance to the disease have been conducted in conjunction with the practical phase of originating new varieties.

Some seedlings that were planted never were evaluated because various unfavorable conditions occurred before the plants fruited. About 350,000 seedlings have been examined for resistance to red stele and for desirable fruit characteristics. Hundreds of selections were made but nearly all were discarded because of undesirable fruit characters, particularly softness. Many selections have been tested extensively both by institutions and private individuals, but relatively few resistant varieties have been introduced for commercial use.

The extent of the work is indicated in the following figures of crosses of strawberry plants made at the Maryland and Oregon stations in cooperation with Department of Agriculture and the number of seedlings grown for stele resistance: