Virus Diseases of the Strawberry

Harold E. Thomas, C. P. Marcus, Jr.

Strawberries are subject to several types of virus diseases. Some of the diseases are of little importance to the crop, but others have seriously interfered with production.

Those of the yellows type had the widest distribution and greatest importance in 1953. They have spread rapidly to practically every strawberry section of the country. In the Pacific Northwest, before yellows appeared, the crinkle disease did great damage. As yellows moved through the area, the two diseases became so closely associated that it was hard to tell one from the other. The combination of the two undoubtedly accentuates the hazard of virus to the strawberry industry there.

If one is to identify accurately a virus disease of strawberry in the field, the symptoms must be clear and definite. But the symptoms often are masked by high or low temperatures, conditions of growth, and peculiarities of the variety. Some other means of identification must be resorted to.

Methods of indexing to susceptible varieties or species, termed indicator plants, often can be used to identify a virus. The insect vector that normally transmits the disease in the field from the diseased to the healthy plant can be used to transmit virus to indicator plants. The grafting technique is employed to graft a stolon runner of the plant being tested to a stolon of the indicator plant. The use of indicator plants has advanced greatly our understanding and classification of strawberry viruses. They enable us to separate and recombine virus entities in a disease complex. They also can help us determine the presence or absence of virus diseases in stocks of commercial varieties. The indicator plant most commonly used is the wild strawberry Fragaria vesca, which grows wild in or around wooded areas.

Investigators have used various selections of F. vesca. The one most widely used is a clone selected by R. V. Harris of East Mailing, England. In California a closely related species, F. bracteata, has been used, along with local clonal selections of F. vesca. Any variety or species that distinguishes the virus may be selected as an indicator, but an advantage lies in the use by all investigators of clones of similar type to permit accurate comparison of results.

DISEASE COMPLEXES in the United States formed by the combination of virus entities were demonstrated through the investigations of Norman Frazier and Harold E. Thomas, of the California Agricultural Experiment Station, and later by J. B. Demaree and C. P. Marcus at the Plant Industry Station at Beltsville, Md. They found that the yellows type of diseases are combinations of at least two virus components. We use the term "yellows type of diseases" in plural form to emphasize that this disease complex has various combinations.

The components obtained from the virus-diseased plants when transmitted to indicator plants fall into broad categories or types in the indicator, but each type shows considerable variation. When these variations combine in nature to produce the disease as observed in the field, it is assumed that the variety of symptoms produced is the result of the varying nature of the components that go to make up the complexes. Climatic conditions affect the symptoms of the disease,but the variation in the components in the complex is thought to be of greater importance.

EVEN A SINGLE VARIETY Of strawberry can show a wide range of virus disease symptoms. That fact and the still greater range of symptoms shown among varieties has caused confusion in attempts to classify the diseases. In other words: The description of a disease in only one susceptible variety is inadequate for general diagnosis in all varieties. Some other means of identification must be used and this is where the indicator plant becomes helpful and necessary. By indexing the diseased plant to the indicator the type of disease can usually be determined.

The symptoms produced in the indicator are so different from those in the variety under test, however, that the description of the disease in the susceptible variety corresponds little with the symptoms produced in the indicator. Therefore a different set of symptoms to describe the two will undoubtedly be necessary. The logical answer to the problem of describing a disease may well rest in the description of the symptoms caused by the entities as they affect the indicator plants and the enumeration of these entities that are resolved from the disease complex. Investigations have not advanced to the point that this type of description is generally possible, but it may well form the basis for future classifications. This type of classification would make it necessary for workers to use a single clone of the indicator, or a specified combination of indicators. These, like varieties, vary to some extent in the symptoms expressed and the degree.

Diseases of the yellows type, the worst of the virus ailments of strawberries, cause a loss of vigor and a marked stunting. Symptoms are cupping, usually upward, of the leaves, a yellowing of leaf borders, and dwarfing of petioles and blades. Stunting is common in all varieties.

Investigators in the California Agricultural Experiment Station, the Department of Agriculture, the Oregon Agricultural Experiment Station, and the University of British Columbia have demonstrated the presence of two entities in the yellows disease complex, as expressed in indicator plants of Fragaria vesca. One causes a mottle type of symptom, which in F. vesca produces small, dark-green, wrinkled leaves, streaked or spotted with yellow. The leaflets vary in size and shape, and often one is extremely small or absent. The entire plant is dwarfed and weak. Thin stolons if any at all form. Dwarfing and some leaf spotting may be the only symptom in milder attacks. The second component of the yellows disease causes moderate stunting and a mild uniform yellowing of the F. vesca plant. No leaf symptom is evident other than yellowing, reduction in size, and occasionally slight puckering or cupping. Fewer stolons are produced.

Another virus that may be related to the yellows type of diseases is found mainly in the eastern United States. Dr. Demaree and Mr. Marcus, who identified it, named it type 2 virus. In F. vesca it causes small, yellowish-green leaves that have a smooth, normal shape and uniform leaflets. The petioles of young runner plants of the infected indicator are much shorter than normal and curve downward. Consequently for several weeks the under sides of the leaflets show from above, but eventually resume an upright position; many buds grow from the base stem and make multicrowned, dwarfed plants.

Single infection in most cultivated Varieties with any one of the three viruses causes only minor effects or none at all. The effects of several together may be entirely different.

In the grouping of virus troubles the Stunt disease falls with the yellows type because of the stunting effect that it causes. Leaf petioles attain one-half to two-thirds of normal length but remain upright. No yellowing occurs, but the upper leaf surface has an unusual flat, dull appearance. Cupping of leaves, upward or downward, is common.

THE CRINKLE TYPE of diseases is second in economic importance. They occur mainly in the Pacific Northwest.

Their chief symptom in cultivated varieties is a crinkling or wrinkling of the leaves. Small, yellowish, pinpoint spots start in the developing leaves and expand somewhat as the leaves grow. The result is a flecked appearance. The faster growth of the healthy tissue around the spots causes a crinkled surface. In the severe form, veins are partly cleared so that the plant gets a somewhat yellowed look. Small sectors in the leaves, narrowing from the margin inward, may become yellow and translucent. There is some reduction in vigor and production of fruit. Some varieties show only the pinpoint mottle spots. Only limited analyses with indicator plants have been reported for this type, but research workers have described mild and severe forms in a single variety. Perhaps, therefore, combinations of virus components do occur.