This disease is especially common and destructive in rainy seasons. It appears to have an extensive range over the country, occurring as far south as Louisiana and throughout the northern strawberry - growing sections. It was especially destructive over its entire range during the seasons of 1914, 1915 and 1916. It is reported as having caused a loss of millions of dollars in the States of Mississippi and Louisiana alone in 1914. Much of this loss resulted from the development of the disease during shipment, though it was prevalent and destructive in the field.
All the above-ground parts are subject to the disease, but the fruit and fruit-pedicels usually suffer most. Small green fruits as well as mature and ripening berries may be affected. The lesions usually appear first on the fruit as small, brown, rotten spots. These rapidly involve the entire berry, from which the fruit-pedicels become affected. Leaves and leaf - petioles may show lesions, especially where they touch rotten fruits. The injury is a dry, brown rot. The fruit shrivels slightly, but retains its form. No exuding of juice is evident. The diseased tissue turns brown, but remains firm. The berry is finally coated with a dense growth of the spore stalks of the pathogene, giving it a grayish brown, moldy appearance.
The fungus responsible for this disease is a species of Botrytis, possibly Botrytis cinerea. Another fungus Rhizopus nigricans( ?) is also known to cause a rot of strawberry fruits which differs strikingly from the Botrytis Rot in that there is a rapid loss of juice from the fruit, giving it the name of "leak." This is accompanied by a collapse or flattening of the fruit. The Rhizopus rot appears more commonly on berries in transportation.
The spores produced in great abundance from the spore stalks on diseased fruits are scattered through the berry patch or field, and many fruits become inoculated. When moisture is abundant and retained on the plants through the day, the spores readily germinate. Whether the germtubes can penetrate the uninjured surface of fruits is not certain, though this is probable. The mycelium spreads rapidly and profusely throughout the tissues of the fruit, between and through the cells, which are rapidly killed. The fungus feeds upon the substance of the invaded tissues and soon sends forth great numbers of brown stalks, branched at the tips and bearing thereon many grape-like bunches of oval thin-walled spores. These spores are readily scattered by the wind and cause new infections. How this fungus winters is not certain, though probably by means of sclerotia. Sclerotia are black tuber-like bodies of densely interwoven mycelium which are formed by all species of Botrytis, in dead host - debris. They pass the winter in a dormant condition, and germinate the following spring, producing tufts of spore stalks bearing spores like those already described. These spores initiate new infections.
Little definite information based upon experimental data or experience can be offered. Losses in shipment may be reduced by careful sorting out of all diseased or injured berries. Discarded fruit should not be allowed to accumulate about the packing house, but should be burned or buried daily. Field losses will be much reduced by growing the plants well apart in order to afford a maximum of light and air.
Stevens, F. L. A destructive strawberry disease. Science n. s.
39: 949-950. 1914. Stevens, N. E. Pathological histology of strawberries affected by species of Botrytis and Rhizopus. Jour. Agr. Res. 6: 361 - 366.1916.
Spring frosts often do damage to strawberry plants. Those varieties in which the seeds are most exposed suffer most. Plants severely affected with Leaf Spot have been observed to be more injured in the winter than those plants free from this disease. Mulching is practiced as a measure against winter injury, while simply covering the plants with straw or blankets is practiced as a protection measure against spring frosts. (Fuller discussion of Frost Injury given under Apple, page 35.)