Relationship between basidiospore dispersal of Thanatephorus cucumeris (Frank) Donk and development of foliage blight of sugar beets

Abstract

Fructifications of an anastomosis group AG 2-2 of Thanatephorus cucumeris on the surface of the petiols of sugar beet plants bearing crown rot lesions and on that of surrounding soil under the oldest leaves of the outer whorls were frequently found in the fields from the later part of June to the middle of September in 1978 and 1979 in Hokkaido. Even in other crops such as corn, wheat and potato, they were sometimes discovered on the surface of the stem, the sheath and the soil under the canopy of the leaves of many plants. No relation was noticed to the kind of plants in the soil heavily infested with AG 2-2. Foliage blight of sugar beets did not occur until the basidiospores were discharged from these fruit bodies. From late July to late August, when the secondary lesions developed and enlarged, many fruit bodies were observed furthermore on the healthy surface adjacent to the lesions in the ventral side of the infected leaves under hot and wet weather conditions. The spore density in the air was closely associated with the number of leaves producing secondary lesions or with the disease severity. In the middle of August, number of spores trapped on an ager plate in the air decreased rapidly with the increase of the distance from the ground, but even at the height of 185cm, 188 spores per 18×18mm² were caught during a night. At the height of 83cm in a bean field which was located 70m apart from the sugar beet field, 10 spores were recorded. Ninety-eight out of the 99 spore cultures which were obtained in the middle of August by the agar trap in the air in a sugar beet field and a bean field belonged to AG 2-2, and the one left belonged to AG 1. In the early part of September if the temperature was low, in addition to spore dispersal not being observed, primary lesions which were formed before that time were chlorolized in the peripheral area and perforated in the middle without forming the secondary lesions.