Occurrence of Soybean Root Rot Caused by Corynespora cassiicola (Berk. & Curt.) Wei in Hokkaido

Abstract

A root rot disease of soybeans (Glycine max (L.) Merr.) was found at Bibai and Memuro, Hokkaido in 1978. At the roots and base of the stems, diseased plants formed reddish brown lesions, which turned violet brown with the dovelopnent of disease. Symptoms of the tops were characterized by stunting, yellowing, and defoliation. The causal fungus was identified as Corynespora cassiicola (Berk. & Curt.) Wei by the similarity to the description of Ellis. Conidia of this fungus were brown, curved, and broad at the base, formed on conidiophores singly or in chain, and varied in morphology: i. e. clavate, cylindrical, and rarely Y shaped. Mean dimension was 155×15.2μm. Each conidium had 2 to 28 septa and a hilum at the base. Conidiophores averaged 83×7.3μm in dimension, were brown and erect, and formed singly or in group. Each conidiophore had 1 to 7 septa and a pore at the end. No stromata were observed. The basal cells of conidiophores were swollen. Young mycelia were white and fluffy and turned gray with age. They become olive to black or violet in appearance when conidia were produced. The fungus produced yellow or pink pigment in PDA. Growth on PDA occurred from 5 to 30 C with an optimum temperature range from 20 to 25 C. The optimum pH for growth was 5.0, and the range was from 4.0 to 8.0. Moist conditions of soil favored disease developement. The optimum soil temperature for disease development was 15 to 20 C. The fungus survived at least one year in soil. Of 13 plant species in 7 families tested, legminous plants (i. e. soybeans, beans, adzukibeans, cowpeas and alfalfa), and sugar beets were found to be susceptible to the fungus by inoculation experiments.