Infection of bean with cucumber powdery mildew, Podosphaera fusca

Since 1995, unusual brownish spots have been observed repeatedly on leaves and stems of bean (Phaseolus vulgaris), cultivars Red Kidney and Pinto, grown in the experimental greenhouse of our institute. A microscopic study revealed hyphae and conidial chains of a powdery mildew fungus covering the spots. Morphological characteristics of the pathogen (shape and dimensions of conidiophores and conidia, presence of fibrosin bodies in conidia, and germination type of conidia) were identical with those of the cucumber powdery mildew fungus maintained in the same greenhouse. This species is often referred to as Sphaerotheca fuliginea or S. fusca, but it was recently reclassified as Podosphaera fusca (Braun & Takamatsu, 2000).

To determine whether the causal agent of the unusual symptoms was P. fusca, seven pots of 3-week-old bean plants, cultivar Red Kidney, were grown from seed in isolation and inoculated by touching cucumber leaves infected with P. fusca onto their leaves. Inoculated plants were kept under a plastic tent in the greenhouse. Three pots of noninoculated plants, placed under another tent, served as controls. Seven days later the first spots appeared on the leaves. The lesions enlarged and merged to cover large areas within two weeks (Figures. 1a,b). The infected leaf tissues became necrotic 6-8 days after the first symptoms appeared. The fungus also spread onto noninoculated leaves and stems.

Sporulation of P. fusca was observed on all infected leaves and stems. No symptoms were seen on the control plants. To confirm pathogenicity, four pots of 2-week-old cucumber plants, grown from seed in isolation, were placed close to the infected bean plants within the plastic tent. Two other pots, under another tent, were used as controls. Five days later, colonies of P. fusca appeared on all cucumber plants, except the controls. The inoculation tests were repeated in the greenhouse with the same results. A field test was also carried out using 4-week-old bean plants, cultivar Red Kidney and an unidentified hybrid. Sporulation and spread of P. fusca was observed on leaves and stems of cultivar Red Kidney. In contrast, only small necrotic spots covered by powdery mildew hyphae were found on the leaves of the hybrid bean plants.

These results show that the unusual necrotic spots on bean leaves were caused by the cucumber powdery mildew fungus, P. fusca. These symptoms have not been described in commercial bean crops. The implications for commercial bean production are as yet unclear. Cultivar Red Kidney was more susceptible to P. fusca than an unidentified hybrid which did not support the sporulation of the fungus. Diseased bean specimens were deposited in the Herbarium at the Dept. of Botany of the Hungarian Natural History Museum (accession number BP 93676). This is the first report of cucumber powdery mildew on bean.

These studies were supported by a grant (OTKA F032931) of the Hungarian Scientific Research Fund and by a János Bolyai fellowship awarded to L. Kiss.