Date of this Version
Published by The American Phytopathological Society September 2002, Volume 86, Number 9, Page 1051 http://dx.doi.org/10.1094/PDIS.2002.86.9.1051B
During July 2001, wilting, dry bean plants (Phaseolus vulgaris L.) at the 3- to 4-leaf stage were observed in a field near Ogallala, NE. Affected plants exhibited dark brown lesions on the hypocotyls and lower stems. Isolations from lesions on diseased plants on potato dextrose agar (PDA) yielded several isolates of an unknown fungus containing binucleate hyphal cells and numerous clamp connections. After growth on PDA for 6 weeks, all fungal isolates remained sterile and did not produce identifying spores, chlamydospores, or sclerotia. Two of the recovered field isolates were randomly selected for pathogenicity tests. Great Northern bean plants (cv. Beryl) were grown in a peat moss/perlite mix (3:1) in 10-cm plastic pots and inoculated separately with each of the two isolates at the two-true leaf stage. Two-week-old cultures grown on PDA (two 100 × 15 mm plates) were ground in 400 ml water in a blender for 3 to 4 min. Approximately 50 to 60 ml of the mycelial suspension was drenched around stems in each pot (two plants per pot), and incubated in growth chambers at 20, 30, and 35°C with a 12-h photoperiod. Six replicates were used for each isolate/temperature combination. After 4 weeks at both 30 and 35°C, fungi were consistently reisolated from stem lesions of wilting plants like those from the field-infected specimens. These fungi were also morphologically identical to the original isolations, thus confirming pathogenicity of the isolates and completing Koch's postulates.