Chlamydospore formation in Fusarium solani f. sp. phaseoli in root extracts of crop plants and their virulence to kidney beans (Phaseolus vulgaris L.)

A simple root extract method was developed for chlamydospore production by three macroconidial isolates (A-1, S-3 and T-1) of Fusarium solani f. sp. phaseoli. All isolates produced significantly (P = 0.05) more chlamydospores in oat root extracts (5 to 7 × 105 ml−1) than those of other crop plants, sterilized soil extracts (5 × 104 ml−1) or salt solutions (9 × 104 ml−1). They produced moderate numbers of chlamydospores (2–5 × 105 ml−1) in kidney beans and fewest chlamydospores (3–6 × 104 ml−1) in tomato root extracts. The fungal conidia in root extracts germinated after 12 h, grew vegetatively for a brief period, and then produced new conidia. Chlamydospores were formed either inside the conidia or from germ tubes or hyphae. Chlamydospore formation was rapid in the beginning and reached a static phase on day 14 of incubation. Chlamydospores produced in bean, pea or cotton root extracts were larger and more virulent to kidney beans than those formed in corn, tomato or wheat root extracts.