Oospore germination of Aphanomyces euteiches in root exudates and on the rhizoplanes of crop plants

A technique was developed that allows prolific production, easy collection, and increased germination frequency of single oospores of Aphanomyces eute iches. The influence of root exudates and roots of various plant species, i ncluding pea, bean, alfalfa, oat, soybean, corn, and tomato, on germination of A. euteiches oospores also was studied. Compared with a sterile, deioni zed water control, root exudates from several hosts were only slightly effe ctive in stimulating oospore germination, since only 0 to 11.1% of oospores exposed to various exudates germinated. By contrast, oospores placed direc tly on plant roots germinated at higher frequencies. Oospores of pea pathot ype isolates P30 and P46 germinated at a greater frequency (30.6 to 61.1%) on pea, bean, and oat roots than on roots of any of the other plant species tested. Oospores of bean pathotype isolates GB33 and GB71 had a higher ger mination frequency (47.2 to 52.8%) on bean roots than on the roots of the o ther plant species tested. A higher percentage of oospores germinated if pl aced on lateral roots as opposed to taproots of pea and bean. A higher perc entage of oospores of bean pathotype isolates and one pea pathotype isolate germinated on 10-day-old rather than on 20-day-old roots of bean. Therefor e, pea and bean roots can be used effectively to germinate oospores of pea and bean isolates of A. euteiches, respectively. This technique will be val uable for studies of sexual reproduction and genetics of A. eureiches.