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 euteiches. The influence of root exudates and roots of various plant species, including pea, bean, alfalfa, oat, soybean, corn, and tomato, on germination of A. euteiches oospores also was studied. Compared with a sterile, deionized water control, root exudates from several hosts were only slightly effective in stimulating oospore germination, since only 0 to 11.1% of oospores exposed to various exudates germinated. By contrast, oospores placed directly on plant roots germinated at higher frequencies. Oospores of pea pathotype 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 germination frequency (47.2 to 52.8%) on bean roots than on the roots of the other plant species tested. A higher percentage of oospores germinated if placed on lateral roots as opposed to taproots of pea and bean. A higher percentage 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. Therefore, pea and bean roots can be used effectively to germinate oospores of pea and bean isolates of A. euteiches, respectively. This technique will be valuable for studies of sexual reproduction and genetics of A. eureiches.