AN ANALYSIS OF SELF-FERTILITY IN FIELD ISOLATES OF PHYTOPHTHORA-INFESTANS

Some field isolates of Phytophthora infestans from England and Wales had a flat, adpressed, waxy appearance when grown in agar culture. These isolates formed very little aerial mycelium and few sporangia, but abundant oospores were formed on or below the agar surface. When these cultures were propagated following single-sporangium and single-hyphal tip isolation, white, fluffy colonies with aerial mycelium and abundant sporangia were formed. These propagations were usually self-sterile of either A1 or A2 mating type and did not form oospores. Occasionally a propagation from a single protoplasmic unit was self-fertile. When grown together self-sterile propagations of A1 and A2 mating type reformed self-fertile cultures. Self-fertile cultures have also been synthesised from A1 and A2 field isolates following co-cultivation on agar. Self-fertile isolates would therefore appear to be intimate mixtures of Al and A2 hyphae. However, results also indicate that heterokaryons/heteroplasmons occurred at a low frequency. Other work on self-fertility reports less strict inhibition of sporangiogenesis and more frequent transmission of the character to asexual progeny.