Control of sexuality in Phytophthora cinnamomi

Abstract

The mating type behaviour of 176 single zoospore isolates collected from 11 dissected sporangia of Phytophthora cinnamomi Rands was examined and the results confirmed previous conclusions from studies of field isolates, that two compatibility types may be involved in the formation of oospores in this fungus. The bilateral formation of sexual spores in paired cultures is believed to occur via the hyphal fusion of two compatibility types and it is thought that chemical stimulation is not involved. Oospore production is also affected by nutrition and innate characters of the isolates used. Determination of the mating reactions of terminal hyphae isolated from mixed A1 and A2 cultures suggested that extensive heterokaryosis of A1 and A2 type nuclei did not occur in the vegetative hyphae of either mating type, and that heterokaryon formation was probably restricted to the site of oospore formation. The absence of 'vegetative' heterokaryosis between the two mating types of P. cinnamomi is postulated and the control of sexuality in this fungus is discussed.