Oospore viability and variation in zoospore and hyphal tip derivatives of the hybrid alder Phytophthoras
Delcan. J Brasier. C. M
Forest Pathology ; 2001 [Vol.31] Pages:65-83
Abstract
Oospore viability, oospore germinability, and phenotypic variation among zoospore and hyphal tip derivatives of the standard and natural variant types of the hybrid alder Phytophthoras were investigated. Oospore viability in the standard hybrid, estimated by the tetrazolium bromide method, was low (» 31–36%). No germination was observed in more than 4000 oospores, although germination did occur in the Phytophthora cactorum, Phytophthora citricola and Phytophthora cambivora controls. This is consistent with the known meiotic irregularities in this hybrid. Mean oospore viabilities in the natural variants were significantly different (p < 0.001), ranging from approximately 24% in the UK variant to approximately 75% in the Dutch variant. Again, no oospore germination was observed. Zoospore and hyphal tip derivatives of the standard hybrid and of the Swedish and Dutch variants resembled the parent isolate in phenotype. The derivatives of the German and UK variants, however, often differed from the parent type. Those of the UK variant were extremely and continuously variable in colony patterns, growth rates, temperature–growth relationships and fertility levels. Although these results do not support the view that the natural variants arise as genetic breakdown products of the standard hybrid, this possibility cannot yet be excluded. The apparent inviability of the oospores suggests that the mycelium and zoospores are mainly responsible for the survival and spread of the alder Phytophthoras in the field.
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