Physical, Enzymic, and Chemical Factors Affecting Viability and Germination of Oospores ofPhytophthora megaspermaf. sp.medicaginis

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Abstract
Oospores of Phytophthora megasperma f. sp. medicaginis were evaluated for viability (activated, dormant or dead) by using tetrazolium bromide, a vital strain, and concomitantly other samples were evaluated for germination in aqueous media. The increase in percentage of activated oospores resulting from different treatments was positively correlated with the increase in the percentage of germination (r = 0.985). Conversely, the percentage of oospores that were dormant or dead was negatively correlated (r = -0.922) with germination. Pretreatments of 40-day-old oospores with 0.05% KMnO4 for 10 min markedly increased oospore activation and germination from 23 to 78% in water and to 84% in root exudate and also killed all contaminating mycelial fragments. Heat shock of oospores at 33 C prior to germination also increased oospore activation and germination. Desiccation and treatment of oospores with 0.5% KMnO4 for 10 min was lethal. Freezing (-15 C) was not lethal, but it reduced viability and germination of oospores. Some, but not all, of the contaminating hyphal fragments were killed by freezing. Sodium periodate at 0.05% had no effect on viability and germination, but at 1.0% oospore viability and germination decreased. Viability and germination of oospores pretreated with either a mixture of .beta.-glucuronidase and aryl sulfatase or cellulase alone did not differ significantly from the untreated control; however, prolonged treatment (48 hr) with the mixture decreased oospore germination. The phytoalexin, medicarpin, added to the growth medium in which oospores were produced or used as a pregermination treatment of oospores, reduced their viability and germination. The percentage of germination was consistently two to three times as great in alfalfa root exudate as in distilled water. Germination of oospores followed by production of a germ sporangium occurred only in root exudate and not in water.