The Relation of the Carbon-Nitrogen Ratio in the Basal Medium to Sexual Reproduction in Species of Phytophthora

Following the discovery by Haskins et al. (1964) that 3-fi-hydroxy sterols are required for the formation of oospores by Pythium species, these compounds were also found to be required for oospore formation by many species and isolates of Phytophthora (Elliott et al, 1964, 1966; Harnish and Merz, 1964; Hendrix, 1964, 1965; Hunter et al., 1965; and Leal et al., 1964). Sterol concentrations of 4-30 mg/liter have been effective. Some species and isolates formed oospores readily on synthetic media to which agar and a suitable sterol were added. Others produced oospores only on certain natural media, and still others produced few or no oospores in the laboratory. Gallegly and Lilly (1964) reported that pairings of A1 and A2 compatibility types of Phytophthora cinnamomi Rands and P. cryptogea Pethyb. & Laff. consistently produced oospores in abundance on hemp seed agar, but not on lima bean agar. Hunter et al. (1965) found that the following species produced no oospores on a medium containing 2 g/liter of D-glucose, 1 g/liter of L-asparagine, and 4 mg/liter of ,/sitosterol: P. hibernalis Carne, P. syringae (Kleb.) Kleb., P. cinnamomi, and two pairings of P. cryptogea. Some isolates of P. citricola Saw., P. ilicis Buddenhagen & Young, and P. megasperma Drechs. produced few or no oospores on the above medium. The influence of the composition of the basal medium (other than the presence of sterol) on the production of oospores by species of Phytophthora has not been studied thoroughly. The concentrations (g/liter) of the carbon and nitrogen sources used by the above investigators have ranged from 2-30 and 1.0-2.5, respectively, to give carbon to nitrogen ratios (C/N) ranging from 3.8:1 to 27.4:1. The major sources of carbon were glucose and sucrose, and of nitrogen were L-asparagine and sodium nitrate.