Differential sensitivity of Phytophthora megasperma f.sp. glycinea isolates to glyceollin isomers

Glyceollin isomers I, II and III were extracted from soybean cotyledons (cv. Harosoy 63) inoculated with Phytophthora megasperma f.sp. glycinea race 1, and separated and purified by column, high performance and thin layer chromatography and quantitated by spectrophotometry. The extinction coefficient of glyceollin I was 10 800, that of glyceollin II and III conformed to published values. Glyceollin I (ED₅₀ approx. 33 μg ml⁻¹) was almost twice as inhibitory as glyceollin II and III to growth of P. megasperma f.sp. glycinea race 1 on V8 juice agar. All three isomers were less active in soybean hypocotyl extract agar. Glyceollin II (ED₅₀ 7 μg ml⁻¹) was the most active against zoospore germination, and caused zoospores to burst; ED₅₀ values for glyceollin I and III were 12·2 and 13·9 μg ml⁻¹ respectively. Three isolates (1·1, 1·2, 1·3) were obtained from sectors in colonies on agar medium amended with glyceollin I, II or III respectively. Isolates 1·1 and 1·3 grew more rapidly on medium amended with glyceollin I, III or a natural glyceollin mixture than did wild type P. megasperma fsp. glycinea race I. All isolates and wild type had similar growth rates on medium amended with glyceollin II. The isolates did not lose their tolerance when grown on control medium and are presumably genetically controlled variants. Tolerance of glyceollin did not increase the aggressiveness of the isolates on soybeans.