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The Role of Pythium in Feeder Roots of Diseased and Symptomless Peach Trees and in Orchard Soils in Peach Tree Decline. S. M. Mircetich, Research Plant Pathologist, Crops Research Division, ARS, USDA, Beltsville, Maryland 20705; Phytopathology 61:357-360. Accepted for publication 19 October 1970. DOI: 10.1094/Phyto-61-357.

Pythium irregulare was isolated most often from decayed peach rootlets, followed by P. ultimum and P. vexans. Twelve other Pythium spp. were isolated occasionally. No relationship between peach tree decline and occurrence of Pythium spp. in peach tree roots or orchard soil was evident. Feeder root necrosis of diseased trees was more than twice that of symptomless peach trees. The number of decayed feeder roots was independent of the number of rootlets invaded by Pythium or the number of Pythium propagules/g soil. Various levels of Pythium in peach orchard soils had no effect on occurrence of peach tree decline during 3 years of observation. Pythium irregulare, P. vexans, and P. ultimum readily invaded excised dead feeder roots of peach trees, suggesting that these fungi may be saprophytic, secondary invaders of peach tree rootlets. An antibiotic selective medium for measuring population of Pythium in soil is described.

Additional keywords: Prunus persica, selective medium for measuring Pythium population in soil.