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Use of Safflower Seedlings For the Detection and Isolation of Phytophthora cactorum From Soil and Its Application to Population Studies. Z. Banihashemi, Associate Professor, Department of Plant Protection, College of Agriculture, Pahlavi University, Shiraz, Iran; J. E. Mitchell, Professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 65:1424-1430. Accepted for publication 9 July 1975. DOI: 10.1094/Phyto-65-1424.

A new method was developed to detect Phytophthora cactorum, an incitant of apple collar rot in, and to isolate it from, naturally infested soil. A 15-mm layer of infested soil was placed around safflower seedlings that had been transplanted into steamed soil. The pathogen caused water-soaking of the hypocotyl at or above the soil line within 4-14 days, after which the affected tissue turned light-brown and collapsed, causing the seedling to topple over. Typical sporangia were formed within 4-24 hours when infected parts were washed and floated on tap or distilled water at 20-22 C. The pathogen frequently could be obtained in pure culture when an infected area occurring above the soil line was plated on a selective medium. All isolates obtained by the safflower method were pathogenic to apple seedlings. Phytophthora cactorum was isolated by this method from soil samples obtained throughout the year around apple trees with and without disease symptoms. The pathogenicity of several species of soil-borne fungi were tested against safflower, but only an infrequently occurring isolate of a fungus which appeared to be a Pythium species caused comparable infection. In a field soil artificially infested with P. cactorum, the activity of the pathogen increased during the first 30 days of incubation at temperatures from 4-28 C, but declined later. After 120 days, the infection was least in soil incubated at 28 C, and remained highest in that incubated at 4 C. The viability of sporangia of P. cactorum introduced into natural soil decreased within 2 weeks at temperatures from 4-28 C, and few sporangia survived beyond 45 days at 20 C or higher.

Additional keywords: soil-borne fungus, Carthamus tinctorius, Malus sylvestris, survival, apple collar rot.