Laboratory Method for Assessing Field Tolerance of Soybean Seedlings to Phytophthora megasperma var. sojae. Benjamin Jimenez, Former Graduate Student, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. J. L. Lockwood, Professor, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. Plant Dis. 64:775-778. Copyright 1980 American Phytopathological Society. DOI: 10.1094/PD-64-775.

Two- to four-day-old seedlings of soybean cultivars with known levels of field tolerance to Phytophthora root rot were planted in slit Styrofoam cups filled with vermiculite. The cups were placed in plastic trays, 17 × 25 × 6 cm, containing 100 g of steamed greenhouse potting mix and 200–250 ml water. Inoculum usually consisted of 4,000 zoospores obtained by flooding lima bean agar cultures of Phytophthora megasperma var. sojae. Plants were grown 15–20 days in a growth chamber at 28 C and a 12-hr photoperiod (17,000 lux). Disease was assessed by measuring dry weights and overall lengths of individual plants. With race 3, lengths of the cultivars Hark, Williams, and Agripro 26 were 36%, 43%, and 55%, respectively, of the uninoculated control plants. Dry weight measurements gave similar results. Results were comparable with race 7 and with a natural soil infested with race 4. The results agree with relative field tolerances reported previously. The response of the field tolerant cultivar Agripro 26 was maintained against eight races of the pathogen. The cultivars Woodworth, Agripro 25, SRF 307 P, and Wayne, also known to have field tolerance, showed less reduction in growth than did varieties without field tolerance.