Effect of Temperature and Wetness Period on Recovery of the Southern Biotype ofDiaporthe phaseolorumvar.caulivorafrom Soybean

Abstract

The effect of temperature and wetness period on the recovery of the southern biotype of Diaporthe phaseolorum var. caulivora was determined using 2-week-old soybean seedlings (cv. Walters) grown in the greenhouse and inoculated with 10(6) ascospores per ml of the fungus. Inoculated seedlings were placed in a dew chamber at either 15, 20, 22, 25, 27, 30, 32, or 35 degrees C. Four randomly selected seedlings were removed after 8, 24, 36, 48, 72, and 96 h, air dried at room temperature, and stems and petioles of each plant were cut into 1-cm sections. Stem and petiole sections were surface disinfested and placed on potato-dextrose agar to determine incidence of recovery of the fungus. The fungus was recovered from a greater percentage of stem and petiole sections of plants incubated at 22 to 30 degrees C than at 20 or 32 degrees C. The lowest recovery was at 15 and 35 degrees C. A multiple regression equation was developed that significantly (R2 = 0.847, P < 0.0001) related temperature and wetness period to recovery. This equation predicted the optimal temperature for recovery to be 25.8 degrees C. In vitro growth and germination of D. phaseolorum var. caulivora was fastest at 25 and 30 degrees C, followed by 20 and 15 degrees C, in that order. At 35 degrees C, in vitro growth did not occur, but in vitro germination was faster than at 20 degrees C but slower than at 25 or 30 degrees C. In vitro growth was significantly correlated to recovery of the fungus at 72 (R = 0.93, P = 0.02) and 96 h (R = 0.90, P = 0.04), but germination was not related to recovery at any time sampled.