Effect of matric potential, pH, temperature, and clay minerals on bacterial colonization of conidia ofCochliobolus sativusand on their survival in soil

Conidia of Cochliobolus sativus were incubated in soil held within filter membrane pockets (FMP), retrieved periodically, examined microscopically for their colonization by bacteria and for lysis, and tested for their ability to germinate in V8 broth. Conidia that had been held within FMP with 10 μm pore size were colonized by bacteria shortly after they were introduced into the soil while those held within FMP with 0.2 μm pore size were colonized only after 40 days in soil. Comparison between the two groups of conidia showed that the former underwent lysis and more rapidly lost their ability to germinate. High temperatures (32°C), alkaline conditions (pH 8.0), and high matric water potentials (0 to 150 cm water suction) enhanced the colonization of bacteria on surfaces of conidia in soil. Under these conditions the conidia appeared to undergo lysis, and to lose more rapidly their ability to germinate. The bacterial colonization was partially restricted at a matric potential of 300 cm water suction or lower but increased sharply after a temporary flooding treatment. When conidia were mixed with clay minerals (bentonite or kaolinite) before they were incorporated into the soil their colonization by bacteria was restricted and only a few conidia were lysed. The results suggest that lysis of conidia of C. sativus in soil may be partially an autolytic process which is intensified by bacteria colonizing them.