FACTORS INFLUENCING SURVIVAL OF PHIALOSPORES OF CHALARA-ELEGANS IN ORGANIC SOIL

The influence of soil moisture, temperature, crop plant, flooding, and addition of plant residues and CaCO3 on survival of phialospores of C halara elegans in organic soil was studied. The extent of survival of phialospores (number of propagules recovered from artificially infeste d soil) was monitored over a 19-wk duration using a semiselective medi um (TB-2RBA). Survival was significantly greater (P less-than-or-equal -to 0.01) in soil maintained at a constant soil moisture level (matric potential of about -50 J/kg) than when moisture level gradually decli ned to about -900 J/kg; propagules could be recovered from both soil m oisture treatments for up to 19 wk. The presence of carrot plants did not significantly influence propagule survival when compared with fall ow soil, and propagules of C. elegans could be detected after 19 wk of soil incubation at 20 C. The presence of onion seedlings, however, de creased recovery of the pathogen to an undetectable level after 15 wk. When soil was flooded and maintained at 4 C, survival was comparable to that in nonflooded, fallow soil. However, with increasing temperatu re (15, 20, or 25 C), recovery of phialospores from flooded soil was s ignificantly reduced (P less-than-or-equal-to 0.01) and no propagules were detected after 12 wk. In nonflooded soil at constant soil matric potential of about -50 J/kg, survival was reduced significantly only a t 30 C and not at any of the lower temperatures tested. Addition of pl ant tissues of alfalfa, carrot, rye, and onion all significantly reduc ed the extent of survival of phialospores compared with the fallow con trol, with onion tissues having the most pronounced effect. The additi on of CaCO3 had no effect. The results from this study indicate that p hialospores of C. elegans can survive in organic soil for periods grea ter than 19 wk, especially under moist and cool conditions. Survival o f phialospores was reduced significantly by the presence of onion seed lings, flooding the soil at 25 C, and addition of plant residues. Thes e findings could have application for reducing inoculum of C. elegans under field conditions.