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Factors Influencing Survival of Phialospores of Chalara elegans in Organic Soil. S. CHITTARANJAN, Graduate Research Assistant, Centre for Pest Management ,Department of Biological Sciences, Simon Fraser University, Burnaby, BC, Canada V5A 1S6. Z. K. PUNJA, Associate Professor and Director, Centre for Pest Management, Department of Biological Sciences, Simon Fraser University, Burnaby, BC, Canada V5A 1S6. Plant Dis. 78:411-415. Accepted for publication 18 January 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0411.

The influence of soil moisture, temperature, crop plant, flooding, and addition of plant residues and CaCO3 on survival of phialospores of Chalara elegans in organic soil was studied. The extent of survival of phialospores (number of propagules recovered from artificially infested soil) was monitored over a 19-wk duration using a semiselective medium (TB-2RBA). Survival was significantly greater (P ≤ 0.01) in soil maintained at a constant soil moisture level (matric potential of about 50 J/kg) than when moisture level gradually declined to about 900 J/kg; propagules could be recovered from both soil moisture treatments for up to 19 wk. The presence of carrot plants did not significantly influence propagule survival when compared with fallow soil, and propagules of C. elegans could be detected after 19 wk of soil incubation at 20C. The presence of onion seedlings, however, decreased recovery of the pathogen to an undetectable level after 15 wk. When soil was flooded and maintained at 4C, survival was comparable to that in nonflooded, fallow soil. However, with increasing temperature (15, 20, or 25C), recovery of phialospores from flooded soil was significantly reduced (P≤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 at 30C and not at any of the lower temperatures tested. Addition of plant tissues of alfalfa, carrot, rye, and onion all significantly reduced the extent of survival of phialospores compared with the fallow control, with onion tissues having the most pronounced effect. The addition of CaCO3 had no effect. The results from this study indicate that phialospores of C. elegans can survive in organic soil for periods greater than 19 wk, especially under moist and cool conditions. Survival of phialospores was reduced significantly by the presence of onion seedlings, flooding the soil at 25C, and addition of plant residues. These findings could have application for reducing inoculum of C. elegans under field conditions.

Keyword(s): black root rot, endoconidia, inoculum density, Thielaviopsis basicola