SOIL POPULATIONS OF RHIZOCTONIA-SOLANI FROM AREAS OF HEALTHY AND DISEASED BEETS WITHIN 4 SUGARBEET FIELDS DIFFERING IN SOIL TEXTURE

Both debris isolation and beet seed colonization methods were used to ascertain Rhizoctonia solani populations in areas of healthy (AH) and rhizoctonia crown rot diseased (DA) sugarbeets within four fields differing in soil texture over a 1-year period (August 1976 to July 1977). Inoculum densities were initially (August-October) higher in DA than in AH, but declined over the winter to levels similar to AH by June. As ascertained by the debris isolation method, AH populations remained low (mostly less than 30 colonies/50 g soil) throughout the year. High (90-422 colonies/50 g soil) DA inoculum densities were apparently sustained by active parasitism. Seasonal R. solani populations were postulated to consist of groupings of propagules differing in capacities for survival. Inoculum densities in AH and DA were similar in June indicating that factors other than inoculum density per se may initiate DA within beet fields. Soil textural differences did not adversely affect R. solani populations. Assays based on debris isolation depicted populations more accurately than did beet seed colonization assays. Most colonies developing in debris isolation plates originated from sclerotia.