QUANTIFICATION OF MONOSPORASCUS-CANNONBALLUS ASCOSPORES IN 3 COMMERCIAL MUSKMELON FIELDS IN SOUTH TEXAS

Monosporascus cannonballus ascospore populations in the soils of three south Texas muskmelon fields were quantified at planting and at harvest in 1992. Sampling was conducted along eight-row segments (transects) within a 100 X 50 m area in each field according to a stratified random design. In a preliminary study (June 1991), ascospore counts were higher in-beds than between-beds and at a moderate depth of 10-20 cm in field A (annually cropped to muskmelon in fixed, plastic-mulched, drip-irrigated beds). Ascospore numbers were significantly lower in adjacent field B (rotated land, bare soil, and furrow irrigation). In 1992, ascospore counts in A were again significantly higher than in B and also higher than in C, a field similar to B. However, root disease severity at harvest was similar in A and B, and few significant positive correlations between ascospore concentration and root disease severity were found in any field. The presence of other root-damaging pathogens or of infective vegetative mycelium of M. cannonballus may explain this lack of correlation, as could inoculum densities exceeding levels necessary for maximum root disease expression. Variance to mean ratios indicated random partitioning of plants with similar root disease severity among transects in each of the three fields.