Effects of Inoculum Concentration on the Apparent Resistance of Watermelons to Fusarium oxysporum f. sp. niveum. R. D. Martyn, Assistant Professor, Plant Sciences Department, Texas A&M University, College Station 77843. R. J. McLaughlin, Research Associate, Plant Sciences Department, Texas A&M University, College Station 77843. Plant Dis. 67:493-495. Accepted for publication 25 September 1982. Copyright 1983 American Phytopathological Society. DOI: 10.1094/PD-67-493.

The apparent wilt resistance of watermelon (Citrullus lanatus) cultivars to Fusarium oxysporum f. sp. niveum was dependent on the concentration of initial inoculum. Four wilt resistance rankings were established: susceptible (>80% wilt), slightly resistant (51–80% wilt), moderately resistant (21–50% wilt), and highly resistant (≤20% wilt). Wilt resistance rankings for most cultivars usually dropped one level as inoculum concentration increased logarithmically. Two cultivars, Dixielee and Smokylee, remained highly resistant up to inoculum levels of 1 × 10⁶ conidia per milliliter. Populations of F. oxysporum f. sp. niveum recovered from soil indicated that planting wilt-susceptible cultivars tends to increase the pathogen population in the soil, whereas planting cultivars highly resistant to wilt decreases it.