Effect of Plant Population and Inoculum Density on Incidence of Sclerotinia Wilt of Sunflower

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Abstract
Eight field experiments were established at different sites with soils naturally infested with Sclerotinia sclerotiorum to study the effect of plant population and inoculum density (ID) on disease incidence (DI) of Sclerotinia wilt of sunflower. Plant populations between 37 to 74.1 .times. 103 plants per hectare were established and replicated at each site and inoculum densities were determined for the upper 16.5-cm of soil. Analysis of covariance indicated no significant difference in DI between plant populations for DI recorded 3 wk after the initiation of anthesis or 17 wk after planting. Inoculation densities for the eight sites ranged from 0.09 to 1.55 germinable sclerotia per 800 cm3 of soil and disease incidence varied from 4.8 to 70.7% and 16.1 to 78.8% at the first and second evaluation dates, respectively. Quadratic regression analysis showed a positive correlation between DI and ID and indicated that as ID increased there was a decreasing effect on DI. Results indicated the importance of inoculum densities of less than one sclerotium per 800 cm3 of soil. The inoculum densities of S. sclerotiorum resulting in incidence of Sclerotinia wilt are the lowest reported for a soilborne pathogenic fungus in terms of propagules per unit of soil and a corresponding DI.

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