The effects of the density of Phomopsis sclerotioides in soil and other environmental factors on black root rot of cucumber were investigated. Cucumber plants were grown in soil containing P. sclerotioides at 1, 10, 100, and 1000 CFU/g. Wilt incidence from 3 to 7 weeks after transplanting was strongly correlated with P. sclerotioides density in soil (P < 0.05). Root rot of squash rootstock occurred in soil with very low inoculum densities (0.1 CFU/g), and was strongly related to P. sclerotioides density (Y = −0.3x + 1.2, R2 = 0.743, P < 0.05) at 8 weeks after transplanting. Cucumber plants showed wilt symptoms in soil containing 1 CFU/g. Wilt symptoms in cucumber plants occurred 4 to 7 days earlier in soil at 22°C than in soil at 27 or 17°C. Root rot development could be predicted from the density of P. sclerotioides in soil and soil temperature. However, further studies on the effects of other environmental factors are required to test the linear model in commercial fields. This information is essential for determining the threshold pathogen density at which most control techniques, particularly those other than soil disinfection, will be effective.