Influence of inoculum density of Verticillium dahliae, temperature and relative humidity on epidemics of verticillium wilt of cotton in northern Iran

During 1992--2003, frequency of Verticillium dahliae propagules, disease incidence and severity of verticillium wilt of cotton were determined in several cotton growing fields in Golestan province, northeastern Iran. Inoculum density varied among fields and different years ranging between 2-47 propagules/g of air-dried soil with an average of 18.96+/-0.73. In addition, the pattern of diseased plants varied with type of field and year. Simple regression analysis showed a linear relationship between inoculum density of V. dahliae at planting time on one hand, disease incidence and severity for all years on the other. The straight line model described the increase in disease intensity index over the accumulated physiological time from sowing. The number of days above 28 degrees C (T) and the area under relative humidity (RH) had significant effects on inoculum density in soil (MS) and final disease development (Y) and fitted the Y = 65.840 - 0.0034 RH + 0.57 MS - 1.7T model with R2 = 0.859 and significant F-function (p<0.0001).