Inoculum Density of Cochliobolus sativus in Soil and Common Root Rot Of Wheat Cultivars in Queensland

Common root rot was assessed in wheat cultivars grown at field sites in Queensland having low, intermediate and high inoculum densities of Cochliobolus sativus. The densities at seeding time in the upper 10 cm of the profile, were 3, 77 and 132 viable propagules per g soil respectively. Both incidence and intensity of disease at tillering, flowering and firm dough stages of plant development appeared closely related to inoculum density. Additional inoculum applied with seed increased disease markedly at the low inoculum density site and slightly at the intermediate site, and had no effect at the high inoculum density site, indicating a threshold level for maximum disease in the test. Reduction in dry weight of plants at the low and high sites paralleled the disease levels at the tillering and flowering stages. The relative reactions of the cultivars were expressed consistently: Songlen and Timgalen invariably were more diseased than Kite and QT2338/9. The results suggest that cultivars may be evaluated for resistance in the field from tillering onward if disease levels are adequate. C. sativus was the dominant fungus isolated from the subcrown internodes of plants from non-inoculated subplots. Its frequency of isolation was much less from the low inoculum density site than the intermediate and high inoculum density sites, and lower from Kite and QT2338/9 than from Songlen and Timgalen. Fursariurn chlamydosporurn was isolated from subcrown internodes with increasing frequency as the season progressed. However, it was considered to be a secondary invader of the plant material.