Assessment of methods for evaluating common root rot in spring wheat and infection of subterranean plant parts by the causal fungus Cochliobolus sativus

Some methods of assessing common root rot of spring wheat, caused primarily by Cochliobolus sativus, were compared. In field trials in Saskatchewan, disease ratings based on the extent of lesions on crowns did not discriminate among cultivar and line reactions to the disease as consistently as did disease ratings based on lesions on subcrown internodes. Further, reductions in fresh weight and grain yield were associated with disease evaluated on the subcrown internodes but not on the crowns. In Saskatchewan fields many lesions occur on lower leaf sheaths and since some sheaths may disintegrate, considerable variation can occur in ratings based on crowns. Disease incidence and disease rating based on subcrown internode lesions in Brazilian material were highly correlated with those based on lesions on all subterranean plant parts. Since reports show that grain yield per plant declines with increasing disease severity evaluated by the subcrown and all basal part methods, both are deemed appropriate for the evaluation of common root rot and their use is discussed. The frequency of isolation of C. sativus was fairly similar from crowns and from subcrown internodes of wheat plants in Saskatchewan. It was lower from crown roots and least from seminal roots. Apparently some isolations from roots represent original infections and do not result from an extension of the fungus from the central axis. C. sativus was recovered at moderate frequencies from subcrown internodes that were devoid of lesions. Its isolation frequency from other basal parts of these same plants was consistently lower than that from parts of plants having subcrown internodes with lesions.