Interaction between turcicum leaf blight and maize streak virus disease in maize

The effects of concomitant infection by exserohilum turcicum and maize streak virus (MSV) were quantified in one susceptible maize inbred line. Infection by MSV masked symptom expression of Turcicum leaf blight. In general, plant leaf area affected (PLAA), area under disease process curve (AUDPC) and number of Turcicum leaf blight lessons were lower in the doubly infected plants. However, infection of maize by MSV had no effect on sporulation of E. turcicum. Similarly, infection of maize by E. turcicum had no effect on MSV titre nor AUDPC of maize streak virus disease. The study also investigated the nature of gene action conditioning resistance to either disease under double infection. Generally dominance gene action seemed to play a greater role than additive gene action. For both diseases, specific combining abilities (SCA) were significant for most of the disease indices although general combining ability was significant for some turcicum leaf blight (TLB) disease indices. For MSVD, genetic variance components followed a similarly trend with non-additive gene action being higher than additive gene action both for the screen house and field trials, indicating that resistance to MSV is conditioned by dominant genes. Resistance to TLB on the other hand was due to non-additive gene action among the seedling, but on older plants, additive gene action was important as indicated by the significant GCA estimates for final severity of TLB taken when plants were approximately 45 days old. However, GCA for older field plants were not significant for all the disease indices of TLB studied. This was attributed to the low TLB levels in the field plants. Correlations between SCA and GCA effects for TLB and MSVD were good implying that selection for plants with high SCA and GCA for MSVD and TLB, respectively, would be effective in offering protection to the plants. In general disease was more severe under screen house conditions than field suggesting that screening for resistance to both TLB and MSV could best be done under screen house conditions. However, since polygenic resistance to TLB is not well expressed in seedling plants, field screening would also be needed.