Interactions of virus accumulation, disease severity, and disease incidence as heritable indicators of host resistance to MDMV-A and their influence on yield

Typescript (photocopy).Significant differences were observed among sorghum cultivars in the amount of MDMV-A accumulated over time. Virus concentration (estimated as ELISA values, A410 nm) increased for 30 days following inoculation and then decreased. ELISA values were significantly different among genotypes depending on i) age of the plant when inoculated, ii) temperature, and iii) leaf position. 'Subliminal" infections of MDMV-A were detected in both symptomless inoculated seedlings and older leaves of mature plants. No virus was detected in the inoculated leaves of the cultivars QL3-Tx, QL3- India. QL11- or their F1 hybrids. Virus accumulation differed significantly within and among cultivars with mosaic or red-leaf symptoms. The association between 'resistance' to MDMV-A with restricted virus accumulation in infected plants was cultivar specific. To reduce the intrinsic variations due to differences in virus accumulation over time, area under the curve of ELISA values versus time (AUViPC) was proposed as an additional parameter for identifying cultivar 'resistance'. Statistical analysis on AUViPC were used to separate cultivars with different levels of resistance to virus accumulation. Virus accumulation, severity of the disease and yield reduction in 12 cultivars were used as parameters for multiple regression analysis. The best model to explain the variation in yield was: Y = 2.66 + 3.51 x1 + 0.137 x2 - 1.20 x3; where x1 = virus accumulation (ELISA values), x2 = square of symptom severity, x3 = interaction of host symptoms and virus accumulation, and Y = proportional decrease in yield/panicle (probit transformation). A diallel analysis of four parents differing in 'resistance' to MDMV-A (as estimated by area under the disease progress curves and virus accumulation) was carried out to study the genetic of 'resistance' to MDMVA. The additive-dominant model for area under the disease progress curve was not adequate because of the presence of extranuclear factors indicated by significant differences (P=0.01) between reciprocal crosses and F2 segregation values differing from the Mendelian ratios; this cytoplasmic effect was detected in RTx430. The diallel analysis for virus accumulation showed that the genetic variation is mainly additive. Dominance for resistance to virus accumulation accounted for the non-additive variation..