Genetic analysis of resistance to maize streak virus among mid-altitude maize inbred lines
Maize streak virus disease (MSVD) is the most destructive viral disease of maize in the Sub- Saharan Africa. In Kenya, the disease results in reduction of crop dry matter and grain yields. Like most foliar disease, the disease is managed by means of quantitative partial resistance. Since breeding for durable resistance is an essential trait for improved maize varieties, it is thus important to understand genetic systems conditioning MSV resistance in diverse sources and also assess the yield damage caused by the disease. This study was designed to determine (i) Mode of gene action of two maize inbred lines, an MSV -tolerant inbred line CML202 and an MSV-immune inbred line Osu23i (ii) allelic relationships of MSV genes found in the two MSV -parental sources. (iii) The effect of MSV infection on dry matter and grain yields. A set of SIX generation designated Parent l, Parent2. FI, F2, Backcrossl:l and Backcross I :2 derived from parental and biparental crosses of MSV susceptible parent EM 11-133 and the MSV tolerant parent CML202 and another set derived from MSV susceptible inbred line EMII-133 and MSV immune parent Osu23i and a set of four generations designated PI, P2, Fl and F2 derived from both parental and biparental crosses of both MSV sources of resistance CML202 and Osu23i were planted in three randomized complete block design experiments. The Means and variances of MSV scores rated on individual plants were fitted onto Hayman's 1958 additive-dominance model to determine the mode of gene action of the two parental sources while allelic relationships of genes in the two sources were determined using means of MSV scores and graphic presentations. Eight varieties including three parentals EM 11-133, CML202 and Osu23 i three FI s from three respective generations, a MSV resistant check WH505 and a MSV susceptible check H614D were planted in a fourth split-plot experiment to assess maize crop yields damage caused by MSVD infection. One main plot was inoculated with MSV using viruliferous leafhoppers while the other main plot was a control experiment. The mode of gene action results indicated MSV resistance 111 CML202 and Osu23 i is controlled by additive gene effects with dominance x dominance epistatic interaction. The numbers of effective factors were estimated to be between 2-7 genes. It appears two separate genetic systems are involved in control ofMSV; MSV is controlled through partial resistance in CML202 while complete resistance is responsible for control of MSV in Osu23i. Allelic relationship studies results revealed the two sources CML202 and Osu23i had different genes. Thus by utilizing CML202 and Osu23i MSV sources, a breeder can attain a robust oligenic or multigenic resistance systems which will be hard for destructive virus isolates to overcome. Results showed that MSV disease considerably reduced (P<O.OO 1) stover dry matter and grain yields. Stover yields losses ranged from 19 -29% while that of grain ranged from 8 to 48 % The susceptible inbred line EMll-133 sustained large reduction in stover yields (29 %) and grain yields (48%). The susceptible check H614D and tolerant check WH505 sustained stover yield reduction of 22 and 25% and grain yield reduction of 25 and 19 % respectively. However, the yields of MSV immune parent Osu23 i, tolerant parent CML202 and EMll-133xCML202 (Fl) progeny were not affected. From the results of this study, it is evident that MSV resistance sources exist and that these sources can be utilized in the formation of hybrids which can be availed to farmers to solve the problem of forage and grain shortage.