Reactions of Sweet Corn Hybrids with Resistance to Maize Dwarf Mosaic

Yield and ear quality of sweet corn (Zea mays L.) can be reduced by maize dwarf mosaic (MDM). When Janson and Ellett (6) first reported the disease from southern Ohio, they observed nearly total losses in fields of late-planted sweet corn. Yield reduction due to MDM and symptom expression are affected by the growth stage at which sweet corn is infected (5,12, 18,25). When late plantings of sweet corn are exposed to large populations of viruliferous aphids, plants can be infected at early stages of host growth. Infection at juvenile growth stages reduces plant height, delays maturity, decreases ear diameter and length, reduces ear weight, and increases the number of missing kernels in the basal end of the ear, which is commonly called butt blanking (5,17,18). Butt blanking results from retarded growth of pollen germ tubes on silks of infected plants (19), and it reduces the marketability of sweet corn used for fresh market and the weight of kernels cut from the cob that are used for processing. Yields of susceptible sweet corn hybrids can be reduced as much as 40% by MDM (5,17,18,21). Reductions of up to 12% were observed when two sweet corn hybrids were inoculated at various growth stages with maize dwarf mosaic virus (MDMV-A) (5). Yield was reduced as much as 36% when hybrids were inoculated with sugarcane mosaic virus (SCMV-MB) at the fourto fiveleaf stage (17). Reductions of 15% occurred when hybrids were inoculated at various growth stages with a mixture of MDMV-A and SCMV-MB (17). Yield reductions were as large as 40% when hybrids were inoculated at the threeto fourleaf stage with MDMV-A or SCMV-MB/ MDMV-A (21). Planting MDM-resistant hybrids is the most feasible method of controlling MDM, because control of aphid vectors is not practical. Resistance to MDM is thought to be expressed through restriction of cell-tocell movement of the virus (11,12,15), interference with systemic movement (14), lower concentrations of virus (1,7,10,15, 21), and hypersensitive reactions that localize viral infection to a few cells (15). Hybrids and inbreds with genes for resistance to MDM do not always exhibit asymptomatic responses after infection (7,12,21, 25). Scott and Rosenkranz (25) suggested that symptomatic reactions may be induced by microenvironmental influences, expression of resistance at late growth stages, inoculum concentrations above a specific threshold, or a combination of these factors. Jones and Tolin (7) observed an unusual reaction in resistant hybrids expressed as distinct sectors of symptomatic and asymptomatic tissue. Concentration of virus was high in chlorotic, symptomatic sectors, but virus was not detected in asymptomatic sectors (7). Sectoring may be the result of restricted cell-to-cell movement of a virus due to a mechanism of resistance (7,12). Sources of resistance to MDM in sweet corn have not been identified or widely used, although some sugary enhancer inbreds may express partial resistance (2). Genes for resistance from dent corn sources, such as Pa405, Ga209, and B68, have been introgressed into commercial and public sweet corn inbreds (1,8,16). The inheritance of resistance to MDM may be controlled by one or more genes, possibly involving modifiers or minor genes (3,4,9,13,14,20,22, 24). A single dominant gene, Mdm1, on the short arm of chromosome 6 appears to be involved in resistance to all viral strains causing MDM (15). Louie et al. (14) postulated a common genetic basis of resistance to all strains; however, Scheifele and Wernham (24) observed different genetic systems controlling reactions to MDMV-A and SCMV-MB. Modifier genes may be associated with specific resistance to MDMV-A and SCMV-MB (14) and with the degree of cell-to-cell movement of virus that can result in sectoring symptoms and systemically symptomatic plants in spite of some resistance genes. Most of the previous research on the effects of MDM on yield evaluated the reactions of susceptible hybrids to MDMV-A or SCMV-MB (5,17,18,21). Although previous studies have demonstrated that MDM reduces yield, it is not apparent that yields of resistant hybrids with varying incidences of symptomatic reactions will be entirely unaffected by infection. The objectives of this study were to compare the incidence of MDM infection and types of symptomatic responses among commercial sweet corn hybrids with resistance to MDM and to determine the effects of MDMV-A and SCMVMB on the yields of these hybrids.