Abstract
Introduction: In the past, SCMV and other SCMD-causal viruses have caused serious losses in various maize and sugarcane-growing regions, including Hawaii, Egypt, Natal (South Africa), Argentina, Puerto Rico, Cuba, Australia, USA (Koike and Gillaspie, 1989; Fuchs and Grüntzig, 1995; Chen et al., 2002) and several other countries in South America (Perera et al., 2012 and references therein). Epidemics have been followed by replacement of susceptible noble-type canes by hybrid canes with tolerance or, better still, resistance and the propagation of resistant maize genotypes (Silva-Rosales et al., 2015 and references therein). The evolution of new strains of SCMV has required a continuing breeding programme to prevent heavy losses. Losses caused by SCMV are mainly (1) a reduced yield of the crop, (2) the need to include mosaic resistance when breeding new cultivars, and (3) the slowing of the interchange of cultivars between countries because of quarantine concerns over the introduction of new strains of SCMV. Crop Losses: Crop losses caused by SCMV depend on many factors, including the susceptibility of the cultivars to the prevailing strains of SCMV, the incidence of infection, the prevailing environmental conditions, the stage of growth when infection occurs, and interaction with other agents affecting the crop. Crop losses can vary from negligible to severe. Some documented instances of heavy losses in sugarcane crops due to mosaic outbreaks are as follows. In the 1980s, losses on some farms in the Isis district of Queensland, Australia, were estimated to be about 50% (Jones, 1987). In some commercial plantings of cv. Q95 from an infected source, the infected plants had fewer tillers and were less vigorous than apparently healthy plants nearby (Ryan and Jones, 1986). In Guatemala in 1974-1976, many stunted stools of mosaic-affected cv. Q83 were responsible for lack of uniformity in fields near Santa Lucia. The cane tonnage in these fields was seriously reduced (Fors, 1978). Estimations of Potential Losses in Experiments: Sugarcane In Natal, South Africa, plots of sugarcane cv. NCo376 (highly susceptible) and N12 (moderately resistant) were established with either infected or healthy cane. The plots were harvested regularly and tested serologically for SCMV to the 6th ratoon. There was a decline in the number of shoots showing mosaic symptoms in both cultivars during the experiment. However, mean yield reductions were 22% for infected NCo376 and 16% for N12 compared with yields of initially healthy cane (Cronje et al., 1994). In Brazil, plots in two locations were planted with 0, 25, 50 and 100% initial SCMV infection. Virus spread was noticeable for cv. CB46/47, but negligible for cv. IAC50/134. For CB46/47 yield losses between initially healthy and 25% infected plots were 27% and 19% in the two locations; with 100% infection, yield reduction was 71% in both areas. For IAC50/134 the only significant difference in yield was between 0 and 100% infection, an 18% reduction in diseased plots in both areas (Matsuoka and Costa, 1974). In Java, Indonesia, field trials with 0 and 100% SCMV-infected seed cane gave sugar yield reductions of 9.3% for POJ3016 and 11.1% for POJ3067 associated with the disease (Kuntohartono and Legowo, 1970). In Spain, when healthy sugarcane was planted between rows infected by SCMV, the cultivars CO62/175 and NA56/79 were sufficiently resistant for commercial production, but losses of 0.4-0.5 t/ha were found for every 1% infection between the 2nd and 4th cutting (Olalla Mercade et al., 1984a). In Pakistan, mosaic-free seed cane gave a significantly higher yield of cane (48.5 t/ha) than mosaic-infected seed cane (44.5 t/ha) (Ahmad et al., 1991). Maize In East Africa, 10 susceptible maize hybrids had yield losses of 18-46% when inoculated with SCMV in the seedling stage (Louie and Darrah, 1980). In Germany, SCMV was more prevalent than MDMV, but had a similar effect on growth and yield of maize. Early infection reduced plant height by 25%, total weight by 38% and ear weight by 27% (Fuchs et al., 1990). Disease Complexes: SCMV and related potyviruses may occur in disease complexes with other plant pathogens; either additive or synergistic effects may occur. In Louisiana, USA, losses in sugarcane caused by Sorghum mosaic virus (formerly called SCMV-H) and ratoon stunting disease (RSD, caused by the bacterium Leifsonia xyli subsp. xyli) were additive in cv. CP67-412, but synergistic (greater than the sum of each disease separately) in CP65-357 (Koike, 1982). In Spain, RSD symptoms were associated with the presence of SCMV, and damage by RSD was greatest in fields with clear mosaic symptoms (Olalla Mercade et al., 1984b). In Thailand, inoculation of the downy mildew-susceptible maize cv. Guatemala with an SCMV-like virus increased susceptibility to Peronosclerospora sorghi only slightly, whereas with the resistant Suwan 1 maize cv., the virus increased susceptibility from 27 to 61% (Sutabutra et al., 1976). In many African (especial East African) countries, SCMV and some of the SCMD-causal viruses may also interact synergistically with Maize chlorotic mottle virus (genus Machlomovirus; family Tombusviridae) to cause maize lethal necrosis disease, an emerging debilitating disease of maize (Niblett and Claflin, 1978; Wangai et al., 2012) that can cause total crop loss.
Genome-Wide Analyses and Prediction of Resistance to MLN in Large Tropical Maize Germplasm
