CHAPTER 1: LITERATURE REVIEW
1.7 Gray Leaf Spot Disease
In sub-Saharan Africa, the gray leaf spot (GLS) has become endemic throughout the humid areas in Western, Eastern and Southern Africa (Menkir and Ayodele, 2005;
Caldwell, 2002). Gray leaf spot is caused by Cercospora zeae-maydis Tehon and Daniels, with two isolates that have been reported in the USA (Lipps et al., 1998).
These isolates vary in their aggressiveness but they do not have physiological specialisation into races (Carson, 1999). Only one isolate is found in Africa, which is similar to the one that is most prevalent in the USA (Lipps et al., 1998). Each isolate was relatively uniform suggesting asexual reproduction (Lipps et al., 1998), which minimises emergence of different races of GLS.
1.7.1 Disease Cycle, Epidemiology and Control
Severe GLS occurs in monoculture situations when the pathogen over-winters in maize debris on the soil. The disease development is highly weather dependent.
Conidiaspores are disseminated by wind or water and require 72 hours of ≥ 95%
relative humidity, 13 hours of leaf wetness and 20 to 28oC to germinate (Lipps et al., 1998; Ward et al., 1993). Lesions of GLS develop on leaves and produce conidia for secondary spread after two to four weeks (Lipps et al., 1998). Caldwell et al. (2002) reported that disease severity increased with increasing levels of soil nutrients.
The tan to brown, and narrow lesions with squared-off ends are visible after tasselling (Wysong, 1996; Ward et al., 1993). These lesions grow together resulting in larger blighted areas and the dense sporulation produces a greyish cast on leaves, ear husk and stalks leading to stem lodging (Ward and Birch, 1993). Early blighting of the leaves above the ear causes grain yield loss but late infection does not cause economic damage (Lipps et al., 1998). Ward et al. (1993) estimated grain yield loss at 50% in the humid and high potential areas. A viable management option would be to delay onset of the disease for as long as possible, through conventional tillage that reduces the level of soil based inoculum (Carson, 1999; Lipps et al., 1998; Ward et al., 1997d). Resourceful farmers can apply foliar fungicide sprays to control GLS (Ward et al., 1996; Ward et al., 1997a, 1997b, 1997c; Lipps et al., 1998; Ward and Newell, 1998; Carson, 1999), but these fungicides are usually not affordable by small-scale farmers in Southern Africa. Thus, planting of resistant cultivars would provide a more sustainable control strategy.
1.7.2 Sources of Resistance to Gray Leaf Spot
Resistant germplasm has been found in the United States and South Africa. In the USA, Ayers et al. (1984) and Freppon et al. (1994) reported fleck-type lesions in the resistant inbreds Pa875, NC264, NC288, Va59 and Oh43. Freppon et al. (1994) also reported some chlorotic lesions on resistant inbreds NC250 and NC288 and some moderately resistant hybrids. These chlorotic lesions provide a mechanism for reducing the number and size of lesions and reduce secondary inoculation.
Resistance was also reported in the inbreds T222 and Mo18W (Ulrich et al., 1990), and some inbreds derived from Lancaster population (Graham et al., 1993). Graham et al. (1993) reported that inbreds derived from Lancaster had better resistance than inbreds from Iowa Stiff Stalk Synthetic (BSSS) population. In South Africa, Gevers et al. (1994) reported GLS resistance in the inbreds K054W and S0507 from the F and M heterotic groups (Section 1.2 in Table 1.1). Gevers et al. (1994) reported that inbred S0713W from the P heterotic group was susceptible to GLS. Thus, GLS resistance is also available in locally adapted maize germplasm.
1.7.3 Gene Action Conditioning Gray Leaf Spot Resistance
Gene action conditioning resistance to GLS has been studied extensively in temperate materials. Bubeck et al. (1993) reported that more than five genes controlled resistance in the line NC250A indicating that resistance was quantitative.
Larger general combining ability (GCA) than specific combining ability (SCA) effects for GLS resistance have been reported (Huff et al., 1988; Thompson et al., 1987;
Ulrich et al., 1990). Predominance of the GCA variance over the SCA variance suggested that additive effects were more important than non-additive gene action in conferring resistance to GLS. However, a study by Elwinger et al. (1990) and Freppon et al. (1994) found that hybrids between resistant and susceptible inbreds were resistant suggesting that dominance gene action was also important in conditioning resistance to GLS. Coates and White (1998) also reported that dominance gene action controlled GLS resistance in the inbred B37HtN. Studies by Lipps et al. (1998) indicated that environmental effects were not significant for GLS resistance because hybrid rankings were similar across 22 environments. In Southern Africa, Gevers and Lake (1994) reported larger GCA than SCA effects, but Hohls et al. (1995) reported complete dominance and minor epistasis in inbreds from the P, M and F heterotic groups. Resistance to GLS could be improved through reciprocal recurrent selection that utilises both GCA and SCA variation.
1.7.4 Selection for Resistance to Gray Leaf Spot
Selection for resistance can be effective because additive genetic effects largely determine GLS resistance and resistance is highly heritable (Donahue et al., 1991).
Coates and White (1998) reported narrow-sense heritabilty of 100% in some inbreds of B73 orientation. Cromley et al. (2002) reported high level of resistance in single crosses between resistant and susceptible inbreds, which was similar to crosses between two resistant parents. Graham et al. (1993) reported that 11 cycles of recurrent selection for grain yield did not compromise GLS resistance in the population BSSS. However, Lipps et al. (1998) reported that some resistant hybrids had lower yield potential than their susceptible hybrids. According to Lipps et al.
(1998), resistant hybrids only had yield advantage under high GLS severity.
Appropriate resistance for use in regional maize would be one that does not result in a yield penalty when the disease is not severe. This is very important because in
Southern Africa GLS severity varies between seasons depending on the highly variable weather pattern.