Management strategies to reduce the rate of disease development are aimed at maintaining the plant, especially the upper canopy above the ear, in a healthy condition until physiological maturity. The upper 8-9 leaves of the maize plant contribute 75-90%
of the photosynthate required by ears during grain fill (Allison and Watson, 1966). For maximum grain yields to be achieved, these leaves must be healthy because grain yield is a function of photosynthesis, and is related to a healthy leaf area and its duration after flowering (Eik and Hanway, 1966).
Chemical control
Research in the USA to determine the efficacy of fungicides and their economic feasibility in controlling GLS were variable (Hilty etal., 1979; Ayers etal., 1985, Smith, 1989; Lipps and Pratt, 1991; Carter, 1992; Carter and Stromberg, 1992a and 1992b;
Riviera-Canales, 1993; Martinson etal., 1994; Wegulo 1994; Martinson and Munkvold, 1995; Wegulo ef a/., 1997).
Wegulo (1994) and Wegulo ef al. (1997) suggested several factors play a role in fungicide control of GLS in maize crops. These include correct timing of fungicide applications, number of sprays, prevailing climatic conditions, efficacy of the fungicide group and the level of host resistance. In RSA, Ward ef al. (1997a) found grain yield response was not necessarily the best parameter to justify spraying and showed that the decision to apply fungicides should be based on the expected added income which should exceed the added costs of fungicide treatments. Fungicides belonging to the benzimidazole and triazole chemical groups provide good control of C. zeae-maydis.
Combinations of these two groups have been registered for use in RSA and have been widely adopted by commercial maize producers in this country (Ward, 1996; Ward et al., 1997c; Nowell, 1997). The timing of application of systemic fungicides is critical.
The most effective time to commence treatments is prior to the start of the logistic phase of the epidemic (Ward ef al., 1997c).
Rotational cropping
'Several studies have shown that even a single year of crop rotation can significantly reduce the initial level of C. zeae-maydis inoculum (Stromberg and Donahue, 1986).
As C. zeae-maydis does not survive longer than two years in maize stover, crop rotation has been recommended as a control measure or an alternative to ploughing (Latterell and Rossi, 1983; Stromberg and Donahue, 1986; Spink and Lipps, 1987; Huff ef al., 1988, Ward ef al., 1993). Although rotations to other crops are likely to have significant agronomic benefits (Palti, 1981), crop rotation is not economically attractive
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to producers when alternative rotation crops are likely to produce a lower income.
Furthermore, the highly efficient windbome nature of dissemination of the pathogen makes it is unlikely that maximum benefits will be obtained from a rotation system as maize grown in rotation may still be at risk as a result of wind-blown inoculum originating from infested maize debris in the area.
Tillage practices
Maize debris from the previous season is the only source of inoculum for subsequent maize crops (Hilty et al., 1979; Beckman and Payne, 1982; Stromberg and Donahue, 1986; Payne etal., 1987, White etal., 1996). The increased use of conservation tillage practices in RSA and USA, has been associated with the increase in incidence and severity of GLS. However, trials in the USA and RSA indicate that tillage is of little value in controlling the disease in areas where GLS is endemic and weather conditions are favourable (Payne et al., 1987; Smith, 1989; Ward et a!., 1997d). Rather, the abundance of external inoculum and the distance for dissemination of inoculum from adjacent fields are more likely to affect GLS epidemics. The benefits from improved moisture conservation, the economic and environmental benefits of conservation tillage are unlikely to be abandoned in favour of conventional tillage practices for the control of GLS (White et al., 1996).
Maturity group and planting dates
In contrast to the findings by Hilty et al. (1979) and Beckman and Payne (1982), Rupe et al. (1982) suggested that plant age is important in GLS development. In general, the period of the season with the highest rainfall will result in the highest incidence of GLS.
Therefore, planting to avoid this peak infection period will be beneficial, provided grain yield potential and reliable grain yields are not compromised. Ward et al. (1997b) confirmed that short-season hybrids planted early in the season are less affected by GLS, as they may reach physiological maturity before significant foliar blighting and loss occur (Stromberg and Donahue, 1986).
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In contrast, long-season hybrids, adapted to a longer growing season, are at greater risk from GLS as they are subjected to longer periods of blighting during a greater portion of the grain-fill period (Stromberg and Donahue, 1986).
Plant density
Beckman and Payne (1982), Payne and Waldron (1983), and Ayers et al. (1985) found that GLS severity was higher in high plant populations. They suggested that this was because of increased relative humidity microclimates which favoured development of the pathogen. This was in contrast to Smith (1989), de Nazareno et al. (1991) and de Nazareno et al. (1993a and 1993b) who proposed that less GLS occurred under high plant densities because of the "shielding effects" from spore interception in the denser canopies compared to more open, lower density maize stands as found in small-scale farming.
Host resistance and tolerance
In general, white-grained hybrids have higher levels of resistance to GLS than yellow- grained hybrids, owing to different genetic backgrounds (Nowell, 1997). However, in both types of grain, both resistant and susceptible germplasm has been found.
Fortunately, approximately half of the maize grown in the RSA is white-grained.
Resistance and susceptibility are the two exremes of a continuous scale. Disease tolerance is defined as a relative measure of the yield response of two or more genotypes under equal levels of GLS (Nutter, et a/., 1993). These definitions of resistance, susceptibility and tolerance have been used throughout this thesis.
To date, most resistance to GLS has been quantitative, with a few exceptions such as those noted by Gevers et al. (1994). Recent studies have shown some high grain- yielding hybrids have good levels of resistance to GLS (Ayers et al., 1985; Roane and Donahue, 1986; Stromberg and Donahue, 1986; Lipps and Pratt, 1989; Coates and
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White, 1994; Gevers and Lake, 1994; Perkins etal., 1995, Ward, 1996; Nowell, 1997).
Although host resistance is the most cost effective and cost-efficient means for managing GLS (Lipps and Pratt, 1989; Graham etal., 1993; Coates and White, 1998), commercial hybrids are not currently available in the USA or RSA with adequate resistance to completely avoid grain yield loss due to GLS (Stromberg and Donahue, 1986; Perkins etal., 1995; Nowell, 1997; Ward etal., 1997d, Coates and White, 1998).
Although some maize hybrids can produce higher yields than other hybrids with similar levels of GLS severity (Ward et al., 1999). Under the definition of tolerance by Nutter ef al. (1993), these hybrids can be defined as tolerant to GLS.
In contrast to the USA, a high frequency of quantitative resistance to GLS has been found in commercial hybrids in the RSA (Ward et al., 1993; Nowell, 1997; Ward and Nowell, 1997). In addition to quantitative resistance, a single gene conferring qualitative resistance to GLS has been identified in a South African inbred (Thompson etal., 1987; Gevers and Lake, 1994).
A number of breeding programmes have directed considerable effort toward discovering resistant or tolerant germplasm. Quantitative trait loci (QTL) with additive gene action (Thompson etal., 1987; Bubeck af a/., 1993; Saghai Maroof ef al., 1996;
Young, 1996) or dominant genes with major effects (Elwinger ef al., 1990; Gevers ef al., 1994) have been implicated in expression of resistance to GLS. Quantitative resistance to GLS has been found to impact on lesion size, latent period and sporulation (Freppon ef al., 1996).
Many commercial farmers still preferto plant higher yielding, susceptible, hybrids rather than hybrids with effective quantitative resistance (Ward ef al., 1999). This has necessitated the use of fungicides to effectively and economically manage GLS epidemics in the RSA.
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Lambert and White (1997), as part of a recurrent selection programme designed to improve grain yield, showed that some hybrids, and their crosses, show multiple disease resistance. Wang et al., (1998) has shown that in the USA, two pathogenic species of GLS exist with unknown abilities to evolve new pathotypes.
So/7 fertility
The literature reflects limited and contradictory findings on the effect of fertility on the incidence and severity of GLS. Smith (1989) found increased levels of GLS in response to increased nitrogen (N) levels. However, Carrera and Grybauskas (1992) found increasing levels of N had no effect on GLS. Smith (1989) found potassium (K) had little effect on GLS but this may have been due to the relatively high levels of soil K at the trial site. Phosphorus was also found to have little effect on GLS severity (Smith, 1989).
Burning
Burning GLS infested maize debris may be effective in reducing inoculum. However, as C. zeae-maydis is a windbome pathogen, an external inoculum source is adequate to cause a GLS epidemic. In addition, the negative aspects, i.e., reduced organic matter of the soil that allows water run-off and erosion during rainfall, far outweigh the potential benefits of using burning to control GLS inoculum levels (Ward and Nowell, 1998).
Silage
Inoculum carry-over may also be reduced by harvesting maize for silage because most of the foliage is removed before GLS becomes epidemic. The effect of this on GLS severity the following season will be similar to practicing rotation and, or, ploughing the field. However, this has not been quantified (Ward and Nowell, 1998).
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Irrigation
Overhead irrigation, particularly the use of centre pivots, can significantly increase the rate of GLS development (Ward et al., 1993; Nowell, 1997). Ward (1996) suggested the judicious timing of irrigation applications, aimed at avoiding any increase in the leaf wetness period. No research has been undertaken in this field.