Introduction

Grey leaf spot (GLS) of maize is caused by the fungus Cercospora zeae-maydis. The disease is now recognised as one of the most significant yield-limiting diseases of maize worldwide and certainly in the province of KwaZulu-Natal (see Table 1). Not only is it a threat to maize production in the commercial farming sector, it also reduces yields of maize on small-scale farms. The disease was first identified in KwaZulu-Natal in 1989/90 and has since spread to neighbouring provinces and most maize producing countries in Africa.

Symptoms

Symptoms are initially first observed on the lower leaves of the maize plant. The immature lesions are similar to lesions caused by other foliar maize pathogens, and first appear as small tan spots about 1 to 3 mm in size and are irregular in shape. The tan spots usually have yellow or chlorotic borders and, are more easily observed when the leaf is held to light

Mature lesions are readily distinguished from other pathogen symptoms and are distinctly rectangular in shape (5 to 70 mm long and 2 to 4 mm wide), and run parallel with leaf-veins.

Lesions, tan in colour, assume a grey sheen or caste when sporulating. As disease progresses, lesions coalesce and blighting of the whole leaf may result.

Under favourable conditions, blighting progresses upwards on the plant.

and the whole plant may die before the crop reaches maturity,

and serious yield losses may result.

Under these conditions, the maize plant may be pre-disposed to stalk-rotting fungal attack and resultant severe lodging adding further to the yield losses.

Disease Cycle

Grey leaf spot is highly dependant on favourable weather conditions. It requires frequent and prolonged periods of high humidity and warm temperatures (20E to 30EC) to complete spore gemination and the infection process. Spores (conidia) are produced from infested residues of previous maize crops in spring under conditions of high humidity and these are windblown to infect the newly planted maize crop. The lower leaves are usually the site of primary infection.

Lesions resulting from the initial infection produce spores that are wind- or rain-splashed to the upper leaves. (Ward et al 1999). Under unfavourable conditions (hot, dry weather), the fungus can remain dormant and then resume rapid development as soon as favourable weather conditions return (Latterall and Rossi, 1988). In mid- to late-season plantings and under favourable conditions, lesions may first appear on the mid- to upper-canopy as a result of wind-blown spores from adjacent infected maize. Such late season infections may be serious because it is the upper canopy that contributes 75 to 90% of the photosynthate for grain fill (Allison and Watson, 1966).

The occurrence of fewer and/or shorter periods of high humidity early in the growing season may account for the slower rate of early-season disease development (during the months of November and December). In contrast, good early season rains and more periods of high humidity (in November and December) have led to a higher frequency of early-season lesions (and more severe disease) (Ringer and Grybauskas, 1995).

Disease Management

As maize is the only known host for GLS, and the pathogen is not known to be seedborne, GLS is only able to survive from one season to the next on maize debris from a previously infected crop. It is spores produced in the infected debris in spring that are wind-blown to the newly planted maize that triggers the new epidemic.

Agronomic Practices
Tillage practices aimed at reducing initial inoculum by burying infested debris are classical methods of control and have been demonstrated to be effective in managing GLS (Latterell and Rossi, 1983). However, ploughing is less effective in managing the disease in areas with high levels of inoculum and where GLS is already established (Perkins et al., 1995). This is because inoculum from neighbouring infected fields, may be wind-blown to infect maize grown under conventional tillage systems. Further, other sources of inoculum may result from production practices used in South Africa. For example the practice of allowing maize to dry down to about 13,5% moisture in the field before harvesting, allows equinoxial winds to remove infected leaf tissue, which may be deposited on contours and headlands in and around maize fields. Such debris, and stubble remaining on the soil surface after ploughing, may act as an important source of inoculum to infect newly planted maize in the late Spring (Ward, 1996). Observations at Cedara have indicated that in dry seasons GLS may be detected three weeks earlier in no-till maize than in conventionally tilled maize. However, the improved moisture conservation under no-till, more than offsets the adverse effects of earlier GLS infection. In seasons favourable for GLS there is little or no difference between no-till and conventional tillage in the time the GLS infects maize.

Crop rotations have shown that even a single year of alternative crops away from maize can reduce initial inoculum. Rotations also provide additional benefits by improving soil quality, conserving soil water content and may reduce maize soil pathogens.

Other practices such as time of planting, plant density and timing of irrigation applications may all play a role in reducing disease severity.

Genetic Resistance
Hybrid resistance is perhaps the most cost-effective strategy of managing GLS. However, few hybrids have sufficient resistance to prevent yield losses due to GLS in commercial maize production. Resistance is due to several genes which are additive in effect, and each of which adds small increments of resistance to the hybrid. Breeders have found that if too high a level of resistance is required, breeding would be time consuming and other genetic characteristics such as yield or growing season length may be sacrificed. This can be observed in Table 1, where the more resistant hybrids, have in general a lower yield potential than hybrids more susceptible to disease. However, each season, more GLS resistant hybrids are being evaluated and their yield potential continues to improve.

Fungicide Control
Although efforts to improve genetic resistance to GLS in maize hybrids, it can be seen that even the most resistant hybrids still respond to fungicide treatment.

Table 1.  Cedara Cultivar Trial: 1999 / 2000