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).
