Fig. 1 Primary infections develop on the lower maize leaves

Pat Caldwell
Discipline of Plant Pathology University of Natal, Pietermaritzburg

Grey leaf spot (GLS), caused by Cercospora zea-maydis Tehon and E.Y. Daniels [1] on maize (Zea mays L.) is recognised as one of the most yield-limiting diseases of maize world-wide. It is estimated to be spreading at a rate of 80-160km each year [2].

The occurrence of this pathogen in KwaZulu-Natal, South Africa (SA) in the late 1980s, was its first official report from the African continent [2]. It is thought that while off-loading imported grain, from the United States, in Durban harbour during the drought years in the early 1980s, infected debris could have been carried by wind, infecting nearby maize. The pathogen has since become pandemic, causing yield losses of up to 60%. It has spread to other provinces in SA as well as other African countries, i.e., Cameroon, Kenya, Malawi, Mozambique, Nigeria, Swaziland, Tanzania, Uganda, Zaire, Zambia and Zimbabwe. It has also been reported to occur in Brazil, Colombia, Costa Rica, Mexico, Peru, Trinidad and Venezuela [2].

The pathogen can only survive from one season to the next if maize debris is present on the soil surface. The fungus in the debris produces conidia in the spring, following periods of high humidity. Conidia are the primary source of inoculum and are wind-dispersed to the newly-planted maize crop [2]. Germinating spores produce appresoria over stomata before penetrating the host tissue [3]. Primary infections usually develop on the lower maize leaves and when lesions mature, conidia are wind-dispersed to infect upper leaves (Fig. 1).

Fig. 2 Conidiophores emerge through stomata

Conidiophores emerge through stomata (Fig. 2)on both the ab- and adaxial leaf surfaces and give rise to conidia (Fig. 3) [3].

Fig. 3 Conidiophores with conidia

Unlike most fungi, C. zeae-maydis can remain dormant during unfavourable environmental conditions (hot, dry weather) and resume rapid development as soon as favourable weather conditions return. Under prolonged favourable conditions, and especially after the canopy has closed, developing lesions may coalesce, resulting in extensive blighting and necrosis of leaf tissue. The disease usually develops from the time of tasselling. Under favourable environmental conditions, (>90% RH; 122-30°C) [4], especially monoculture maize, it may occur before tasselling [2].

If GLS infected debris remains on the soil surface from winter to spring, spores will be produced which will infect the new maize crop. In this way, the disease cycle is repeated.

Early symptoms are pin-point spots surrounded by a yellow halo which turns tan later in the season. These lesions are easily observed when the leaf is held against the light. Lesions take about 7 days to elongate and develop into the typical rectangular lesions symptomatic of GLS. Mature lesions are sharply rectangular, long and narrow, run parallel to the leaf veins and are grey to tan in colour. Under severe disease pressure, lesions may coalesce and blight the whole leaf (Fig.4) [2].

Fig. 4 Leaf symptoms

Disease severity is unpredictable and might vary from year to year, field to field and from one cultivar to another. The pathogen can also cause extreme water loss from the plant. Sugar production is affected, resulting in reduced ear size, lower grain yields and sometimes premature death. Maize grown for silage is also affected by the lower nutritive value of the crop. Disease is most severe in warm (20-28°C), humid areas and is favoured by prolonged overcast, rainy or misty days which provide enough free moisture on the leaves essential for disease development [5].

Although early findings suggested high plant populations were conducive to creating high humidity micro-climates for disease, this has more recently been disputed as it was found that in dense populations disease decreased, possibly because their canopies provide more of a shield to wind-born spores than provided by open canopies [6].

Tillage operations aimed at complete burial of debris are one way of managing GLS. Discing provides insufficient burial of residues and ploughing can leave as much as 10% residue on the land surface. This could, provide sufficient inoculum to start an epidemic; subsequent tillage would have to bury residual debris [6].

Most maize grown for silage in SA are long season hybrids and often the disease is severe when the crop is cut. However, removal of the crop for silage does reduce inoculum carry-over [6].

The pathogen does not survive beyond a year in infected maize debris and, because it is host specific, rotating maize with soybeans, dry beans and cereals is feasible. Unfortunately, rotations are not always economically attractive and historically this is the case in SA [6].

Host resistance is considered one of the best options for managing GLS [7]. Several commercially available hybrids are now available which offer some resistance to GLS. In SA, an unexpectedly high frequency of quantitative resistance to GLS has been found already present within commercial hybrids. In addition, a single gene conferring qualitative resistance to GLS has been found in one SA maize genotype [2].

Experience in KwaZulu-Natal has shown significant increases in severity of GLS in maize grown under irrigation compared to dry land production. It is thought that irrigation extends the leaf wetness period, enhancing disease development [6].

Fungicides are used to effectively and economically manage GLS in SA. Systemic benzimidazole and triazole fungicides have been found to be most effective and are currently registered for the control of GLS. Fungicides delay leaf blighting, especially during grain fill. The cost-effectiveness of treatments allows for economical use of fungicides in commercial maize. However, to achieve maximum effect, fungicides must be applied at the correct stage of GLS development. Research has shown that the best time for initial spraying is when disease severity levels reach 2 to 3% of the leaf area blighted and when lesions are restricted to the basal five leaves of the maize plant. Highest grain yields are achieved with treatments providing disease control until the crop is physiologically mature [8].

Trials conducted in SA showed that GLS increased in severity with increasing applications of nitrogen and potassium [9].

Blighting and premature death of leaves severely limits yield production. This is especially true for the upper eight or nine leaves which contribute 75-90% of the photosynthate for grainfill [2]. Leaves of susceptible hybrids may become severely blighted or killed as early as 30 days prior to physiological maturity. Additional losses are incurred when photosynthate is diverted from the stalk to the roots, which then predisposes the tissue to stalk and root rots resulting in stalk lodging. Losses due to lodging are often worse when maize is mechanically harvested, as opposed to hand-harvested fields [2].

1. Tehon, L.R., and Daniels, E. 1925. Notes on the parasitic fungi of Illinois. Mycologia 17:240-249.
2. Ward, J.M.J., Stromberg, E.L., Nowell, D.C., and Nutter, F.W. Jr. Gray leaf spot - a disease of global importance in maize production. Plant Disease 83: 884-895.
3. Beckman, P.M. and Payne, G.A. 1982. External growth, penetration, and development of Cercospora zeae-maydis in corn leaves. Phytopathology 72: 810-815.
4. Thorson, P.R. and Martinson, C.A. 1993. Development and survival of Cercospora zeae-maydis germlings in different relative humidity environments. Phytopathology 83: 153-157.
5. Rupe, J.C., Siegel, M.R., and Hartmann, J.R. 1982. Influence of environment and plant maturity on gray leaf spot of corn caused by Cercospora zeae-maydis. Phytopathology 72: 1587-1591.
6. Ward, J.M.J., and Nowell, D.C. 1998. Integrated management for the control of gray leaf spot. Integrated Pest Management Review 3: 1-12.
7. Bubeck, D.M., Goodman, M.M., Beavis, W.D., and Grant, D. 1993. Quantitative trait loci controlling resistance to gray leaf spot in maize. Crop Science 33: 838-847.
8. Ward, J.M.J., Laing, M.D., and Rijkenberg, F.H.J. 1997. Frequency and timing of fungicide applications for the control fo gray leaf spot. Crop Protection 16: 265-271.
9. Caldwell, P.M., Ward, J.M., Miles, N., and Laing, M.D. 2002. Assessment of the effects of fertilizer applications on gray leaf spot and yield in maize. Plant Disease. In press.