5.1.6
INTERACTION OF FERTILIZERS AND GREY LEAF SPOT ON MAIZE YIELDS FOR COMMERCIAL AND SMALL-SCALE FARMERS

P CALDWELL, J WARD, N MILES and M LAING

University of Natal, Scottsville, Pietermaritzburg, South Africa

Background and objectives
The beneficial effects of reducing the incidence and severity of plant pathogen diseases by the manipulation of the supply of inorganic nutrients are widely recognized and well documented in the literature [1]. However, there is a paucity of information on the nutritional effects on the development and severity of grey leaf spot (GLS), caused by the fungus Cercospora zeae-maydis in maize (Zea mays). The objectives of the trial were to investigate the effects of nitrogen and potassium on the rate of disease development, final disease severity and yield, and the economic implications of GLS on maize for the commercial and small-scale farmer.

Materials and methods
The trial was carried out in KwaZulu-Natal, South Africa over two seasons (1996-97 and 1997-98). A randomized complete block design with 12 plots replicated three times was used. Each plot comprised 8 rows, 8 m long-spaced 750 mm apart. Plots were split for sprayed and unsprayed fungicide treatments. Normal weed and pest control practices were followed. Fertilizers were applied annually by hand and disked in prior to planting. Basal nutrients applied to the trial as a whole at planting were phosphorus (P) as double superphosphate, 20% P; sulphur (S) as calcium sulphate, 18% S; and zinc (Zn) as zinc sulphate, 23% Zn; at 105, 50 and 30 kg/ha, respectively. Nitrogen (N) treatments were 0, 60 and 120 kg/ha applied as a split application, with limestone ammonium nitrate (28% N) being the fertilizer source. Potassium (K) as potassium chloride, 50% K was applied at planting at 0, 25, 50 and 150 kg/ha. A high-yielding maize cultivar (RS5206), which is highly susceptible to GLS, was used to ensure GLS infection in the trial.

Results and conclusions
Increases in N and K applications resulted in an earlier appearance of GLS and increased final disease severity levels. In the unsprayed plots, yields in the unfertilized N plots were higher than in the 60 and 120 kg N/ha plots, possibly because of the lower disease severity in these plots, whereas yields in the K-treated plots were highest at the higher K application levels. The differences in yield between the sprayed and unsprayed treatments were greatest in the 150 kg/ha plots, compared to the lower and untreated K plots which could also be accounted for by the greater disease severity at the higher fertilization levels. Profits increased linearly with increased K applications in the sprayed trials. However, in the unsprayed trials there was a limited yield and profit increase at the lower K application levels. These results have far-reaching implications for the small-scale maize farmer who is encouraged to fertilize for maximum yields but often does not have the financial resources or technical skills required to apply costly fungicide treatments for the control of increased GLS at higher fertilizer applications [2].

References
1. Huber DM, 1980. Plant Disease 5, 381-406.
2. Ward JMJ, Darroch MAG, Laing MD et al., 1997. South African Journal of Plant and Soil 14, 43-48.