No abstract

Timing and frequency of fungicide treatments for management and control of gray leaf spot of maize, caused by Cercospora zeae-maydis, were quantified with the logistic model and area under disease progress curve (AUDPC). Control was most effective when spraying commenced as disease severity levels reached 2 to 3% of the leaf area blighted and when lesions were restricted to the basal five leaves of the maize plant. Highest grain yields were achieved with treatments providing disease control until the crop was physiologically mature. To provide this length of control, the frequency and number of fungicide applications varied with the stage of host development when disease was first apparent; with early infections, more fungicide treatments were necessary to provide protection until physiological maturity. Yield responses to fungicides appeared to be a function of the growth stage of the host when sprays were initiated, the amount of disease at spray date, the length of fungicide control, and effective control through to physiological maturity.

The beneficial effects of stubble tillage on soil and water conservation are widely recognized, but surface stubble also increases the potential for crop damage by fungal pathogens that overwinter in the previous season's debris. In recent years, gray leaf spot (GLS) has become a major yield‐limiting disease, resulting in grain yield losses as high as 60% in high yield potential maize (Zea mays L.) production areas. A study was launched to investigate strategies that could be adopted to facilitate the continuation of conservation tillage practices without exposing maize to unnecessarily high risk from GLS. The aim of the study was to investigate the interactive effects of four tillage practices leaving varying amounts of surface residues and fungicide treatments for the control of stubble‐associated pathogens. In the hot, dry season, unfavorable for GLS, the onset of disease was 23 d earlier in no‐till with higher disease than conventional tillage. The benefits of conserved soil moisture under stubble tillage with concomitant higher grain yields than conventional tillage offset the detrimental effects of higher disease. Results from the study indicate that tillage practices are unlikely to have a major impact in managing GLS; since the mean yield of conventional tillage with minimal stubble (3% residue), during four seasons, was between 28 and 209 kg ha−1 lower than tillage treatments leaving 82 and 26% stubble on the soil surface, respectively. During the four seasons of the study, grain yield responses to fungicide treatment ranged from 477 kg ha− in unfavorable seasons to 3830 kg ha− in seasons favorable for GLS. The judicious application of fungicides will reduce the risk of financial loss from GLS and will allow the continuation of the desirable stubble tillage practice in sustainable farming systems.

No abstract

The effects of the application of 0, 60, and 120 kg N ha-1 and of 0, 25, 50, and 150 kg K ha-1 on final disease severity, standardized area under disease progress curve, and grain yield were investigated at Cedara, South Africa, on a maize (Zea mays) hybrid susceptible to gray leaf spot (GLS), caused by Cercospora zeae-maydis. The trial was a randomized 3 × 4 factor design, split for fungicide treatments, and replicated three times. With increased N and K levels, final percent leaf blighting and the standardized area under disease progress curve were higher. In fungicide-treated maize, grain yields increased with increasing levels of N and K, as expected. In non-fungicide-treated maize, grain yield increased significantly with increased levels of N, despite increased disease severity. This was in contrast to small increases in grain yields from non-fungicide-treated maize with increased K levels, which were not significant. This was probably because grain yield response, which should have occurred at higher K applications, was reduced by increased disease severity. The effect of N, P, and K on GLS wasinvestigated at Ahrens. Maize was grown in a 4 × 4 × 4 N-P-K factorial, in a randomized complete block design. Fertilizer was applied at 0, 60, 120, and 180 kg N ha-1, 0, 30, 60, and 120 kg P ha-1, and 0, 50, 100, and 150 kg K ha-1. No fungicides were applied. A single disease assessment at physiological maturity showed that final disease severity increased with increasing levels of N, P, and K. These results have implications for small-scale farmers who are encouraged to fertilize for increased grain yields but may not have the resources to apply fungicide sprays to control fungal diseases.