Studies on Cercospora zeae-maydis, the cause of grey leaf spot of maize in KwaZulu-Natal.
In 1983, Latterell and Rossi described grey leaf spot (GLS) of maize (Cercospora zeae-maydis Tehon and Daniels) as "a disease on the move". This pathogen has more than lived up to its reputation. It is estimated to be spreading at a rate of 80-160 km each year, and is recognized as one of the most grain yield-limiting diseases of maize worldwide. The occurrence of the pathogen in the Province of KwaZulu-Natal (KZN), Republic of South Africa (RSA), in 1988, was its first official report from the African Continent. It has since become pandemic, causing grain yield losses of up to 60%. It has spread to other provinces in RSA as well as other African countries, namely Cameroon, Kenya, Malawi, Mozambique, Nigeria, Swaziland, Tanzania, Uganda, Zaire, Zambia and Zimbabwe. It has also been reported to occur in Brazil, China, Columbia, Costa Rica, Mexico, Peru, Trinidad, and Venezuela. The use of soil macro- and micronutrients in the management of fungal plant pathogens is widely documented in the literature. Specific nutrients are known to increase or decrease disease resistance in plants. However, each host-pathogen interaction must be considered on an individual disease basis, together with environmental and soil variables. Although few diseases can be eliminated by a corrective fertilizer regime, the severity of a disease can be reduced by specific nutrients, particularly when used in conjunction with other cultural practices. However, the economic implications, and not grain yield alone, of different control measures should be considered; i.e., farmers must compare the expected added gross margin ha -1 (added income minus added costs) with the potential variability in expected added gross margin ha -1 (upper and lower limits) of each treatment when deciding on which fertilizer applications and/or fungicide treatments to use. Literature reviews were undertaken on both GLS and the use of soil nutrients to control fungal plant pathogens to provide the necessary background technical information in order to conduct research under local conditions, and to assist in interpretation of results of experiments. Nutrient trials to control GLS were conducted at two sites in KZN, i.e., Cedara (1995/96, 1996/97 and 1997/98) and Ahrens (1995/96). Research at Cedara showed that with increased applications of nitrogen (N) at 0, 60 and 120 kg N ha -1 and potassium (K) at 0, 25, 50 and 150 kg K ha -1, leaf blighting occurred earlier, and final percentage leaf blighting and the standardized area under disease progress curve were higher. The Ahrens trial also showed that with increased applications of N (0, 60, 120 and 180 kg N ha -1) and K (0, 50, 100 and 150 kg K ha -1), there were also increases in final percentage leaf blighting. Increasing phosphorus levels of 0, 30, 60 and 120 kg P ha -1 did not have any effect on final percentage leaf blighting. The application of systemic fungicides to GLS-susceptible maize was highly effective in controlling GLS and increasing grain yields substantially with increased N and K applications. In the non-fungicide treated plots, grain yields did not increase with increased applications of K in all three years of the trial. This was probably because grain yield response, which should have occurred at higher K applications, was reduced by increased GLS severity. Similarly, grain yields did not increase significantly with N application in 2 of the 3 years of the trial. At Cedara, non-fungicide treated maize produced a financial loss of -R165 and -R48 with 25 and 50 kg K ha -1 respectively, relative to 0 kg K ha -1. However, increasing N applications resulted in increasing grain yields, and added gross margins of R714 ha -1 and R536 ha -1 with applications of 60 and 120 kg N ha -1, respectively. The drop in added gross margin at 120 kg N ha -1 was probably because of increased GLS levels at higher fertiliser rates, resulting in reduced grain yields. In fungicide treated maize, added gross margin relative to 0 kg K ha -1 increased from R851 to R1212 ha -1. However, there was a loss of -R133 ha -1 in added gross margin relative to 0 kg N ha -1 at 60 kg N ha -1 as increased grain yields did not offset the added cost of N fertilizer and fungicide applications. At 120 kg N ha -1 added gross margin relative to NO was R423 ha -1. Highest grain yields and gross margins in fungicide treated maize were obtained with 120 kg N ha -1 and 150 kg K ha -1, as expected. However, in non-fungicide treated maize, highest grain yields and gross margins were obtained using 60 kg N ha -1 and 50 kg K ha -1. This was because of higher GLS severity at the higher N and K application rates. Yields of wheat grown in soils with residual fertilizers after non-fungicide treated maize were higher (4.21 ha -1) compared to yields (3.61 ha -1) grown on residual fertilizers after maize that had been sprayed to control GLS. This was probably as a result of GLS reducing the photosynthetic area of maize leaves, causing premature death with a concomitant reduced uptake of nutrients by roots. This resulted in higher residual levels of fertilizers in soils where fungicide applications were not used to control GLS on maize compared to soils planted with maize where GLS was controlled through the application of fungicides. In KZN there are approximately 350,000 small-scale farmers. The same diseases that affect commercial agricultural production also affect the small-scale farmer, the major difference being in the methods of disease control employed. At the commercial level, most farmers rely on the use of agro-chemicals, which are often not available to the small-scale farmer due to the relatively high cost of agro-chemicals, application methods, and the non-availability of products in the rural areas. The level of illiteracy of the small-scale farmer may also inhibit the use of agro-chemicals. In many African countries, the per capita consumption of maize may be as high as 100 kg per year. Production of cereals in Africa has fallen in the past 25 years. This, together with yield reductions of maize caused by GLS, is likely to contribute to an even greater food deficit in many African countries. At present, low soil fertility and pH levels are a problem among small-scale farmers both in the RSA and other parts of Africa. In the RSA, government policy is to increase maize production by small-scale farmers through improved agronomic methods, including increased fertilizer application. Appropriate and affordable rotations and other improved agronomic practices need to be developed and promoted to ensure food security and sustainable systems for smallscale farmers. The results from the nutrient trials presented in this thesis have practical applications for the small-scale farmer who does not have the option of controlling GLS through the use of agrochemicals. The small-scale farmer will be able to attain a maximum gross margin from his maize crop by applying 60 kg N ha -1 and 50 kg K ha -1, if no fungicides are applied. However, comparative analyses of manure showed that a small-scale farmer would have to apply 1-3 tonnes of manure in order to achieve similar nutrient levels - a procedure that would be impractical. Comparative financial analyses of aerial and knapsack fungicide applications showed that it would be uneconomical for the small-scale farmer to apply fungicides using a knapsack sprayer. A simple spreadsheet has been created to help farmers make the best choice of N (0, 60 or 120 kg N ha -1) and K (0, 25, 50 or 150 kg K ha -1) and the number of fungicide application (O, 1, 2 or 3). This will eliminate the guesswork needed for farmers to maximize gross margins, based on a specific amount of money available. The resistance expressed by different hybrids on conidial germination of C. zeae-maydis at varying temperatures, desiccation periods and interrupted dew periods was investigated using the susceptible ZS 206 and the less susceptible SC 625 maize cultivars. Germination of conidia was maximized at 28°C on both cultivars by 48 hr with ZS 206 showing 100% germination, in contrast to only 63% germination in SC 625. As the number of days (1-5) of desiccation increased following inoculation, germination decreased from 100 to 47% in ZS 206 and from 62 to 0% in SC 625, respectively. The observation that C. zeae-maydis is able to tolerate unfavourable conditions and resume germ tube growth when favourable conditions return was confirmed in interrupted dew period studies. There was no change in percentage germination after 48 hrs., when plants were subjected to interrupted dew periods of 2-36 hrs, following a 6 hr period at 95-100% RH at 28 °C in a dew chamber. However, germination was lower (64%) on SC 625 than ZS 206 (90%). The wider range of temperature conditions favourable for conidial germination of ZS 206, and the fact that it was less affected by desiccation and interrupted dew periods than SC 625, could account for the different susceptibility levels of these two hybrids to GLS. Peak daily conidial catches were found to be between 1200 and 1400 hrs when temperatures and vapour pressure deficits were highest and leaf wetness lowest. Multiple regression analyses identified high evaporation over a 24 hr period, low temperatures over a 48 hr period and wind over a 72 hr period as the weather variables most strongly associated with high conidial releases. Rain, high vapour pressure deficit values and temperatures between 20-30 °C with leaf wetness over a 72-day period, together with prolonged high evaporation over a 48 hr period were identified as limiting factors in conidial release. These results indicate that temperatures (< 20 °C) and moisture 24-48 hrs prior to release is required for production of conidia. However, dry air and leaf surfaces are required for conidia to break off conidiophores at the point of attachment, i.e., a hygroscopic process is involved in release of conidia in C. zeae-maydis. In general, the process of conidiogenesis in C. zeae-maydis is similar to that observed on C. beticola. Successive formation of conidia on the same conidiophore are in accord with previous observations on C. zeae-maydis. Conidial measurements are also similar to other taxonomic descriptions of C. zeae-maydis. Hyphae aggregate in the substomatal cavity and give rise to fascicles of 1-2 septate conidiophore initials which emerge through the stoma. A single, aseptate conidium develops from the conidiogenous cell of the conidiophore initial. Extension growth of the conidiogenous cell from the base and one side of the terminal conidium, leads to the lateral displacement of the conidium on the conidiophore. After conidial secession, the conidiophore continues to grow, producing a second conidium from the conidiogenous cell at the apex of the extended conidiophore. This sympodial and successive proliferation of the fertile conidiogenous cell results in the formation of a characteristic 1-3 geniculate, occasionally 4, conidiophore, bearing a single conidium at each apex. This body of research has added information that was previously missing in the lifecycle of C. zeae-maydis. However, this additional information has, in turn, led to other yet unanswered questions which need to be addressed in the future, particularly under southern African conditions. A thorough knowledge and understanding of the epidemiology of this pathogen can result in more effective control strategies with increased yields for both commercial and small-scale farmers in KZN.