Reaction of maize genotypes in young stages to Cercospora zeae-maydis and influence of infested maize residue on progress and spread of gray leaf spot in central Uganda

Two studies were conducted to investigate the reaction of young and adult maize plants to Cercospora zeae-maydis in greenhouse and field conditions at two conditions at two locations in central Uganda. The greenhouse experiments were conducted at Ohio Agricultural Research and Development Centre, Ohio, USA to develop protocols for inoculating young maize plants at three-leaf stage with C. zeae-maydis in Uganda in the greenhouse. In the greenhouse study, four methods of minoculation (hypodermic injection, conidial suspension, stem inoculation and agar plug inoculation) were tested on two maize genotypes of known reactions to C. zeae-maydis (susceptible and resistant). Distinct qualitative (lesion type) and quantitative (lesion length and percentage leaf area affected) differences were detected among the genotypes used. The inoculation methods were also tested on seven genotypes (2 resistant, 3 moderate resistant and 2 susceptible) under uniform environmental conditions. Incubation period was a consistent component of resistance. Of the four inoculation techniques tested, hypodermic injection of spore suspension through the whorls and stem were consistently reliable and therefore more effective as they produced symptoms produced on most plants inoculated. However, injection was less laborious and symptoms produced by stem inoculation were localized. Results indicated that high humidity and presence of free water was essential to produce substantial symptoms. Also significant interactions between genotypes and inoculation techniques indicated the possibility that the techniques varied in their effectiveness. Studies were also conducted to investigate the effect of C.zeae-maydis infested maize residue on the development of gray leaf spot under tropical conditions in two locations of Central Uganda using a local susceptible variety, LP16. Infested maize residue collected the previous season was used as inoculum source in varying amounts s high (80% coverage) ,a medium (40% coverage) and control (0% soil beverage) to stimulate no tillage, minimum tillage and maximum tillage with complete burial residues. Results obtained showed that distance form residue source significantly affected foliar damage but direction form the residue foci did not significantly affect disease level nor was distance by direction interactions significant. The present study indicated that the gradients in both seasons were not affected by wing direction in any location and were steeper in Kabanyolo than Namulonge. However, residue level (percentage soil cover) by distance interactions were significant. The rate of disease increase was highest on plants closest to the residue, indicating that it was most important source of inoculum. This was especially prominent in Kabanyolo that had minimum background contamination. Gray leaf spot severity, area under disease progress curves and disease intercepts decreased with distance but apparent infection rate appeared to increase with distance form residue source in most cases.