Zonate leaf spot of Sorghum bicolor (L.) Moench, resistance screening and factors influencing Gloeocercospora sorghi propagule germination

Abstract

Zonate leaf spot of sorghum occurs throughout the wet sorghum growing regions of the world (10, 20, 100). The causal fungus, Gloeocercospora sorghi Bain and Edgerton, produces both conidia and sclerotia. The conidia are rain-splash disseminated; sclerotia are survival propagules and germinate via sporodochial formation (10, 100) or germ tube. No sexual stage is known. Sclerotia are formed subepidermally in infected leaf tissue and are released into the soil during leaf decomposition. Fungistatic inhibition of sclerotial germination has not been investigated. In addition, the influence of host and nonhost derived nutrients on Gloeocercospora sorghi propagule germination is not known. As sorghum plants age, susceptibility to zonate leaf spot increases. A brief stage of susceptibility is also observed during the 2-3 leaf stage of seedling development. Information necessary for screening different sorghum genotypes for resistance or susceptibility and for understanding the nature of propagule germination is lacking. Field inoculation methods were developed in order to screen sorghum lines and hybrids for zonate leaf spot reactions. Also, various environmental factors that influence the germination of conidia and sclerotia of Gloeocercospora sorghi were investigated. Conidial suspension (100,000/ml) application to whole plants following the flagleaf stage of development was the most effective inoculation method. Application of inoculum prior to the flagleaf stage resulted in levels of infection insufficient for resistance screening. Conidia require free moisture for germination. The survival rate of conidia exposed to less than 75% relative humidity at 25 C was low and decreased with decreasing relative humidity and increased time exposures. Conidia do not survive freezing; however, sclerotia do survive freezing and exposures to low relative humidities. Root exudates, leaf guttations, and seed exudates stimulated sclerotial germination. The sugars sucrose, glucose, fructose, and mannose stimulated conidia and sclerotia to germinate. Germination of the sclerotia was dependent on the nutrient concentration. Sclerotia and conidia did not germinate on nonsterile soil. The ability of the above nutrients and nutrient sources to overcome fungistasis was concentration dependent.