Ambient Stresses Regulate the Development of the Maize Late Wilt Causing Agent, <i>Harpophora maydis</i>

Abstract

Late wilt, a severe vascular disease of maize caused by the fungus Harpophora maydis , is characterized by relatively rapid wilting of maize plants before tasseling and until shortly before maturity. In Egypt and Israel, the disease is considered to be a major problem. The pathogen is currently controlled using cultivars of maize having reduced sensitivity, but the fungi can undergo pathogenic variations and become a threat to resistance cultivars as well. The abiotic and biotic factors influencing the infection and disease development are not fully determined. To impose stress in a uniform and chronic manner, we expose the Israeli H. maydis isolates colonies or spores to light, different pH, ionic and hyperosmotic pressures (induced with KCl or sorbitol) or oxygen-related stresses (induced with oxygen enrichment, menadione or peroxide). The optimum pH for both hyphal development and spore germination was pH = 5 - 6, similar to reports for the Egyptian, Indian and Hungarian isolates of H. maydis . In the hyperosmotic regime, hyphal growth was affected in a dosage-dependent curve. Although inoculation under high salt stress also inhibited spore germination, the spores were relatively resistant to this stress in comparison to the hypha. An opposite picture was revealed under menadione/peroxide stress: under high dosage of these compounds, the spore germination was virtually abolished while the colony growth was moderately affected. A daily oxygen enrichment of liquid medium cultures caused an increased growth in the pathogen wet and dry biomass, but daily double treatments led to growth suppression. These findings are a preliminary step towards the inspection of the fungal-host interaction under these different stressful environments. This is important for the future development of new strategies to restrict the disease burst and to protect field corps.