pH modulation of the environment by Stagonosporopsis cucurbitacearum, an important pathogen causing fruit rot in Cucurbitaceae

Abstract

The Cucurbitaceae is a genetically diverse group of plants containing several important commodity crops in many parts of the world such as cucumber, pumpkin and melon. In the last decades, fruit rot caused by Stagonosporopsis spp. became a major disease in both field grown and greenhouse grown cucurbits. Yield losses due to Stagonosporopsis can show seasonal peaks up to 30%. Despite its economic importance, only limited information is available about growth characteristics of Stagonosporopsis cucurbitacearum. Our in vitro studies with different media indicated an optimal growth rate of the fungus within the range of pH 5 to pH 6. Independent of the carbon source (sucrose, glucose, dextrose, fructose) alkalization of 1-3, 5 pH units was noticed under both carbon deprivation and excess. The observed pH modulation could not always be related with a more favourable growth environment. The key factor influencing both pH modulating capacity and growth showed to be the nitrogen source. Supplying nitrate, ammonium or a combination of both, the environmental pH respectively increased, decreased or remained stable. In addition to a pH-elevating effect, nitrate supply also stimulated growth whilst growth on ammonium containing media was seriously affected. This research highlights the importance of the nitrogen source in the growth and regulation of environmental pH by fungi and adds in our understanding of S. cucurbitacearum pathogenicity.