Effects of soil temperature and moisture on the pathogenicity of fungi associated with root rot of subterranean clover

Abstract

The effects of soil temperature (10, 15, 20 and 25¦C) and moisture (45% water holding capacity (WHC), 65% WHC, and flooding) on the pathogenicity of five fungi, both alone and in combinations, were investigated to determine the involvement of these fungi in a severe root rot disorder of subterranean clover in Western Australia. Fusarium avenaceum, Pythium irregulare, and Rhizoctonia solani were highly pathogenic while Fusarium oxysporum and Phoma medicaginis, particularly when used singly, were only weakly pathogenic. Compared with individual fungi, fungal combinations increased the severity of root disease and decreased plant survival and plant fresh weight. While the fungi investigated caused root rot over the range of soil temperatures and moisture conditions of this investigation, the most severe root rot occurred at 10¦C, with less at 15 and 25¦C, and least at 20¦C. Temperature had a marked effect on the disease severity and its effect varied with individual fungi and their combinations, in particular, combinations involving P. irregulare (severest root rot at 10 and 15¦C). The most severe root rotting, compared with the control, occurred at 65% WHC, with less at 45% WHC, and least under flooding conditions. There was often a significant interaction between temperature and moisture for the various fungi and fungal combinations tested.