Effects of soil moisture, organic matter and grass mulching on the carpogenic germination of sclerotia and infection of bean by Sclerotinia sclerotiorum

Microplot experiments were carried out during the autumn and winter of 1995 to determine the effects of soil moisture and levels of organic matter on the carpogenic germination of Sclerotinia sclerotiorum and subsequent infec tion of Phaseolus vulgaris. Soil moisture treatments were established by ir rigation to field capacity: (a) when soil had dried to - 25 KPa, (b) when s oil had dried to - 54 KPa, and (c) eight days after the soil had dried to - 54 KPa. Three levels of organic matter were established by mixing compost and yellow-red latosol in the following proportions (by volume): (a) 0% com post and 100% soil, (b) 50% compost and 50% soil, and (c) 100% compost and 0% soil.,More apothecia and diseased plants were observed in the plots with higher moisture levels. Drier treatments resulted in a reduction in number (in the winter experiment) or elimination (in the autumn experiment) of ap othecia. Other experiments were designed to examine the effect of grass mul ching and organic matter levels on the carpogenic germination of S. sclerot iorum. In one experiment, grass mulching was compared with no mulching, and in another, the effects of different depths of mulch on the soil surface ( 0, 1.5, 3, 6 and 9 cm) were examined. Fewer apothecia were formed in mulche d soil and the effect was most pronounced with 6 and 9 cm of mulch. The hig hest level of mulch (9 cm) impaired the vigour and yield of bean plants. In these experiments, carpogenic germination was increased by high soil organ ic matter content. Control of soil moisture and grass mulching may reduce t he production of apothecia of S. sclerotiorum even in soils rich in organic matter.