Effect of organic matter on soil inoculum potential and soil suppressiveness to Gaeumannomyces graminis var. tritici and Rhizoctonia solani

It is widely recognized that organic amendments may reduce crop losses caused by soil borne plant pathogens. Most of the studies conducted so far were related to short term effects observed in vegetable cropping systems. The long term effects of compost or manure amendments in field cropping systems has been investigated in Australia and, concerning Rhizoctonia solani diseases on wheat, inconsistent results were reported. Two large field experiments were set up in 1996, at Le Rheu and la Jaillière (France), to study the effects of organic fertilization in wheat and maize production. In 2006, at the end of the experiments, soil was sampled to assess the effects of the organic matter fertilization on the soil inoculum potential and soil suppressiveness to diseases caused by Gaeumannomyces graminis var. tritici (Ggt, take all) and R. solani. Bioassays were set up in growth chamber, using wheat and carrots to measure soil inoculum potential and soil suppressiveness to Ggt and R. solani, respectively. A pathological index was used to evaluate the disease severity caused by Ggt on wheat and area under the disease progress curve (AUDPC) was calculated to quantify the disease severity caused by R.solani. ANOVA was used to compare the data. At both locations, the inoculum potential of the soils naturally infested by Ggt was low and similar in plots amended with different organic matters and mineral fertilization. The level of soil suppressiveness to take-all was high in plots amended with organic matter compared to the low level after mineral fertilisation. This suggests that organic amendments preserved the suppressive ability of the soil towards take all when a wheat/maize rotation is performed. Concerning R. solani, the soil inoculum potential was reduced in plots amended with organic matters, but only at La Jaillière not at Le Rheu. At both 2 locations, soil suppressiveness to R. solani damping-off was not affected by organic amendments. There were neither clear positive nor detrimental effects of organic fertilisation in a wheat/maize rotation. Therefore organic matter can be used to improve soil fertility without any negative effects on the phytosanitary status of soil.