Epsilon Open Archive

Abstract

Root diseases caused by soil-borne pathogens are often main constraints in legume crop production. Changes towards organic farming practices have recently contributed to an increase in legume cropping, mainly for nitrogen supply purposes, and these have raised concerns about unacceptable build-up of soil-borne pathogen inocula. This study aimed to evaluate the impact of frequent legume cropping on such inocula build-up, and had emphasis on Aphanomyces euteiches, an important pathogen causing pea root rot in Sweden. Field experiments with legume monocultures were established, and the effect of these monocultures on disease development and yields in subsequent pea, broad bean and snap bean was measured. Isolates of Aphanomyces spp., from several legumes, were tested for host ranges and characterised by means of morphological, biochemical and molecular methods. A survey of legume-specific soil-borne pathogens in fields under frequent legume cropping in northern Spain was also undertaken. Several legumes were found to be hosts for A. euteiches, and this pathogen was isolated from field-grown alfalfa, snap bean, pea, sweet clover and vetch. The Swedish isolates of A. euteiches were assigned two putative pathotypes, pea- and vetch-specific. Other species of Aphanomyces had a wide host range among legumes, but these did not induce disease symptoms. Sequencing of ITS1 - 5.8S - ITS2 rDNA region and RFLP of AT-rich DNA allowed appropriate delineation of these Aphanomyces spp. Monocultures of the tested legume crops affected the inoculum potential of A. euteiches differently. Pea, broad bean, snap bean, vetch and sweet clover were almost equally efficient in inoculum build-up and markedly affected subsequent pea and broad bean yields. Monocultures of alfalfa, birdsfoot trefoil, red, white, and Persian clover had lower impact on disease development and yields in subsequent crops. Thielaviopsis basicola and Rhizoctonia solani were the most prevalent pathogens in pea and snap bean fields in Spain and significantly affected yield. Climatic factors and soil properties favoured prevalence of these pathogens in Spain, whereas A. euteiches was most prevalent under Swedish conditions. It is concluded that intensive legume cropping will, on many soil types not be sustainable in the long-term due to the build-up of soil-borne pathogen inoculum.