Root rot of peas in the Netherlands: fungal pathogens, inoculum potential and soil receptivity.

Fungi associated to pea (Pisum sativum L.) root rot were studied. Fusarium and Oomycetes were most common. Fusarium solani f. sp. pisi, Fsp, was widely distributed and the most frequent fungus in roots of diseased crops. The pathogens Thielaviopsis basicola , Tb, and Aphanomyces euteiches, Ae, were new records for pea in The Netherlands. These pathogens showed physiological specialization on pea. Mycosphaerella pinodes and Phoma medicaginis var. pinodella were of ten found, mainly as blight diseases. For several root rot pathogens, seed infection was a source of primary inoculum. The disease pressure in soil, due to these pathogens, called inoculum potential of the soil (IPS), was determined by bioassay. The IPS of fields, determined before growing peas, was the best predictor of disease intensity in crops. The bioassay is now being used to advise pea growers. A field survey of about 200 pea fields showed that root rot could develop in crops on fields with marked differences in cropping history and agronomic qualities. Root rot severity in crops correlated with the cropping frequency of legumes, though this variable explained only a fraction of the variation in severity. The effect of soil-habitat on root rot pathogens, called soil receptivity (SR), was examined. In SR assays, inoculum is added to the soil. A computerized equipment was developed to automatically control the water potential and temperature during the SR experiments. In 1991, soil samples were taken from 50 commercial fields and 5 experimental plots, and SR to Fsp, Tb, and Ae was assayed. About 20 samples showed increased IPS since earlier assessments, mainly due to sof t rot. Pathogenic activity significantly depended on soil. Most soils were conducive to Ae, intermediate for Fsp and suppressive to Tb, In biocontrol experiments, isolates of non-pathogenic F.oxysporum and Gliocladium roseum reduced Fusarium root rot. The suppression in soils where Fsp was naturally present depended on the dose and the soil tested. The antagonism of Actinomyces spp. and fluorescent pseudomonads was independent of their origin from suppressive or conducive soils. Soil nutrients, soil physical properties and kind and size of microbial populations correlated with receptivity to Tb, Ae or Fsp. Soil sterilization rendered all soils conducive, suggesting that relations between abiotic properties and SR are mediated by microbial activities. The value of SR as a tool in integrated disease management is discussed