COMPARISON OF SOIL RECEPTIVITY TO THIELAVIOPSIS-BASICOLA, APHANOMYCES-EUTEICHES, AND FUSARIUM-SOLANI F-SP PISI CAUSING ROOT-ROT IN PEA

Soil receptivity as a quantifiable characteristic ranging from conduci veness to suppressiveness to soilborne pea pathogens Thielaviopsis bas icola and Aphanomyces euteiches was determined by analysis of differen ces in disease response curves obtained by artificial introduction of inoculum into natural field soil samples. Several parameters, includin g maximum root rot severity, the area under the health index curve, sc ores on the first axis of a principal component analysis (PCA) on dose responses, and Weibull model fitting were used to describe the diseas e responses. In all cases, the Weibull model gave satisfactory fits. P CA yielded a first axis that comprised 86% of the variance found when using Weibull predicted responses for T. basicola and 74% of the varia nce found for A. euteiches. This PCA axis essentially represented the average increase in disease severity due to the addition of increasing doses of inoculum to the soil. The Weibull scale parameter B, which r epresents the amount of inoculum necessary to increase root rot severi ty by 63% with respect to the level caused by pathogens naturally pres ent in the soil, is another means of quantifying the receptivity of so ils to these plant pathogens. Weibull parameter B, maximum root rot se verity, the area under the health index curve, and the scores on the f irst PCA axis were strongly correlated for each of the pathogens teste d individually. To compare the extent and behavior of soil receptivity responses to different pathogens, Weibull parameters B and C (slope a t dose B) were chosen because of their universal definition, in contra st to PCA scores. Comparison of the average levels of Weibull paramete rs B and C indicated significant differences between the pathogens. Ye t, no significant similarity in the ranking of the soils was found for the three pathogens, demonstrating that individual soils may interact with different pathogens in totally different ways. In general, soils were suppressive to T. basicola but conducive to A. euteiches, wherea s their response to Fusarium solani f. sp. pisi ranged from conducive to suppressive. Therefore, risk assessment of soils prior to planting may require different strategies for each pathogen. Bioassays with soi l samples taken before the last pea crop in 1987 and 1991 revealed a s ignificant increase in the natural inoculum potential of soils that ma inly was accounted for by A. euteiches and Pythium spp. These results strongly indicate that A. euteiches must be considered one of the most threatening pathogens to pea crops in the Netherlands.