DYNAMICS OF THE SURVIVAL AND INFECTIVITY TO POTATO-TUBERS OF SPORANGIA OF PHYTOPHTHORA-INFESTANS IN 3 DIFFERENT SOILS

Changes affecting sporangia of Phytophthora infestans in three soils from Brittany were monitored at nine dates over a 60 day period, using buried filter membranes on which sporangia had been deposited. Germination was poor on all dates (< 30%), but significant differences between soils were found both in germination rates and in the proportion of germlings producing secondary sporangia. Lysis was active in all three soils (up to 40-60% of sporangia affected), and was associated with colonization of the pathogen organs by bacteria or fungi. Infectivity to potato tubers persisted for 15-45 days depending on soil. No single index describing inoculum state could be correlated with infectivity to tubers. One of the three soils was suppressive to P. infestans, and was characterized by very low germination rates and intense lysis of the pathogen by fungal hyphae.