SURVEY OF SOILS SUPPRESSIVE TO 3 SPECIES OF PHYTOPHTHORA IN TAIWAN

A total of 486 soil samples collected from various locations throughout the island of Taiwan was tested for ability to suppress sporangial germination of three species of Phytophthora. Sporangia were kept in 50% V-8 juice for 1 h to ensure uniform direct germination and to overcome the general soil fungistasis. The numbers of soils supporting germination of sporangia of P. parasitica, P. palmivora and P. capsici at 20% or less were 101, 113 and 80, respectively, and at >80% were 105, 62 and 185, respectively. Generally, the three species of Phytophthora showed a similar pattern of sporangial germination in the same soil. Most soil samples were either suppressive or conducive to each of these three fungi. Only a few soils supported a high germination rate of one or two Phytophthora species and a low germination rate of the remaining species. The correlation of sporangial germination on soil samples among the three species of Phytophthora were positive, high (r = 0.85 for P. parasitica vs P. palmivora, 0.84 for P. parasitica vs P. capsici, and 0.87 for P. palmivora vs P. capsici) and very significant (P < 0.001). The degree of pathogen suppression was not correlated with soil texture, soil colour or vegetation. In general, soils with a pH lower than 5.0 were more suppressive than those with a higher pH, although a number of soils with a pH lower than 5.0 were conducive and some soils with a pH higher than 7.0 were suppressive. The quadratic relationship between soil pH and sporangial germination of all three Phytophthora were correlated significantly (P < 0.001) and highly (r = 0.70 for P. parasitica, 0.66 for P. palmivora, and 0.65 for P. capsici). Most suppressive soils with a pH lower than 5.0 remained suppressive to Phytophthora after being steamed at 100-degrees-C for 15 min, while most suppressive soils with a pH higher than 7.0 become conducive.