CLASSIFICATION AND CHARACTERISTICS OF PHYTOPHTHORA/-SUPPRESSIVE SOILS IN TAIWAN PJ ANN Department of Plant Pathology, Taiwan Agricultural Research Institute, Wu-feng, Taichung, 413, Taiwan, R.O.C. Background and objectives In Taiwan, Phytophthora diseases are especially serious because of frequent heavy rainfall favorable to spread of sporangia and zoospores, and lack of cold temperature in the winter to kill the pathogens. However, plants grown in certain areas with much lower disease incidence than those in the nearby areas have been reported occasionally. Therefore, soils suppressive to Phytophthora
  • species were investigated and the characteristics and mechanisms of the Phytophhtora-suppressive soils were studied. Results and conclusions When spore germination was used to assay pathogen suppression, soils suppressive to Phytophthora capsici, P. paimivora, and P. parasitica were found to be widely distributed in Taiwan. Based on analyzed results of 25 suppressive soils and 25 conducive soils, Taiwanese Phytophthora-suppressive soils could be classified into three groups. Of the 25 test suppressive soils, 15, 8 and 2 soils belonged to group I, II, and III, respectively. The group I suppressive soils were acidic with pH value equal to or lower than 5.0. Meanwhile, concentration of exchangeable aluminum ion (AI+3) of all the kind soils was higher than 100 ppm. Whereas, aluminum ion concentration in other group suppressive soils as well as in all of the conducive soils studied was less than 50 ppm. When the pH was adjusted to 6-8, most group I soils became conducive. However, they still remained suppressive to Phytophthora after being steamed at 100 C for 15 min. The group 11 suppressive soils were acidic, neutral, or alkaline, with organic contents higher than 4% and also contained higher phosphorus, potassium, calcium, and magnesium ions, ranging from 500 to 3000 ppm, respectively. These soils still kept suppressive after acidic treatment, but all of them become conducive to Phytophthora after steam treatment. Two suppressive soils (one with pH 4.5 and one 8.5) contained zinc and copper ions higher than 100 ppm, respectively, were categorized belonging to group III. Aluminum ion was studied and was considered to be the main mechanism involved in pathogen-suppression in the group I suppressive soils. When 100 ppm of Ai+3 was added to the conducive soils tested, 2 conducive soils (pH<5.0), but not soils (pH>5.5), increased their ability in suppression of sporangial germination of Phytophthora dramatically. However, conducive soils (pH>5.5) still could to converted to Phytophthora-suppressive soils after acidification plus aluminum amendment. Concentration of Ai+3 ion higher than 25 ppm was effective in inhibition of germination of sporangia and zoospores of the three Phytophthora species in 2% V-8 juice. The same concentration of aluminum ion was also effective to kill more than 95% sporangia and 99% zoospores in V-8 juice after 24 hr. References 1. ANN PJ. 1994. Soil Biol. Biochem. 26,1239-1248. 2. Lu SC, Chuang TY. 1989. Plant Prot. Bull. 31, 225-237.