Department of Plant Pathology, Taiwan Agricultural Research Institute, Wu-feng, Taichung, 413, Taiwan, ROC

Background and objectives
In Taiwan, Phytophthora diseases are especially serious because of frequent heavy rainfall favourable 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 occasionally been reported. Therefore, soils suppressive to Phytophthora species were investigated and the characteristics and mechanisms of the Phytophthora-suppressive soils were studied.

Results and conclusions
When spore germination was used to assay pathogen suppression, soils suppressive to P. ;capsici, P. ;palmivora, and P. parasitica were found to be widely distributed in Taiwan. Based on results of 25 suppressive soils and 25 conducive soils analysed, 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 aluminium ion (Al3+) of all the kind soils was higher than 100 p.p.m. Whereas, aluminium ion concentration in other group suppressive soils, as well as in all of the conducive soils studied, was less than 50 p.p.m. 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 100C for 15 ;min. The group II 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 p.p.m. 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 as belonging to group III. Aluminium ion was studied and was considered to be the main mechanism involved in pathogen suppression in the group I suppressive soils. When 100 p.p.m. of Al3+ was added to the conducive soils tested, two conducive soils (pH<5.0), but not soils (pH>5.5), dramatically increased their ability in suppression of sporangial germination of Phytophthora. However, conducive soils (pH>5.5) still could to converted to Phytophthora-suppressive soils after acidification plus aluminium amendment. A concentration of Al3+ ion greater than 25 p.p.m. was effective in inhibition of germination of sporangia and zoospores of the three Phytophthora species in 2% V-8 juice. The same concentration of aluminium ion was also effective in killing more than 95% sporangia and 99% zoospores in V-8 juice after 24 ;h.