1Abashiri Farm, Tokyo University of Agriculture, Abashiri, Hokkaido 099-3501, Japan; 2Rakunou Gakuen University, Ebestu, Hokkaido 069-0836, Japan

Background and objectives
According to previous studies [1, 2] the cause-and-effect relationship between disease development of common scab of potato and soil conditions can be explained by the aluminum ion concentration of soil. We have developed a useful method to judge whether common scab of potato is easily controlled by simply decreasing soil pH. An experiment was conducted to determine differences in growth of Streptomyces scabies in culture media containing soils of Andosols in Hokkaido and Kyusyu areas, the main potato production areas in Japan.

Materials and methods
Soil samples were collected from the surface soil of Andosols in Hokkaido and Kyushu areas. Streptomyces scabies strain SSY8011-3 (culture collection of Dr Tanii) was used. YME medium was adjusted to pH 4.2 with 0.02 M citric acid. This medium was then divided into 100-ml aliquots and poured into bottles each containing 1.0 g soil samples and 2.0 g agar powder. These media were adjusted to pH 4.6 and 4.8 by 1 and 0.1 M NaOH and then autoclaved at 121C for 20 min. Each medium was then poured into three petri dishes. A 3-ml distilled water suspension of spores and mycelium fragments was made and 0.05 ml then spread uniformly on the plate as inoculum. The cultures were incubated at 29C for 15 days, then the number of colonies on the plate was counted every 5 days. The pH value of each medium was measured before S. scabies was inoculated. The amounts of pyrophosphate-extractable aluminum (Alp), acid oxalate-extractable aluminum (Alo) and acid oxalate-extractable silica (Sio) of the sample soils were determined by the Blakemore method.

Results and conclusions
The pH of YME agar media containing autoclave-sterilized soil varied in each of the sample soils. The number of colonies of S. scabies on the YME agar media increased in high-pH media. There was a negative correlation between Alp/Alo of soil and the increasing ratio of medium pH. Moreover, there was a logarithmic correlation between Sio in soil and the increasing ratio of medium pH. The growth of S. scabies was inhibited in media containing soil with an Alp/Alo of more than 0.22%. The growth of S. scabies was inhibited in media containing soil with less than 0.54% Sio. These findings suggest that the growth of S. scabies is affected by Alp/Alo and Sio of soil in the media, and are in agreement with results reported previously [1]. Therefore our method of using media containing soils is useful to judge the ability of soil to suppress common scab of potato caused by S. scabies.

1. Mizuno N, Yoshida H, 1993. Plant and Soil 154, 505-508.
2. Yoshida H, Ushiki J, Mizuno N, Tadano T, 1997. Plant Nutrition for Sustainable Food Production and Environment, pp. 757-758.