Abstract
Zoosporic fungi such as Pythium, Aphanomyces and Phytophthora depend on high soil moisture for infection, disease development, and rapid spread. The invasion of root tissues by these fungi is made by motile zoospores which are produced in water. This report is dealing with the occurrence, and control (soil solarization) of soil-borne diseases of spinach caused by zoosporic fungi, especially outbreaks mechanisms of root rot caused by Aphanomyces cochlioides Drechsler which is one of the most serious diseases of spinach in growing areas in Hokkaido, Japan. Outbreaks mechanisms of spinach root rot in relation to soil moisture and nitrate nitrogen are
summarized as follows.
1) High soil moisture conditions from rainfall or excess irrigation promote the production of zoospores of A. cochlioides, and such conditions are conducive to the spread of secondary zoospores.
2) Due to leaching and dilution by stagnant water, high soil moisture also causes decrease in mineral salts, especially nitrate nitrogen that inhibits zoospore production from zoosporangia, and causes the encystment and death of secondary zoospores. Consequently, the inoculum potential remains high, and disease outbreaks occur.
3) Severe outbreaks occur in fields of sandy soil where nitrate nitrogen is readily leached, and of heavy clay soil or soils with compact subsoil where nitrate nitrogen concentration is readily diluted by stagnant water.
4) The outbreaks mechanisms of spinach root rot apply to cases of the diseases caused by some zoosporic fungi, A. euteiches and P. aphanidermatum.
