Activity of Phytophthora infestans in soil in relation to tuber infection

Abstract

In vitro studies of Phytophthora infestans (Mont.) de Bary on chick-pea-sucrose agar showed optimum growth at 20° C. with a range from 4° C. to less than 30° C. Most sporangia in pure culture lose viability within 3–4 weeks, depending on temperature and relative humidity, although some viable sporangia may still be found after 100 days. Reduction of the oxygen content of the air by half had no effect on germination of sporangia, but high concentrations of carbon dioxide impeded germination of sporangia and zoospores. The levels of oxygen and carbon dioxide present in the soil are probably not sufficiently different from those in the atmosphere to affect germination. Tuber infection may occur through eyes, lenticels, or wounds. The resistance of lenticels and the susceptibility of eyes increases with maturity and storage. A method of detecting infectivity by inoculating non-sterile soils into slices of potato tuber is described. Maximum persistence of infectivity of sporangia was 77 days in a clay soil at 25% water-holding capacity and 15° C. Persistence increased with decreasing temperature, and was greatest at 15–25% water-holding capacity. The fungus appears to persist by sporangia which germinate and form short germ tubes directly or through an intermediate zoospore stage. The effect of secondary bodies on persistence is doubtful. Depth of penetration of sporangia or zoospores varied with soil type and the amount of water added. The maximum thickness of soil through which penetration was confirmed was 6 in. in black fen soil and 8 in. in sand.