The influence of environmental factors on the persistence of Zoophthora radicans conidia

The ability of Zoophthora radicans conidia to infect and induce mycosis in Plutella xylostella larvae was dramatically reduced after just 24 hr field exposure in the Cameron Highlands, Malaysia. In laboratory studies some conidia retained infectivity for up to 16 days on the surface of foliage or soil. The rate at which conidia lost infectivity was greater on the adaxial leaf surface than on the abaxial leaf surface and greater on soil with a low moisture content than on soil with a higher moisture content. One hour of simulated heavy rainfall had no impact on the ability of primary conidia on either the adaxial or abaxial leaf surface to induce mycosis in test larvae; 3 hr rainfall significantly reduced the level of infection induced in larvae bioassayed on the adaxial leaf surface. When primary conidia were incubated on leaf surfaces under conditions of high humidity to encourage capilliconidia development and then subjected to heavy rainfall, 1 hr of exposure significantly reduced the level of mycosis induced in test larvae bioassayed on adaxial but not abaxial leaf surfaces. Exposure of primary conidia to natural, temperate solar radiation for 4 hr did not affect the level of mycosis induced in test larvae. Ultraviolet and simulated tropical solar radiation were both damaging to conidia, although capilliconidia were significantly more tolerant to radiation than primary conidia. The ability to induce mycosis in test larvae was not affected by 1 hr of exposure to simulated tropical solar radiation but 4 hr exposure significantly reduced the level of mycosis induced in test larvae. Eight hours of exposure had no further affect on the level of mycosis induced by secondary capilliconidia but did reduce the level induced by primary conidia. Solar radiation is considered to be the major environmental factor affecting Z. radicans persistence. The results are discussed in relation to the use of Z. radicans for P. xylostella control.