Effect of additives on the efficacy of microencapsulated Hirsutella rhossiliensis controlling Heterodera schachtii on sugar beets

The nematophagous fungus Hirsutella rhossiliensis is a common antagonist of a number of important plantparasitic nematodes including the beet cyst nematode Heterodera schachtii. Although H. rhossiliensis is highly pathogenic, slow growth characteristics and weak competiveness under field conditions limit its biocontrol potential. Cellulose-based microcapsules were tested for their ability to improve fungal competitiveness in the soil and nematode parasitism. In a first approach microcapsules containing 15% fungal mycelium plus 15% corn gluten and 0.5% yeast extract as additives gave low levels of H. schachtii control. Fungal parasitism was not observed and indicated that nematode mortality was most likely caused by non-specific accumulation of nematicidal compounds associated with decomposition of the additives in the microcapsules. Removal of the corn gluten additive resulted in a minor increase of 20% parasitism of H. schachtii juveniles. Furthermore, the yeast extract that acted as nutrient source for the fungus diffused through the semi-permeable membrane of the capsules during the formulation process and was ineffective. Replacing the corn gluten and yeast extract with 3% autoclaved baker’s yeast and using finely dispersed mycelium instead of pelletized mycelium increased juvenile parasitism in the soil up to 90% and decreased number of juveniles per cm root length by over 80%. Cellulose based microcapsules supplemented with autoclaved baker’s yeast, therefore, proved to be an effective delivery system for H. rhossiliensis to control H. schachtii.