Mechanisms of action of Akanthomyces lecanii on peanut rust: Ultrastructural investigations

Peanut rust caused by Puccinia arachidis is one of the most important peanut foliar diseases in the world, particularly in Burkina Faso. For sustainable peanut production, ecological disease management strategies are needed to limit the overuse of synthetic fungicides in the control of peanut rust. Thus, the use of Akanthomyces lecanii , a hyperparasite of rust fungi, may be an alternative to control peanut rust. However, investigations of the mechanisms of action of A. lecanii are essential before its development as a biocontrol agent. To do so, peanut leaves bearing rust pustules were detached, and each leaf was sprayed with 500 µl of a suspension of A. lecanii conidia (10 6 conidia/ml) at the lower surface. Then, each leaf was incubated in a Petri dish on moist blotting paper, and kept in the laboratory at 20 °C, under a 12:12 light/dark photoperiod. After 15 days incubation, the inoculated and uninoculated rust pustules were observed under photonic and electron microscopes Pictures revealed that A. lecanii colonized the urediniospores of inoculated sori. Our results showed that the mechanisms of action of A. lecanii on P. arachidis could involve the following events: (i) attachment of the antagonist to urediniospores mediated by a mucilaginous extracellular matrix; (ii) penetration due to mechanical pressure and enzymes action on the cell-wall; (iii) active growth of the antagonist inside of urediniospores and digestion of the cell content. From our study, the use of suspensions of A. lecanii spores is a promising tool for biocontrol of peanut rust.