Potential of Lecanicillium species for dual microbial control of aphids and the cucumber powdery mildew fungus, Sphaerotheca fuliginea

Abstract

Detached leaf disc bioassays were conducted against cucumber powdery mildew and three species of aphid with three entomopathogenic species of Lecanicillium; Lecanicillium longisporum (Vertalec^®), Lecanicillium attenuatum (CS625), and an unidentified isolate (DAOM198499). The three Lecanicillium species had high virulence against the aphids Myzus persicae, Macrosiphum euphorbiae and Aulacorthum solani with the exception of DAOM 198499, which demonstrated reduced virulence to A. solani with an LT₅₀ of 6.4 days. Otherwise, LT₅₀ ranged between two and four days. Suspensions of conidia and blastospores of the Lecanicillium species were also applied onto 15 mm leaf discs dissected from cucumber plants previously inoculated with Sphaerotheca fuliginea. Powdery mildew did not develop when the Lecanicillium applications were made one and eight days after S. fuliginea inoculations. When Lecanicillium was applied to highly infected leaf discs 11 and 15 days after S. fuliginea inoculation, the application suppressed subsequent production of S. fuliginea spores as compared to the controls. These results suggest the potential of a dual role for Lecanicillium spp. as microbial control agents against aphids and powdery mildew.

Introduction

Aphids and powdery mildew are two of the most serious insect pests and plant diseases in greenhouses (Kiss, 2003, Milner, 1997). Chemical pesticides have been the main control methods for these pests in crop production. To date, several mycopesticides have been developed and used in several countries including the United Kingdom and the United States (Goettel et al., 2005, Kiss, 2003). These include Vertalec^® based on Lecanicillium longisporum (Petch) Zare & Gams (formerly known as Verticillium lecanii (Zimm.) Viegas) against aphids, Botanigard^® based on Beauveria bassiana (Bals.) Vuill. against aphids and whitefly, and biofungicides, for example, AQ 10 biofungicide^® based on Ampelomyces quisqualis (Schlechtend.: Fr.) Polacci, and Sporodex^® based on Pseudozyma flocculosa (Traquair, Shaw & Jarvis) Boekhout & Traquair against powdery mildew. Although these products have the advantage of a restricted host range, this specificity is also one of the limiting factors for their commercial use (Ownley et al., 2004). Therefore, a mycopesticide with a wider host range but with little to no influence on natural enemies or beneficial organisms may have a commercial advantage if it simultaneously controls various pests and/or plant diseases.

Mitosporic fungi such as B. bassiana, Lecanicillium spp., and Paecilomyces fumosoroseus (Wize) Brown & Smith that are generally known as entomopathogenic fungi have recently been discovered as having activity against fungal plant pathogens (Askary et al., 1998, Benhamou and Brodeur, 2000, Benhamou and Brodeur, 2001, Dik et al., 1998, Ksvková and Čurn, 2005, Miller et al., 2004, Ownley et al., 2004, Verhaar et al., 1997, Verhaar et al., 1998) which suggests that such entomopathogens could be developed as mycopesticides with a dual role.

Lecanicillium is the only major mitosporic fungus routinely found to affect naturally occurring populations of aphids (Milner, 1997). Many isolates of Lecanicillium demonstrate high pathogenicity to several species of aphids such as Aphis gossypii (Glover), Macrosiphum euphorbiae (Thomas) and Myzus persicae (Sülzer) (Alavo et al., 2001, Askary et al., 1998, Kim et al., 2001) and a commercial product, Vertalec™ has been developed. In addition, it has been recently demonstrated that Lecanicillium spp. have activity against powdery mildew (Verhaar et al., 1997, Verhaar et al., 1998, Askary et al., 1998, Dik et al., 1998, Miller et al., 2004). Although such studies have demonstrated activity of Lecanicillium species against aphids or powdery mildew, only one (Askary et al., 1998) tested activity of several isolates against both targets. In addition, recent reclassification of Verticillium lecanii into several Lecanicillium species (Zare and Gams, 2001) brings forth the possibility that several different species were actually involved in the previous studies.

The aim of this study was to test the control potential of three Lecanicillium species for simultaneous control against aphids and powdery mildew. We chose a commercialized isolate (L. longisporum from Vertalec), an isolate previously studied for its dual activity (Lecanicillium spp. DAOM 198499; Askary et al., 1998) and Lecanicillium attenuatum CS625, with previously demonstrated activity against aphids (Kim et al., 2001). We used a detached leaf disc bioassay with these three species of Lecanicillium against three economically important aphid pest species, Aulacorthum solani (Kaltenbach) (fox glove aphid), M. euphorbiae (potato aphid) and M. persicae (green peach aphid) and cucumber powdery mildew (Sphaerotheca fuliginea (Schlechtend.: Fr.) Pollacci). Since the commercial product Vertalec is based on blastospores, and the previous studies used either blastospores or conidia, we also compared the activity of both blastospores and conidia in our assays. All species demonstrated dual activity against aphids and powdery mildew.