Effects of selected fungicides on in vitro spore germination and vegetative growth of Ascosphaera aggregata

Abstract - Five fungicides were studied for their in vitro effects on spore germination and vegetative growth of Ascosphaera aggregata, the causative organism of chalkbrood in the alfalfa leafcutter bee, Megachile rotundata. The compounds were incorporated directly into liquid or agar growth medium. EC₅₀s for inhibition of vegetative growth on an agar surface were estimated as follows: enilconazole, 0.034; Rovral (iprodione), 1.10; Benlate (benomyl), 1.16; and Ascocidin (N-glycosylpolifungin), 135.6 ppm (m/v). DFMO (α-DL-difluoromethyl omithine) stimulated growth resulting in an approximately 50% increased radius at 10 000 ppm compared to the control. Benlate inhibited spore germination; at the highest concentration only, Rovral inhibited germination while Ascocidin and enilconazole exhibited a slight effect. DFMO did not affect spore germination. Ascocidin and Benlate exhibited most effect on germling viability at 48 h followed by Rovral and enilconazole. DFMO had no effect on germling viability. Benlate, Rovral and enilconazole had the most pronounced effect on germ tube growth at 96 h. Ascocidin affected germ tube growth at the higest concentration; DFMO exhibited little to no effect. Comparisons between in vitro results obtained in the present study with previously published in vivo results indicate that in vitro effects on spore germination most closely correlated with efficacy against chalkbrood in vivo, suggesting that compounds with activity against spore germination or germling viability may be the most promising candidates for use against chalkbrood.