Evaluation of adjuvants added to fungicides for controlling Cercospora leaf spot on sugar beet

Cercospora Leaf Spot (CLS) is a destructive foliar disease of sugar beet caused by Cercospora baticola. Recent development of resistance to several classes of chemicals by C. beticola coupled with untimely applications and/ or frequent rainfall after fungicide applications have resulted in reduced efficacy of fungicides in controlling CLS. Adjuvants are typically included in fungicide formulations as inert ingredients to improve retention, delivery, and efficacy against targeted pathogens. In the greenhouse trials, three recommended fungicides (Penncozeb (R), Inspire (R) XT, and Badge (R) SC) were applied and evaluated alone or in mixtures with four adjuvant types (Complex (R), Preference (R), Cerium Elite (R), and Transfix (R)) before and after inoculation with C. beticola. Disease severity was recorded 14 days post-inoculation. Greenhouse trials showed that application of fungicides before disease onset resulted in reduced disease severity. Inspire XT had lower disease severity than Penncozeb or Badge SC. Adjuvants alone were not effective at controlling CLS. Among all fungicide-adjuvant combinations, only Complex and Inspire resulted in lowest disease severity. In the field trials, fungicides were applied alone or with three adjuvants. Disease severity was evaluated at 14 days intervals and recoverable sucrose (i.e., sugar yield) was measured at harvest. Field trials showed significant variations between years in disease severity and sugar yield (P < 0.001). These variations may be caused by lack of rain events immediately after the fungicide application or during the following two days. The finding that addition of adjuvants did not improve the efficacies of fungicides at controlling CLS was because if there was no rain, then the adjuvants should not be any better than formulated fungicide. Thus, more experiments are necessary with similar and different adjuvant fungicide combinations.