Field evaluations of plant defense activators and sulfur as alternatives to copper bactericides for the management of cabbage black rot in Florida

The bacterium Xanthomonas campestris pv. campestris causes black rot, one of the most important diseases affecting brassica production worldwide. Typically, the foliar application of copper-based bactericides is used to manage the disease. However, reports of copper tolerance among xanthomonads, expanded production of diverse brassica crops, and inherent climate challenges have motivated the brassica industry in Florida to evaluate alternatives to copper for improved black rot management. Across nine cabbage trials conducted in 2021 and 2022 in Florida, twelve alternatives were evaluated individually or as tank mixtures within a program. Each trial included a copper hydroxide standard (Kocide 3000) and a nontreated control. Based on the area under disease progress curve (AUDPC), solo applications of the plant defense activators Actigard (acibenzolar-S-methyl), Howler (Pseudomonas chlororaphis), Regalia (Reynoutria sachalinensis extract), ReyZox (Reynoutria sachalinensis extract + azoxystrobin), Theia (Bacillus subtilis), and Vacciplant (laminarin) reduced disease severity compared to the nontreated control and performed equivalent to or better than the Kocide 3000 standard across trials. Howler, Regalia, and Kocide 3000 increased cabbage yields compared to the non-treated control in at least one trial, while Actigard failed to improve yield in any trial. Applications of Microthiol (sulfur), either alone or as a tank mixture, failed to reduce disease severity or improve yield. Regardless of the program, maximum disease control at the final rating was limited to 8% compared to the nontreated control, indicating a need to improve integrated approaches for black rot management.