Black Rot of Brassica Crops

By Cody Molnar and Nick Volesky et.al

Overview

Black rot (Xanthamonas campestris pv. campestris, Xcc) is an uncommon but devastating bacterial disease in South Dakota. Typically, South Dakota’s climate is not conducive to development of this disease, but when periods of hot and wet weather are sustained, it can lead to rapid and widespread crop loss. Black rot can develop at any stage of growth, though it is more commonly seen in the spring on new transplants or late summer. It is most severe on broccoli, cabbage, kale and cauliflower, but can infect all members of the brassica family, including Brussels sprouts, bok choy turnips, kohlrabi, radishes, mustards and others, including forage and ornamental brassica. Bacteria enter pores on the leaf margins, called hydrathodes, through water droplets via splashing, dew condensation, rain or irrigation water. They can also enter through sites of damage from insect feeding, hail, or other mechanical damage to the plant. Black rot can be spread through infected plant residue from the current or previous years. It is also seed-borne. Once a plant is infected, large yellowing and dying lesions will spread down the leaf and eventually result in severe damage or plant death.

Symptoms

Black rot can present differently on various crops. In cauliflower, broccoli, and cabbage and other members of Brassica oleracea, it begins as yellowing wedges on the leaf margins that spread down the leaf (Figure 1). Veins within the affected area will turn brown or black, with leaf tissue surrounding the affected areas turning yellow. Eventually, large portions of the leaf necrose as the disease progresses into the stalk, which will cause head or stalk rot and death of the entire plant (Figure 2). At this stage, stems and stalks will often have black discoloration in the vascular tissue (Figure 3). 

In bok choy, turnips, and other members of Brassica rapa, black rot can appear as necrotic speckles on the leaves, which can resemble some leaf spot diseases, but will eventually resemble symptoms on other brassicas and cause head rot and plant collapse as well. Many other soft rots, fungi, and opportunistic pathogens will often colonize the dying plant as well, which can make recognizing this disease difficult.