Farmers have always had to deal with unpredictable weather. Erratic weather patterns are increasingly occurring, and dealing with the weather is simply getting more confounding as climate change becomes a day-to-day reality.
While hot and humid conditions favor the development of some fungal pathogen such as powdery mildew, the steamy, humid, rainy weather much of the Eastern seaboard and a few other regions have recently experienced can be ideal for the development of Xanthomonas spp. and their resulting diseases. Xanthomonas are plant pathogenic bacteria which can cause extensive crop losses in an array of vegetables.
For those growing cole crops (kale, cabbage, broccoli, cauliflower, kohlrabi, Brussels sprouts), black rot is a concern when the weather gets hot, wet and humid. Black rot can expose the crop to other rots later in the season, when these cool season crops are ready for harvest, even if it does not cause extensive losses in the growing season itself. Storage rots are a concern on crops harvested from fields infested with black rot.
Unlike many diseases caused by fungal pathogens which thrive in moist conditions, black rot is caused by the bacterial pathogen Xanthomonas campestris pv. campestris. This specific Xanthomonas species only infects cruciferous plants, but be aware there are some common weeds which fall into that category which can harbor the pathogen. Black rot in cole crops is also known as bacterial blight, black stem, black vein or stem or stump rot.
The pathogen can survive and overwinter in plant debris, although the most common route of introduction is believed to be via seeds and transplants, so clean propagation methods are needed. Irrigation water, dew, wet leaves and rain can all transport the bacteria and rapidly spread the disease. Working among wet plants is one way to spread this serious pathogen.
Yellow V-shaped regions on leaves are the primary signs of infection in cole crops, with yellowing often occurring before the leaf wilts and dies. Leaf veins become black as the plant’s vascular system will turn black. Even if the crop is not totally destroyed, plants with black rot will be small and low quality. Secondary rots can develop and storage life is short.
Researchers have recently discovered the means at which X. campestris impacts and invades cells on the molecular level. These finding can, perhaps, lead to innovative means of preventing disease development.
Bacterial leaf spot of pepper and tomatoes is caused by X. campestris pv. vesicatoria, of which four separate species have been found: X. euvesicatoria, X. vesicatoria, X. perforans and X. gardneri. The pathogen is most often spread via contaminated seeds or transplants, but overwinters in plant debris and weed hosts. Working when plants or fields are wet will help to spread the disease. Overnight temperatures of 75º F or higher, combined with relative humidity levels of 85% or greater, favor development.
High temperature/high humidity levels and moisture are a perfect storm for this disease. For those growing in certain regions, such as the much of the East Coast, bacterial leaf spot is regularly controlled for in spray programs, as hot, humid and rainy are normal summer weather conditions.
Researchers at Rutgers University recently found that X. campestris pathogens have developed resistance to copper, which is the treatment of choice for controlling disease development. Ongoing testing at a small number of farms has shown that various strains of the pathogen are present in peppers, and that approximately 50% of the time, these pathogens show resistance to copper.
Andy Wyenandt, Extension specialist in vegetable pathology at Rutgers, wrote in a 2020 bulletin, “It is extremely important to know what races of BLS are present so you can chose the proper cultivars to grow. Choosing the proper cultivar will do two things: significantly reduce the chances of BLS development and significantly reduce the number of copper applications on your bell pepper crop. As a note, there are a few non-bell peppers available with BLS resistance packages.”
The disease affects all portions of the plant. It begins with small spots, which can then coalesce and may cause holes to develop as the disease advances. The spots can also appear to scab. The pathogen is spread via moisture and contaminates plants via natural openings or wounds.
Working in and among wet plants or in wet field conditions can rapidly spread the pathogen. Site selection for fields with good air movement, and which don’t remain wet, can help to prevent disease development or severity. Rotations should be at least three years, and crop debris must be destroyed. Jimsonweed, horesnettle and other solanaceous weeds act as host plants. Scouting and the early removal of any infected plant is warranted, according to the New York State IPM Program.
Since the disease’s causal agent is a bacterium, antibiotics would be effective, but are extremely limited in use to prevent resistance. Bacteriophages have been shown to have effectiveness; Bacillus spp. and Streptomyces spp. are also available for use to prevent bacterial leaf spot, although research shows varying results.
Other X. campestris variations can – and do – infect many other crops. These too can cause devastating effects, including total crop loss. Xanthomonas also causes citrus canker and leaf spot on other tree fruits, bacterial blight of beans, attacks many ornamental and woody plants and is known to cause over two dozen diseases on more than 400 species of plants.