Black rot is the most damaging bacterial disease of Brassica oleracea, a species that includes common veggies like cabbage, cauliflower, broccoli, kale and Brussels sprouts. Imagine Xmas without Brussels sprouts! Other species of cultivated brassicas, like B. napus (oilseed rape and swede), and some ornamental plants and weeds can also be susceptible to this disease.
Black rot causes ugly yellow and black lesions on the leaves and can cause complete yield losses (Cover Image). In humans, we typically treat bacterial diseases with antibiotics. However, using antibiotics in agriculture is strictly regulated or banned as it may lead to the emergence of antibiotic resistance. Instead, breeding brassicas to be resistant to black rot is a more environmentally friendly solution. However, this is a challenging task because, like in any society, the causal agent, Xanthomonas campestris pv. campestris (Xcc) is a genetically diverse bacteria that can be classified into numerous different races and phylogenetic clades (Fig. 1). Different Xcc races/clades can be prevalent in different crop types and rotations, and resistance to one Xcc isolate may not necessarily be effective against others.
To breed resistance, we first need to identify plant lines with the trait of interest, in this case black rot resistance. In this latest study, both vegetable B. oleracea and B. napus diversity fixed foundation sets (DFFSs) were screened for resistance to the most common Xcc races 1, 4, 5 and 6 (Fig. 2). B. napus is closely related to B. oleracea and can be screened in the search for black rot resistance. What are DFFSs? These are a unique resource developed at the University of Warwick that have been designed to capture the genetic diversity of >6000 brassicas in a smaller subset of 100-200 plant lines per set – a much more manageable number to screen!
Screening of the DFFSs showed that resistance to Xcc races 1 and 4 was rare in B. oleracea but more common in B. napus. Conversely, resistance to races 5 and 6 was more common in B. napus than in B. oleracea. A subset of black rot-resistant DFFS lines was then challenged with additional Xcc isolates representing the different clades of races 1, 4, 5 and 6 (Fig. 1), allowing identification of brassicas that had genuine race-specific resistance (e.g. to all isolates of a given race), clade-specific resistance (e.g. to isolates belonging to the same clade but they could be different races) and broad-spectrum resistance (e.g. to all or most Xcc isolates tested) (Fig.2).
These black rot-resistant brassicas now provide a starting point for breeding programs aimed at controlling this damaging disease.
Figure 1. Phylogenetic tree showing the genetic diversity of the black rot pathogen Xanthomonas campestris pv. campestris races and clades.
Figure 2. Graphical abstract summarising the identification of black rot resistant brassicas.
Shannon F. Greer, Jamie Harrison, Daisy Bown, Maria Serrano, Rana Muhammed Fraz Hussain, Srayan Ghosh, Graham R. Teakle, Vardis Ntoukakis, David J. Studholme, Joana G. Vicente, Murray Grant published this study in Plant Pathology Journal:
Identification of Race-Specific, Clade-Specific and Broad-Spectrum Resistance to Xanthomonas campestris pv. campestris in Brassica Diversity Fixed Foundation Sets
Cover Image: Symptoms of black rot disease in brassicas. Left to right kale, cabbage and cauliflower leaves infected with black rot.
