Erwinia caratovora subsp caratovora Pseudomonas marginalis and P fluorescens soft rots

Cultivars vary in susceptibility to soft rots; the broccoli (calabrese) cvs Marathon, Headline and Trixie appear to have some tolerance to P. fluorescens. Major efforts are being made to breed for resistance to soft rot in Chinese cabbage in Japan, Taiwan and other parts of Asia. In studies of >800 accessions, only low levels of tolerance were found, but three cycles of recurrent selection starting with the best 23 accessions improved this tolerance significantly (Ren et al., 2001) (Fig. 7.21). Resistance appeared to be controlled by additive genetic effects with narrow sense heritability of 42%. From this research, resistance was suggested to be controlled by two major partially dominant genes and associated minor genes. It is also suggested that Erwinia was more virulent than Pseudomonas.

Broccoli (calabrese) cvs Shogun, Green Defender and related hybrids are quite tolerant, and some cauliflowers had higher levels of tolerance in both seedlings and mature plants. When cv. Shogun was crossed with the best Chinese cabbage lines using fusion, higher levels of resistance were obtained in the back-cross than in either fusion parent (Ren et al., 2000). Improved resistance to bacterial soft rot following fusion between B. rapa and B. oleracea has been recorded. Field trials of Chinese cabbage (B. rapa var. pekinensis) in Ontario, Canada identified that the cvs Yuki, Manoko and Summer Top had

Pseudomonas Marginalis
Fig. 7.21. Development of resistance to Pseudomonas fluorescens. The effects of three cycles of recurrent selection of broccoli (calabrese, Brassica oleracea var. italica) on resistance (M.H. Dickson).

the least wastage (<10% plant loss) from bacterial soft rot (E. caratovora subsp. caratovora) (Warner et al., 2003).

Cultural controls involving the avoidance of excessive applications of nitrogenous fertilizers and encouraging aeration through the crop canopy by wider spacing can help minimize the impact of these pathogens. With broccoli (calabrese) in particular, crop spacing is used to determine head size according to market demands, and this leaves little option for change. Straw or plastic mulch placed around the base of plants minimizes the splash transference of bacteria from the soil on to foliage and helps reduce disease incidence.

Where permitted, copper oxychloride or similar compounds can provide some control, except under conditions of high inoculum potential and moist weather favouring the pathogen. Two applications are required for broccoli (calabrese), the first when spears are 10 mm diameter and the second after 7 days.

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