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- Author or Editor: M. S. Chiang x
Clubroot, caused by Plasmodiophora brassicae Wor., is a major disease of crucifers. The pathogen, which is highly mutable, can survive in the soil for many years, and often occurs in mixed races or populations (9). Of the several races that Williams (10) has identified, the most common and virulent one observed world-wide is race 2 (E.C.D. 16/02/31) (3) to which there are limited sources of resistance within the Brassica oleracea L. group. However, a number of cultivars and lines of rutabaga (B. napus L.) are resistant to P. brassicae race 2 (1). Therefore, in 1974, an interspecific hybridization program was initiated with the purpose of transferring the gene(s) responsible for resistance to P. brassicae race 2, from rutabaga (B. napus L.) to cabbage (B. oleracea L. ssp. capitata L.). Recently, we found that dominant genes from rutabaga responsible for resistance to both P. brassicae races 2 and 6 (E.C.D. 16/02/30) were transferred successfully to cabbage (5 and unpublished data). Both resistances were transferred later form resistant BC3 cabbages (Fig. 1) to broccoli (B. oleracea L. var. italica Plenck) and to cauliflower (B. oleracea L. var. bo-trytis L.). The single dominant gene inheritance to race 2 (5) should simplify the production of race 2-resistant F1 hybrids.
Thiocyanate ion (SCN−) content, derived from indolylglucosinolates, was analyzed in the head and roots of 3 cabbage lines (80–5, 80–38, and 80–35, all derived from interspecific hybridization between Brassica napus L. and B. oleracea L. Capitata group), segregating for degrees of susceptibility to clubroot disease incited by Plasmodiophora brassicac Wor. There was a linear increase in SCN− content in heads with increasing clubroot severity of both susceptible lines 80–5 and 80–38. The head SCN- content of line 80–35, resistant to clubroot, was not influenced by clubroot severity and was negatively correlated with head SCN− content. Root fresh weight increased with increasing clubroot severity and was positively correlated with root SCN− content.
Broccoli (Brassica oleracea Italica Group) plants resistant or susceptible to clubroot (Plasmodiophora brassicae Wor. races 2 and 6) produced from 10 segregating breeding lines were analyzed for their composition of glucosinolates as determined in terms of their hydrolytic breakdown products: goitrin, volatile isothiocyanates, and thiocyanate ion. Unlike goitrin and volatile isothiocyanates, the average content of thiocyanate ion was significantly lower (548 μg·g-1 dry weight) in resistant plants than in susceptible ones (712 μg·g-1). Of 15 resistant plants derived from 10 breeding lines containing zero goitrin and/or relatively low concentrations of one or more glucosinolate products, 10 plants were free of goitrin, three were low in volatile isothiocyanates (≤100 μg·g-1), two were low in thiocyanate ion (≤200 μg·g-1), and two were low in total glucosinolates (≤600 μg·g-1).