Hiroaki Yoshikawa

Variation in virulence of Plasmodiophora brassicae in Japan tested with clubroot-resistant cultivars of Chinese cabbage (Brassica rapa L. ssp. pekinensis)

The differential hosts of Williams (1966) and the European Clubroot Differential (ECD) (Buczacki et al., 1975) have been used commonly to identify populations of Plasmodiophora brassicae, which causes clubroot disease in Brassica crops. However, some of these hosts showed intermediate and fluctuating scores to most populations from Japan. Therefore, these hosts could not be used to provide a clear classification in Japan. We have tried to clarify the genetic diversity in pathogenicity of P. brassicae in Japan using Japanese clubroot-resistant (CR) F1 hybrid (F1) cultivars and lines of Brassica rapa. The responses of some CR F1 cultivars were very clear. Four groups of field populations in Japan were recognized using the CR F1 cultivars. The clear response obtained here may depend largely on the genetic purity of the F1 cultivars. Moreover, it is possible to classify some of these Japanese populations into the same race using the Williams set and ECD 01 to ECD 05. The present differential hosts may be useful in the study of European populations of P. brassicae. The response of the differential hosts suggests that there are several major CR genes in B. rapa. It is suggested that pyramiding CR genes would be useful in breeding CR cultivars that can overcome the breakdown of the present CR cultivars of Chinese cabbage.

RAPD markers linked to a clubroot-resistance locus in Brassica rapa L.

Linkage of random amplified polymorphic DNA (RAPD) markers with resistance genes to clubroot (Plasmodiophora brassicae Wor.) in Brassica rapa L. was studied in a doubled haploid (DH population obtained by microspore culture. Thirty-six DH lines were obtained from F1 plants from a cross between susceptible ‘Homei P09’ and resistant ‘Siloga S2’ plants. ‘Homei P09’ was a DH line obtained by microspore culture of the Chinese cabbage variety ‘Homei’, which is highly responsive in microspore culture. The resistant line ‘Siloga S2’ was obtained by two rounds of selfing of the fodder turnip ‘Siloga’. Three RAPD markers, RA12-75A, WE22B and WE49B, were found to be linked to a clubroot-resistance locus. These three markers were linked in the DH lines and an F2 population and should be useful for marker-assisted selection in breeding programs.

Bolting resistant breeding of Chinese cabbage. 1. Flower induction of late bolting variety without chilling treatment

SummaryWhen a local slow bolting variety ‘Osaka Shirona Bansei’ (Brassica rapa L. ssp. pekinensis, syn. B. campestris L. ssp. pekinensis) was grown in a phytotron (25°C, 16 hours day length without chilling treatment), one third of the plants bolted and flowered. In order to clarify the different flowering responses in the variety, a progeny line (FNC) of the flowering plants was chilled for 4 different periods (0, 22, 36 and 53 days) in a chamber of 2 ∼ 7°C, then transplanted to three different conditions, i.e. PHY: 25–20°C day and night temperatures, 16 hours day length, GHL: 10 ∼ 25°C, 16 hours day length with supplementary light and GHN: 10 ∼ 25°C, natural day length (10 ∼ 15 hours). In PHY, FNC bolted and flowered with almost the same leaf numbers in all 4 different chilling treatments. This means that FNC has very low sensitivity and no requirement to low temperature for its reproductive growth. In GHN (short day length), FNC bolted very slowly. Then the bolting and flowering of FNC were promoted by both long day length and high temperature. The newly found bolting characteristics of ‘Osaka Shirona Bansei’ could be applied to breed unique slow bolting Chinese cabbage (B. rapa L. ssp. pekinensis) which might be non-sensitive to low temperature and its bolting and flowering would be induced with the combination of long day length and high temperature. Using the unique variety, it might be also possible to establish a new cropping type of Chinese cabbage (late autumn sowing, spring harvest).

Leaf morphology and soft rot resistance in offspring of a somatic hybrid between Chinese cabbage and kale (Cruciferae)

SummaryVariations in leaf morphology and soft rot resistance were investigated in offspring of a somatic hybrid between Chinese cabbage (2n=20) and wild kale (2n=18). Concerning leaf morphology, self-fertilized offspring of the somatic hybrid (2n=38) had a mean value intermediate between Chinese cabbage and kale in only one of the three indices used for comparison. Backcrossed offspring of the somatic hybrid with Chinese cabbage had intermediate mean values between Chinese cabbage and self-fertilized offspring of the somatic hybrid, but there was much larger variation within the backcross population. As to soft rot, Chinese cabbage was susceptible, and wild kale resistant. Self-fertilized and open-pollinated offspring of the somatic hybrid had intermediate mean values of the parental species and large variations of the symptoms. The backcrossed offspring were rather susceptible but showed continuous variation. In the backcrossed progeny, no correlation was observed between resistance and leaf morphology. It is hypothesized that recombinants having soft rot resistance from kale in combination with leaf morphology of Chinese cabbage will segregate in later generations.