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Jianping Ren and Michael H. Dickson

More than 700 accessions of Brassica vegetables were screened for resistance to bacterial (Erwinia) soft rot disease using a newly developed testing procedure. Dipping a needle in 2-day-old bacterial culture and pricking petioles of plants gave the most-consistent and distinguishable results in both seedling greenhouse and mature plant field tests. High humidity (100%) and warm temperature (higher than 23°C) are the two essential conditions for this test to be successful. So far, immune material has not been found. In B. rapa, less than 7% of the accessions showed some degree of resistance. High correlation was found between seedling greenhouse tests and mature plant field tests. Genetic study showed that soft rot resistance in B. rapa is a quantitative trait. The broad-sense heritability was 60% and narrow-sense heritability was 42% in the tested population. Following three cycles of recurrent selection, the resistance level in cycle three population was improved by 2.4 disease score points (1–9 scale) compared to the original parental population and the disease score of the best line in cycle 3 was 2.7 compared to a susceptible check on which the disease score was 8 under greenhouse conditions. Under field conditions, the best cycle three line scored 2.0 in comparison to the susceptible check, which scored 7. From our study, the recurrent selection works well for improving the resistance level to the soft rot disease in B. rapa.

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Jianping Ren, Warren F. Lamboy, lames R. McFerson, Stephen Kresovich and Jianping Ren

Fifty-two germplasm accessions of Chinese vegetable Brassicas were analyzed using 112 random amplified polymorphic DNA (RAPD) markers. The array of material examined spanned a wide range of morphological, geographic, and genetic diversity, and included 30 accessions of Brassica rapa (Chinese cabbage, pakchoi, turnip, broccoletto), 18 accessions of B. juncea (leaf, stem, and root mustards), and 4 accessions of B. oleracea ssp.alboglabra (Chinese kale). The RAPD markers unambiguously identified all 52 accessions. Net and Li genetic similarities were computed and used in UPGMA cluster analyses. Accessions and subspecies clustered into groups corresponding to the three species, but some accessions of some subspecies were most closely related to accessions belonging to another subspecies. Using genetic similarities, it was found that Chinese cabbage is more. likely to have been produced by hybridization of turnip and pakchoi, than as a selection from either turnip or pakchoi alone. RAPD markers provide a fast, efficient technique for diversity assessment that complements methods currently in use in genetic resources collections.

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Jianping Ren, James R. McFerson, Rugang Li, Stephen Kresovich and Warren F. Lamboy

Fifty-two germplasm accessions of Chinese vegetable brassicas were analyzed using 112 random amplified polymorphic DNA (RAPD) markers. The array of material examined spanned a wide range of morphological, geographic, and genetic diversity, and included 30 accessions of Brassica rapa L. (Chinese cabbage, pakchoi, turnip, and broccoletto), 18 accessions of B. juncea (L.) Czern. (leaf, stem, and root mustards), and four accessions of B. oleracea L. ssp. alboglabra (Chinese kale). The RAPD markers unambiguously identified all 52 accessions. Nei-Li similarities were computed and used in unweighed pair group method using arithmetic means (UPGMA) cluster analyses. Accessions and subspecies were clustered into groups corresponding to the three species, but some accessions of some subspecies were most closely related to accessions belonging to other subspecies. Values for Nei-Li similarities suggest that Chinese cabbage is more likely to have been produced by hybridization of turnip and pakchoi than as a selection from either turnip or pakchoi alone. RAPD markers are a fast, efficient method for diversity assessment in Chinese vegetable brassicas that complements techniques currently in use in genetic resources collections.