Eighty-four isolates of Sclerotinia sclerotiorum from four cabbage production fields in North Carolina and 16 isolates from an experimental cabbage field plot in Louisiana were DNA-fingerprinted and tested for mycelial compatibility. In a comparison with 594 unique DNA fingerprints of S. sclerotiorum from Canadian canola, no fingerprints were shared among Canadian, North Carolina, and Louisiana populations. DNA fingerprints from the North Carolina sample were distinctive from those of the Canadian and Louisiana samples, with significantly more hybridizing fragments in the 7.7- to 18-kilobase range. Forty-one mycelial compatibility groups (MCGs) and 50 unique DNA fingerprints were identified from the North Carolina sample. Three MCGs and three fingerprints were identified from the Louisiana sample. From the North Carolina sample, 32 MCGs were each associated with a unique fingerprint; of these, there were 11 clones (i.e., cases in which two or more isolates belonged to the same MCG and shared the same DNA fingerprint). Six clones sampled from two or more fields represented approximately 29% of the total sample (24 of 84 isolates), with six clones recovered from fields 75 km apart. There were 10 cases in which one MCG was associated with more than one DNA fingerprint and two cases in which one DNA fingerprint was associated with more than one MCG. The small sample from Louisiana was strictly clonal. The North Carolina sample had a clonal component, but deviated from one-to-one association of MCG with DNA fingerprint to an extent consistent with more recombination or transposition than the other two populations sampled.

A semi-selective medium was used to examine the aerobiology of ascospores of Sclerotinia sclerotiorum in five commercial cabbage fields in eastern North Carolina. Ascospores were present in all five fields from 26 September to 30 November. However, numbers of ascospores varied greatly depending on location, sampling date, and time. In general, peak ascospore deposition occurred between 11:00 A.M. and 1:00 P.M., with the number of colonies recovered ranging from 3 to 55/dish (9 cm in diameter). Peak ascospore numbers at all locations were found from mid- to late October, but a second, smaller peak was also evident at each location in late November. Information obtained was employed to evaluate the role of wounding in infection of cabbage by ascospores of S. sclerotiorum in controlled environmental chambers. A method for production and release of ascospores of S. sclerotiorum was employed in controlled-environment chambers for the inoculation of cabbage plants with one of three representative foliar wounds: a bruise, a cut, or a non-lethal freeze. Wounding treatments were applied to 7-week-old cabbage plants, misting was added to maintain continuous leaf wetness, and ascospores were released from apothecia twice daily for four consecutive days. Spore trapping with a semi-selective medium indicated that inoculum was evenly distributed within the chambers and deposition was similar to levels recorded in the field. At 31 days after inoculation, disease incidence ranged from 0% on the control to 96% on the freeze treatments. Freeze-treated plants showed the highest disease severity throughout the entire incubation period. Mean area under the disease progress curve of severity values were 0, 0.2, 34 and 60 for the control, cut, bruise, and freeze treatments, respectively. Results indicate that freeze and bruise injuries are important factors associated with infection of cabbage by S. sclerotiorum.

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Clubroot, caused by Plasmodiophora brassicae Woronin, has occurred for at least 50 years in three counties in northwestern North Carolina, but has not been reported previously from eastern North Carolina, where most commercial cabbage is produced. In the fall of 1995, clubroot was observed in a direct seeded, commercial cabbage field in Plymouth, NC. Diseased cabbage plants were stunted and roots exhibited clublike swellings. Clubs were randomly harvested from roots of five plants to obtain a composite isolate to determine which race(s) of P. brassicae are infecting cabbage in eastern North Carolina. Three experiments were conducted, using the procedure of Williams (2). Four replicates of 10, 1-week-old seedlings of eight different crucifer cultivars were inoculated by dipping in a spore suspension (1 × 108 cysts/ml) of P. brassicae and planted in pasteurized potting mix. Seedlings dipped in sterile water served as controls. Inoculated seedlings were incubated in a greenhouse at 18 to 28°C for 6 to 8 weeks and assessed for clubroot incidence and severity. The isolate of P. brassicae from eastern North Carolina was most virulent on cabbage (Brassica oleracea var. capitata cv. Jersey Queen), collard (B. oleracea var. acephala cv. Vates), and wild mustard (B. nigra); moderately virulent on canola (B. napus cv. Brutor) and rutabaga (B. napus cvs. Laurentian and Wilhelmsburger); and least virulent on cabbage (cv. Badger Shipper). Canola (B. napus cv. Nevin) and control seedlings were not infected and exhibited no symptoms. Similar results were obtained for all experiments. Based on these results, the isolate of P. brassicae from eastern North Carolina was designated as race 6 and pathotype 5 according to Williams (2) and Some (1), respectively. However, further experiments with single-cyst-derived isolates from individual clubs obtained from different geographic locations are needed to accurately characterize field populations of P. brassicae on cabbage in eastern North Carolina. References: (1) A. Some et al. Plant Pathol. 45:432, 1996. (2) P. H. Williams. Phytopathology 56:624, 1966.