1a. Objectives (from AD-416):
Identify new sources of resistance in the Citrullus germplasm collection to Pseudoperonospora cubensis [downy mildew] (Berk. & Curt.) Rostov.
1b. Approach (from AD-416):
Screen 1600 accessions in the USDA germplasm collection of Citrullus for sources of resistance to Pseudoperonospora cubensis [downy mildew] (Berk. & Curt.) Rostov. Retest selected accessions to validate earlier results. Select and self-pollinate resistant plants and produce seed from them.
3. Progress Report:
Identifying new sources of resistance is conducted in support of inhouse project #6607-21000-011-00D Objective 3: Conduct genetic characterizations and phenotypic evaluations of the preceding crops and related wild species for priority genetic and agronomic traits. The watermelon industry has dealt with bacterial fruit blotch (BFB)ever since the 1990s. Although a large effort was made to control the disease, it is difficult to eradicate the incidence due to the seed-borne nature and imperfect detection methods. The objectives of this study were to screen the USDA watermelon germplasm collection for resistance to bacterial fruit blotch and to identify the most resistant accessions for use in inheritance studies. Also, we will release the self-pollinated progeny of the most resistant plants from the most resistant accessions for use by industry in the development of resistant cultivars. The 1,689 PI accessions were directly seeded in the field, with 2 to 10 seeds/hill based on seed availability and seed germination rate. Plots were thinned to 1 plant/hill for evaluation of disease resistance in the main screening test. A retest was run using the 20 most resistant accessions, along with 1 susceptible PI accession and 1 check cultivar ('Charleston Gray'). The experiment was a randomized complete block design with 2 replications for germplasm screening and 4 replications for the retest. Raised beds were made up with drip irrigation tubes and black polyethylene mulch. Plants were in rows 3.1-m apart with plants 1.2 m apart. A foliar application was made on the experiment when plants were at the 4 to 6 true-leaf stage with a BFB suspension of strains AAC 001 and AAC 948. The two isolates were grown on nutrient agar (VWR, Radnor, PA) for 48 h and washed from the agar surface with deionized water. In the field, the suspension was diluted to 106 cfm/ml. Silwet L-77® (MomentiveTM, Albany, NY) surfactant was added to the suspension at 0.03% to lower the leaf surface tension. A total of 10 ml suspension was applied to plants in the field with a hand sprayer. Overhead irrigation applied twice a week to increase the severity of disease. A disease rating was made weekly, starting one week after inoculation. The rating was on a 0-9 scale where 0 = no symptoms and 9 = severe large leaf lesion and dead plants. The most resistant accessions from initial data analysis are PI 186490 (Nigeria), PI 271770 (South Africa), PI 482316 (Zimbabwe), PI 482246 (Zimbabwe), PI 482252 (Zimbabwe), PI 482265 (Zimbabwe), PI 482283 (Zimbabwe), PI 482284 (Zimbabwe), PI 482322 (Zimbabwe), PI 482331 (Zimbabwe), PI 482342 (Zimbabwe), PI 485583 (Botswana), PI 560006 (Nigeria), PI 560008 (Nigeria), PI 596666 (South Africa) with a rating less than 3, compared to the check PI 635598 (United States) having a rating range of 5 to 8.