@article{gusmini_song_wehner_2005, title={New sources of resistance to gummy stem blight in watermelon}, volume={45}, ISSN={["1435-0653"]}, DOI={10.2135/cropsci2005.0582}, abstractNote={Gummy stem blight, caused by Didymella bryoniae (Auersw.) Rehm, is a major disease of watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai]. Plant breeders need sources of resistance that can be incorporated into adapted breeding lines to help control the disease. We tested all the available accessions from the USDA‐ARS watermelon germplasm collection, including C. lanatus var citroides (L.H. Bailey) Mansf., for resistance to gummy stem blight. The experiment was a randomized complete block with 1332 cultigens, two sites (field and greenhouse), two or four replications, and two to six plants per plot. The resistant check was PI 189225 and the susceptible check was ‘Charleston Gray’. PI 279461, PI 482379, PI 254744, PI 526233, PI 482276, PI 271771, PI 164248, PI 244019, PI 296332, and PI 490383 were selected as the most resistant cultigens to be used in future breeding efforts. The most susceptible cultigens were PI 183398, PI 169286, PI 223764, PI 226445, PI 525084, PI 534597, and PI 278041.}, number={2}, journal={CROP SCIENCE}, author={Gusmini, G and Song, RH and Wehner, TC}, year={2005}, pages={582–588} }
 @article{song_gusmini_wehner_2004, title={Screening the watermelon germplasm collection for resistance to gummy stem blight}, ISBN={["90-6605-667-3"]}, ISSN={["2406-6168"]}, DOI={10.17660/actahortic.2004.637.6}, abstractNote={All available accessions from the USDA watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai) germplasm collection, including C. lanatus var. citroides, were screened for resistance to gummy stem blight (Didymella bryoniae, anamorph Phoma cucurbitacearum). The experiment was a randomized complete block with 1,325 cultigens (elite cultivars, obsolete cultivars, breeding lines, and PI accessions), two locations (field and greenhouse), and two or four replications. Isolates used were collected from cucurbits and verified for virulence on watermelon. The most resistant cultigens were significantly better than the check, ‘Charleston Gray’, and the most susceptible cultigens were significantly worse. The most resistant and most susceptible cultigens were retested, along with check cultivars (including a set of cucumber cultigens with known characteristics of resistance and susceptibility), to verify their reaction. The retest was a randomized complete block with 75 (38 in 2000) cultigens, two locations (field and greenhouse), and three or four replications. The most resistant cultigens were PI 279461, PI 254744, PI 482379, PI 244019, PI 526233, PI 482276, PI 164248, PI 482284, PI 296332, PI 490383, PI 271771, and PI 379243. The most susceptible cultigens were PI 226445, PI 534597, PI 525084, PI 223764, PI 169286, and PI 183398. INTRODUCTION Gummy stem blight (Didymella bryoniae (Auersw.) Rehm) is one of the most destructive diseases of watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai), a major vegetable crop in the U.S. (Schenck, 1962; Keinath, 1995). Gummy stem blight causes crown blight, extensive defoliation and fruit rot, and can cause severe losses in the field. For example, in 1991 over 15% of the watermelon crop in South Carolina was abandoned before harvest (Power, 1992). The disease also causes loss of fruit during storage and transportation (Leupschen, 1961; Sowell and Pointer, 1962; Norton, 1978). Didymella bryoniae is seed-borne (Lee et al., 1984), air-borne (Van Steekelenburg, 1983), and soil-borne (Keinath, 1996; Bruton, 1998). The fungus persists in crop residue even at extreme temperatures, such as –9°C for 14 days (Van Steekelenburg, 1983). D. bryoniae is a facultatively necrotrophic fungus (Svedelius, 1990); thus, wounding the leaves, particularly the old ones (Van Steekelenburg, 1985a), helps disease initiation because the production of exudates from the lesions favors the fungus in its growth and infection. High relative humidity and the presence of free water on the plants are required for the fungus to induce large lesions on leaves and stems (Van Steekelenburg, 1981, 1984, 1985a, b). Finally, there is no evidence of race specialization for this pathogen (St. Amand and Wehner, 1995). Genetic resistance has received attention in the last 50 years. Differences in resistance to gummy stem blight have been demonstrated among cultivars of watermelon. ‘Congo’ was the least susceptible, ‘Fairfax’ was intermediate, and ‘Charleston Gray’ was the most susceptible (Schenck, 1962). PI 189225 was the most resistant accession of 439 evaluated from the USDA watermelon germplasm collection (Sowell and Pointer, 1962). Proc. XXVI IHC – Advances in Vegetable Breeding Eds. J.D. McCreight and E.J. Ryder Acta Hort. 637, ISHS 2004 Publication supported by Can. Int. Dev. Agency (CIDA) 64 Several years later, PI 271778 was identified as an additional source of resistance (Sowell, 1975). It had a disease response that was intermediate between PI 189225 and ‘Charleston Gray’. A later screening effort of 138 watermelon accessions showed that PI 500335, PI 505590, PI 512373, PI 164247, and PI 500334 were resistant to gummy stem blight (Boyhan et al., 1994). Resistant cultivars were developed from two crosses (‘Jubilee’ x PI 271778 and ‘Crimson Sweet’ x PI 189225) by selecting disease-resistant seedlings from backcrossed families that produced high yield of excellent quality fruit (Norton et al., 1986). ‘AU-Jubilant’ and ‘AU-Producer’ (Norton et al., 1986), ‘AUGolden Producer’ (Norton et al., 1993), and ‘AU-Sweet Scarlet’ (Norton et al., 1995) were released with moderate to high resistance to anthracnose, fusarium wilt, and gummy stem blight in greenhouse screening tests. Seedling screening methods are the most commonly used among breeders to test new cultivars and lines for resistance to gummy stem blight in cucurbits (Zhang et al., 1995; Dias et al., 1996; Zhang et al., 1997; Wehner and Shetty, 2000). Tests involve spraying seedlings with a water suspension of spores collected from in vitro cultures of the pathogen. However, in field tests the gummy stem blight resistance of these cultivars has not proven useful. So far, no cultivars of watermelon (Sumner and Hall, 1993) have been released that have high resistance to gummy stem blight. The objective of this study was to identify new and useful sources of resistance to gummy stem blight by screening the USDA watermelon germplasm collection. The ultimate objective was to develop resistant and adapted cultivars. MATERIALS AND METHODS Locations and Seed Sources All experiments were run at the Plant Pathology greenhouses in Raleigh, N.C., and at the Horticultural Crops Research Station, Clinton, N.C. All Citrullus Plant Introduction (PI) accessions were obtained from the Southern Regional Plant Introduction Station, Griffin, Ga. The accessions originated from 66 different countries. The checks were 51 watermelon cultivars, along with a set of seven cucumber cultivars, to provide reference points for gummy stem blight resistance. The checks were obtained from public and private plant breeders in the U.S. Inoculum Preparation For all tests, D. bryoniae was increased on petri plates containing 15 ml potato dextrose agar (PDA). Inoculated plates were incubated for 2 to 3 weeks at 24 ± 2°C under alternating periods of 12 h fluorescent light (40 to 90 μmol.m.sec PPFD) and 12 h darkness until sporulating pycnidia formed. For all inoculations, a spore suspension was prepared by flooding the culture plates with 5 to 10 ml of sterile, distilled water and scraping the surface of the agar using a finger. The liquid from each plate was filtered through 4 layers of cheese-cloth to remove mycelia, pycnidia and dislodged agar. The final pH of the inoculum was unadjusted. Spore concentration was measured with a hemacytometer and adjusted to a concentration of 5x10 spores/ml adding deionized water. Immediately before inoculation, Tween 80 (2 drops/L) was added to the inoculum. Inoculation Procedure In the greenhouse test, plants were inoculated at the second true leaf stage, after the leaf surface was damaged by brushing it with a wooden stake. Inoculum was applied using a hand-pumped spray bottle (EcoLogical, Sprayco, Mich.). Immediately after inoculation, plants were placed in a humidity chamber with clear-plastic walls (top open during the summer, top closed during the winter). Humidifiers were used in the chamber (Model 500, Trion, Sanford, N.C.) running continuously for the treatment time (1 day before inoculation through 3 days after inoculation) to keep the relative humidity close to 100% day and night. Plants in all treatments were watered daily using overhead sprinklers, except when humidifiers were running.}, number={637}, journal={ADVANCES IN VEGETABLE BREEDING}, publisher={Leuven, Belgium : International Society for Horticultural Science}, author={Song, R and Gusmini, G and Wehner, TC}, year={2004}, pages={63–68} }