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Joseph N. Wolukau, Xiaohui Zhou, and JinFeng Chen

Gummy stem blight (GSB) caused by Didymella bryoniae (Auersw.) Rehm is one of the most important soilborne diseases of melon and causes significant economic losses (30% to 60%) ( Frantz and Jahn, 2004 ). Although chemical control has been

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Gabriele Gusmini, Luis A. Rivera-Burgos, and Todd C. Wehner

Gummy stem blight is a major disease of watermelon [ C. lanatus (Thunb.) Matsum. & Nakai]. It is caused by three genetically distinct Stagonosporopsis species, S. cucurbitacearum (syn. Didymella bryoniae ), S . citrulli , and S . caricae

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Joseph N. Wolukau, Xiao-Hui Zhou, Ying Li, Yong-Bin Zhang, and Jin-Feng Chen

Melon production is severely constrained by several soil-borne disease pathogens. Of these pathogens, Didymella bryoniae (Auersw), Rehm that causes gummy stem blight (GSB) is one of the most destructive resulting in substantial economic losses

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D.J. McGrath, L. Vawdrey, and I.O. Walker

Resistance to gummy stem blight [Didymella bryoniae (Auersw.) Rehm] was evaluated in two accessions of Cucumis melo L., PI 266935 and PI 266934. Based on disease reaction scores and dry weights, PI 266934 possessed much greater resistance than PI 266935. The quality of resistance of PI 266934 was unaffected by the ranges of seedling ages and inoculum concentrations used. No melon cultivar, to our knowledge, is highly resistant to gummy stem blight in the field, and alternative sources of superior resistance are potentially useful for breeding.

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Luis A. Rivera-Burgos, Emily Silverman, Nebahat Sari, and Todd C. Wehner

Gummy stem blight (GSB) is a major disease of watermelon [ Citrullus lanatus (Thunb.) Matsum. & Nakai] that leads to significant economic losses. This disease is caused by three genetically distinct Stagonosporopsis species, S . cucurbitacearum

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Paul C. St. Amand and Todd C. Wehner

Leaf and stem resistance to gummy stem blight [Didymella bryoniae (Auersw.) Rehm.] in five resistant by susceptible crosses of cucumber (Cucumis sativus L.) was investigated using generation means analysis. No single gene of major effect controls either leaf or stem resistance to gummy stem blight in these five crosses. The mean number of effective factors controlling leaf resistance in the cross `Slice' × `Wis. SMR 18' was estimated to be at least five. Estimates of broad- and narrow-sense heritabilities indicated that environmental effects were larger than genetic effects. In general, additive variance was the larger component of genetic variance. Epistasis was significant in most crosses, and dominance was present in several crosses. Additive gene effects contributed more to resistance than to susceptibility in contrast with dominance gene effects. Reciprocal differences for leaf rating were detected in the crosses M 17 × `Wis. SMR 18' and `Slice' × `Wis. SMR 18'. Phenotypic correlations between leaf and stem ratings were moderate (r = 0.52 to 0.72). Estimates of genetic gain for resistance to gummy stem blight ranged from low to moderate. Breeding methods that make best use of additive variance should be used because much of the variance for resistance is additive, and dominance effects, at least in these crosses, tended to contribute to susceptibility.

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Todd C. Wehner and Nischit V. Shetty

Gummy stem blight (Didymella blight), caused by Didymella bryoniae (Auersw.) Rehm and its anamorph Phoma cucurbitacearum (Fr.:Fr.) Sacc., is the second most important disease of cucumber (Cucumis sativus L.) in North Carolina after root knot nematodes Meloidogyne sp. Both Didymella blight and Phoma blight, caused by Phoma exigua Desm., have similar symptoms and control practices, and are generally referred to as gummy stem blight. In order to determine whether resistance existed to North Carolina isolates of D. bryoniae, 851 cultigens [cultivars, breeding lines, and plant introduction (PI) lines] were evaluated in the field. Plants were inoculated with one selected isolate (highly pathogenic in preliminary greenhouse tests) at the vine tip-over stage. They were rated for foliage lesion size and number on a 0 to 9 visual scale (0 = no disease, 9 = plant killed) and average ratings for 10 plants per plot were analyzed. The ratings ranged from 2.0 (highly resistant) to 8.5 (highly susceptible) with a mean of 6.2. The most resistant breeding lines and PI accessions were PI 200815, PI 390243, `LJ 90430', PI 279469, and PI 432855. The most resistant cultivars were `Homegreen #2', `Little John', `Transamerica', and `Poinsett 76'. The most susceptible cultigens in the study were PI 288238, PI 357843, PI 357865, and PI 167134. Two popular cultivars in North Carolina, `Calypso' and `Dasher II', were moderately resistant.

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Yiping Zhang, Molly Kyle, Konstantinos Anagnostou, and Thomas A. Zitter

Greenhouse and field evaluations of melon (Cucumis melo L.) for resistance to gummy stem blight, caused by the fungus Didymella bryoniae (Auersw.) Rehm, were conducted on 798 U.S. Dept. of Agriculture Plant Introduction (PI) accessions and 24 related Cucumis species. Plants were inoculated at the three to four true-leaf stage with a virulent isolate of D. bryoniae collected from Onondaga County, N.Y., and disease indices were calculated based on foliar and stem symptoms. In greenhouse screens, 43 C. melo accessions showed a high level of resistance. Results were consistent between the optimized greenhouse screening procedure described and inoculated replicated field tests. Of these accessions, a Chinese group, PIs 157076, 157080, 157081, 157082, 157084; another group from Zimbabwe, PIs 482393, 482398, 482399, 482402, 482403, 482408; and some others from different origins, PI 255478 (Korea) and PI 511890 (Mexico), showed high levels of resistance, at least equal to that in PI 140471, the leading source of resistance to date.

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Todd C. Wehner and Paul C. St. Amand

Gummy stem blight [Didymella bryoniae (Auersw.) Rehm] is the second most important pathogen of field-grown cucumbers (Cucumis sativus L.) in North Carolina and a severe problem for greenhouse-grown cucumbers worldwide. To determine whether resistance exists under North Carolina field conditions, 83 cultigens [cultivars, breeding lines, and plant introduction (PI) accessions] were evaluated in the field for 4 years for their resistance to a mixture of D. bryoniae isolates. Plants were inoculated at the vine tip-over stage and rated for foliar lesion size and number. Cultigens identified as resistant in Wisconsin and The Netherlands were not resistant in North Carolina. When averaged over years and locations, the most resistant C. sativus cultigens were PI 164433, `Slice', PI 390264, M 17, and M 12. Several accessions of related Cucumis species were highly resistant: PI 299568 (C. myriocarpus Naud.), PI 282450 (C. zeyheri Sond.), PI 299572 (C. myriocarpus), and PI 233646 (C. anguria L.). The most susceptible cultivars were `Colet', `Meresto', `Supergreen', `Dura', `Pioneer', `Marketmore 76', `Pickmore', and `Addis'. `Calypso' and `Dasher II', popular cultivars in North Carolina, were moderately susceptible.

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Paul C. St. Amand and Todd C. Wehner

Gummy stem blight (Didymella bryoniae (Auersw.) Rehm) is one of the major cucumber diseases, causing the second highest loss of any disease in North Carolina. Published methods of screening for resistance to this fungus are poorly correlated with field resistance. The objective of this study was to develop seedling or detached-leaf screening methods that are correlated with field resistance. Seedling tests examined the effects of: seedling age (1, 2 or 3 true leaves), days in humidity chamber, inoculum concentration (1×105, 1×106 or 1×107 spores per ml), time of inoculation (am vs. pm), fungal isolates, and cultigens. Detached leaf tests examined the effects of leaf age (1st, 2nd or 3rd true leaf), inoculum concentration (1×104, 1×105 or 1×106 spores per ml), and light levels during incubation (dark vs. 12h light/12h dark). Correlations between seedling tests and field data were moderate to high (r = 0.5 to 0.7). However, the coefficients of variation were also high. Correlations between detached leaf tests and field data were very low or negative.