HORTSCIENCE, VOL. 35(6), OCTOBER 2000 1132 BREEDING, CULTIVARS, ROOTSTOCKS, & GERMPLASM RESOURCES HORTSCIENCE 35(6):1132–1140. 2000. Gummy stem blight of cucumber is caused by Didymella bryoniae [synonyms: Myco- sphaerella citrullina (C.O. Sm.) Gross., and Mycosphaerella melonis (Pass.) Chiu and J.C. Walker] and its anamorph Phoma cucurbitacearum (Farr et al., 1989) (syn- onyms: Ascochyta cucumis Fautr. and Roum., and Phyllosticta cucurbitacearum Sacc.). The pathogen attacks several other genera of Cucurbitaceae such as melon (Cucumis melo L.), watermelon (Citrullus lanatus Matsum. & Nakai) and squash (Cucurbita pepo L.). The pathogen is attracted to plants by volatile compounds produced by Cucumis and Cucurbita species (Pharis et al., 1982). The incidence and severity of damage due to gummy stem blight on cucumber is second only to those of root knot nematode in North Carolina (St. Amand and Wehner, 1991). Gummy stem blight is a serious disease of greenhouse cucumber in The Netherlands, where it causes fruit rot (black rot) (Punithalingam and Holliday, 1972; Van Steekelenburg, 1985a). Cucumber plants may be infected at any growth stage from seedling to mature fruit, and infection can occur on all plant parts, including roots (Thinggaard, 1987). Didymella blight and Phoma blight have similar symptoms and control practices and, together, cause gummy stem blight. Al- though chemical control is available, resis- tant cultivars would be preferable. Infection caused by D. bryoniae is depen- dent on relative humidity (RH), with more infection occurring at 95% than at 50% RH. Greatest infection is also produced by free- standing water on leaves. In the case of older leaves, wounding is essential for infection (Van Steekelenburg, 1985b). St. Amand and Wehner (1995a) reported that susceptibility of cucumber plants increased with increased guttation. Detached-leaf and greenhouse seed- ling tests both indicated that old leaves were more susceptible to the pathogen than young leaves. Field tests also revealed that leaves and stems were more susceptible after the vine tip-over stage (about eight true leaves) (St. Amand and Wehner, 1995a). Cucumber beetles ( Diabrotica undecimpunctata howardi Barber and Acalymma vittatum Fab- ricius) and powdery mildew [ Erysiphe cichoracearum DC. or Sphaerotheca fuliginea (Schlect.) Poll.] may predispose the plant to infection or contribute to pathogen spread (Bergstrom et al., 1982). Van Steekelenburg (1985a) reported that late-morning transition to day temperature settings in the greenhouse caused a higher incidence of main stem lesions and internal fruit rot from D. bryoniae on cucumber compared with early-morning transition. This was attributed to longer duration at high humidity, and to water condensation on fruits. St. Amand and Wehner (1995a) re- ported that dawn inoculation during field or greenhouse tests increased leaf susceptibility compared with dusk inoculation, possibly because of free water and nutrients provided by guttation. The typical symptoms of gummy stem blight on cucurbits are circular, tan to dark brown spots that appear first at the leaf mar- gins and rapidly spread over the entire leaf. Circular, black or tan spots also appear on the cotyledons and stems of young plants. Stem cankers develop in the cortical tissue, and a brown, gummy exudate is produced on the surface. On fruits, the disease causes a soft rot with black discoloration of the epidermis (exocarp). Small, water-soaked spots develop that later enlarge to an indefinite size, with a gummy exudate. In the lesions of different organs, pycnidia of the anamorph stage (P. cucurbitacearum) and perithecia of the teleomorph (D. bryoniae) are observed as abundant black dots (Zitter et al., 1996). Seedling tests for gummy stem blight have been developed (Abad and Wehner, 1992; Van Steekelenburg, 1981; Wyszogrodzka et al., 1986), and disease ratings from seedling tests described by St. Amand and Wehner (1995a) were found to be highly correlated (r = 0.82 to 0.96) to disease ratings in field tests. There have been no reports of race spe- cialization for D. bryoniae. In eight geo- graphically diverse isolates tested for differ- ences in pathogenicity to cucumber, there was no significant interaction of cultivar with isolate, and no differences were observed among the eight isolates for pathogenicity based on cluster analysis (St. Amand and Wehner, 1995b). However, two isolates (one from The Netherlands and one from Spain) were found to be more pathogenic than the isolates from the United States. Resistance in cucumber to D. bryoniae appeared to be non- specific or horizontal in pattern, indicating that breeders could use a single pathogenic isolate of D. bryoniae when screening for resistance. The objective of this experiment was to test all available plant introduction (PI) ac- cessions from the U.S. National Plant Germ- plasm System, cultivars, and breeding lines (hereafter collectively referred to as culti- gens) of cucumber for gummy stem blight resistance under field conditions in North Carolina. Screening the Cucumber Germplasm Collection for Resistance to Gummy Stem Blight in North Carolina Field Tests Todd C. Wehner 1, 2 and Nischit V. Shetty 2 Department of Horticultural Science, North Carolina State University, Raleigh, NC 27695-7609 Additional index words. Ascochyta cucumis, Colletotrichum lagenarium, Cucumis sativus, Cucurbitaceae, Didymella bryoniae, disease resistance, Mycosphaerella melonis, Phoma cucurbitacearum, Phoma exigua, Phyllosticta cucurbitacearum, vegetable breeding Abstract. 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. Received for publication 30 Aug. 1999. Accepted for publication 14 Feb. 2000. The research reported herein was funded in part by the North Carolina Agricultural Research Service (NCARS). Use of trade names in this publication does not imply endorsement by the NCARS of the products named, or criticism of similar ones not mentioned. We gratefully acknowledge the technical assistance of Z.G. Abad and R.R. Horton, Jr. The cost of publish- ing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. 1 To whom requests for reprints should be addressed. E-mail address: todd_wehner@ncsu.edu. 2 Professor and Graduate Research Assistant, respectively.