Screening the Cucumber Germplasm Collection for Resistance to Gummy Stem Blight in North Carolina Field Tests

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.