by Luis Rivera, Gabe Gusmini and Todd C. Wehner
Department of Horticultural Science
North Carolina State University
Raleigh, NC 27695-7609
Gummy stem blight caused by Didymella bryoniae (Auersw.) Rehm [=Mycosphaerella citrullina (C.O.Sm.) Gross. and Mycosphaerella melonis (Pass) Chiu & Walker] and its anamorph Phoma cucurbitacearum (Fr.:Fr.) Sacc. [=Ascochyta cucumis Fautrey & Roum] is one of the most destructive diseases of watermelon. Resistance to gummy stem blight was ranked for several years by U.S. watermelon researchers as the third most important trait for germplasm evaluation (after bacterial fruit blotch and Fusarium wilt). Gummy stem blight was first described by Fautrey and Roumeguere in France as the disease caused on cucumber by Ascochyta cucumis in 1891. In 1917, gummy stem blight was reported for the first time in the U.S. affecting watermelon fruit from Florida, where it is still an important limiting factor for the watermelon industry. One severe gummy stem blight epidemic on watermelon was reported in the Southeastern USA, with over 15% of the watermelon crop in South Carolina abandoned before harvest. In addition, severe economic losses have been reported during transportation and in storage due to the disease commonly known on fruit as black rot.
Relevant characteristics of these disease/fungus are:
- Symptoms are evident as crown blight, stem cankers, and extensive defoliation, with necrotic areas on the cotyledons, hypocotyls, leaves, and fruit.
- D. bryoniae is a seed-borne, air-borne, or soil-borne fungus capable to survive in vegetable debris in the field during the winter
- D. bryoniae is a facultative necrotroph: the production of exudates from lesions helps the fungus in its growth and infection
- There is no evidence of race specialization for this pathogen and breeders and pathologists usually use a mixture of isolates to test cucurbits for resistance to D. bryoniae
Methods of seedling screening for resistance to gummy stem blight are based on spraying the seedlings with a water suspension of spores collected from in vitro cultures of the pathogen.
Among commercial cultivars of watermelon, ‘Congo’ is the least susceptible to gummy stem blight, ‘Fairfax’ intermediate, and ‘Charleston Gray’ the most susceptible. Dr. J.D. Norton in the years between 1978 and 1995 developed resistant watermelon cultivars from two crosses (‘Jubilee’ X PI 271778, ‘Crimson Sweet’ X PI 189225) by selecting disease-resistant seedlings from backcrossed families. ‘AU–Jubilant’ and ‘AU–Producer’, ‘AU–Golden Producer’, and ‘AU–Sweet Scarlet’ were released with moderate to high resistance to anthracnose, Fusarium wilt, and gummy stem blight. However, they were much less resistant to gummy stem blight than the resistant parents PI 189225 and PI 271778. To date, no cultivars of watermelon, muskmelon, or cucumber have been released that have high resistance to natural epidemics of gummy stem blight in the field.