Identification and characterization of Didymella bryoniae causing gummy stem blight disease of watermelon (Citrullus lanatus) in Turkey

Watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai) is an important plant in the family Cucurbitaceae. Didymella bryoniae (Auersw.) Rehm [anamorph Phoma cucurbitacearum (Fr.) Sacc.] is the causative agent of gummy stem blight (GSB), a disease affecting members of the family Cucurbitaceae. Watermelon seedlings showing varied degree (20–90%) of suspected GSBs were collected from seedling companies in Antalya province, Turkey. The symptoms included angular water-soaked lesions, which turned tan over time; defoliation; dry, pale green stem; stem necrosis; gummy exudates; wilt; and the eventual death of the upper parts of the seedling plants. The growth of the fungal isolate in vitro showed white aerial and olivaceous mycelium, and olive to dark green or black substrate mycelium at the latter period of fungal growth. The colony surface was rough and undulated. The conidia were round-ended, cylindrical, monoseptate, and hyaline. The conidia ranged from 6.4 to 13.6 μm in length and 3.69–4.68 μm in diameter. The isolates were subjected to PCR using three primers, namely, DB-F3/DB-R3, GSBF1/GSBR1, and ITS1/ITS4, which produced fragment sizes of approximately 120, 780, and 560 bp, respectively. The real-time PCR using DB-F3/DB-R3 primer and SYBR Green fluorescence dye produced positive result for the samples tested. The D. bryoniae isolate was pathogenic to all the four different cucurbits tested and were consistently re-isolated from the diseased seedlings confirming their pathogenic status and thereby satisfying Koch's postulates. This is the second report confirming the presence of D. bryoniae in Turkey after its first detection in cucumber in 2015.