Identification and Characterization of the Causal Agent of Gummy Stem Blight from Melon in East China

Melon (Cucumis melo) is one of the most important fruits for summer season in the world. With the dramatic increasing planting area of melon in protected facilities in China, gummy stem blight is now becoming the most serious disease in melon production. To understand the pathogen which causes this disease, morphological characteristics and rDNA internal transcribed spacer (ITS) sequences as well as their pathogenicity were analyzed. 43 isolates were identified from the sample stem collected from Jiangsu, Zhejiang, Anhui and Jiangxi Provinces of the East China. The mycelia from the PDA culture were white to dark gray on top and black on the bottom. After 2 days, the colonies on the PDA varied in diameter from 2.5 to 3 cm and extended to the edge of the petri dish. The rDNA-ITS sequences were amplified from 43 isolates and sequenced. Based on the blast search and alignment analysis, the pathogen of melon gummy stem blight was Didymella bryoniae. The similarity of these 43 isolates and other D. bryoniae from BLAST searches of GenBank was 100% or 99%. Phylogenetic analysis, based on rDNA ITS sequence, clearly distinguished D. bryoniae and Didymella spp. from the 10 other species studied. A pathogenicity test was conducted in a greenhouse on watermelon cvs. Sugar Baby, Sugarlee and Charleston Grey and melon cvs. Tongtian and Zhengzhu per isolate. Eighteen isolates showed highly pathogenic to both watermelons and melons. Together, according to the morphological characteristics and rDNA ITS sequence analysis as well as pathogenic test, we concluded that the pathogen of melon gummy stem blight in East China was D. bryoniae. Further studies are undertaking to distinguish the physiological races of D. bryoniae which have not been reported yet. This project was supported by National  Industrial Technology System for Watermelon & Melon (CARS-NO.8).