Morphological and Molecular Characterization of <i>Alternaria</i> Species Causing Leaf Blight on Watermelon in China

Abstract

Watermelon is an economically important crop in China and is commonly affected by <i>Alternaria</i>-like leaf blight that can result in significant economic losses. In this study, 830 <i>Alternaria</i> isolates, recovered from symptomatic watermelon leaves, were identified based on morphological traits, pathogenicity, and multilocus sequence analyses of glyceraldehyde-3-phosphate dehydrogenase (<i>GAPDH</i>), histone 3 (<i>HIS3</i>), the internal transcribed spacer of ribosomal DNA (rDNA ITS), and the RNA polymerase II second largest subunit (<i>RPB2</i>). Watermelon isolates grouped to five <i>Alternaria</i> species and one unclassified <i>Alternaria</i> species. They were <i>A. tenuissima</i>, <i>A. alternata</i>, <i>A. cucumerina</i>, <i>A. infectoria</i>, <i>A. gaisen</i>, and <i>Alternaria</i> sp. Notably, <i>A. tenuissima</i> was the most prevalent (73.5%) of the six isolated species, followed by <i>A. alternata</i> (25.0%), <i>A. cucumerina</i> (1.1%), <i>Alternaria</i> sp. (0.2%), <i>A. infectoria</i> (0.1%), and <i>A. gaisen</i> (0.1%). Pathogenicity tests demonstrated that all six <i>Alternaria</i> species could produce brown necrotic lesions on detached leaves of watermelon. The average disease incidence (75.1%) and average disease index (60.8) of watermelon resulting from inoculation of leaves with <i>A. cucumerina</i> were significantly higher than levels resulting from <i>A. alternata</i> (52.9% and 37.2) and <i>A. tenuissima</i> (47.5% and 30.8). Inoculation with <i>Alternaria</i> sp. resulted in a disease incidence (70.0%) and disease index (51.5), which were lower than those of <i>A. cucumerina</i>. The disease incidence and disease index in watermelon leaves inoculated with the one isolate of <i>A. infectoria</i> and the one isolate of <i>A. gaisen</i> present in the inoculated leaves were 28.9% and 16.4, and 48.9% and 31.4, respectively. Results of the study indicate that <i>Alternaria</i> species associated with watermelon leaf blight in China are more diverse than that has been previously reported. This is the first report globally of <i>A. infectoria</i>, <i>A. gaisen</i>, and an unclassified <i>Alternaria</i> species as causal agents of leaf blight on watermelon.