Stemphylium botryosum and Cladosporium cucumerinum: A new challenge in watermelon production in Egypt

<p>Farag Mohamed</p>

Stemphylium botryosum and Cladosporium cucumerinum: A new challenge in watermelon production in Egypt

Farag Mohamed

Agricultural Research Center, Egypt

: J Plant Physiol Pathol

Abstract

In February 2017, a severe disease with typical symptoms of small brown spots (1 to 2 mm in diameter) was observed on the leaves of watermelon. On the other hand, different lesions were observed on leaves and petioles brown to dark brown in color with or without a chlorotic halo. Shape of lesions was circular to oval and on leaves they were generally 1 to 14 mm in diameter in Minia County, Egypt. The pathogens were consistently isolated from leaf lesions on Potato Dextrose Agar (PDA) incubated at 25 °C for 7 days. Identification of the isolated fungi was verified at Assiut University Mycological Center based on their morphological characteristics. Microscopic observations revealed that conidia of Stemphylium botryosum were muriform, mostly oblong to ovoid but occasionally nearly globose, subhyline to variant shades of brown, mostly constricted at the median septum and measured 12 to 14×8 to 10 μm (average 13.4×8.9 μm). On the other hand, Cladosporium cucumerinum conidia measured 2 to 8 × 1 to 3 μm (average 4.94×1.94 μm). Pathogenicity tests were performed by spraying a conidial suspension (105 conidia ml-1) on healthy watermelon (cv. Giza 1), plants, at the 5-true-leaf stage. Disease symptoms appeared on watermelon, which were similar to those observed under natural infection conditions. S. botryosum and C. cucumerinum were consistently re-isolated from artificially infected watermelon tissues, thus confirming Koch's postulates. For the diseases reported here, we suggest the name Stemphylium leaf spots and Cladosporium leaf spot. This is the first report of a disease of watermelon caused by a species of Stemphylium and Cladosporium.