First Report of Leaf Spot and Stem Canker on Watermelon Caused by Paramyrothecium foliicola in Mexico.

Abstract

Watermelon is an important vegetable crop in Mexico and produced on 358,105 ha, with nearly 1.5 x 106 tons. In September 2019, brown, irregular shape to round lesions with concentric rings were observed on the leaves and stems of watermelon plants in Sonora State. The surface of the lesions contained abundant cup-shaped sporodochia covered by masses of olive-green to black conidia. Edge sections of symptomatic tissues were cut from the leaves, disinfected in 70% ethanol for 1 min and subsequently washed twice with distilled water. Disinfected tissue samples were transferred to PDA medium and incubated at 27°C for 15 days. White colonies were observed with spordochia arranged in concentric rings with characteristic of greenish-black masses of conidia. Spore masses stained with lactophenol blue were examined microscopically. Conidia were nonseptate and rod-shaped with rounded ends that measured 6.65 ± 0.54 x 1.56 ± 0.25 μm (n = 100). The characteristics of the fungus were similar to those reported for Paramyrothecium foliicola (Rennberger and Keinath, 2020). Molecular identification was performed on a representative isolate. RNA polymerase II second largest subunit (RPB2), calmodulin (CmdA) and the β-tubulin (B-tub) genes were amplified and sequenced with the primer sets RPB2-5F2-RPB2-7cR, CAL228F-CAL737R and Bt2a-Bt2b, respectively. These sequences were submitted to GenBank with the acc. nos. MW116070 for RPB2, MW116071 for CmdA and MW116072 for B-tub. BLASTn analysis of the sequences demonstrated 99.34 to 100% identity with Paramyrothecium foliicola (acc. nos. MN398043, MN593713 and MN398138). Koch's postulates were verified on 15-day-old watermelon seedlings and mature fruit. One point of each of ten watermelon seedlings and six points of each of five fruit were marked for inoculation. A plug of mycelium obtained from a monosporic pure culture (grown for 15 days in PDA) was applied to each point without wounds. PDA only medium was included as the controls. The pathogenicity tests were repeated twice. Treated seedlings and fruit were kept in plastic bags at 27°C for 15 days. The first symptoms appeared 4 days after inoculation on the seedlings and 3 days after inoculation on the fruit. At the end of the test, the symptoms were similar to those observed initially in the field. The pathogen was re-isolated from lesion edges, and the morphological characteristics of the pathogen were determined to correspond with those of the inoculated fungus. Control seedlings and fruits remained healthy. P. foliicola has been reported to cause leaf spot disease on wild rocket and basil (Matić et al., 2019) and, recently, on watermelon in South Carolina (Rennberger and Keinath, 2020). To the best of our knowledge, this report is the first to describe P. foliicola causing leaf spot and stem canker on watermelon in Mexico.