Differential Susceptibility of Two Citrullus amarus Pollenizer Watermelons to Five Species of Pythium and Globisporangium

Abstract

Pollenizer watermelon (Citrullus amarus) must be planted alongside triploid watermelon cultivars to provide a source of pollen. Early season death of pollenizer watermelon cultivars SP-6 and SP-7 was observed in a research field in Charleston County, SC, in April 2022, 3 to 4 weeks after transplanting. Disease incidence was 15 and 12% for the two cultivars, respectively. Two species of Pythium (P. myriotylum and P. aphanidermatum) and three species of Globisporangium (G. ultimum, G. irregulare, and G. spinosum) were recovered from 13 of 17 plants cultured and identified based on the sequences of the cytochrome c oxidase subunit I gene. Pythium spp. were recovered from both cultivars, while Globisporangium spp. were recovered only from ‘SP-7’. In pathogenicity tests, ‘SP-7’ had lower area under the disease progress curve (AUDPC) values than ‘SP-6’ with one isolate each of four species and lower AUDPC values than seedless watermelon ‘Estrella’ with one isolate each of two species. Mean AUDPC values did not differ between ‘Estrella’ and ‘SP-6’ with any isolate. AUDPC values were greater with G. ultimum on all three cultivars than with one isolate of G. spinosum and both isolates of P. myriotylum and P. aphanidermatum. Susceptibility of the new cultivar SP-7 to Pythium and Globisporangium spp. differs from susceptibility of ‘SP-6’. These results will help growers choose which pollenizer cultivar to plant.