Cucurbit Rootstocks Resistant to f. sp. Remain Resistant When Coinfected by in the Field.

Interspecific hybrid squash ( × ) rootstocks used to graft watermelon ( var. ) are resistant to f. sp. , the fungus that causes wilt of watermelon, but they are susceptible to , the southern root knot nematode. A new citron () rootstock cultivar Carolina Strongback is resistant to f. sp. and . The objective of this study was to determine if an interaction between and f. sp. race 2 occurred on grafted or nongrafted triploid watermelon susceptible to f. sp. race 2. In 2016 and 2018, plants of nongrafted cultivar Fascination and Fascination grafted onto Carolina Strongback and interspecific hybrid squash cultivar Carnivor were inoculated or not inoculated with before transplanting into field plots infested or not infested with f. sp. race 2. Incidence of wilt and area under the disease progress curve did not differ when hosts were inoculated with f. sp. alone or f. sp. and together. wilt was greater on nongrafted watermelon (78% mean incidence) than on both grafted rootstocks and lower on Carnivor (1% incidence) than on Carolina Strongback (12% incidence; ≤ 0.01). Plants not inoculated with f. sp. did not wilt. At the end of the season, Carnivor had a greater percentage of the root system galled than the other two hosts, whereas galling did not differ on Fascination and Carolina Strongback. f. sp. reduced marketable weight of nongrafted Fascination with and without coinoculation with . reduced marketable weight of Fascination grafted onto Carnivor compared with noninoculated, nongrafted Fascination. In conclusion, cucurbit rootstocks that are susceptible and resistant to retain resistance to f. sp. when they are coinfected with .