Powdery mildew resistant rootstocks impart tolerance to grafted susceptible watermelon scion seedlings

Authors: Kousik, Chandrasekar - Shaker; MANDAL, MIHIR - Oak Ridge Institute For Science And Education (ORISE); HASSELL, RICHARD - Clemson University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/30/2017
Publication Date: 6/28/2018
Citation: Kousik, C.S., Mandal, M., Hassell, R. 2018. Powdery mildew resistant rootstocks impart tolerance to grafted susceptible watermelon scion seedlings. Plant Disease. 102(7):1290-1298. https://doi.org/10.1094/PDIS-09-17-1384-RE.
DOI: https://doi.org/10.1094/PDIS-09-17-1384-RE

Interpretive Summary: Watermelon is an important vegetable crop grown in 44 states in the U.S.A. Many diseases and pests attack watermelon seedlings and plants and reduce their yield resulting in monetary loss for growers. One such disease, known as powdery mildew can infest watermelon seedlings and can cause reduced vigor or death of seedlings. ARS scientists in Charleston SC developed watermelon rootstocks that are resistant to powdery mildew. Watermelon seedlings that were not resistant to powdery mildew when grafted on resistant rootstocks showed less severe powdery mildew. The resistant rootstocks can help reduce powdery mildew severity on commercial watermelon cultivars. The results of this study will be useful to watermelon growers, extension workers, seed industry and University researchers to help manage powdery mildew in the greenhouse where watermelon seedlings are generally grown.

Technical Abstract: Powdery mildew (PM) caused by Podosphaera xanthii is an important foliar disease affecting cucurbit crops grown in the United States and commonly occurs on seedlings, foliage, petioles, fruit and stems. The disease is known to cause serious economic losses and can be difficult to manage. Grafting watermelon (Citrullus lanatus) on resistant rootstocks for managing soilborne disease has been gaining popularity in the U.S.A. Experiments to determine if PM resistant rootstocks could impart resistance to susceptible scion seedlings were conducted for three years from 2012 to 2014 in a greenhouse. A PM susceptible watermelon cultivar Mickey Lee (scion) was grafted on 25 watermelon and 4 bottle gourd (Lagenaria siceraria) rootstocks. Grafted plants were inoculated with a suspension of PM conidia (104 conidia ml-1) and rated 14 days after inoculation. Mickey Lee grafted on six resistant watermelon rootstocks had significantly lower PM severity on the cotyledons, second true leaf and upper leaves. In contrast, Mickey Lee grafted on susceptible watermelon (USVL677-PMS) or bottle gourd (USVL848-PMS) rootstocks had significantly greater PM severity. However, not all PM resistant watermelon rootstocks imparted resistance to the scion. Two PM resistant bottle gourd rootstocks (USVL351-PMR and USVL482-PMR) and a F1 hybrid of the two (USVL482351-PMR) provided significantly greater levels of resistance to the susceptible watermelon scion cotyledons and true leaves compared to some of the PM resistant watermelon rootstocks and the susceptible bottle gourd rootstock (USVL848-PMS). Grafting watermelon on resistant rootstocks may help mitigate the effect of PM on the susceptible scion seedlings.