Submitted to: Crop Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 2, 2012
Publication Date: May 2, 2012
Citation: Kousik, C.S., Donahoo, R.S., Hassell, R. 2012. Resistance in watermelon rootstocks to crown rot caused by Phytophthora capsici. Crop Protection. 39:18-25. Interpretive Summary: Grafting watermelon onto other cucurbit (cucumber family) rootstocks for the control of soil borne diseases has been practiced in the far-east for many decades. However, it is not known how the rootstocks developed in Asia will fare against the diseases and pests prevalent in the United States. Phytophthora capsici is a pathogen of cucurbits that is very prevalent in the Southeastern United States and it causes severe crown and fruit rots. Researchers at the U.S. Vegetable Laboratory, Charleston, SC, tested the available commercial rootstocks for resistance to this pathogen. Several commercial bottle gourd rootstocks with resistance to this pathogen were identified. These results indicate that the resistant bottle gourd rootstocks will be useful for growing watermelon in fields where there is a known presence of the disease causing pathogen.
Technical Abstract: Phytophthora crown and fruit rot caused by Phytophthora capsici is becoming an important and emerging disease of watermelon (Citrullus lanatus) in south eastern United States. In recent years, the practice of grafting seedless watermelons (triploids) onto rootstocks belonging to other Cucurbitaceae genera has been slowly gaining importance in the United States. However, it is not known how these commercial rootstocks, developed mainly in Asia will respond to the diseases prevalent in the local production areas in the U.S. We evaluated the available commercial rootstocks for tolerance to Phytophthora crown rot by inoculating them with a zoospore suspension (25,000 zoospores/plant) consisting of a mixture of isolates of P. capsici in the greenhouse. Disease development on the rootstocks was rated on a 1-9 scale (1= no symptoms, 9=plant dying or dead) for four weeks. Data analysis using non-parametric repeated measures indicated that the commercial Lagenaria (bottle gourd, Lagenaria siceraria) rootstocks were tolerant to Phytophthora crown rot when compared to susceptible watermelon controls. All the Cucurbita inter-specific hybrid (Cucurbita maxima x Cucurbita moschata) rootstocks evaluated were extremely susceptible to P. capsici. Similarly, the wild watermelon rootstock ‘Ojakkyo’ was also susceptible. A diagnostic PCR assay specific for assessing the presence of P. capsici confirmed infection of all the susceptible Cucurbita rootstocks and watermelon checks. Real-time quantitative PCR using a SYBR green based assay indicated the presence of significantly more P. capsici DNA in crown tissue of the susceptible Cucurbita inter-specific hybrid rootstocks and watermelon compared to the tolerant Lagenaria rootstocks. Our results suggest that watermelon grafted on Lagenaria rootstocks will be useful in fields with a known presence of P. capsici.