Submitted to: International Phytophthora Capsici Conference
Publication Type: Proceedings
Publication Acceptance Date: October 21, 2009
Publication Date: December 1, 2009
Citation: Kousik, C.S., Donahoo, R.S., Hassell, R. 2009. Determining Tolerance in Commercial Watermelon Rootstocks to Crown Rot caused by Phytophthora Capsici using Real-Time PCR. International Phytophthora Capsici Conference. pp 17-18. Technical Abstract: Phytophthora crown and fruit rot caused by Phytophthora capsici is becoming an important and emerging disease of watermelon (Citrullus lanatus) in southeastern United States. Various rootstocks have been used for grafting watermelon in Asia and Europe to manage soil-borne diseases such as Fusarium wilt. In recent years, the practice of grafting seedless watermelons (triploids) onto rootstocks belonging to other Cucurbitaceae genera is gaining acceptance in the United States. However, at this point, it is not known how these rootstocks will respond to the diseases prevalent in the local production areas. We evaluated seventeen commercial rootstocks for tolerance to Phytophthora crown rot by inoculating them with a zoospore suspension (10,000 zoospores/plant) consisting of a mixture of isolates of P. capsici in the greenhouse. Disease development on rootstocks was rated on a 1-9 scale (1= no symptoms, 9=plant dead) for four weeks and the data was analyzed using non-parametric repeated measures. All commercial Lagenaria hybrid rootstocks evaluated were tolerant to Phytophthora crown rot when compared to susceptible watermelon controls. All the Cucurbita inter-specific (Cucurbita maxima x Cucurbita moschata) rootstock hybrids evaluated were extremely susceptible. 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 rootstocks. Even though the Lagenaria rootstocks were tolerant, some of the plants (13-31%) tested positive for the presence of P. capsici in the PCR based assays; however, the pathogen could only be isolated from a few plants (3-7%) on semi-selective media. Significant positive correlations (P=0.0001) between disease ratings and pathogen isolations (r=0.94), disease rating and PCR assay (r=0.91), and pathogen isolations and PCR assay (r=0.86) were observed in this study. Real-time quantitative PCR using a SYBR green based assay indicated the presence of more P. capsici DNA in the susceptible Cucurbita inter-specific hybrid rootstocks and triploid watermelon, compared to the tolerant Lagenaria rootstocks. Based on the results of this study and the increase in the interest for using grafted watermelons, the use of tolerant Lagenaria rootstocks may be more appropriate in fields with a known presence of P. capsici.