IDENTIFICATION, ELUCIDATION, AND DEVELOPMENT OF DISEASE AND NEMATODE RESISTANCES IN VEGETABLE CROPS
Location: Vegetable Research
Title: Grafting Seedless Watermelons, How and Why? Will Grafting Help With Disease Control?
Submitted to: Atlantic Coast Agriculture Conference and Trade Show
Publication Type: Proceedings
Publication Acceptance Date: December 8, 2009
Publication Date: January 11, 2010
Citation: Kousik, C.S., Thies, J.A., Donahoo, R., Hassell, R. 2010. Grafting Seedless Watermelons, How and Why? Will Grafting Help With Disease Control?. Atlantic Coast Agriculture Conference and Trade Show. pp 38-39.
In recent years, the practice of grafting seedless watermelons (triploids) onto rootstocks belonging to other Cucurbitaceae genera has gained importance in the United States. Grafting vegetable crops, especially cucurbit’s, is very common in Europe and Asia. In these regions, the practice of crop rotation is difficult, as land available for farming is limited and under intense use. The continuous use of land eventually leads to the increase of soil borne pathogens such as those causing Fusarium and bacterial wilts. Watermelons are grafted on diverse rootstocks in many parts of the world primarily for managing Fusarium wilt (Fusarium oxysporum f. sp. niveum), which is a major limiting factor. The practice began in the 1920s in Japan, where today over 90% of the watermelons grown are grafted. Grafting has been reported to provide other benefits such as: tolerance to drought, high water table, low temperatures, and high winds; it is also known to provide improved nutrition uptake, increased plant vigor and yield, firmer fleshed fruits and the ability to thrive in a wide range of soils. There are several methods of grafting available such as: tongue approach grafting, hole insertion grafting, one cotyledon grafting, and side grafting. In some countries, robotic machines are available to help make the grafts. Currently researchers across several universities and the USDA are evaluating the benefits of grafting in the United States using commercially available rootstocks. However, at this point, it is not known how these rootstocks will respond to the diseases prevalent in the local production areas. Phytophthora crown and fruit rot caused by Phytophthora capsici is emerging as an important disease of watermelon in south eastern United States. We evaluated seventeen commercial rootstocks for tolerance to Phytophthora crown rot by inoculating them with a zoospore suspension consisting of a mixture of P. capsici isolates in the greenhouse. Several commercial bottle gourd (Lagenaria siceraria) hybrid rootstocks (e.g. Macis, Emphasis, FR-Strong, WMXP-3944, and WMXP-3938) were tolerant to Phytophthora crown rot when compared to susceptible watermelon controls (Mickey Lee). All the Cucurbita inter-specific rootstock hybrids evaluated (e.g. Strong Tosa, WR-15006 and WMXP-3943) were extremely susceptible to P. capsici. Similarly, the wild watermelon rootstock ‘Ojakkyo’ was also susceptible. Seedless watermelon grafted on bottle gourd rootstocks Emphasis or Macis appeared to be tolerant compared to susceptible watermelon cultivars. Real-time quantitative PCR using a SYBR green based assay indicated the presence of more P. capsici DNA in crowns of the susceptible Cucurbita inter-specific hybrid rootstocks and seedless watermelon, compared to the tolerant bottle gourd rootstocks. All the currently available bottle gourd or Cucurbita rootstocks we evaluated were susceptible to root knot nematode. We are now evaluating and developing bottle gourd and other cucurbit germplasm for resistance to P. capsici, root knot nematode, powdery mildew and other diseases to be used as rootstocks for grafting watermelon. The ultimate success of grafting as a tool to manage soil borne diseases of watermelon in the United States will depend upon the appropriate rootstock/scion combination used in any given location.