Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Research on grafted watermelons has expanded in the last 5 years on the US East Coast. There are two main reasons for using grafted watermelon plants: 1) to manage soil-borne diseases such as Fusarium wilt and 2) to increase plant productivity by providing a more vigorous root system.
Grafted watermelons are widely used throughout the other major watermelon producing areas of the world including southern Europe, the Middle East, and the Far East. However, use in the US has been limited, largely due to wider availability of land for production and the added cost of grafting plants. Grafted plants are currently 4 times more costly than non-grafted plants.
Watermelon are commonly grafted onto interspecific hybrid squash (Cucurbita maxima x Cucurbita moschata) or bottle gourd (Lagenaria siceraria). These will have Fusarium wilt resistance. Interspecific hybrid squash rootstocks have chilling tolerance in addition to the disease resistance, but some rootstocks may be so vigorous that they delay flowering if fertilization is not managed properly. Another issue with interspecific squash rootstocks in some cases is a reduction in watermelon sugar content (Brix) as well as off flavors. Bottle gourd rootstocks have chilling tolerance and less vigor then squash rootstocks, and have little effect on fruit quality or flowering.
Seedless watermelon plant grafted onto interspecific squash rootstock. Arrow points to the graft union and plastic grafting clip.
Close up of graft union. Watermelons are more difficult to graft than other vegetables.
Neither bottle gourd nor interspecific squash rootstocks have root knot nematode (RKN) resistance and are very susceptible to RKN injury. Work at the USDA in Charleston, SC has identified some wild watermelon rootstocks with root knot resistance and these may be available as rootstocks in the future.
With wider availability of grafted watermelons, costs per plant have been reduced. Studies also have shown that productivity of grafted watermelons is 30-50% higher than non-grafted plants, that grafted plants can be planted at two thirds of the population of non-grafted plants to achieve those yields and that grafted plants require much less fertilizer to produce those results. Partial budget economic analyses have shown economic advantages using grafted watermelon plants. In one analysis, the above effects lead to a net change in profit of over $1300 per acre using grafted plants.
In 2016 trials in Delaware, the seedless variety “Fascination” that was grafted using interspecific Cucurbita rootstock, and planted at 78% of population of ungrafted Fascination, yielded 22% higher. Fruits were heavier and there were significantly more fruits in the second and third harvests compared to ungrafted Fascination.
2017 trials will look at reducing populations further and reducing nitrogen fertilizer with grafted plants.