2017 ASHS Annual Conference

Crop rotation cycles for seedless watermelon produced in the US can be 3 years or longer, and are necessary to diminish the presence of soil borne diseases such as Fusarium by planting non-host crops (such as cotton in Arizona) between watermelon crop production years, potentially limiting the amount of watermelon a grower can produce. However, watermelon is a minor crop in Arizona and faces competition for use of arable land with other crops, making rotation more difficult. Grafting of cucurbits has been considered as a solution to allow production without rotation and Arizona melon industry recently began testing grafted watermelon in the field with limited rotation. In addition to soil-borne disease resistances, many rootstocks used for cucurbits, especially interspecific rootstocks, are chilling tolerant, while scion watermelon cultivars are highly sensitive. Watermelon transplants are traditionally planted in mid-late March in central western Arizona, sometimes with row cover to protect the young plants from chilling and wind damage. Grafting onto cold tolerant rootstock may provide additional advantage by advancing planting time to allow an earlier crop. During 2015 and 2016 we planted grafted and non-grafted seedless watermelon ‘Tri-X-313’ on an early date of March 1 to test if use of a cold tolerant rootstock ‘StrongTosa’ could compensate for low night and soil temperatures. Treatments included grafted and non-grafted plants combined with covering with frost protection and non-covered. The results of these two years showed grafted plants had more advanced vine growth and flower development, and 115% more cumulative yield (25.2±3.1 kg/plant or 50.6±6.3 tons/acre) than non-grafted plants (11.7±1.5 kg/plant or 23.5±3.1 tons/acre) by both increased number and size of fruits. No particular soil-borne disease incidence was observed during our trials. Grafting of seedless watermelon is a promising tool to potentially allow earlier planting and therefore earlier harvests, increased yields, as well as allowing reduced land rotation.