This article addresses considerations of using grafted watermelon plants in Indiana.
Choosing rootstocks
Interspecific hybrid squash is the most widely used watermelon rootstock. Many cultivars of the hybrid squash rootstocks that have been developed by different companies, share similar genetic background and thus similar characteristics. All of them have outstanding resistance to Fusarium wilt, but, unfortunately, highly susceptible to root-knot nematode. A newly developed ‘Carolina Strongback’ rootstock from a wild watermelon, citron, has resistance to both Fusarium wilt and root-knot nematode, providing a much better option when both pathogens are present in one field.
Using grafted watermelon plants is increasingly being accepted if a field has a known history of Fusarium wilt and, perhaps, root-knot nematode. However, the disease history of a field is generally unknown in most cases. In the absence of diseases, do grafted plants have a higher yield than nongrafted plants? This is a question most growers would ask considering the cost of grafted plants is much higher than nongrafted watermelon transplants.
Do grafted plants have higher yield without Fusarium wilt pressure?
Many factors play a role, thus there isn’t a simple answer to this question. Environmental condition is one of them. We saw the most dramatic yield increase of using grafted plants in a field without Fusarium wilt pressure in 2019. 2019 season was marketed with high precipitation. In our research plot, soil (sandy loam) water contents ranged from 0.25 to 0.30 m3/m3 in the entire vegetative growth and fruit development stages. Watermelon plants do not perform well under the wet condition, but it seems this was not the case for the grafted plants. Grafted plants increased yield by 60% compared to the yield of non-grafted plants. Comparing between the rootstocks, watermelons grafted onto squash interspecific hybrid had a slightly higher yield (not significant) compared to the ‘Carolina Strongback’ rootstock.
It has been observed in many cases that grafted plants delayed fruit set. But the vigorous and healthy vines and strong root systems can support a longer harvest, which could contribute to a higher overall yield (Figure 1). In the above experiment conducted in 2019, the plants were harvested for one and a half months, and the yield advantage of using grafted plants was more pronounced in later harvest events.
Figure 1. Vine vigor of non-grafted ‘Fascination’, and ‘Fascination’ grafted onto interspecific hybrid squash rootstock and ‘Carolina Strongback’ rootstock in the 2019 experiment.
In 2020, our results of the same experiment were opposite. Yield of non-grafted plants were higher than that of grafted plants regardless of rootstocks. An important reason is due to an influence of a combination of Phytophthora blight and late-season vine decline, which destructively ended the trial early. The take-home message from this story is that growers who use grafted plants should be very careful in controlling diseases and pests in order to support an extended crop season and longer harvest. If severe pest problems result in a short season, growing grafted plants may not have a positive effect on yield or may even have a negative effect. Another important consideration when growing grafted plants is fertilizer application. Using fertigation rather than applying fertilizers all preplant is more favorable for grafted plants as fertigation reduces the risk of plant nutrient deficiency toward the later part of the season.
Can planting density be reduced to one third when growing grafted plants?
This recommendation was developed partially from an economic standpoint by comparing cost of using grafted plants to cost of using soil fumigation in controlling Fusarium wilt. It would be less expensive for using soil fumigation to control Fusarium wilt with the typical plant spacing (around 1800 plants/acre) and current prices of grafted plants and soil fumigation. Cost of the two disease-control practices is becoming comparable if plant number reduced to 1/3 or more.
Do experimental data support this statement? Across experiments conducted by several research groups including the ones conducted at SWPAC, we noticed the yield of grafted plants was generally maintained when plant number was reduced. But interestingly, if the non-grafted watermelon was included in the experiments, their yield normally was also maintained when plant number reduced. What these results tell us is that it might be safe to use fewer plants per acre. Growers may be reluctant to reduce the plant number because there are so many uncertainties that could cause plant loss after planting. However, when using grafted transplants that cost 3-5 times more than normal watermelon transplants, it makes sense to cut the plant number, and reducing the number to 1/3 is a reasonable choice.
Do grafted watermelon plants better survive transplant shock?
A favorable feature of the grafted watermelon plants, especially the ones grafted with hybrid squash rootstock, is a better tolerance to low temperatures comparing with normal watermelon plants. Because it is not unusual to have cold days after planting, this is a great feature for watermelon production in Indiana. To maximize the benefits of using grafted watermelon plants, we recommend planting grafted plants early, assuming last frost has passed. Grafted plants are more likely to survive soil temperatures in the lower 50 °F range than nongrafted plants. If soil temperatures remain below 70 °F for an extended period, grafted seedless watermelons grow faster than non-grafted ones.
How about the quality of fruit from grafted watermelons?
Last but not least, I want to address the question about grafting effects on fruit quality. There are many researches targeting different aspects of fruit quality such as size, appearance, sugar content, acidity, firmness, color, antioxidants, aroma volatiles, etc. I would like to highlight a few points here that I think growers should be aware of and take into consideration when growing grafted watermelon plants. Firstly, grafted plants may delay fruit set that results in delayed fruit ripeness, especially when hybrid squash rootstock was used. If fruits are harvested before it is fully ripe, many quality attributes are sacrificed regardless of whether the fruit is from grafted or non-grafted plants. When the price is high, there is always a temptation to start harvest as early as possible. There is nothing wrong with this approach, but additional caution should be applied when harvesting grafted plants. It is always a good idea to cut a few fruit open to fully evaluate the ripeness especially for fruit from the grafted plants. On the other hand, if multiple fields are ready to be harvested but the harvesting crew is limited, it is okay to harvest the grafted field the last. Studies have found fruit from grafted plants can stay longer in the field, which might be associated with a firmer flesh of these fruits. Grafting may also affect fruit size, studies have found fruit from grafted plants tend to be bigger compared to fruit from non-grafted plants. If the market has a strict requirement for 45-count-sized fruit, growers should avoid using varieties that tend to develop large fruit as the scion variety.