Grafting Watermelon for Managing Fusarium Wilt in Southern Maryland
Fusarium Wilt, caused by the soil borne pathogen Fusarium oxysporum f. sp. niveum is becoming more problematic in seedless watermelon production in Southern Maryland. Unfortunately, there are few effective management options for this soil borne disease. New races of Fusarium Wilt are now present in the area that can overcome traditional cultivar resistance. Effective fungicides are limited and do not provide season-long control at labeled rates. In many cases, once a field is infested with Fusarium Wilt, watermelon production is no longer a viable option. One option that has been effective in other areas is grafting of susceptible cultivars onto fusarium resistant rootstocks of interspecific hybrid squash or citron species. Watermelon grafting is more difficult than tomato grafting and is normally done by outside companies who specialize in the technique.
During the 2020 growing season, a field research and demonstration trial was conducted at three farms with a history of fusarium wilt to evaluate the efficacy of grafting for fusarium management. Grafted plants of the seeded cultivar Jubilee were used as the pollinizer cultivar and seedless cultivar Fascination were used as the seedless cultivar. Tri-Hishtil (25 School House Rd, Mills River, NC 28759 (P) 828-620-5020), a commercial firm in North Carolina specializing in grafting donated the plants for the trial. Both Jubilee and Fascination were grafted to either an interspecific squash rootstock or “Carolina Strongback” Citron rootstock. At each location, three non-grafted Jubilee and three non-grafted Fascination plants were planted at four or five replications (locations) throughout the field and flagged. Each farm used conventional management practices, including black plastic mulch, drip irrigation, and fertigation. Each site used different in-row and between-row spacing.
The 2020 season was not favorable for main season watermelon production. The season began normal enough, though late-season frost in May resulted in damage to early planted fields. All trial fields were planted after any frost and established well. Unfortunately, rains began in July and continued with the remnants of Hurricane Isaias dropping up to 12 inches of rain over the region in early August. Rains continued through the harvest period. Rain totals for the season were recorded at 80.26 inches, a record for the year and far beyond the normal of 45 inches per year. As a result of the extremely wet season, Phytophthora fruit and root rot was widespread, particularly on two of the farms with heavier Beltsville silt loam soil. The third location was located on sandy loam soil and did not exhibit heavy phytophthora losses through most of the field.
Grafted and non-grafted plants were examined throughout the season. Data on root viability, root knot nematode presence, and vine condition was able to be recorded.. At harvest, 3 plants of cultivar/rootstock combination from each replication was evaluated for viable root count, vine condition, root knot nematode presence and other comments. Due to phytophthora presence, yield data was only collected from one farm. Yield data from plants grafted to Citron and plants grafted to interspecific squash was collected from sections representing a 175 sq. foot area (25 feet long by 7 feet wide). The section harvested included one Jubilee plant and 4 Fascination plants. All fruit was picked and weighed. The yield was collected from four representative areas throughout the field. Misshapen or non-marketable immature small fruit were not tallied in total yield. Yield data was not collected from nongrated plants as those plants had diminished root systems and did not yield marketable fruit. Fruit quality data was also collected at this site from a representative subsample of fruit harvested during yield evaluation. These fruits were quartered and data including fruit weight, rind thickness, fruit length and width, pH, Brix, and comments on taste and other fruit features such as hollow heart or pips was recorded.
What we found
Plants of either cultivar grafted to either interspecific squash or citron rootstock performed better than non-grafted plants in terms of viable roots present at harvest and vine condition. Fusarium Wilt was confirmed in non-grafted plants after vine run. Grafted plants did not exhibit any symptoms of Fusarium Wilt throughout the season at any location. However, grafted plants were susceptible to phytophthora root and fruit rot. Foliar diseases including powdery mildew and gummy stem blight were also present on grafted and non-grafted plants. On average across all three sites, the Fascination plants grafted to either the citron or hybrid squash rootstocks exhibited 96.7% healthy viable roots at harvest compared to only 6.7% healthy viable roots for Fascination own-rooted plants. On average across all three sites, the Jubilee plants grafted to citron had 69.2 % healthy roots, and plants on hybrid squash rootstocks exhibited 74.4% healthy viable roots compared to only 7.7% healthy viable roots for Jubilee own-rooted plants.
Root Knot Nematodes (RKN): Plants were evaluated at harvest for root galling with a value of 0 being no galling present and 100 being severe infestation with all root stems affected. Root knot nematode was only present on one farm with sandy loam soil. The plants from either rootstock on the farms without nematodes present did not show RKN symptoms. However, on the infected farm, the interspecific squash rootstock exhibited severe root galling, with an average rating of 100 for Fascination and 75 for Jubilee. The Citron rootstock did not exhibit any root galling with an average rating of 0. In terms of yields, both rootstocks performed well. One explanation may be the high level of attention and management for this field with fertigation and irrigation conducted as needed on a daily basis. Even with severe galling, interspecific squash were able to take up needed water and nutrients to achieve high yields.
The average yield of the citron rootstock was 78.3 tons per acre whereas the average yield of the interspecific squash rootstock was 73.8 tons per acre. Therese yield differences are not significantly different. Own rooted plants did not maintain viable root systems and thus yield data was not collected. The other consequence of poor root systems is the vines from grafted plants eventually “overran” the non-grated plants in the fields.
This article appears on April 15, 2021, Volume 12, Issue 1 of the Vegetable and Fruit News