Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] cultivars exhibit diverse phenotypic traits, yet are derived from a narrow genetic base. Heirloom cultivars, and to a lesser extent modern open-pollinated (OP) cultivars, are perceived to contain vital genetic variation that is critical for biodiversity conservation and crop improvement. The objective of this study was to characterize the diversity of six heirloom and open-pollinated watermelon cultivars that are popular among U.S. organic, direct-market, and home gardeners. An additional evaluation was conducted to determine whether significant phenotypic and genotypic variation existed among seed lots sourced from different commercial seed vendors. Important horticultural traits such as days to germination, days to first flower, yield, and fruit quality were measured over two field seasons. Genetic diversity was estimated using 32 simple sequence repeat (SSR) markers. Significant differences in horticultural traits among seed lots in both years were observed only in days to germination and first male flower, which may be a consequence of vendor differences in seed storage and quality control. Heirloom ‘Moon and Stars’ and modern OP ‘Sugar Baby’ were the most genetically distinct from the other cultivars and heirloom ‘Georgia Rattlesnake’ was determined to be highly related to the modern OP ‘Charleston Gray’. The two heirloom cultivars were observed to have lower average gene diversity than the modern cultivars. Heirloom ‘Moon and Stars’ contained significant genetic variation among seed lots, yet heirloom ‘Georgia Rattlesnake’ contained none. These findings suggest that genetic variation can be more accurately attributed to pedigree and foundation seed maintenance practices than to the “heirloom” designation per se. The variation reported in this study can be used to inform conservation and breeding efforts.
Suzanne Stone, George Boyhan and Cecilia McGregor
George E. Boyhan, Suzanne O’Connell, Ryan McNeill and Suzanne Stone
Organic production is a fast-growing sector of agriculture in need of variety evaluations under their unique production systems. This study evaluated 16 watermelon (Citrullus lanatus) varieties for their performance characteristics under organic production practices. Plants were grown on plastic mulch-covered beds on land that had been certified organic in accordance with the U.S. Department of Agriculture National Organic Program. Six of the entries were F1 hybrids; the remaining entries were open-pollinated (OP) varieties. Of the 10 OP varieties, three were considered heirloom varieties, including Cream of Saskatchewan, Georgia Rattlesnake, and Moon & Stars. ‘Georgia Rattlesnake’ was the highest yielding variety and had the greatest average fruit weight. Along with ‘Georgia Rattlesnake’, ‘Nunhems 800’, ‘Nunhems 860’, ‘Orangeglo’, and ‘SSX 8585’ were included in the top five yielding varieties. The top five yielding varieties had fruit size that averaged more than 20 lb. Fruit size correlated with rind thickness, with lighter fruit having thinner rind (Pearson’s correlation, r = 0.779), which is not unexpected. ‘Sangria’ had the greatest average soluble solids content at 11.2%, which was greater than all entries with soluble solids less than 10%.
George E. Boyhan, Julia W. Gaskin, Elizabeth L. Little, Esendugue G. Fonsah and Suzanne P. Stone
Certified organic production is challenging in the southeastern United States due to high weed, insect, and disease pressure. Maintaining and building soil organic carbon in midscale organic production systems can also be difficult due to the warm, moist conditions that promote decomposition. Focusing on cool-season cash crops paired with warm-season cover crops may help alleviate these production problems. This 3-year study (2011–13) evaluated two vegetable rotations of cool-season crops with cover crops for their productivity, disease management, and soil building potential in Watkinsville, GA. In the first rotation, cool-season cash crops included onion (Allium cepa), strawberry (Fragaria ×ananassa), and potato (Solanum tuberosum). These crops were rotated with green bean (Phaseolus vulgaris), oats/austrian winter pea (Avena sativa/Pisum sativum ssp. arvense), southernpea (Vigna unguiculata), and sunn hemp (Crotalaria juncea). In the second rotation, cool-season cash crops included onion, broccoli (Brassica oleracea Italica group), lettuce (Lactuca sativa), and carrot (Daucus carota ssp. sativus). These were rotated with millet (Urochloa ramosa), sunn hemp, egyptian wheat/iron clay pea (Sorghum sp./Vigna unguiculata), and sorghum × sudangrass (Sorghum bicolor × S. bicolor var. sudanese)/iron clay pea. Onion yields in both rotations were at least 80% of average yields in Georgia. Lettuce yields were at least double the average yields in Georgia and were comparable to national averages in the 2nd and 3rd years of the study. Strawberry yields in these rotations were lower than Georgia averages in all 3 years with a trend of lower yields over the course of the study. By contrast, potato, although lower than average yields in Georgia increased each year of the study. Broccoli yields in the first year were substantially lower than average Georgia yields, but were comparable to average yields in the 2nd year. Carrot remained less than half of average Georgia yields. Green bean were half of average Georgia yields in the 2nd year and were comparable to average yields in the 3rd year. As expected from what is observed in cool-season organic vegetable production in Georgia, disease pressure was low. Cover crops maintained soil organic carbon (C) with a small increase in active C; however, there was a net loss of potentially mineralizable nitrogen (PMN). Active C averaged across both rotations at the beginning of the study at 464 mg·kg−1 and averaged 572 mg·kg−1 at the end of the study. On the basis of this study, using cover crops can maintain soil carbon without the addition of carbon sources such as compost. Finally, longer term work needs to be done to assess soil management strategies.
Suzanne P. Stone, George E. Boyhan and W. Carroll Johnson III
The southeastern United States produces 50% of U.S. conventional watermelon (Citrullus lanatus) but only 7% of U.S. organic watermelon. Weeds are a major threat to watermelon yield in the southeastern United States, and organic weed control is estimated to cost 20-times more than conventional herbicide programs. The objectives of this study were to determine the optimal weed control regime to reduce hand-weeding costs while maintaining yield and to compare the weed suppression of two watermelon types with differing growth habits in an organic system. In 2014 and 2015, watermelon plots were randomly assigned to the following treatments in a factorial arrangement: vine or compact growth habit; 1.0- or 0.5-m in-row spacing; and weekly weed control (kept weed-free by hoeing and hand-pulling weeds) for 0, 4, or 8 weeks after transplanting (WAT). At the time of the watermelon harvest, not weeding resulted in average total weed densities of 86.6 and 87.0 weeds/m2, and weeding for 4 WAT resulted in average total weed densities of 26.4 and 7.0 weeds/m2 in 2014 and 2015, respectively. Nonetheless, weeding for 4 WAT resulted in watermelon yields and fruit counts comparable to those of weeding for 8 WAT during both years. This partial-season weeding regime resulted in 67% and 63% weeding cost reductions for vine and compact plants, respectively, in 2014, and a 43% reduction for both growth habit types in 2015. In 2015, a separate experiment that evaluated weeding regimes that lasted 0, 1, 2, 3, 4, and 8 WAT found that yields resulting from weeding for 3 WAT were greater than those resulting from weeding for 2 WAT. However, the yields did not differ when weeding was performed for 4 WAT and 8 WAT.