Triploid Watermelon Variety Evaluation, Spring 2002^[1]

Diploid (seeded) watermelons generally weigh from 18 to 35 lb and represent at least half of the commercial crop grown in Florida. Icebox watermelons weigh 6 to 12 lb each and are grown on a very small acreage. Triploid (seedless) watermelons usually weigh 15 to 22 lb and are grown in Florida on perhaps 40% of the acreage. The proportion of the Florida crop devoted to triploid production is increasing each year. Florida produced 8.6 million cwt of watermelons of all types from 24,000 harvested acres in 2000-2001, which provided an average yield of 310 cwt/acre. The average price was $5.70/cwt resulting in a crop value of over $42 million which accounted for 2.5% of the gross value of the state’s vegetable crops (Fla. Agr. Stat. 2002).

The concept of triploid (seedless) watermelons was first described in the U.S. literature by Kihara (1951) based on experimentation that began in 1939 in Japan. Seed for planting seedless watermelons results from a cross between a tetraploid female parent, developed by treating diploid lines with colchicine or by other means, and a diploid (normal) male parent. The resulting triploid plants are sterile and do not produce viable seed. However, small, rudimentary seeds develop which are eaten along with the flesh just as immature seeds are eaten in cucumber.

Fruit enlargement in seeded fruit, including watermelon, is enhanced by growth-promoting hormones produced by the developing seed. Growth hormones are lacking in seedless watermelons so those agents must be provided by pollen. Since flowers of triploid plants lack sufficient viable pollen to induce normal fruit set, diploid seeded watermelons are interplanted with triploids to serve as pollenizers. An adequate bee population is necessary to insure that sufficient transfer of pollen occurs. Seedless fruit (from triploid plants) tend to be triangular shaped without sufficient pollination.

Although the procedure for production of seedless watermelons has been known for about 50 years and commercial varieties have been available for many years, the interest in and acreage of seedless watermelons has remained small in Florida until recently. Erratic performance, poor seed germination, high seed costs, and inadequate varieties resulted in lack of interest in seedless watermelon production in the past, but most of the deterents have now been overcome. It is estimated that seedless watermelons now represent about 40% of the total production in Florida.

Specialty vegetables are in high demand and seedless watermelons offer an attractive alternative for discriminating consumers and the food service industry. Seedless watermelons are being actively promoted by marketing organizations and seed companies to stimulate demand. At the same time, new varieties are being developed that are superior to those previously available. Seedless watermelons have been evaluated at this location annually since 1988.

The objective of this trial was to evaluate the performance of triploid watermelon cultigens under west-central Florida conditions.

Seeds of 42 triploid watermelon varieties or experimental hybrids (Table 1) were planted in a peat-lite growing mix in planter flats (1 ¼ x 1 ¼ x 2 ¼ in. cells) on 22 January. The watermelon transplants were grown by a commercial plant grower.

Soil samples from the experimental area obtained before fertilization were analyzed by the University of Florida Extension Soil Testing Laboratory (Hanlon and Devore, 1989): pH = 7.2 (target pH is 6.0) and Mehlich I extractable P = 105 ppm (very high), K = 12 ppm (low), Mg = 54 ppm (high), Ca = 552 ppm (adequate), Zn = 9.7 ppm (adequate), Cu = 4.4 ppm (adequate), and Mn = 4.9 ppm (adequate).

The EauGallie fine sand was prepared in late January by incorporation of 0-0.8-0 lb N-P₂O₅-K₂O per 100 linear bed feet (lbf). Beds were formed and fumigated with methylbromide:chloropicrin, 67:33 at 2.3 lb/100 lbf. Banded fertilizer was applied in shallow grooves on the bed shoulders at 3.1-0-4.3 lb N-P₂O₅-K₂O/100 lbf after the beds were pressed and before the black polyethylene mulch was applied. The total fertilizer applied was equivalent to 150-40-208 lb N-P₂O₅-K₂O/A. The final beds were 32-in. wide and 8-in. high, and were spaced on 9-ft centers with four beds between seepage irrigation/drainage ditches, which were on 41-ft centers.

Transplant return, the percentage of seeds that developed into acceptable transplants, was determined for each entry. The transplants were set in holes punched in the polyethylene mulch at 3-ft in-row spacing on 1 March. The replicated plots were 24 ft long and had eight plants each and were repeated three times in a randomized, complete block design. Diploid watermelons that were being evaluated were direct seeded in beds on each side of two triploid watermelon beds on 20 February to serve as diploid pollenizers. Plant stands recorded just before vines grew together showed no significant differences among plots. Weed control in row middles was by cultivation and applications of paraquat. Pesticides were applied as needed for control of gummy stem blight (chlorothalonil, azoxystrobin, and maneb), and lepidopterous larvae (Bacillus thuringiensis and spinosad).

Watermelons were harvested twice during the 17-23 May and 30 May - 5 June periods. Marketable (U.S. No.1 or better) fruit according to U.S. Standards for Grades of Watermelons (U.S. Dept. Agr., 1978) were separated from culls and counted and weighed individually. Fruit 12 lbs and larger were assumed to be marketable. Tetraploid fruit, where they occurred, were not included in the marketable category because they are not seedless. Six fruit from each entry at each harvest were used to determine soluble solids (a measure of sweetness) with a digital, hand-held refractometer, polar and equatorial dimensions, rind thickness, flesh color, and the incidence and severity of hollowheart. Where possible, data were subjected to analysis of variance and mean separation was by Duncan’s multiple range test.

Maximum temperatures (Table 2) were higher than average in March and April and minimum temperatures were above average in March, April, and May. This resulted in an earlier than usual crop. Rainfall was near normal throughout the period.

Transplant return ranged from 43% in HSR 2402 to 100% in HA 6033 (Table 3). Transplant return was 90% or higher in 30 of the 42 entries. Differences in seed performance may be related to seed quality as influenced by production techniques, seed storage, or characteristics of the individual hybrid.

Total yields (Table 3) ranged from 375 cwt/acre for ‘Amarillo’ to 1253 cwt/acre for HA 6033. Only one other entry produced yields significantly similar to those of HA 6033. Average fruit weight for the entire season varied from 14.8 lbs for ZG 8825 to 22.9 lbs SW 493. The number of fruit per plant ranged from 1.5 for ‘Amarillo’ to 3.6 for HA 6033. Soluble solids concentrations varied from 11.9% for HSR 2402 to 13.9% for HA 6033. Accordingly, soluble solids in all entries far exceeded the 10% specified for optional use in the U.S. Standards for Grades of Watermelons to describe very good internal quality (U.S. Dept Agr., 1978). The incidence of hollowheart in the fruit sampled ranged from 0% in 11005031, ‘Omega’, ‘Trillion’ and XWT 8706 to 75% in HSR 2402.

The distribution of fruit into weight classes is shown in Table 4. When triploid fruit are packed in cartons, there is an advantage in having a high proportion of fruit in the 15 to 18 lb category. ‘Red Sweet’, ‘Revolution’, ‘Slice & Serve 830', ‘Sugar Slice’, Super Seedless 7167, ‘Sweet Slice’, ‘Tri-X-Carousel’, ‘Tri-X Palomar’, and ZG 8820 produced over 40% of their fruit in this weight range. Fruit are graded into two or more sizes when they are shipped in bins. Large fruit are useful for food service or as a precut product. For example, HA 6033, ‘Samba’, and SW 493 produced 52, 44, and 60% of their fruit, respectively, that weighed more than 22 lbs whereas many entries produced smaller fruit.

Seedless watermelon variety trials have been conducted at this location each spring season since 1988. The highest yields ranged from 507 cwt/acre in 1996 to 1253 cwt/A in 2002 which greatly exceeded the 871 cwt/acre average high during the entire 15-year period.

Variety shape and rind patterns and flesh color, based on observations in this trial, are shown in Table 1. Varieties producing oval to oblong fruit may be more suitable for boxing than varieties producing round melons. Generally, the striped melons are more attractive for the U.S. market than those with dark stripes on a very dark green background, or those with a solid dark green rind. Flesh color was outstanding in ‘Fandango’, ‘Freedom’ and ‘Seedless Sangria’.

Based on results of this and previous trials, triploid hybrids, in alphabetical order, that should perform well in Florida include ‘Freedom’, ‘Genesis’, ‘Millionaire’, ‘Omega’ (for trial), ‘Revolution’, ‘Seedless Sangria’, ‘Sugar Shack’ (for trial), Super Seedless 7167, 7177, 7187 (for trial), ‘Summer Sweet 5244’, ‘Summer Sweet 5544’, ‘Tri-X 313’, ‘Tri-X Carousel’, ‘Tri-X Palomar’, and ‘Tri-X Shadow’. ‘Triton’, a yellow-flesh variety should be evaluated for that niche market. Other varieties may perform well on individual farms.

The information contained in this report is a summary of experimental results. No discrimination is intended and no endorsement is implied where trade names are used.

We are grateful to the following firms for their financial and material support of vegetable variety evaluation during 2001-2002. Abbott & Cobb; Agrisales, Inc.; BHN Research; Fafard Inc.; Harris Moran Seed Co.; Hazera Quality Seeds; Hollar Seeds; Paramount Seeds, Inc.; Sakata Seed America; Shamrock Seed Co.; Seminis Vegetable Seeds, Inc.; Southwestern Vegetable Seed Co.; Sugar Creek Seeds, Inc.; Sunseeds; Syngenta Seeds; U.S. Seedless, LLC; Willhite Seed, Inc.; and Zeraim Gedera Ltd.

Florida Agricultural Statistics Service. 2002. Vegetables, acreage, production, value. Orlando, Fla.

Hanlon, E. A. and J. M. DeVore. 1989. IFAS extension soil testing laboratory chemical procedures and training manual. Fla. Coop. Ext. Circ. 812.

Kihara, H. 1951. Triploid watermelons. Proc. Amer. Soc. Hort. Sci. 58:217-230.

U. S. Dept. Agr. 1978. U.S. standards for grades of watermelons. AMS, Washington, D.C.

Florida Agricultural Experiment Station Journal Series No. T-

[1] GCREC-Bradenton Research Report BRA-2002-3.  This research was supported by the Florida Agricultural Experiment Station and approved for publication as Journal Series No. T-00563

[2] Professor

^[3] Biological Scientist

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