Nov 29, 2024

UGA trials examine variety performance to help improve watermelons

With consumers going from seeded to seedless watermelons and desiring darker rinds and smaller size melons, breeders are upping their game and producing better varieties that aren’t seeded, too big or light- skinned and possess characteristics other than disease resistance.

Since 2021, Ted McAvoy, assistant professor of horticulture and vegetable Extension specialist for the University of Georgia, has researched large and mini seedless watermelons in variety trials for yield, size distribution and defects, and screens varieties for anthracnose disease resistance. He also studies grafted watermelon plant spacing.

“We can find varieties that adapt through changing our planting dates, selecting varieties that are more adapted to our area and coming up with production practices with fungicides and varieties to handle diseases that may be exacerbated by different weather conditions,” McAvoy said.

At the UGA Cooperative Extension College of Agricultural & Environmental Sciences campus in Tifton, Georgia, McAvoy conducts trials testing many commercial varieties.

Historically, one seed company dominated the watermelon market. When that company began selling direct and ending its distributor relationships, that left a gap for distributors that needed variety trial information, McAvoy said.

The trials confirm what growers have been seeing in the fields. “The big picture is pretty much all the seed companies have a competitive melon now and it’s not dominated like it was in the past,” he said.

Variety performance

For large size melons, McAvoy recommends Excursion, Sierra Nevada and El Capitan, varieties that produce 30- to 36-count sizings. For smaller sizings, the 45- to 60-count melons, Miramonte and Jet Ski varieties are recommended.

“The varieties have performed well for two years in a row,” he said. “The good thing about big melons is they have large, bigger melons and in general, larger weight per acre. But, on the flip side, the growers are not getting good prices. They might be too big in actuality.”

Because anthracnose has developed resistance to the Quadris fungicide, disease resistance screenings show anthracnose is becoming a bigger problem for Georgia watermelons.

A USDA grant is funding involving researchers in Florida, Georgia, South Carolina and New York exploring management methods. Pathologists, including Bhabesh Dutta, UGA plant pathology professor and Extension vegetable disease specialist, are looking at fungicide screenings and collecting isolates to determine if the disease populations are fungicide resistant populations.

Last year was the first year of trials, which include known susceptible and resistant seeded and seedless varieties to determine which varieties perform best. Extension personnel are screening varieties and evaluating irrigation management practices for anthracnose control. While analysis isn’t complete, McAvoy said the tests show differences in varieties for anthracnose occurrence and severity.

That study is also examining irrigation and fungicides for cucumber varieties. Anthracnose on cucumbers is a separate pathogen species than anthracnose on watermelon. McAvoy said he hopes the research will produce recommendations on varieties and irrigation scheduling growers can utilize.

Georgia growers produce watermelons on small narrow plastic beds during warm spring temperatures for earlier production on agronomic fields, large scale production of cotton and peanuts, and are irrigated via the same overhead systems. Researchers are examining irrigating in the morning, midday or at the end of the day. A hypothesis holds if water is applied at the end of the day, disease could increase through moisture remaining on the leaves through the night.

Spacing factors

Because grafted watermelons are four to five times more expensive per plant than non-grafted plants, McAvoy is studying how more expensive grafted plants can be planted further apart without reducing yields.
Many growers will grow on a configuration of 8 feet between rows and 3-4 feet between plants, but at McAvoy’s farm, he grows on 6 feet between rows. On 6-foot centers, McAvoy placed 4-8 feet spacing between the plants in one foot increments. He decreased spacing by half at the largest spacing of 8 feet between plants.

Typically, Extension recommends 24 square feet per plant. McAvoy tested the grafted plants at his standard planting density of 1,815 plants/acre as well as reduced planting densities of 1,452, 1,210, 1,037 and 907 plants/acre.

Preliminary findings show wider spacings that use fewer plants per acre are feasible in reducing per-acre planting costs. McAvoy said he’s not seeing a reduction in yields or size.

“These are promising preliminary results where it seems like we may be able to reduce plant populations without reducing yields or having an effect on fruit size,” he said. “Basically, these plants were able to compensate.”

Besides disease resistance, many of the rootstocks are more vigorous and provide larger plants and better root systems that consume water and nutrients more efficiently.

Dutta is working with grafted watermelon to control fusarium wilt, using resistant rootstocks and then grafting the seedless watermelon on top as the scion. Researchers have examined citron and squash rootstocks for fusarium wilt resistance.

They discovered Carolina strongback, a citron, appears to be growers’ best option to manage fusarium wilt.

“Some of the other squash rootstocks are resistant against fusarium, but they’re also very susceptible to root knot nematodes, whereas the citron Carolina strongback seems like it performs well against fusarium wilt,” McAvoy said.

As opposed to Florida’s hot and humid weather that attracts low levels of numerous diseases, Georgia diseases are seasonal. McAvoy notes cool and dry springs don’t attract many foliar diseases until later, typically in June when the region begins receiving summer rains. Early season fungicides mainly consist of weekly applications of Bravo chlorothalonil. Toward the end of season, growers must control gummy stem blight, phytophthora, downy mildew — diseases that like moisture — before switching to heavier fungicides.

“In general, up until June, you could almost not apply anything and not have a disease because the weather conditions aren’t right and we don’t have the spores,” McAvoy said.

Disease is the primary concern of watermelon. Because of Georgia’s cooler temperatures and drier climate, the state attracts pests like aphids, thrips and squash bugs, which are not big watermelon threats and are typically easily managed.

“We don’t encounter insecticide resistance in these insects like we do on other crops,” he said. “Insects are relatively easy to control and with low pest pressures, especially in the spring. Now, fall — forget it, as whiteflies eat our lunch.”

Written by Doug Ohlemeier, Assistant Editor