What Have We Learned From Two Decades of Onion Cultivar Research at Penn State?

Pennsylvania Onion Production

Many Pennsylvania onions are marketed through the PA Simply Sweet Onion Program, the state’s only trademarked crop, established in 2002 by the Pennsylvania Vegetable Growers Association (PVGA) in collaboration with Dr. Mike Orzolek at Penn State. There are also an increasing number of non-program onions being grown for direct retail markets, including roadside stands, farmer’s markets, or CSAs.

Onions marketed in the PA Simply Sweet program are yellow or red, intermediate day, Spanish-type onions that meet two criteria:

1. A minimum bulb size of 2.5 inches in diameter (medium, jumbo, and colossal-sized bulbs)
2. High sugar (at least 6% soluble solids)

Low pungency (concentrations between 1-4 μmol pyruvic acid/kg bulb tissue) is also a desired trait of sweet onions, but not a required standard of the PA Simply Sweet program. The PA Simply Sweet program has approved cultivars that meet these criteria, but the predominant cultivar grown is currently Candy. The onion transplants are primarily sourced from producers in Arizona or Texas or locally from growers who start their seeds in greenhouses in December–January. In recent years, cv. Candy has become increasingly difficult to source from transplant producers, so growers are starting to shift to growing the cv. Expression as an alternative.

Onion Cultivar Evaluations

Fortunately, Penn State researchers have spent two decades evaluating over 70 alternative yellow, sweet Spanish cultivars for the PA Simply Sweet program, thanks to the efforts of Dr. Mike Orzolek. Of these cultivars, Expression, Exacta, Spanish Medallion, and Aruba consistently produced high yields, high sugars, and low pungency comparable to Candy (Table 1). Tom Butzler, Penn State Extension, collaborated with Orzolek in 2018 and 2019 and then continued the evaluation in 2020 with Dr. Tim Elkner, Penn State Extension, at the Russell E. Larson Agricultural Research Center (Rock Springs) in Centre County and a commercial grower field in Lancaster County, PA. The ten cultivars with the highest yields from the Lancaster County location are presented in Table 2, many of which produced higher yields than Candy.

Table 1. The average marketable yield, bulb weight, pungency, and % sugar of the top five onion cultivars evaluated by Orzolek at the Horticulture Research Farm at Rock Springs over the past 15 years.

Table 2. The average marketable yield, % jumbo and colossal-sized bulbs, pungency, and % sugar of the top 10 onion cultivars evaluated by Butzler and Elkner in 2020 in a commercial grower’s field in Lancaster Co. and Rock Springs in Centre Co., PA.

* Yield and % jumbo and colossal sized bulb data not presented for Centre County due to poor planting conditions that resulted in significant losses.

Cultivar Susceptibility to Center Rot Evaluations

Another area of interest was onion cultivar disease tolerance or resistance. Penn State researchers Jennie Mazzone and Dr. Beth Gugino evaluated 13 onion cultivars in 2015 and 2016 for center rot disease susceptibility and selected horticultural characteristics. Center rot, caused by the pathogens Pantoea ananatis and P. agglomerans, is considered a significant bacterial disease of onion in PA and other onion growing regions in the U.S. (Figure 1).

Figure 1. Characteristic bleaching of interior onion leaves and individual scale necrosis caused by center rot. Photo: Jennie Mazzone, Penn State

Since cv. Candy is very susceptible to center rot, Mazzone and Gugino’s two-year study aimed to identify cultivars less susceptible to center rot while still meeting the criteria of the PA Simply Sweet program. At harvest, cvs. Blush, Red Sky, Dulce Reina, Expression, Crockett, Spanish Medallion, and Sedona had lower disease incidence than Candy (Table 3). Of these cultivars, only Expression and Spanish Medallion had high yields, large bulbs (jumbo and colossal sized), low pungency, and high sugar, similar to Candy (Table 3). Cultivars Blush, Red Sky, Dulce Reina, and Crockett had considerably lower yields and bulb sizes than Candy. After three months of storage, no cultivar had a lower disease incidence than Candy, showing that the standard Candy remained competitive post-harvest compared to the others evaluated.

Table 3. The average % center rot incidence at harvest and post-harvest (after three months in storage), marketable yield, % jumbo and colossal-sized bulbs, pungency, and % sugar of the 13 onion cultivars evaluated at Rock Springs (2015 and 2016) and Southeast Agricultural Research and Extension Center (Landisville) in Lancaster Co., PA (2016). Results are an average of the three trials unless otherwise stated.

Gugino continued the center rot evaluation of cvs. Spanish Medallion and Candy in 2017 on two grower farms in Lancaster County, one farm in Centre County, and at Rock Springs. There was no significant difference in total marketable yield between Candy and Spanish Medallion at any of the grower farms, and center rot disease incidence was lower in cv. Spanish Medallion at two of the three grower farms (Table 4). Under high disease pressure due to toothpick inoculating the onions at Rock Springs, cv. Spanish Medallion yielded significantly higher than Candy and had substantially lower center rot at harvest.

Table 4. Comparison of average marketable yield (%) and center rot incidence between cvs. Candy and Spanish Medallion at three commercial farm locations in Lancaster and Centre counties and the Plant Pathology Research Farm at Rock Springs in Centre County, PA. Natural inoculum was relied upon at the grower farms while the Rock Springs trial was inoculated.

Alternative Cultivars to Candy

This research demonstrates that there are promising alternatives to the standard onion cv. Candy. Orzolek showed that Expression, Exacta, Spanish Medallion, and Aruba had desirable horticultural characteristics, including high yields, that met the requirements of the PA Simply Sweet program. Expression and Spanish Medallion continued to be competitive in Butzler and Elkner’s 2020 trial, along with several other cultivars. Mazzone and Gugino found additional promise with Expression and Spanish Medallion maintaining desirable horticultural characteristics while having reduced center rot losses at harvest compared to cv. Candy. Candy also maintained its desirability with low post-harvest disease, although a similar low post-harvest disease pressure was present in Spanish Medallion and Expression.

Although post-harvest losses for cv. Candy were limited in these trials, the losses at harvest, as seen in Mazzone and Gugino’s center rot incidence at harvest data, ultimately negatively impacted the total marketable yield of cv. Candy. Relying on predominantly one cultivar is not sustainable, especially as cv. Candy transplants become more difficult to source. Across all the Penn State onion cultivar evaluations, cvs. Expression and Spanish Medallion consistently produced large bulbs with low pungency, high sugar, and low center rot disease incidence, making them a viable alternative for PA growers.

Figure 2. The representative appearance of onion cultivars Candy (A), Expression (B), and Spanish Medallion (C). Photo: Jennie Mazzone, Penn State

Acknowledgments

Research data contributions by Dr. Mike Orzolek, Tom Butzler, and Dr. Tim Elkner. Funding for these evaluations is provided by the Pennsylvania Vegetable Marketing and Research Program, Pennsylvania Vegetable Growers Association, and the Pennsylvania Department of Agriculture Specialty Crop Block Grant Program.