Purpose: Onions are an important crop to producers in the Tri River Area. These trials were conducted to provide commercial onion growers research based information on the yield of selected onion varieties growing under Western Colorado conditions. Their susceptibility to Fusarium basal rot caused by Fusarium oxysporum f. sp. cepae was also studied.

Materials and Methods: Commercial onion fields farmed by Jerry Hines (Delta county) and Duane Homewood (Montrose county) were used in this study. The Hines field (field #1) consisted of a heavy soil with soluble salts (white alkali) evident on the soil surface. The Homewood field (field #2) was sandy; the presence of soluble salts was not evident.

Each variety (Table 1) was planted four times in a randomized complete block design. Plots were planted on 26 (field #1) and 27 April (field #2). Beds were on 34 and 30 inch spacings, respectively, with two lines of onions per bed, each planted with a different variety. Thirty feet of row was planted to each variety. Population data was collected monthly during the growing season and the highest population count for each plot used to determine the percentage of onion bulb loss (Table 3). Loss of onion bulbs was attributed to Fusarium basal rot caused by Fusarium oxsporum f. sp. cepae.

Onions were harvested and graded on 14 September (Field #1) and 18 September (Field #2). Ten feet of row was harvested, graded, and weighed. Onion yields ranged from 209 to 803 hundred weight per acre (cwt/a). MSTAT-C, The Michigan State MicroProcessor Statistical Program, was used to evaluate statistical differences among cultivars. All data in Table 2 and 4 have been converted to hundred weight per acre unless otherwise indicated. The statistical difference between onion varieties is indicated by the letter(s) which follow the yield per acre (cwt/a). Varieties followed by the same letter are statistically identical in regard to yield. The level of significance is given at the bottom of each table. A significance of 0.01 means the same results will occur 99 times out of a 100 times this trial is planted. A significance of 0.05 means the same results will occur 95 times out of 100. Where no statistical difference exists, no letter evaluation is given and the note in the respective LSD box is Ans@ for no significance.

Results

The total marketable yields for varieties in Field #1 (Table 2) ranged from 209 to 803 cwt/a. Superchief was the outlier in this trial. Without Superchief, yields for Field #1 would range from 209 to 638 cwt/a. The total marketable yield of varieties in Field #2 (Table 4) ranged from 464 to 712 cwt/a.

Superchief (Table 2) had the highest yield of 803 cwt/a as well as the lowest number of Fusarium infected bulbs at harvest of all other varieties examined. Tamara (Table 2) had the lowest yield at 209 cwt/a as well as the greatest number of Fusarium basal rot infected onion bulbs (1201) per acre at harvest. Spinnaker, with a total marketable yield of 263 cwt/a, had the second highest level of Fusarium basal rot infected onion bulbs per acre at 1133. Other varieties with an apparent high susceptibility to Fusarium basal rot include Pinnacle with 1049 infected bulbs per acre, X351 with 879 infected bulbs per acre, and Regiment with 862 infected bulbs per acre.

A comparison of the highest plant population during the growing season with the number of non-Fusarium infected bulbs at harvest was conducted for Field #1. The percentage of bulbs lost to Fusarium during the growing season ranged from 13% to 51%.

Discussion

Growers should take into account marketable yield as well as the size bulb they plan on marketing when selecting varieties to plant. For example, growers wishing to produce mostly colossal size bulbs should select those varieties where the number in the colossal column is followed by an Aa@.

Fusarium basal rot, caused by Fusarium oxysporum f. sp. cepae, is a serious problem in some commercial onion fields in western Colorado. Field #1 had a high level of Fusarium infected bulbs at the time of harvest. In addition, up to 51% of the onion plants in some plots in this field were lost during the growing season due to Fusarium basal rot. However, no significant difference existed among varieties in susceptibility indicating all the varieties examined are equally susceptible to the causal agent of this disease (Table 3). No Fusarium infected bulbs were recorded at harvest in Field #2 (Table 4).

No significant difference was found in either field in regard to culls (splits, bolts, and misshapen bulbs) (Table 3 and 4).

Additional research needs to be conducted in fields contaminated with high levels of F. o. cepae to determine the level of susceptibility of other onion varieties to this disease pathogen. Variety trials to determine differences in yield should continue in western Colorado. This research is necessary for growers to select the highest yielding varieties. Without high yields of quality onions, area growers will not be able to compete with other onion producing areas, and the onion industry in western Colorado will cease to exist.

Acknowledgments:

Planting could not have been done without the assistance of CSU Master Gardeners in cooperation with the growers. Additional support provided by Avelino Hernandez and Mondo Arreola was invaluable. George Novotny, Olathe, Colorado, was instrumental in arranging for seed for these trials. Dr. Howard F. Schwartz, Professor, Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins, CO, provided guidance and direction on methodology for the collection of Fusarium basal rot data. Their support is greatly appreciated.

Table 2: Yield of onion bulbs in Field #1

Table 3: Culls and Fusarium infected bulbs - Field #1

Table 4: Yield of onion bulbs in Field #2

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