Purpose: Pink root, caused by Phoma terrestris (syn. Pyrenochaeta terrestris) can be a serious problem in commercial onion fields. Pink root is reported to reduce yields and bulb size. In this study, twenty six onion varieties were examined for their susceptibility to pink root.

Materials and Methods: Two 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 (see 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. All bulbs in eight feet of row were hand dug and evaluated as to the percentage of roots infected by pink root, as indicated by color.

All percentages were summed and divided by the number of bulbs to give an average percentage of roots infected per plot (Table 2). MSTAT-C, the Michigan State MicroProcessor Statistical Program, was used to evaluate statistical differences among cultivars. The statistical difference of pink root susceptibility is indicated by the letter(s) which follow the percentage of infection. Percentage followed by the same letter are statistically identical in regard to incidence of pink root. In this trial, Aa@ following the percentage of pink root indicates the highest level of pink root infection; Ad@ in Field #1, and Ac@ in Field #2 following a percentage indicate the lowest level of infection.

Results: Table 2 shows the percentage of pink root for each variety from both fields. Field #1 had a high incidence of Pink Root throughout the planting, with up to 52 % of the roots infected (Gallatin). Field #2 had a much lower overall level of root infection with a maximum infection of 11 %, again in Gallatin. The level of significance is given at the bottom of each table. A significance of 0.01 means the same results would occur 99 times out of a 100 times this trial was planted; 0.05 means the same results would occur 95 times out of 100 trials.

Table 3 is a compilation of the total marketable yield and the percentage of pink root for each variety examined. There are several instances where the percentage of pink root and yield are both high, indicating tolerance to the disease organism. PS 456994 had an average pink root infection of 41% yet still yielded 536 cwt/a. XPH 15120, 15122, and 15232 had 23, 25, and 28% root infection, respectively, with yields in excess of 500 cwt/a. Superchief with a pink root percentage of 17, yielded 803 cwt/a. Sharon, however, at a pink root infection of 37%, yielded only 246 cwt/a.

Discussion: Onion varieties are said to have differences in pink root resistance. While none of the varieties of onions tested were found to be completely resistant to this disease organism, there appears to be a difference in the ability of a given variety to endure infection while maintaining a high yield.

In fields known to have high levels of Phoma terrestris growers should consider planting onion varieties shown to yield well even under such conditions. These varieties include Superchief, T433, and PS 456994. The yield of Tamara, Sharon, Spinnaker, Gallatin, and Outrigger are more negatively affected and would not be a good choice for planting in fields known to have high levels of Phoma terrestris. Varieties such as Gallatin appear to be more susceptible than the other varieties examined even when the field has a low level of pink root inoculum.

This research provides insight into the variability of onion varieties to Phoma terrestris as it affects yield. Additional research needs to be conducted in fields known to produce high levels of pink root to compare the degree of susceptibility of other onion varieties.

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 pink root data. Their support is greatly appreciated.

Table 1: Varieties included in this trial:

Table 2: Percentage of Pink root in Field #1 and Field #2

Table 3: Total Marketable Yield vs Percentage of Pink root