Pink Root Practices

Pink root in an onion bulb. Photo by Howard F. Schwartz, Colorado State University, Bugwood.orgPink root in an onion bulb. Photo by Howard F. Schwartz, Colorado State University, Bugwood.org
Mike Thornton speaks about managing pink root at the University of Idaho field day.

Mike Thornton speaks about managing pink root at the University of Idaho field day.

In Idaho’s Treasure Valley, pink root is one of the main diseases in onions. At the University of Idaho field day held in June at the Parma Research and Extension Center, Mike Thornton shared his research regarding alternatives to pink root control and explained the three main methods used to manage the soil-borne disease.

Crop rotation to non-host crops is effective, and the longer the rotation between onions, the less disease pressure. However, with an over 50-year history of onions in the Valley, this method may be impractical. There is only so much ground to plant onions.

Varieties resistant to pink root can help control the disease. Thornton recommends growing cultivars that have been bred to better tolerate the disease so there is less impact on the crop.

Figure 1: A comparison of fumigation to Fontelis applied via drip at two-leaf stage.

Figure 1: A comparison of fumigation to Fontelis applied via drip at two-leaf stage.

Fumigation is the third way of controlling pink root. Traditionally, growers have applied fall treatments of chloropicrin or metam sodium. Thornton thinks there is a better option: Dupont Fontelis. This fungicide can be applied in-furrow or with water through drip irrigation, placing it closer to the root zone where the pathogen is found. However, Fontelis is not nearly as effective as chloropicrin (Fig. 1), suppressing pink root rather than controlling it.

Thornton’s future trials will focus on ways to make Fontelis more effective, stretching it out over a longer time with a lower rate, using it in combination with a fumigant and timing drip applications. Perhaps Fontelis alone will work if used on a highly-resistant variety or if a field has low disease pressure, he said.

Thornton is also researching a very different idea to control pink root. The disease nibbles at the roots all season, reducing the plant’s ability to take up water and nutrients, and he is hoping to show that plant health will help with pink root. To test whether a healthier onion bulb will help ward off pink root, he is using targeted applications of Redox and Winfield foliar and drip-applied fertilizers.

Attendees prepare to roll out at the field day held at the Parma Research and Extension Center.

Attendees prepare to roll out at the field day held at the Parma Research and Extension Center.