Managing Fusarium Dry Rot In Clearwater Russet

Main Points:
1. Seed source was not a primary determinant of dry rot development in storage.
2. Seed treatments were generally effective in reducing Fusarium seed piece decay.
3. Archive as a post-harvest application was effective in reducing Fusarium dry rot in storage.

The high susceptibility of Clearwater Russet to Fusarium dry rot (Novy et al., 2010) is one of the few negative characteristics of this variety. Some growers have reported the need to increase the planting rate by decreasing seed spacing because of the higher risk that seed will decay prior to plant emergence.

In addition, storing Clearwater Russet potatoes has been challenging due to the greater risk for dry rot decay. Specific industry driven questions have centered on improving management options to minimize seed decay, the need to clarify if there is a relationship between seed decay and harvested tuber decay, and establishing recommendations for seed and post-harvest treatment options. Since Fusarium storage decay is increased by any tuber injury, there is a need to address and identify the potential impact of plant maturity on shatter bruise susceptibility. Shatter bruises can provide entry points for Fusarium dry rot and subsequent decay in storage. 

Seed Source

We conducted a two-year project aimed at improving the management of Fusarium dry rot in Clearwater Russet. We evaluated 7 (2021) and 8 (2022) different Clearwater seed lots, each originating from a different county in Idaho. In 2021 the lots ranged from 3-32 percent (box test) and 13-50 percent (field test) for dry rot incidence. In 2022, the seed lots ranged from 0.3-70 percent incidence (box) and 25-42 percent incidence (field). However, in both years the incidence and severity of dry rot were not correlated with emergence, yield, tuber quality, or storage disease incidence. Dry rot incidence after storage was low in both years. Seed source was not a primary determinant to dry rot development in either year of the study.

There were limited differences in the incidence of bruise between seed lots, but some differences in yield per plant were observed between seed lots. Clearwater Russet grown at Kimberly showed greater late season bulking whereas there was limited increase in yield per plant at Miller Research late season. The differences in location, soil type and management on late season bulking is interesting and warrants further investigation. 

Seed Treatments

Isolates of Fusarium sambucinum in southern Idaho with resistance to fludioxonil have been identified in the PNW (Christian et al., 2020). This resistance may lessen the efficacy of some seed treatments for managing dry rot. Research results from the first year of the project in 2021 showed most seed treatments were effective in reducing dry rot incidence in Clearwater Russet based on a box test (Figure 1). Maxim alone (active ingredient = fludioxonil) was not effective against a fludioxonil-resistant isolate (FR, red bars). The fludioxonil-sensitive isolate (FS, blue bars) was more aggressive than the fludioxonil-resistant isolate (FR) as demonstrated by the increased seed infection in the UTC (untreated check). 

MZ (Mancozeb) dust alone was more effective against the FR isolate (the less aggressive isolate) than the FS isolate (the more aggressive isolate). Most of the other treatments were similar in dry rot control. It was encouraging to see CMP Extreme, CMP Vibrance, and Emesto Silver were equally effective with and without MZ dust (CMP = CruiserMaxx Potato). This effective control supports the idea that MZ dust may not be needed with a highly effective companion seed treatment product for increased dry rot control. 

Figure 1 – Effect of seed treatments on Fusarium dry rot; 2021 box test.

The trial was repeated with different treatments in 2022 (Figure 2). Unlike 2021, fludioxonil alone (Maxim) was not as effective against the fludioxonil-sensitive isolate as it was in 2021. Salient (difenoconazole) was effective against both fludioxonil-sensitive and fludioxonil-resistant isolates. CruiserMaxx Vibrance Potato (CMV Potato) also contains difenoconazole and is applied at a similar rate of active ingredient as Salient, but it was not as effective against the fludioxonil-sensitive isolate. MZ dust in 2022 followed a trend similar to 2021. Unlike 2021, adding MZ dust improved dry rot control with both CMV Potato and Emesto Silver for the fludioxonil-sensitive isolate. The fludioxonil-sensitive isolate (FS, blue bars) was similar to the fludioxonil-resistant isolate (FR) as demonstrated by the seed infection in the UTC (untreated check). 

Figure 2 – Effect of seed treatments on Fusarium dry rot; 2022 box test

In both years, all seed treatments except Maxim alone reduced Fusarium dry rot seed piece decay under inoculated conditions. Many seed treatments are available for managing Fusarium and selected examples are provided in Table 1. It is a good practice to use a product with two modes of action or to mix products with a single mode of action. 

Products in the M3 category target multiple sites in the pathogen and are not considered at risk for the resistance development. Currently, mancozeb is the only product in this category. MZ dust has been an effective seed treatment for a long time, but the worker safety concerns about using dust treatments has prompted growers to find alternatives to dust. Liquid mancozeb formulations such as STartUP MANZB are available, but they are more difficult to use due to the relative higher rates of application.

Table 1 – Selected seed treatment options for managing Fusarium dry rot in cut potato seed.

Post-Harvest Treatments

There are limited post-harvest treatments for Fusarium dry rot. Stadium (a mixture of azoxystrobin, fludioxonil, and difenoconazole) has shown efficacy against Fusarium dry rot in post-harvest tests (unpublished data from authors). The difenoconazole portion of this mixture has created export restrictions, which has limited the use of Stadium in the PNW. Recent research shows that the fungicide Archive (labeled as Graduate A+ in other crops) may also be effective in reducing Fusarium dry rot. Archive is azoxystrobin and fludioxonil without the difenoconazole.

In 2021, both Stadium and Archive were similar in their ability to reduce Fusarium dry rot development in storage (Table 2, Figure 3). Both products reduced dry rot incidence in 2022, but Stadium was more effective than Archive. This response is not unexpected since the difenoconazole in Stadium adds an additional active ingredient to target dry rot, so removing it from the product may alter the efficacy. Regardless, both years Archive was effective in reducing dry rot. 

Table 2 – Efficacy of post-harvest fungicides on Clearwater Russet potatoes inoculated with F. sambucinum and stored 97 (2021) or 95 (2022) days at 45 degrees F.

Figure 3 – Fusarium dry rot infection in Clearwater Russet potatoes inoculated, treated and stored 95 days at 45 degrees F and 95 percent RH.

Summary

Fusarium dry rot management for Clearwater required a multi-pronged integrated approach. Our work shows that seed source may not be as important compared to using an effective seed treatment. Many effective seed treatments are available and growers should use products that have multiple modes of action. Finally, the use of Archive as a post-harvest application can be part of a program to reduce dry rot in storage.