Oats: Managing Crown Rust and Targeting Fungicide Use

May 12, 2021 / 
by Margaret Smith, PhD, Agronomist, Albert Lea Seed and
Chaunce Stanton, Marketing and Communications, Albert Lea Seed

In years with high crown rust disease pressure, the timely use of a fungicide has been proven to protect grain yields by as much as 19 percent. However, the integration of cultivar selection, timing planting, and disease scouting is as important—and cost-effective—as a pre-emptive prophylactic fungicide application. Begin scouting for indications of crown rust when oats are in the 2- to 6-leaf stage.

Oats can be affected by several fungal disease, including stem rust and leaf spot, but crown rust (Puccinia coronata f. sp. Avenae) is the most detrimental disease in oats, affecting crops in some years from the Dakotas to New York. Crown rust growing and reproducing on oat leaves can reduce the leaf’s photosynthetic capacity by up to 30 percent, yields by as much as 50 percent, and test weight by up to five percentage points.

Managing crown rust takes a multi-pronged approach. First, weather conditions and crown rust spores must be present for disease to infect plants. Not all years have these environmental conditions conducive to infestation. Given the unpredictability of weather, oat growers should manage each year for the potential for crown rust.

Six steps for oat growers to consider for managing crown rust:

STEP 1: SELECT OAT CULTIVAR for DISEASE RESISTANCE & MATURITY

The most important strategy to avoid the crown rust of oats is to plant resistant (R) and Moderately resistant (MR) varieties. There are about 500 unique races of the (crown rust) pathogen, and no single oat cultivar is immune to all races. Research suggests that for oat cultivars with race‐specific resistance, the resistance does not remain effective for more than about five years, because the crown rust population evolves to more readily infect and thrive on their host. Crown rust evolution to overcome single-gene resistance is more rapid than to overcome multiple sources of resistance.

In our 2021 oat line-up, we have several cultivars that are rated with good or average resistance to crown rust. Topping our list for resistance is Deon, a high yielder with a heavy test weight released by the University of Minnesota. Other cultivars with average resistance include: Sumo, Saddle, Esker2020, and Rushmore.

In addition to disease resistance, consider oat cultivars based on their maturity. Earlier-maturing cultivars may escape damage from crown rust as they complete their vegetative growth stages before experiencing very warm temperatures, conducive to disease spread. Albert Lea Seed’s early-maturing oat line-up include: Sumo, Saddle, and Reins.

STEP 2: PLANT AS EARLY AS POSSIBLE

Getting the oats in the ground as soon as soil conditions allow. This pushes growth and development earlier in the season before conditions are favorable for crown rust infection and growth before crown rust may infect the plants. Rust spores proliferate in warm weather conditions (65° F to 80° F), especially in the presence of moisture on leaf surfaces due to rain, dew, high humidity, or fog.

Early planting also can help prevent the onset of barley yellow dwarf virus, which is vectored by aphids. Once oats plants are infected with barley yellow dwarf virus, they cannot be treated.

STEP 3: KNOW YOUR TARGET MARKET

Farmers raising food-grade oats for the milling market will be working to achieve high test weights. Steps 1 and 2, variety selection for high test weight and early planting are crucial to achieve milling standards of 38 lb. test weight/bu. Fungicide treatment, when warranted, may also increase test weight. For farmers raising oats for feed, variety selection and fungicide use will likely differ.

STEP 4: SCOUT for DISEASE

Farmers should begin scouting oats at about the two-leaf stage, which can be as early as late April in some parts of the Midwest. When scouting for crown rust, look for small oval or oblong orange-yellow pustules, primarily on the leaves; however, the infection may also express pustules on leaf sheaths, stems, and panicles. If rust pustules are observed, farmers may consider immediate fungicide application. Once the disease becomes established,  fungicide applications are ineffective.

STEP 5: (If scouting indicates a need) CHOOSE A FUNGICIDE & TIME THE APPLICATION

In years with high crown rust disease pressure, the timely use of a fungicide has been proven to protect grain yields by as much as 19 percent. However, the integration of cultivar selection, timing planting, and disease scouting is as important—and cost-effective—as a pre-emptive, prophylactic fungicide application.

In general, a one-time application at the time of flag-leaf (the last leaf) emergence is most widely recommended.  Results of fungicide treatments on oat varieties differed with: varying levels of susceptibility to crown rust, year, and location (Table 1).  Not all years studied had conditions conducive to crown rust infestation.

 

Table 1. Effect of fungicide application on oats at the flag leaf stage from nine public trials conducted from 2009 to 2017.  Susceptibility ratings for varieties differ by year, due to typical increase in susceptibility to crown rust over time. Green results indicate positive results from the treatment, yellow indicates a neutral result, and red indicates a negative result.

Table One. *Crown rust resistant ratings scales vary somewhat by state and research group. R = resistant; MR = moderately resistant; IR = Intermediate resistance; PR = partial resistance; MS = moderately susceptible; S = susceptible; VS = Very susceptible

 

Bruce Roskens, retired small grains agronomist from Grain Millers, Inc., suggests that farmers keep a close eye on oats from the two-leaf stage to heading. At the first indication rust pustules on the middle leaves, he recommended farmers consider an immediate application of multi-ingredient fungicide, such as Stratego, with a possible second application, if needed, when the flag (last) leaf has emerged.

Farmer Experiences with Fungicide use on Oats

How are oat seed producers putting these recommendations into practice? Several Minnesota growers shared their experiences. (Table 2.) None of these farmers seed a legume or grass with their oats.

 

Table 2. Fungicide use programs for oats on three Minnesota seed production farms.

Table Two

 

Our reporting farmers also related that their fungicide treatments delay oat maturity, which makes it difficult to harvest them standing. All of these farmers swath and windrow their oats for three to four days before combining. Though this requires additional equipment–a swather and pick-up combine head–it has the advantage of letting the oats go through their sweat in the windrow and decreases the need to have to aerate them in bins.

The University of Wisconsin provides fungicide recommendations for small grains in their crop management guide, Pest Management in Wisconsin Field Crops, 2021.

STEP 6: REMOVE ALTERNATE HOSTS

Buckthorn and more than a hundred perennial grass species (worldwide) serve as alternate hosts for crown rust. Although it’s not always practical to eliminate plants that host the disease, consider removing the primary alternate, buckthorn, immediately adjacent to crop fields. Completely eliminating alternate hosts is a big task, though, as viable spores can be carried by wind up to a half-mile.

WHAT ABOUT FORAGE OATS & FUNGICIDES?

Although fungicides have little effect on forage yield in oats, the chief reason to consider a targeted application is to protect forage quality. Late-summer plantings of forage oats are especially vulnerable to crown rust infection because rust spores proliferate throughout the summer. Growers with late summer, forage oat plantings should scout fields carefully.

Canadian researchers observed positive yield results with fungicide treatment of fall-planted forage oats with a crown-rust susceptible variety, but not with a resistant variety.

As with grain oats, forage oat fungicide treatment should be used only when needed, with the right product, and at the right time. Select a product (such as strobiurin-containing products) that is labeled specifically for control of rust in forage oats. In addition, check the fungicide label to note harvest restrictions for that product. Once applied, farmers may have to wait at least one to two weeks to harvest oats for forage.

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