Section 5 - Disease Management

Rust Diseases in Wheat

Wheat is attacked by three different rust diseases. Wheat stem rust is caused by the rust fungus Puccinia graminis f. sp. tritici; wheat stripe rust is caused by P. striiformis f. sp. tritici; and wheat leaf rust is caused by P. triticina. All three rust diseases occur in the United States, however in the spring wheat region of Minnesota and the Dakotas, leaf rust is by far the most common and economically important rust on wheat.

Infections of leaf rust appear as small powdery pustules on the lower and upper leaves of wheat plants (see leaf rust photo on page 48). The rust spores that are produced in the pustules are reddish in color, which gives heavily infected leaves a brown-red appearance, hence the name of rust for these diseases. A single large rust pustule can produce hundreds of spores every day. The spores are wind blown into the atmosphere and carried by the prevailing winds for hundreds of miles and are deposited on wheat crops in rain. Each of these spores has the potential to infect wheat. Since leaf rust needs only 7-9 days from initial infection to production of spores, there can be up to four generations of rust increase in a crop in a single growing season, which results in an exponential increase of leaf rust infections. Rust infections are highest in years with good temperature and moisture conditions for the wheat crop. The red pustules of wheat leaf rust are usually first observed on winter wheat in Minnesota and the Dakotas in mid-to-late May. In some years leaf rust infects winter wheat in the fall, survives the winter on wheat, and begins to increase very rapidly in the spring when daytime temperatures increase to 70-75 degrees. Leaf rust infections on spring wheat are usually first seen in the Red River Valley in mid-June. Initial infections on spring wheat are caused by spores that have been carried in the winds from the leaf rust infected winter wheats in the southern Great Plains. Leaf rust infections in spring wheat reach maximum levels in mid-to-late July.

Yield losses in wheat caused by leaf rust occur annually in the winter and spring wheats grown in Minnesota and the Dakotas. The spring wheats vary greatly for genetic resistance to leaf rust. Some cultivars are highly resistant. On these cultivars very few leaf rust pustules can be found, and the flag leaves stay green for a longer period, thus increasing kernel size and weight. On susceptible cultivars, the flag leaves are covered by thousands of pustules. Heavily infected flag leaves dry down prematurely, reducing kernel size and weight. Planting resistant cultivars is critical to reducing losses due to leaf rust. Losses caused by leaf rust in spring wheats can be as high as 40% in susceptible cultivars. In recent years many growers have also sprayed fungicides on wheat in order to reduce losses due to Fusarium head blight, leaf spots, and leaf rust.

Many different races of leaf rust are found every year in the U.S. These races differ their ability to attack different resistance genes in wheat. As a result, wheat cultivars that are highly resistant when first released, often become susceptible within a few years of cultivation due to the increase of races that can overcome its resistance genes. New cultivars with new and effective resistance genes are essential to maintain leaf rust resistance and reduce yield losses.

In recent years stripe rust has also occurred in Minnesota and the Dakotas. Stripe rust is a disease that is most common in areas where wheat develops and matures in cool environments. Stripe rust is characterized by strips of very small yellow pustules that grow between the leaf veins (see stripe rust photo on page 48). In most years stripe rust may not occur at all in this region. However years with cool and wet weather in the spring and summer (such as 2003 and 2004) may be conducive for the onset and development of this disease. In years when stripe rust does occur, the infections on winter wheat can be seen in early-mid May in southern Minnesota and South Dakota. In recent years stripe infections have reached damaging levels in winter wheats in western and central South Dakota. Stripe rust infections on spring wheats are most common and evident in early July. Many winter and spring wheats have good resistance to stripe rust. This resistance combined with the normal hot weather in July limits development of stripe rust infections in years when the rust is present in the upper Midwest.

From 1900-1950s stem rust was a major disease of spring and winter wheat. Infections of stem rust are now rarely seen in the U.S. because most wheat varieties are highly resistant to prevalent races of this fungus. However, most U.S. wheat cultivars are susceptible to a fast-evolving race of stem rust designated Ug99. This race is currently confined to parts of Africa and southwest Asia, but breeders and pathologists are preparing for its possible spread to North America. Stem rust pustules are dark red and are found on stems (see stem rust photo below)and occasionally on leaves. The alternate host for wheat stem rust, the common barberry, has largely been eradicated in the spring wheat region. Elimination of the barberry plants removed the sexual cycle of the fungus, which has greatly reduced the number of stem rust races that occur in the U.S.

Information on the spread of wheat rusts in the current crop year can be found at the USDA-ARS Cereal Disease Laboratory website (www.ars.usda.gov/mwa/cdl) in the cereal rust bulletins section. Information on the rust resistance in wheat cultivars can be found in varietal trial guides published online by the Minnesota Agricultural Experiment Station (www.maes.umn.edu) and the North Dakota Agricultural Experiment Station (www.ag.ndsu.nodak.edu).

Models to Help Fungicide Decisions On Small Grains

The decision for fungicide application is based on multiple components and include yield potential, market price, presence of disease in canopy, disease susceptibility of variety, climatic conditions (extended dew periods increase risk of leaf and head diseases) and previous crop (a previous wheat or barley crop increases the risk of fungal leaf spots and scab, but not leaf rust).

SDSU has developed a Small Grain Fungicide Decision Guide as well, in the form of a Microsoft Excel spreadsheet. Go online to http://plantsci.sdstate.edu/planthealth. Click on “Small Grains” then “Small Grain Fungicide Use Decision Guide.”

Managing Disease in Winter Wheat

Current winter wheat varieties are moderately susceptible to disease and have responded well to fungicides. Timely fungicide applications to winter wheat planted in spring wheat stubble controls foliar and head disease resulting in higher yields and better grain quality. NDSU research trials indicate that split fungicide applications have been the most consistent in increasing winter wheat yields. The first fungicide application is tank mixed with the herbicide and the second treatment applied at early flower or Feekes 10.51 stage of growth. The early fungicide application is usually not required for winter wheat planted in a broadleaf crop residue.

Control of volunteer winter wheat is critical following the winter wheat crop to prevent the spread of wheat streak mosaic and other green-bridge diseases. Following winter wheat with a broadleaf crop can also reduce the risk of wheat streak mosaic.

Wheat streak mosaic virus is a disease spread by a tiny mite that thrives on grassy weeds and volunteer grass crops, including corn and wheat. Typical symptoms are stunted plants, yellow streak Leaf RustStripe RustStem Rusting and green/yellow mosaic discoloration of leaves. Severe outbreaks are almost always associated with volunteer wheat in which mites and virus have survived and multiplied.

The mite has a quick life cycle (egg to adult takes only 7 to 10 days) and needs green plants for feeding and reproduction. If no green food hosts are available after hatching, the mite does not survive. Thus, a glyphosate burndown about two to three weeks prior to planting winter wheat will destroy grassy weeds and greatly reduce the threat of wheat streak.

More background on the wheat streak mosaic virus can be found on the NDSU extension web site, www.ag.ndsu.edu/pubs/smgrains.html. See the link “wheat streak mosaic.” Also go to http://prairie.ducks.org – click on ‘Agronomy News’ (direct link to DU HRWW management info: http://prairie.ducks.org/ag_news/winterwheat2.pdf)

Foliar Fungicides, Liquid N May Not Tank Mix Well

Interest in application of foliar N to enhance wheat proteins often results in questions about tank mixing with foliar fungicides. Research on foliar N application (without added fungicide) has indicated that foliar N applied to boost grain protein should be applied shortly after flowering, to avoid injury to the flower and for better uptake of the nitrogen into the grain. Application after flowering rules out the use of Tilt fungicide as a tank mix partner.

NDSU studies show Puma or Discover plus Bronate Advanced applied with the strobilurin fungicides (Quadris, Quilt, Headline) caused severe leaf burn on wheat plants. New tissue that emerged was unaffected. Bronate, and generic formulations plus strobiluron fungicides may also cause similar injury.

Tank mixing of mancozeb fungicide and 28-0-0 was tested at NDSU in the past, with results indicating that leaf burn was severe if a surfactant was added to this tank mix. Leaf burning also was compounded if temperatures at spraying were greater than 90° F. The Carrington study and the Crookston study indicated leaf burn was less using 20-0-0-3 than with 28-0 -0.

Innovations such as liquid N stream bars, specialized streamer nozzles, and air spreaders for applying dry urea are essential tools to maximize the potential of foliar N while minimizing leaf burn. The purpose of applying fungicides is to protect the leaf from diseases, and any practice that burns the leaves negates the purpose of adding the fungicide. Product labels usually give possible registered herbicide/fungicide combinations and application practices to follow or avoid.

Tips for Better FHB Suppression with Fungicides

• Consider seeding early flowering varieties first and late flowering varieties last to spread out weather risks, and spread flowering across more days to allow more time for protective fungicide treatments.

• Scout fields every day when wheat begins to head. It’s time to spray when wheat heads have fully emerged and 25% of main stem heads have started to flower. For many varieties, it may take only four to five days from the time the first spikelets are visible in the boot to growth stage Feekes 10.51.

• Spray hard red spring wheat and durum at early flowering (Feekes 10.51). Spray barley at early heading (Feekes 10.3-10.5); Split, multiple applications improves control in barley under weather conditions that favor multiple infections.

• For ground application, angle spray toward grain heads, using forward nozzle angled at 35 to 40 degrees downward from horizontal, or forward and backward mounted XR8001 nozzles or nozzles that have a two directional spray, such as Twinjet nozzles.

• When using XR flat fan tips, use 40 psi with 9-10 gpa, and 90 psi with 18 gpa.

• NDSU data indicates that fungicide efficacy using 10 gallons of water per acre is similar to 20 gallons of water per acre, although increased spray volume for durum and barley may be considered to improve head coverage and disease control. Take advantage of dew on plants as additional water.

• Large fine droplets and small medium droplets leave more fungicide residue in the spikelets than coarse droplets, which are larger and have a better chance of falling off the plant.

• Use a good adjuvant. Research at NDSU has indicated that Folicur and Tilt perform better with a non-ionic surfactant than without.

• When using aerial application, spray in evening or early morning to capture dew as extra water volume, and use a small droplet size.

FHB, Disease Management Online

NDSU: www.ag.ndsu.edu/pubs/plntdise.html and www.ag.ndsu.edu/extplantpath

NDSU Small Grain Disease Forecasting web site: www.ag.ndsu.nodak.edu/cropdisease/cropdisease.htm. Recorded updates and information by phone: 1-888-248-7382, 231-6467 in Fargo area.

SDSU: http://plantsci.sdstate.edu/planthealth and http://plantsci.sdstate.edu/smallgrainspath

UM (NWROC- Crookston): www.nwroc.umn.edu – Click on “Research Areas” then “Plant pathology” or http://mawg.cropdisease.com

DON (Vomitoxin) in Wheat – Basic Questions and Answers

This is an NDSU publication on what DON is, how it is measured, how to deal with grain with elevated levels of DON, and how to prevent it in the future: www.ag.ndsu.edu/pubs/plantsci/pests/pp1302w.htm.

Testing for DON

A number of laboratories test for DON. Contact your local grain elevator, agronomist, or county extension agent/educator for specific details on how to collect, pack and send the sample for analysis.

Sclerotinia Risk Map for Canola Online

Sclerotinia risk forecasting maps for North Dakota and northwest Minnesota can be found online: www.ag.ndsu.nodak.edu/aginfo/sclerotinia/sclerotinia.htm. The forecasting system is based on soil moisture as it is required for the apothecia (small mushroom structures) to grow from the sclerotia in the soil. The apothecia produce the spores that infect canola petals. It is important to know the growth stage of canola fields when checking the risk map. Canola is only susceptible to Sclerotinia infection when it is flowering.