Disease Management - Section 5

Fusarium Head Blight (FHB, Scab) Epidemic Risk Forecasting System

Minnesota’s State System -- Management of FHB includes an integrated approach such as growing cultivars with resistance, applying fungicide at the early flowering growth stage, and managing infested crop residue with rotation and/or tillage. Epidemic risk predictions of FHB are used as decision aids to determine whether a fungicide application should be made to protect the crop. Since 2004, a statewide FHB epidemic risk forecasting system has been operational and free of charge at http://mawg.cropdisease.com.

The Minnesota FHB epidemic risk forecasting system website is funded by the Minnesota Wheat Research and Promotion Council, and maintained by Meridian Environmental Technology, Inc. (Meridian) in cooperation with the University of Minnesota. Meridian maintains a comprehensive weather database that integrates data collected from surface-observed stations and remotely-sensed information from weather radars and satellites. Observed weather conditions throughout Minnesota are recorded by federal and state agencies such as the National Weather Service and the Federal Aviation Administration (>82 stations), the Minnesota Department of Transportation, Road Weather Information System (93 stations), and the North Dakota Agricultural Weather Network (NDAWN; 10 stations). Remotely-sensed information originates from six NEXt Generation Weather RADar (NEXRAD) weather radars. The outcome is a 4-km resolution composite of hourly precipitation, temperature, and humidity data which is used to update the forecasting system every three hours for a total of eight times a day.

National Forecasting System -- Deployed since 2004, the national FHB epidemic risk forecasting system website located at www.wheatscab.psu.edu is supported by the U.S. Wheat and Barley Scab Initiative. The system services the central and eastern U.S. wheat community located in 24 states, including Minnesota. A number of weather data sources are used by the national model system, with the most substantial data contributions provided by federal agencies. The national epidemic risk forecasting system relies primarily on remote-sensed weather data devices (e.g.: radar, satellites), whereas the Minnesota relies primarily on observed station data. A disease forecasting component based on 24 hour and 48 hour weather forecasts was added to the national model in during 2006. The Minnesota system also incorporated disease forecasts into its risk forecasting website during 2008.

NDSU’s Wheat Disease Forecasting Web Site

Links to small grain disease forecasting, Sclerotinia disease forecasting, and potato disease forecasting will be available at one site: www.ag.ndsu.nodak.edu/cropdisease.

A person seeking information specifically on wheat disease risk, including Fusarium head blight (scab) may go directly to the following web site: www.ndsu.edu/scabforecast

This scabforecast web site is being reformatted for easier use. It will provide information on the risk of Fusarium head blight (scab), as well as risk of the wheat leaf diseases, tan spot, Septoria blotch, and leaf rust. The risk of these four diseases is based on environmental conditions as recorded by the North Dakota Agricultural Weather Network (NDAWN) stations. This network consists of 67 stations distributed across North Dakota, the Red River Valley, and border regions of surrounding states.

The risk map for Fusarium head blight is based on a model developed by plant pathologists at Penn State University, a model which incorporated considerable ND disease and weather data for verification of the model. The scab model also has the opportunity for the producer to indicate whether the variety grown is very susceptible, susceptible, moderately susceptible or moderately resistant to Fusarium head blight. Risk information is available in a chart format and state map format for Fusarium head blight.

The information in the models for the leaf diseases was developed at ND, and provides information in a chart format indicating risk of the three leaf diseases over the past 12 days. Such information is another tool that producers may use to determine the need for fungicide application at critical growth stages to control leaf and head diseases in wheat. Statistics on use of the small grain disease forecasting site indicates that June and July are the heaviest use period for this information.

Submitted by Marcia McMullen, NDSU Extension Plant Pathologist

Diagnosing Leaf Diseases in Wheat

Tan Spot -- The fungus that causes tan spot overwinters on aboveground crop stubble, and infects plants and causes disease over a wide range of temperatures as long as adequate moisture is present on leaves for an extended period (12-24 hours).

Tan spot and Septoria/Stagonospora leaf spotting diseases produce similar leaf symptoms. Tan spot produces oval-shaped, tan lesions that enlarge with age. Lesions are surrounded by a yellow halo, while dark areas eventually form on tissues that were initially infected. Severe disease development causes spots to expand until most, if not all, plant leaf tissues are killed. Yield and kernel weights are reduced if the seed-filling ability of diseased plants is limited early enough.

Wet years tend to favor tan spot epidemics, with the result often being yield loss. Planting wheat into fields with infected small grain stubble is especially risky, since it increases the likelihood for seedling infection early in the growing season. If the disease is established prior to flag leaf emergence and extended wet weather is forecast, fungicide application provides a viable control alternative. Some spring wheat varieties have differing levels of resistance to the disease.

Septoria/Stagonospora Leaf Spot Complex

Septoria/Stagonospora leaf spot can occur across a range of temperatures (50-81° F). At these temperatures, plant leaves must remain moist for plant infection, disease development and spread. Plant leaves must remain moist for at least six hours for infection to occur. If newly infected plants remain wet, more spores will be produced in 10 to 20 days. Wind and rain disperse spores to other leaves and plants. Disease development stops when conditions turn dry.

Septoria/Stagonospora leaf spot diseases look much like tan spot early on. Young lesions appear oval-shaped and have yellow halos. As lesions mature, either tiny black fungal fruiting bodies (black flecks) form, or lesions develop grey centers as light-colored spores are produced. Specific plant symptoms are dependent on which fungi are responsible for causing the disease. Fungicides may be necessary if wet weather is forecast and the disease is present.

Bacterial Stripe

The bacterium that causes bacterial stripe, also known as black chaff, needs wet field conditions for plant infection and disease development. It’s less dependent on specific temperatures than on extended leaf wetness and high humidity. A primary means of spread is through infected wheat seed (bacteria have been found in the seed coat). Bacteria are also suspected to overwinter on aboveground crop stubble in the field.

Plant leaf symptoms appear initially as thin, light brown to golden stripes that appear water-soaked. The bacteria follow plant veins, eventually expanding lengthwise from sheath to tip.

Infection of heads produces black streaks on glumes and can result in a range of symptoms from complete head sterility to kernel discoloration/contamination with bacteria.

Symptom severity depends on when the infection takes place. If flag leaf tissue is killed early on, more yield loss will result. Yield reductions are generally 10% or less.

Crop rotation may help manage this disease, as would tillage, since bacterial populations are known to drop when infected stubble is incorporated into the soil. Infected seed provide the primary means of inoculum. Spraying the crop with fungicides won’t help; the chemistry isn’t effective against bacteria.

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 38). 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 38). 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 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 on the next page)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 streaking 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.

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.

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

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.edu/sclerotinia. 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.

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