Disease Management

Diagnosing Leaf Diseases in Wheat
Many small grain diseases are favored by wet weather, including tan spot, Septoria leaf and glume blotch, scab and net blotch. However, some small grain diseases – such as common root rot, wheat streak mosaic virus, and barley yellow dwarf virus –  can even result in losses in yield and quality when both soil moisture and humidity are low.  More information about diseases in dry weather by Dr. Marcia McMullen at NDSU can be found online at www.ag.ndsu.nodak.edu/drought/ds-01-02.htm .

Following are diseases that generally prefer more humid conditions to develop. One tool to help scout for the formation and outlook of leaf diseases is the NDSU disease forecasting system, online at www.ag.ndsu.nodak.edu/cropdisease/cropdisease.htm .

Leaf rust
Rust spores produced in southern states are carried by northerly winds to our area. 

The fungus that causes leaf rust of wheat (Puccinia triticina) needs warm days and cool nights with six to eight hours of leaf moisture. Plant leaves in the lower canopy are generally the first to become infected since they stay wet longer.  Under the right environmental conditions, masses of spores that are capable of spreading disease can be produced about one week after plants become infected.

Plant symptoms differ depending on their disease resistance levels.  Bright orange to reddish spots, called pustules, tear leaf tissues as spores multiply within the leaves of plants and push outward.  Moderately resistant plants have smaller erupting pustules than those that are susceptible, and have a narrow band of yellow plant tissues, called a halo, surrounding the pustule. Resistant plants can be identified by small off-white to yellow spots, called flecks, on the leaf where infections have been stopped.

Yield losses from susceptible varieties can occur if infection takes place before flowering.  Disease development is greatest when spores are plentiful and weather conditions are optimum for the fungus.

Timely disease management is important since spore populations can build quickly. Fields should be scouted at regular intervals so timely application of fungicides can be made to protect the crop. The USDA Cereal Disease Lab in St. Paul monitors and reports rust development in the southern and central Plains, which in turn can help us keep on top of rust potential in our region.  You can find the CDL’s rust bulletins online at www.cdl.umn.edu . Click on the link “Cereal Rust Bulletins.”

Tan Spot
The fungus that causes tan spot (Pyrenophora tritici-repentis) 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.

Management options are similar to those with Fusarium head blight, since both pathogens survive on aboveground crop stubble.  Crop rotation and residue management can prevent most early-season disease development.  However, as the growing season progresses, spores are produced locally on diseased, non-cropped plants, and spores produced on infected stubble can travel long distances with the help of gusty winds.

Even if growers routinely practice all known management options, it’s likely they could still find tan spot lesions on wheat plants.  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
Three fungi are involved in this leaf spot complex (Stagonospora nodorum, Stagonospora avenae f. sp. triticea, and Septoria tritici). The disease can be spread by
(1) fungi overwintering on crop stubble,
(2) infected seed, and
(3) volunteer wheat plants. 
The pathogens can survive on crop stubble above the ground for three years and in, or on, wheat seed for at least one year.

Septoria-Stagonospora Leaf Spot Complex (top) Bacterials

Septoria/Stagonospora leaf spot can occur across a range of temperatures (50-81˚? F). Like leaf rust and tan spot, a period of leaf wetness is essential 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.  If it turns dry, not only do infections end, disease development is also stopped dead in its tracks.

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 (pycnidia) 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.

Rotating out of small grains for two years reduces residue-borne inoculum of Septoria/Stagonospora leaf spot complex.  In addition, volunteer wheat, rye, barley and wild grasses should be destroyed before rotating the field back into wheat. Spring wheat varieties have different resistance levels for the disease. Varieties with moderate resistance to one or more of the fungi should be used if there is a history of the problem in the field.  Fungicides may be necessary if wet weather is forecast and the disease is present.

Bacterial Stripe
Bacterial stripe, also known as black chaff, is a periodic disease of wheat. Like many diseases, it’s extremely dependent on weather conditions for disease development. 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.

The bacterium that causes bacterial stripe (Xanthomonas t. pv. translucens) 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. Even so, bacteria have been found to multiply much faster at warm rather than cool temperatures.  The disease is spread by raindrop splash.  Plant-to-plant contact during windy periods or during field cultivation activities also spreads the disease.

All aboveground portions of a plant can be attacked, from the first leaf all the way up to the head.  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.

If the weather remains wet, leaf stripes multiply and entire leaves can be killed.  Sometimes, before dew dries in the morning, you can see slimy, thick, creamy-yellow colored droplets at wounds. Droplets contain masses of bacteria that can be spread to healthy plants via rain, wind, insects, tractors, etc. Infection of heads produces black streaks on glumes and can result in a range of symptoms from complete head sterility to kernel contamination with bacteria.

Symptom severity depends on when the infection takes place.  Generally, if 50% of the flag leaf is killed, it results in a 20% average reduction in yield.  However, yield reductions are more commonly 10% or less.

Management strategies have been difficult to evaluate since bacterial stripe is a periodic problem. Crop rotation may help, 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.  If the disease was a problem in your field last year and you saved wheat for seed, it’ll be contaminated. Spraying the crop with fungicides won’t help; the chemistry isn’t effective against bacteria. 

—Charla Hollingsworth,
extension plant pathologist,
University of Minnesota, Crookston

Foliar Fungicides, Liquid N May Not Tank Mix Well
Interest in application of foliar N to enhance wheat proteins has resulted in questions about tank mixing with foliar fungicides. Studies on foliar N application (without added fungicide) by Greg Endres and Blaine Schatz at the NDSU Carrington research station and by John Wiersma at the U of M Crookston, Minn. station have indicated that foliar nitrogen 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.

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. 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 all possible registered herbicide/fungicide combinations.

– Terry Gregoire, NDSU
Extension Service

Small Grain Fungicide Use Decision Guide Online
SDSU extension plant pathologist Marty Draper offers a Small Grain Fungicide Decision Guide designed to help the producer or applicator determine if a foliar fungicide applied to the crop will return an economic benefit.

The decision guide is in the form of a Microsoft Excel ver. 4.0 spreadsheet, which is used by inserting the values appropriate for your operation. The spreadsheet will make all the necessary calculations. Examples are presented and a guide to possible values is included.

Generally, says Draper, a yield increase of 10-15% will be attained from a fungicide application. The cost of the fungicide product, application costs, and the market price for the commodity are the greatest variables that will influence profitability of fungicide application.

The fungicide decision guide can be downloaded from the web site http://plantsci.sdstate.edu/planthealth/Scab/fungguid.htm or contact Draper, ph 605. 688.5157, email draper.marty@CES.sdstate.edu .

White Wheat Heads May Stem From Root Rot
White heads in wheat at the milk to soft dough stage can occasionally be found in wheat fields. Some instances may be due to wheat stem maggot injury, others to root rot infections. When root rot causes white heads, the whole plant generally turns whitish to pale grey-green and the plant easily pulls from the soil. Roots and crowns often are discolored brown on these symptomatic plants, and the crown area is brown and “cheesy,” instead of healthy crowns, which are cream to white colored with the cell structure intact. An NDSU bulletin online has more information on common root rot: Go to www.ext.nodak.edu/extpubs/plntdise.htm then click on the link, “Root and Crown Rots of Small Grains.”

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.

FHB Forecasts Predict Scab Risk
Forecasting models are available again this growing season to enable wheat and barley growers to track development of Fusarium head blight (FHB, scab) and time fungicide treatment for scab suppression if needed.

The NDSU Small Grain Disease Forecasting System can be found online at www.ag.ndsu.nodak.edu/cropdisease/cropdisease.htm. A similar forecasting system for Minnesota can be found online at http://mawg.cropdisease.com The web site includes a plant growing degree information, current weather conditions, and a short-term weather forecast.

The FHB forecasting systems are based on locally available weather information such as relative humidity, temperature, and hours of rainfall. The forecasting information attempts to alert producers to the potential for an environment that promotes Fusarium spore production and spread beginning seven days prior to wheat flowering. Growers are encouraged to use the models as another management tool, but cautioned against considering predictions as absolute.

FHB forecasting is made possible in part by the U.S. Wheat and Barley Scab Research Initiative.  The forecasting model was made available in Minnesota by Wheat Checkoff dollars, the Northwest Research and Outreach Center small grains pathology project, Meridian Environmental Technology, Inc., and the NDAWN system.

Folicur Available for FHB Suppression
The EPA in April granted another Section 18 label for the use of Folicur® 3 .6 F fungicide on wheat and barley to control Fusarium head blight or scab.  One application containing a maximum of 4 fl. oz. per acre of the product can be applied by ground or air on either crop for scab control. The required plant growth stage for application on wheat is beginning flowering (Feekes 10.5.1) and for barley is 50% heading (Feekes 10.5). Section 18 labels for Folicur® can be found at www.nwroc.umn.edu by going to ‘Research areas’ then ‘Plant pathology’ then ‘Section 18s’ or www.smallgrains.com . Producers must have a copy of the applicable label with them during application. Please read and follow all label directions.

Fungicides Differ in Suppressing FHB, DON
Fusarium head blight wasn’t a widespread production problem for wheat growers in the Northern Plains last year.  Have newer, more scab-tolerant wheat varieties (such as Alsen and Hanna) and use of fungicides won the battle in preserving yield and kernel quality? Certainly they help, but scab still has the potential to cause large losses in small grains if an extended period of humid, warm weather precedes and overlaps the critical growth stages when plants are susceptible to infection.

Nevertheless, fungicides are another tool that producers can use to manage scab.  Growers have the freedom to choose from one protective and two systemic fungicide chemistries (triazole and strobilurin) for a wheat head application. Folicur, a Bayer CropScience product with a triazole-based active ingredient, remains the most effective fungicide treatment for controlling scab and preserving grain quality available to producers.  Research has indicated that other triazole-based fungicides such as Tilt (Syngenta), Bumper (Makhteshim Agan of North America), and PropiMax (Dow AgroSciences) are known to be less effective than Folicur, but are more effective than strobilurin-based chemistries such as Quadris (Syngenta) and Headline (BASF) for managing kernel quality issues resulting from scab.

Strobilurins are effective at preserving yield, and they visually appear to reduce the disease symptoms of scab on wheat heads.  However, when exposed to certain environmental conditions, which are yet to be determined, research indicates that the chemistry can cause increased levels of DON, as was seen last year in Crookston.

Producers should avoid applying strobilurin products to wheat when heads are emerging or exposed, since environmental conditions may contribute to decreased kernel quality.  The strobilurin chemistry offers effective control of foliar diseases, however, and can be used without risk of increasing DON levels at earlier plant growth stages.

– Charla Hollingsworth, U of M small grains extension plant pathologist

• Tips for Better FHB suppression
  For ground application, angle spray toward grain heads, using 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.
• Increase spray volume for durum and barley to improve head coverage and disease control.
• 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.
• Use a good adjuvant. Research at NDSU has indicated that Folicur and Tilt perform better with a non-ionic surfactant than without.
• Use dew as additional water.
• When using aerial application, spray in evening or early morning to capture dew as extra water volume, and use a small droplet size.
• Use NDSU's disease forecasting system (www.ag.ndsu.nodak.edu/cropdisease/cropdisease.htm, call toll free 1-888-248-7357 <231-6601 in Fargo area> for recorded updates) for determining the need for fungicide application, or use general guidelines based on yield potential, price of crop, disease presence, past week’s weather and two-week weather forecast.

 –Marcia McMullen, NDSU
extension plant pathologist