Disease Management

Diagnosing Leaf Diseases in Wheat

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.”

Cultivars of spring wheat in recent years showing considerable susceptibility to current races of leaf rust include 2375, Gunner, Ingot, Forge, Russ, Oxen , Wallworth, Parshall, and Argent, to name a few. Susceptible varieties in growth stages up through flowering may benefit from fungicide use, as long as the flag leaf still has relatively few to no pustules of infection.

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 above ground 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).

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. 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 (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 above ground 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 above ground 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, equipment, 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. If flag leaf tissue is killed early on, more yield loss will result.  Yield reductions are generally 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.

Managing Disease in Winter Wheat
Current winter wheat varieties are moderately susceptible to disease and have responded positively 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 conducted from 2001 to 2004 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.ext.nodak.edu/extpubs/smgrains.htm . See the link “wheat streak mosaic.” Ducks Unlimited supports the production of winter wheat for duck habitat, and has compiled excellent northern-grown winter wheat production research data over the past few years. See the information online, http://prairie.ducks.org/Agronomy_News.

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.

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. 

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).

To help producers with “should I or shouldn’t I” spray questions, extension plant pathologists Marcia McMullen of NDSU and Roger Jones, UM, developed a point-based fungicide decision aid for wheat fungal leaf diseases a number of years ago.  It can be found online at www.smallgrains .org/Springwh/jun98/wiersma.htm or under “Production and Research Info” at www.smallgrains.org

As well, 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/. Click on “Small Grains” then “Small Grain Fungicide Use Decision Guide.”

Tips for Better FHB suppression

• 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 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.
• 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.

FHB, Disease Management Online

NDSU -- www.ext.nodak.edu/extpubs/plntdise.htm and www.ag.ndsu .nodak.edu/extplantpath/

SDSU -- http://plantsci.sdstate.edu/planthealth/

UM (NWROC- Crookston) -- www.nwroc.umn.edu – Click on “Research Areas” then “Plant pathology.” Here, there is fungicide label information, disease forecasting, and an FHB management bulletin released last year (Direct link: http://www.nwroc.umn.edu/Fusarium.pdf )

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

The NDSU Small Grain Disease Forecasting System can be found online at www.ag.ndsu.nodak.edu/cropdisease/cropdisease.htm . (Recorded updates and information by phone: 1-888-248-7382, 231-6601 in Fargo area)

A similar forecasting system for Minnesota can be found online at http://mawg.cropdisease.com and South Dakota at http://plantsci.sdstate.edu/smallgrains path – Click on “SD Scab Advisory.”

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, as weather conditions and disease levels can change daily.

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.