Section 5 - Disease Management

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.

During 2004, five on-farm disease management experiments were conducted within commercial production fields located from north to south in the Red River Valley (near Kittson, Strathcona, Oklee, Perley, and Fergus Falls). During 2005, experiments were again planted at similar locations (near Kittson, Strathcona, Oklee, Perley, and Rothsay). Unfortunately, stand losses due to severe weather and flooding resulted in tests at the Kittson and Strathcona locations being discontinued. During 2006 and 2007, tests were planted near Oklee and Fergus Falls while in 2008 trials were established near Fisher and St. Hilaire. In total, from 2004 to 2008, disease severity from 25 environments (location + variety flowering period) was tracked and ground-truth data were collected from replicated, small-plot experiments consisting of data from five to 15 wheat varieties.

During 2004, the FHB risk forecasting model predicted an elevated (moderate to high) risk that an FHB epidemic would occur in two of the five environments, but none were observed. During 2005, it predicted an elevated epidemic risk in one of six environments. An epidemic occurred at that location, as well as at another location where the system predicted a low risk for an epidemic. During 2006-2008, the model predicted low risks in all environments, and no epidemics were observed.

During the last five years, Minnesota’s forecasting system has been incorrect three out of 25 situations. It predicted two epidemics which didn’t occur, and failed to predict one that did.

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.ndsu.edu/ndsu/diseaseforecast

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 eventuallyform 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.

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