Rear Admiral Grace M. Hopper, the grandmother of the computer age, once said that one accurate measurement was worth a thousand opinions. Plant pathologists took Grace’s words to heart and have developed many disease management decision support systems, including ones for the economically important diseases in wheat and barley.
Leaf Wetness Duration Period is Key
Many of these disease management decision-support systems use weather data to calculate leaf wetness duration periods (the consecutive number of hours with free water on the plant’s (leaf) surface) and compare that value with the minimum length of time needed for a fungal spore or bacterium to germinate and grow/penetrate the plant's epidermis (skin).
Why Not Look for Fungal Spores?
Most systems DO NOT measure the presence of the fungal spores. This may seem counterintuitive or even negligent, but it is not as disastrous as you might think. While it is technically feasible to sample air for spores, it is extremely time-consuming to correctly identify the fungal species based on the trapped spores. Also, depending on the pathogen, dispersal of the fungal spores varies from just a few feet to hundreds of miles. This means that you would need multiple spore traps in every field to get an accurate measurement to make the data useful.
Luckily, diseases like tan spot and leaf rust need to go through at least two, if not three, generations to reach economically damaging levels. This means that you have time to scout and accurately determine whether the disease is even present in your field. This does not hold true for Fusarium head blight (FHB), which only needs one generation per season to create damage. Spore traps are, however, not useful to predict the risk of this disease either. Fusarium graminearum has a wide host range, and we have learned over the years that spores are nearly everywhere every single season. We just need the right weather conditions for troubles to start.
Where Can You Find Weather-Based Models for Small Grain Diseases?
• NDSU Small Grains Disease Forecasting Model (for North Dakota and Minnesota): https://www.ag.ndsu.edu/cropdisease
• National Fusarium Head Blight Prediction Center: https://www.wheatscab.psu.edu/
How Do You Interpret and Use These Models?
The small grains disease management decision support systems report the daily risk for successful infections. How do I use that data to decide whether or not to apply a fungicide?
Tan spot
Start scouting for the disease after you have accumulated 6 to 8 days in an 8 to 10-day period favorable for disease development. There will be three growth stages to consider a fungicide application. First, add a fungicide to your herbicide program when you can find the disease before jointing and the wheat variety is (moderately) susceptible (a rating of 5 or higher) to tan spot. Apply a fungicide when the flag leaf is fully extended (Feekes 9) and you find tan spot lesions on the penultimate leaf or the 6th leaf, and the wheat variety is (moderately) susceptible (a rating of 5 or higher) to tan spot. Any fungicide application made at the beginning of anthesis (Feekes 10.51) will control tan spot for the remainder of the season. Know that the tan spot model also works reasonably well to predict the risk of infection for stripe rust in wheat.
Leaf rust
Start scouting for the disease after you have accumulated 6 to 8 in an 8 to 10-day period favorable for disease development. There will be two growth stages to consider a fungicide application. Apply a fungicide when the flag leaf is fully extended (Feekes 9) and you find sporulating leaf rust in the field, and the wheat variety is (moderately) susceptible (a rating of 5 or higher) to leaf rust. Any fungicide application made at the beginning of anthesis (Feekes 10.51) will control leaf rust for the remainder of the season. Know that the leaf rust model also works reasonably well to predict the risk of infection for crown rust in oats.
Stagonospora nodorum blotch and Septoria leaf blotch
Start scouting for the disease after you have accumulated 6 to 8 days in an 8 to 10-day period favorable for disease development. There will be two growth stages to consider a fungicide application. Apply a fungicide when the flag leaf is fully extended (Feekes 9) and you find lesions on the penultimate leaf, or the 6th leaf, and the wheat variety is (moderately) susceptible (a rating of 5 or higher) to Septoria. Any fungicide application made at the beginning of anthesis (Feekes 10.51) will control Septoria for the remainder of the season.
Fusarium Head Blight
Apply a fungicide labeled for suppression of FHB when barley (and oats) has fully headed (Feekes 10.5) or when wheat (and rye) has reached the beginning of anthesis or pollen shed (Feekes 10.51) any time the risk model has indicated moderate to high risk for 3 or more consecutive days just before and during the application window. The National Fusarium Head Blight Prediction Center’s model forecasts the risk for 2,4, and 6 days out. Include the number of days the national model indicates moderate to high risk in your count of cumulative days for risk for FHB. In other words, spray a fungicide to suppress FHB if the risk was low in the days leading up to the optimum timing to apply the fungicide, but the forecast indicates 3 or more days of moderate to high risk.
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