Damon L. Smith, Extension Field Crops Pathologist, Department of Plant Pathology, University of Wisconsin-Madison
Diseases of wheat in Wisconsin have basically been non-existent this season. Dry weather is leading to virtually no disease issue with the exception of one disease. Like 2021, powdery mildew is starting to show up on susceptible varieties. This one fungal disease likes to break the rules of cool and wet. Let’s discuss this disease further and then dig in a bit on what you should do for disease management as we move through the rest of the 2023 winter wheat season.
So, what’s up with powdery mildew
Powdery mildew of winter wheat is caused by the fungus Blumeria graminis f. sp. tritici. The most notable sign of powdery mildew is the white, fluffy fungal growth that occurs on the surface of leaves (Fig. 1). Yellow spots may be present on the underside of the leaf. The white “tufts” might also have very small black pepper-like structures in them. Generally, the disease will start in the lower canopy, and if weather is favorable, will move up the canopy eventually reaching the flag leaf and even infecting heads on susceptible varieties.
The reason that powdery mildew has been an issue this year, despite the dry weather, is that it happens to like cool night-time conditions combined with high humidity and dew events. Warm days and cool nights often lead to dew and extended periods of leaf wetness (think semi-arid climates). This combined with temperatures less than 80 F, means the fungus can thrive on susceptible varieties where humidity has been high. Excessive rain events actually deter this particular fungus, as heavy rain events can wash spores from the leaf. So, it isn’t surprising that we are seeing powdery mildew right now given the weather we have had in parts of the state.
Should you spray fungicide for powdery mildew?
Most of the time I would say no. Often in Wisconsin, the weather begins to turn much warmer as we approach heading and the fungus will stop spreading and remain a novelty in the lower canopy. Remember, once daytime temperatures get above 80 F, the fungus will stop or slow in progression. The key in making the fungicide spray decision is to know the susceptibility of the variety you planted and watch the weather. If the weather remains conducive (temps below 80 F, no rain, but dew) and the variety is ranked susceptible, then spraying around flag leaf emergence might be warranted. You can consult the “Fungicide Efficacy for Control of Wheat Diseases” table for products rated with the best efficacy for powdery mildew. Note that most of the higher rated products are triazole compounds, or compounds with a triazole in their mix. No need to be fancy here, find something that fits your budget and has good efficacy. There should be ample choices.
I don’t care about powdery mildew, but what disease should I keep an eye on next?
Fusarium head blight (Fig. 2) has been a perennial problem for us in Wisconsin over the last few years. Not only have we seen significant damage and yield reductions due to the disease, but we have seen significant discounts at the elevator for levels of deoxynivalenol (DON or Vomitoxin) above 2 ppm. The one exception was 2021, which is sort of shaping up similar to 2023. However, it remains important to manage this disease actively here in Wisconsin.
Be sure you know the relative susceptibility of the varieties you have planted. We have excellent data showing significant reductions of FHB where we use a resistant variety and then layer a fungicide application on top. In 2019 we evaluated the susceptible variety, Hopewell, against the resistant variety, Harpoon. Figure 3 shows the FHB levels for the two varieties which were also subjected to a fungicide application. Clearly variety resistance works.
When it comes to fungicides for FHB, there are a few to choose from. These are Caramba, Prosaro, Miravis Ace, Prosaro Pro, and Sphaerex. Again the “Fungicide Efficacy for Control of Wheat Diseases” table shows the efficacy ratings of these products against FHB. Timing is everything when using a fungicide for FHB management. Be sure to time applications at the start of anthesis or within 5-7 days after the start. This is the ideal window of opportunity to control FHB and reduce DON levels in the finished grain. Spraying earlier than anthesis or later than about a week after the start of anthesis will result in lost efficacy, or no control of FHB. Also, these fungicides are effective against powdery mildew, so if that disease happens to be an issue for the variety you have chosen, a single application of fungicide at the anthesis timing should take care of both problems.
Should I spray for Fusarium head blight in 2023?
The answer to that question is a little complicated. However, there is a tool that can help with this decision. You can find the FHB prediction tool at http://www.wheatscab.psu.edu.This tool should be monitored frequently as your crop approaches anthesis and soon after. It can help you determine if your crop is at risk, based on the weather conditions. We are quickly approaching anthesis in the southern portion of Wisconsin. As of May 31, 2023, the risk for FHB in the whole state of Wisconsin is low, even on susceptible varieties. Again, dry weather leading up to this week has not been favorable for FHB. The 7-day forecast is also not looking conducive. Thus, the likelihood of a significant return on the fungicide investment is likely to be low this season. However, if you are risk adverse and would like to put an anthesis application of fungicide on, I would not be fancy with my choice. You might choose the cheapest product you can get ahold of, that is rated at least “G” on the fungicide efficacy table. This is not the season where you need to spend a lot of money on a fungicide, as the yield benefits are going to be lower due to the dry weather at heading. If you would like to study the performance of fungicide in wheat trials in Wisconsin, you can find trial data from my research going back 10 years by CLICKING HERE. Be sure to study multiple years and make sure a product was consistent in performance.
The ‘Take Home’ for wheat management over the next couple of weeks.
This can be cooked down to two main points. Here they are:
- Don’t get too concerned about powdery mildew unless your variety is rated suscpetible
- If that is the case, then plan to apply a fungicide at anthesis for FHB (see below)
- If you plan to apply an FHB fungicide application – especially on susceptible varieties
- Shoot for Anthesis or up to 5-days after the start of anthesis for any of the fungicides that are rated “G” in the fungicide efficacy table
- Can go slightly earlier (Feekes 10.5; Efficacy slightly reduced compared to typical timing) up to 5-days after the start of anthesis for Miravis Ace
- Watch the “Scab Alerts” – it isn’t perfect, but can help you make a decision (http://www.wheatscab.psu.edu)