Heavy rains can increase the sclerotinia stem rot risk but they also make fields too wet for ground sprayers. This can delay timely applications, especially because demand for aerial applicators can make for long wait times. If you don’t have time to get to all crops, take care of those at the proper stage for application as they generally offer the best return on investment.
Here are scenarios that may help to work through this situation:
Applications at 50% flower: Spraying at the end of the window may be effective — especially if branching or strong plant recovery from heat or drought stress extends the flowering period.
But in general, late applications are not as effective as applications at 20-30% flower on the main stem since the main stem flowers first, and these flowers are more likely to drop on main leaves and against the main stem. These main-stem petals lead to infection that rots the main stem. Also, earlier infections have more time to develop and cause more damage to the plant.
Fungicide does not provide a curative benefit, so any infection present before application will not be stopped. A late application may stop subsequent infection, but infection that is already present can spread throughout the plant and from plant to plant, particularly if the crop is lodged, reducing the effectiveness of this late application.
Applications after 50% flower: The damage is usually done in this situation. If conditions have been moist and yield potential is high, early petals that dropped into the canopy and led to infection will likely lead to yield loss — even if the later petals are sprayed. Also important, applications after 50% flower are not on fungicide labels and may be inside the pre-harvest interval for some fungicides, which could result in fungicide residue on harvested seed.
Second applications: Split applications — two lower rate applications 7 to 14 days apart — can be worthwhile if conditions are good for fungal growth and the crop flowers for a long period. Split applications might also be effective in crops with plants at multiple stages. The first application can be made when first plants reach recommended staging and the second when the remaining plants are ready to be sprayed. If the second is not made, remember that the first lower rate application in the split will have less contact with the target fungus and less persistence on the plant than one full rate application, therefore less reduction in disease. Not all products are registered for a split application. (Find the list of fungicides at the bottom of this article.)
Hail delays/extends flowering: Canola beat down by hail at early flowering can recover, and start to bolt and re-flower again. In that case, growers may see a benefit from spraying for sclerotinia again if yield potential is above 25-30 bushels per acre, and conditions are moist (rain, humidity and/or dew). Keep in mind that some fungicides are registered for only one application per year. If you already sprayed that product before the hail, switch to a different one.
Moisture changes economics:
—Wet to excessively wet. Some growers with crops hit with heavy rains are pulling back on applications while waiting to see if the crop recovers. This could be the right strategy if yield potential has dropped, but note that in this case, the crop may have a dense canopy that is conducive to disease. Often the low-yield potential used to make the sclerotinia spray decision is due to a thin canopy.
—Dry to wet. It can also happen that dry conditions at the beginning of flowering change to wet conditions during flowering, which — with good yield potential — may increase disease risk and justify an application near the end of the spray window. This can be a good strategy in high yield conditions and high sclerotinia risk situations.
—Wet to hot/dry. Hot and dry conditions can slow the progression of sclerotinia. If timing is late, you still haven’t sprayed, the canopy had dried out and there is no rain in the forecast, then maybe the late-window application isn’t worth it.
Alternaria black spot spin-off benefit: Alternaria rarely warrants a fungicide application on its own, but the disease does seem to be a little more common this year. If alternaria lesions are present lower in the canopy and you’re thinking about a sclerotinia fungicide application, it could stop the progress of alternaria as well.
*Updated June 23, 2016. Email firstname.lastname@example.org if you have further updates.