Sclerotinia: Does fungicide at 50% flower make sense?

July 15, 2020 – Issue 18

The application window for most fungicides closes at 50 per cent flower. (See the table at the end of the article.) This is peak flower when the field is at its yellowest. In stands that have low plant counts or are uneven, this stage can last for a week or more.

In general, late applications are not as effective as applications at 20 per cent flower because early infection tends to cause the most yield loss. But fungicide applied late in the window can provide valuable protection from sclerotinia stem rot if flowering is extended or if conditions become more conducive to disease.

Scenarios

Single first applications at 50% flower. Spraying at the end of the window may be effective — especially if branching or strong plant recovery from heat stress extends the flowering period.

Fungicide does not provide a curative benefit, so any infection present before application will not be stopped. Also, 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.

Second applications. Split applications seven to 14 days apart might be best for crops with a lot of plants at different stages or crops at particularly high risk for losses from sclerotinia stem rot (good yield potential, good moisture, longer flowering period). In an uneven crop, the first application can be made when the first plants reach 20 per cent staging. If necessary, a second application can be made when the remaining plants are ready to be sprayed. Even if most of the crop seems to be less mature, an application when those earlier plants are at the right stage may pay off – if the crop is moist and yield potential is good – because those spore-infested petals can land on less mature (pre-flowering) plants and cause infection. Early infection on these less mature plants could cause higher yield loss because the disease will have that much longer to grow. Not all products are registered for two applications. (Find the list of fungicides at the bottom of this article.) Note, two applications does not mean a “half rate” applied twice. This rate is not on label and cutting rates of pesticides can promote resistance.

Hail delays/extends flowering. Canola beat down by hail at early flowering can recover, and start to bolt and re-flower again. Once flowering resumes, 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 the crop received one of these applications before the hail, switch to a different for the post-hail application.

Applications after 50 per cent flower. By this time, infection that will cause the major economic damage is already done. Also, and this is very important, applications after 50 per cent flower are not on fungicide labels and may be inside the pre-harvest interval for some fungicides. This could result in fungicide residue on harvested seed. Check the interactive Spray to Swath Interval Calculator for pre-harvest intervals for each product.

Moisture situations that can change the economics of a spray:
–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. Regular heavy rains can also wash off petals and other flower debris from plants, which can reduce infections.
–Dry to wet. Dry conditions at the beginning of flowering can 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. Keep in mind that it most likely will take two to three weeks for sclerotia to germinate and produce spores and at this point the canola may no longer be flowering, which is necessary for the spores to propagate.
–Wet to hot/dry. Hot and dry conditions can slow the progression of sclerotinia stem rot. 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.

Dig deeper

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