How late is too late for nitrogen top dress?

Can it be too wet for sclerotinia stem rot infection?
A lot of Western Canada has the moisture for good canola yields and for sclerotinia stem rot infection. It will be a busy season of fungicide spraying. But can it be too wet? Not really.

Rain events that reduce sclerotinia stem rot risk would be unusual. It could take three weeks of saturated soils to kill the sclerotia resting bodies in the soil. And if some parts of the field are in this situation, most of the field could have exceptional growing conditions and high disease risk. Spores from apothecia germinating everywhere else will be moving freely throughout the fields.

Lygus large enough to pierce into pods are the focus for scouting and, if necessary, spraying. The most vulnerable crop stage for lygus feeding is after flowering and when seeds are enlarging on lower pods.

We don’t have lygus thresholds for the bud and flower stages because economic loss is rare at these stages. Research shows that low-level lygus feeding at flowering can actually stimulate yield. Spraying at flowering stages may be warranted in the rare situation where high numbers prevent canola from flowering. Lygus – timing, scouting and thresholds | Research update on thresholds

Will boron tank-mixed with fungicide help protect canola from heat blast?
There are two points here: 

Can you tank-mix boron with fungicide? Generally yes, but check with the fungicide provider. They probably have not tested every boron product. 
Does boron protect canola from heat blast? Boron deficiencies are not that common on the Prairies, but can occur in light soils with low organic matter, high pH (8+) and excess moisture. While most studies and site-years show no economic benefit to boron fertilizer, some fields may have a stack of risk factors this year. On the specific scenario of protecting canola flowers from heat blast, again the research is not supportive. Most site years show no economic benefit.
If trying boron, leave a few test strips. However, with a tank mix of fungicide and boron, it will be difficult to determine which product made the difference. It would require strips with no spray, just fungicide and just boron. Canola Encyclopedia on boron

How late is too late for nitrogen top dress?
Canola with higher yield potential could need more nitrogen. And excess moisture can increase nitrogen losses to the air or through leaching.

The ideal timing for nitrogen top dress in canola is before the five-leaf stage. That gets nitrogen into the root zone before peak uptake. Later applications could still help. AAFC research out of Indian Head, Saskatchewan, showed that nitrogen top dress as late as 5-10% flower can still provide a yield benefit.

A nitrogen top up could extend canola’s vegetative period and delay maturity, but probably not by much. We checked on this with Sally Vail with Agriculture and Agri-Food Canada, Jeff Schoenau with the Univeristy of Saskatchewan and Mario Tenuta at the University of Manitoba. Based on research and in-field experience, each suggested minimal delay in canola maturity, even with a later-than-recommended top up. 

How about sulphur? “I’m seeing patchy sulphur deficiency in fields in southern Saskatchewan that received four or more inches of rain in one shot earlier in season, especially sandy fields,” Schoenau says. “Late application of ammonium sulfate can be of benefit.” Tips to apply nitrogen and sulphur in season