Protect Your Fungicides!
2008 Cercospora Leaf Spot Management Strategies

Janice LeBoeuf, OMAFRA Vegetable Crop Specialist
Dr. Ron Pitblado, Weather INnovations Incorporated

Here in Ontario, as growers well know, we have fewer registered fungicide choices for Cercospora leaf spot control than do our US colleagues. This means that we need to take extra care to preserve the effectiveness of the fungicides we have. I would suggest that this should be one of our top priorities in sugarbeet production in Ontario. We have received an emergency-use registration for Tilt (propiconazole), which gives us a valuable tool to manage disease resistance for 2008.

Why is this so important? Until now, our two strongest fungicides for Cercospora control have been Senator and Headline. It is very easy for resistance to develop to these products. For example, in potatoes, decreased sensitivity of the early blight pathogen to Headline was detected after only two years of use. Resistance of the Cercospora pathogen to thiophanate-methyl (Senator in Canada; Topsin in US) has been documented in the Red River Valley for over a decade. Senator resistance has also been documented in some areas of Michigan, and may be present at some level in Ontario.

Critical point: If we do not rotate fungicide groups in our Cercospora program, we could lose the effectiveness of Headline, Senator, and/or future new chemistries (due to resistance development) within even a year or two.

We expect to have new fungicides registered over the next couple of years, however, if our other strong fungicides have already become ineffective due to overuse, can we ensure that the same thing won’t happen with the new products? We need to protect our existing fungicides so that we have something to rotate with when we get new fungicide registrations.

Critical point: Never, ever use Headline back to back or Senator back to back. Rotate to other fungicide groups. Headline, Senator, and Tilt are in different chemical families, so there is no reason to use any of them back to back. For 2008, we are recommending Headline for the first application, Tilt for the second application, and Senator if a third application is required.

We can still use Senator. Even in Michigan, where some resistance to it has been documented, it is still recommended for use, although only in a tank mix with one of the mancozeb-type fungicides (eg. Manzate, Penncozeb, Dithane, Polyram/metiram).

Critical point: Educate your sugarbeet growing neighbours about the importance of rotating fungicides. If they use the same product back to back, the resistant fungi won’t stay confined to their farm! It will become YOUR problem, too!

With the emergency use registration of Tilt, we can rotate among effective products to safeguard these fungicides against resistance. Check with OMAFRA or Michigan Sugar Company for updates on fungicide registrations.