Features Agronomy Diseases
Manitoba potato disease report: July 20

The late blight strain in Idaho has been confirmed as US#8, which is A2 mating type and mefanoxam/metalaxyl insensitive.

Wisconsin’s five reports have all been US#23 (mefanoxam/metalaxyl resistant).

Extensive rains from July 13 to 17 have taken the disease severity risk values over the critical threshold of 18 in many places in the province. We will now change the risk calculation model from Wisdom to modified Tomcast. The DSVs accumulation on a seven-day period will now be more critical for fungicide scheduling. A seven-day change of 0-3 is considered low risk, a change of 4-8 is moderate risk, and a change of 9 or more is high risk.

With a few sunny days break, rains are forecast again for July 23 and 24. Use of systemic (translaminar) fungicides along with a protectant fungicide will be a good strategy to stay ahead of the late blight disease.

The crops in general are doing well, with good soil moisture and temperatures. The crops are settling down, creating wet conditions under the canopy in many low-lying areas. This may be time for scouting for sclerotinia stem rot in such areas. Even though the disease is minor on potatoes, it can create messy vines for harvest. Fluazinam fungicides have activity against late blight and sclerotinia mold.

Scouting for late blight in fields and home garden potato and tomato plants is critical now that the weather is going to be wet for quite a few days.

Early blight disease has started showing up (low incidence) in some RB fields; Ranger Russets are showing a slightly higher level of incidence. With lots of in-canopy humidity and warm temperatures, and occurrence of morning dew, scouting for the progress of early blight is important in early maturing and susceptible varieties. Fungicides and combinations with dual activity against late blight and early blight should be considered, keeping in mind the fungicide resistance management strategy.