Jul 16, 2017 - MB Potato Disease REPORT #6

Dr. Vikram Bisht,
Plant Pathologist, Potato & Horticultural Crops,
Manitoba Agriculture

There were scattered rain events across Manitoba on July 11 & 12, ranging from 2 to 18 mm. The accumulated precipitation is still below normal (50-80% of normal) in most of the potato producing areas of the province (Fig. 1).

The accumulated P-Days has gone over 300, a value above which the early blight disease could often appear. Plants under stress of moisture and nitrogen or early maturing varieties should be scouted for symptoms of early blight. Preventative contact fungicides, mancozeb and chlorothalonil are effective when the disease pressure is low.

Fig 1. Accumulated precipitation for the 2017 growing season (% of normal)
Fig 2. Potato heat units (P-Days) accumulation (% of normal) for mid-May emerged crops.

Temperatures had gone up to 30-32oC in many potato growing areas. Such temperatures may cause added stress on plants in moisture stress patches and often lead to heat runners, new stolon formation from already formed tubers; or sometimes chain of tubers. (Photo 1).

Photo 1. Due to heat stress some tubers are producing new stolons or elongating. (Photo courtesy – Darren White, Delta Ag).

So far, no late blight reported in Manitoba. Hot and dry conditions in the last few days are not favourable for late blight disease. The DSVs (late blight risk values) are still low, even with significant increase on July 12. Scattered rains are forecast in the coming week.

Late blight has been reported in Chatham-Kent Division, Ontario (Strain? On tomato, July 04), Michigan (US-23 on tomato, July11), Washington (US-8 on potato, July 6).

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