Late blight was reported in southern Washington’s Walla Walla County Wednesday. The weather this spring has been highly favorable for the establishment of late blight. This, combined with the presence of volunteer potatoes in southern Idaho in areas where late blight was present in 2015, make the risk for late blight in the Pacific Northwest high this year.
Mefenoxam (Ridomil-based products) is highly effective against the US-23 strain of late blight, which was the only strain found in Idaho in 2015. If this product was used in-furrow, or if foliar applications are being made at the time of dime-sized tubers and two weeks later, then the potato crop will have effective protection. Potato growers are advised to apply protectant fungicides weekly beginning just prior to row closure. This is critical to protecting the foliage during the time of rapid foliage growth and high irrigation. If blight has not developed by the end of July, then programs can be relaxed.
Source: Miller Research