In dry range conditions avoid spraying when very hot and dry as products like Tordon photo decomposes quite rapidly. If your near ground water the best is 2,4-D amine. If the water table is 10 to 25 feet deep Curtail is preferred and if the water table is deeper than 25 feet Tordon is preferred. Need to pre-bait to improve performance. Call Jim Knight at 994-5579 for additional suggestions.
Late blight is caused by a residue or seed borne fungus.
Montana has not yet had serious problems with the disease but the potential is always there if mild humid conditions exist during the growing season. All gardeners in Montana can help prevent the introduction by planting only certified seed potatoes.