In the late 1990s, summer blight (Phytophthora infestans) devastated tomatoes in the Pacific Northwest. This disease is worst in cool, wet summers and least troublesome in hot, dry years. When late blight strikes, the symptoms first appear on leaf veins, stems, and green fruit as water-soaked spots that quickly enlarge into dark or purplish black areas that kill the affected plant parts. On ripening fruit, cream-colored concentric areas grow and merge over the whole fruit. Late blight can affect potatoes as well; it's the same disease that caused the Irish potato famine of 1845--46.
One way to protect tomatoes against this disease is to erect plastic-covered shelters over plants. The shelters keep tomato leaves dry ― and spores of late blight must land on wet leaves to infect the plant. However, shelters like this aren't effective in areas that get soaking fogs. Also, overhead watering defeats the shelter's purpose.
These shelters, built by Chuck and Mona Pinches on Thetis Island, British Columbia, use 2-by-2 stakes to support gables covered with 6-mil, ultraviolet-resistant plastic. The tops can be lifted off to make it easier to harvest fruit in the center of the 3- to 4-foot-wide rows. The Pincheses found that besides saving their crop from blight, the shelters also prevented cherry tomatoes from splitting after late summer rains.