Some variants cause more severe problems on tomatoes others are more of a problem on potatoes. There are several variants of this organism. Where does late blight come from? Late blight is caused by the fungus-like water mold Phytophthora infestans. Other tomato and potato diseases such as Septoria leaf spot (see University of Wisconsin Garden Facts XHT1073), early blight (see University of Wisconsin Garden Facts XHT1074) and Verticillium wilt may be misidentified as late blight. Affected leaf, stem, fruit or tuber tissue often eventually develops a white-gray, fuzzy look as the late blight organism begins to reproduce. Potato tubers with the disease develop a reddish-brown discoloration under the skin and these areas may become sunken. Tomato fruits with late blight develop large, often sunken, golden- to chocolate-brown, firm spots with distinct rings. If weather conditions are cool and wet, entire plants can collapse and die from late blight in seven to 10 days. Stems can also exhibit dark-brown to black areas. What does late blight look like? On leaves of tomato or potato, late blight begins as pale-green or olive-green areas that quickly enlarge to become brown-black, water-soaked, and oily-looking. More recently, the disease caused severe losses for fresh market tomato producers and home gardeners in Wisconsin. Historically, late blight was responsible for the Irish potato famine, which led to the death of at least one million Irish, and the emigration of another one million Irish to Great Britain, the United States, Australia, Canada and other countries. This disease also affects, although typically to a lesser extent, eggplants and peppers, as well as related weeds such as nightshade. What is late blight? Late blight is a destructive disease of tomatoes and potatoes that can kill mature plants, and make tomato fruits and potato tubers inedible. Hopefully, the nights and days in the coming weeks will be in the normal range, and you will see more fruit set on your plants.Late blight can decimate tomatoes and potatoes in seven to 10 days if weather conditions are cool and wet.Īmanda Gevens, Anna Seidl, Brian Hudelson, UW-Madison Plant Pathology I think the late March planting and cool nights are major factors as to why you have fewer tomatoes. Tomatoes tend to drop their blossoms when the night temperatures are consistently below 55 degrees. If you recall, a couple of weeks back (in April), we had several nights cooler than normal. Tomatoes grow best at daytime temperatures of 70 to 80 degrees and nighttime temperatures between 60 to 70 degrees. Since you planted in late March, that could be part of the problem. For those who planted in February, we had some unseasonably warm weather and those gardeners are reporting lots of fruit. We also discussed planting times and the weather. And, you tell me that they are getting ample sunlight and sufficient water. After discussing the growing conditions of your tomatoes in detail with you, I don't think this is the problem. Excessive nitrogen application can lead to large, green, healthy plants but little to no fruit. When I am asked the question about the absence or sparseness of tomatoes, my first thought is that too much nitrogen fertilizer has been applied. My tomato plants are healthy, but don't seem to have as many tomatoes as past years.
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