Are tomatoes and potatoes with late blight safe for eating and canning?

Late blight on tomatoes and potatoes is a devastating fungal disease that has surfaced in Michigan.

Late blight is affecting Michigan tomato and potato growers and home gardeners alike. The late blight has already been reported in Allegan, Clinton, Gratiot, Ingham, Ionia, Isabella, Macomb, Montcalm and St Joseph counties on USA blight. USA blight is a national website project on tomato and potato late blight in the United States. At this site you can report disease occurrences, submit a sample online, observe disease occurrence maps and sign up for text disease alerts. There are also useful links to a decision support system, and information about identification and management of the disease.

Late blight in both tomatoes and potatoes is caused by the oomycete Phytophthora infestans, and is a worldwide destructive disease.

Under favorable weather conditions, tomato and potato crops can be destroyed within days. Cool, moist conditions are considered most favorable for late blight to develop and spread. This blight is highly contagious to other plants in home gardens and commercial fields.

Do not can tomatoes with late blight. Tomatoes showing signs of late blight disease have firm, dark brown lesions that quickly become large, wrinkled and somewhat sunken. Diseases such as this may lower the acidity of the tomato flesh to a level that makes it unsafe for canning. Even tomatoes with the infected parts removed should not be canned. The United States Department of Agriculture Complete Guide to Home Canning recommends that only disease-free, preferably vine-ripened, firm tomatoes be used for canning.

According to Dr. Barbara Ingham, food safety specialist with the University of Wisconsin Extension, you can safety eat and preserve unblemished tomatoes growing on plants with leaves, stems or adjacent fruit showing signs of infection. However, these tomatoes are at a higher risk for developing the late blight lesions after they are harvested, so make sure to eat or process these tomatoes as soon as possible after harvesting them.

Michigan State University Extension always informs home canners of the importance of acidifying tomatoes. Adding bottled lemon juice or citric acid to your disease-free tomatoes raises the acidity level enough that Clostridium botulinum spores cannot survive, and produce a deadly toxin that causes botulism. Add two tablespoons of bottled lemon juice or a half-teaspoon of citric acid to each quart of tomatoes. For pints, use one tablespoon of bottle lemon juice or one-quarter teaspoon citric acid. Just add the lemon juice or citric acid before filling the jars with tomatoes or tomato juice.

It is also not recommended to eat fresh or frozen diseased tomatoes even if the diseased parts have been cut out. The disease organism by itself is not harmful but the tissue damage causes the tomatoes to have lower acidity and creates conditions that promote the growth of other potentially harmful microorganisms. The tomatoes may or may not have an off flavor.

The exact same advice is given for potatoes. Only use firm, disease free potatoes for eating, canning or freezing. Never use potatoes showing sign of late blight. Discard the whole potato rather than cutting off diseased parts in case it has spread to the inside of the potato. Potatoes are a low acid food and should be pressure canned.

Up to date recipes for canning can be found at the National Center for Food Preservation.

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