Safe Preserving: Eating and Preserving Tomatoes Affected by Late Blight

canned tomatoesLate blight has hit parts of Wisconsin hard again this year. The home gardener can be worried because it’s a disease that affects tomatoes and potatoes just at the peak of harvest, and some may be wondering whether ripening tomatoes or freshly harvested potatoes are safe for eating or preserving.

Late blight is a common disease in tomatoes and potatoes caused by the fungus Phytophthora infestans. According to Dr. Luke LaBorde, Pennsylvania State University, “The disease thrives in cool, moist conditions and can wipe out an entire crop within just a few weeks of infestation. In tomatoes, the fruits may become infected initially with firm, dark brown lesions that rapidly become enlarged, wrinkled, and somewhat sunken. The rotted areas are usually located on the top of the fruit and may remain firm or become mushy.”

Both green and ripe tomatoes can be infected. Potatoes can become infected both before or after harvest, with the disease appearing as brown, dry and sunken areas. Dr. Margaret McGrath, a plant pathologist at Cornell University, has suggested that tomatoes and potatoes – those harvested from diseased plants but without blight symptoms on the vegetable flesh – likely do not pose a health risk to the consumer.

Since there is no documented harm from eating blight-infected fruit, it may be tempting to simply cut off the infected portion. But resist that temptation because the fruit will taste bitter and may be harboring other organisms that could cause food-borne illness. And resist the temptation to can or preserve diseased tomatoes or potatoes, even those where the infected portion has been removed. The virus affecting the plant tissue can cause changes in the biochemistry of the vegetable tissue.

What if you have unblemished tomatoes growing on plants with leaves, stems or adjacent fruit showing signs of infection? These may be safely eaten, and even preserved. As always, it’s important to use up-to-date, research-tested recipes to avoid the risk of botulism poisoning from home-canned tomatoes.

Here are some tips for safe canning of tomatoes.

Always add acid to tomato products. Whether pressure canning, boiling-water canning, or steam canning, research published in the 1990s shows that tomatoes may not have sufficient acid to avoid botulism toxin from forming, so a small amount of acid is always added.

Add acid to tomatoes in the proper form. When adding acid, use bottled lemon juice because it has a standard level of acidity. Add 2 tablespoons bottled lemon juice per quart and 1 tablespoon per pint. Another option is to add citric acid, ½ teaspoon per quart or ¼ teaspoon per pint. Citric acid is less widely available, but is used mainly by large commercial canneries. Other acids such as ascorbic acid (vitamin C; Fruit Fresh) or acetic acid (vinegar) are not recommended.

Always follow a research-tested, up-to-date recipe. The University of Wisconsin-Extension publication Tomatoes Tart and Tasty (B2605) was updated in 2008 to incorporate recent changes in the USDA Complete Guide to Home Canning.

 And what about potatoes? Potato ‘fruits’ showing signs of late blight infection should not be used for home canning. Discard the whole potato rather than cutting off diseased portions since the fungus may spread to the interior. However, if only the plant is diseased, and not the potato flesh that is eaten, the potatoes may be harvested and preserved.  Since potatoes are a low-acid food, they must be pressure processed. Up-to-date recipes for vegetable canning in Wisconsin can be found in the UW-Extension publication Canning Vegetables Safely (B1159). Safe preserving! Barb