To can or not to can – the tomatoes are looking a bit spotty are they safe to preserve?

Blight, frost? Tomatoes need to be disease-free and in good shape for food preservation.

Tomato season is upon us, if the weather cooperates, the tomatoes will begin to ripen and canners will begin the summer ritual of processing many jars to enjoy in the months to come. But what happens if you are one of the gardeners who experience some kind of fungus infestation in your crop, like blight? Or your last few tomatoes are exposed to frost at the end of the season? Are these tomatoes safe to process and consume?

The United States Department of Agriculture (USDA) Complete Guide to Home Canning recommends that canners select only disease-free, preferably vine-ripened, firm tomatoes for canning. It is also recommend not to use tomatoes from dead or frost killed vines. Tomatoes that have been affected by blight or other fungus related infestations may have a higher pH (decrease in acidity) of the tomato flesh to a level that makes it unsafe for canning. Tomatoes that have been damaged by frost are not recommended for canning because the soft tissue of the tomato may be quickly invaded by fungus or other bacteria.

It is recommended that any tomato showing signs of late blight disease should not be used for canning. This includes tomatoes with only minor lesions as well. There is no way to tell that the infestation has spread to the interior of the fruit and the extent of the internal infestation is not always clearly visible.

If only the plant leaves or stems have been impacted and the tomatoes are unblemished they are safe to process. Keep in mind that these tomatoes are at a higher risk for developing late blight lesions after they are harvested. It is important to consume or process these tomatoes as soon as possible after harvesting. If green tomatoes are picked to ripen indoors, monitor closely for signs of disease.

When canning tomatoes to ensure safe acidity in whole, crushed or juiced tomatoes, the National Center for Home Food Preservation and the USDA recommend:

1 Tablespoon bottled lemon juice in each pint jar

2 Tablespoons bottled lemon juice in each quart jar


1/4 teaspoon citric acid per pint jar

1/2 teaspoon citric acid per quart jar

Michigan State University Extension does not recommend freezing or consuming fresh diseased tomatoes. There is no proof that the disease organism by itself is harmful to humans, but the tissue damage and a rise in pH can create conditions that promote the growth of other potentially dangerous microorganisms. Whenever food is being prepped for long storage whether it is canning or freezing, it is important to always choose the freshest, highest quality product. The condition of the food item will not improve once it has been canned or frozen. To learn more about food preservation visit the National Center for Home Food Preservation or the Michigan Fresh website. For preventing blight and other tomato diseases, follow the recommendations by Mary Hausbeck, from MSU Extension Department of Plant, Soil and Microbial Sciences. A final thought regarding garden produce, “When in doubt, throw it out.” Don’t take any chances especially if someone in your family is immune compromised, on special medications, going through cancer treatments, has AIDS, asthma or other allergies.

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