What is causing black spots on my tomatoes?

October 17, 2023 Ask the Expert, Fruit Trees and Small Fruits, Managing Insect, Diseases, and Animal Pests

Question:

I grow Juliet tomatoes each year primarily for fresh eating and freezing for sauce. This year I planted late due to weather. Of interest, I use no chemicals/sprays etc, only addition is bone meal. My question is…what is causing black spots and are they safe to freeze/ process? Should I treat the soil in my beds with something? 

Answer:

Jonathan Foster, Special Projects Assistant

I’m sorry to be the bearer of bad news, but your tomatoes appear to be infected with anthracnose, a fungus-caused ailment of the fruit. The sunken spots are a tell-tale sign. The fruit should be ok to eat if no symptoms have developed, and we recommend harvesting slightly before full ripeness and eating the fruit just as soon as it’s ready. Sources vary on whether it’s safe to eat fruit if infected portions are cut away (contrast with the link below), so I would try to eat them fresh and unblemished if at all possible. I wasn’t entirely sure about the potential for freezing such infected fruits safely, though, so I reached out to Dr. Beth Calder, a UMaine Cooperative Extension professor and an expert on food safety. Her reply: “I like the advice from this Lancaster Farming article: https://www.lancasterfarming.com/country-life/gardening/is-that-tomato-rotten-or-is-it-safe-to-eat/article_41c8278a-35f6-11ee-8f76-c753548c4540.html.  Although the spot itself may not be a potential food safety problem, a blemish or crack can certainly allow for potential pathogens to enter the produce and that could be a potential food safety problem.  It would be safer to throw away diseased produce.” Tragic news for your harvest, but it’s better to be safe than sorry….

For next year, you will want to practice crop rotation and plant your tomatoes in a different location, as the pathogen can overwinter in your soil. As you will read in the Clemson link above, there are also chemical treatments available if you choose to go that route. Best practices for tomatoes will help, too–consistent irrigation, well-drained soil, watering the soil rather than overhead through the leaves, pruning the plants to allow airflow at the base, and promptly disposing of any debris (no composting, as any fruit tissue that survives may carry the spores). Your goal is to prevent warm, moist, still air from sitting among the leaves, which promotes growth of all fungal pathogens (anthracnose among them).