Hello,

Thank you for using the Ask an Expert service. A lot of info coming at you below! Hope some of it helps.

It is hard to say for sure what your issue is without photos or a sample. What does the problem on the fruit look like? It is possible that the spotting is caused by a biological plant pathogen, but it could also be caused by an abiotic issue like a nutrient deficiency. Or multiple issues.

First, let’s think about some general plant health concerns. It’s good that you know to avoid overhead watering! Do your tomatoes get consistent watering, so that the soil is moist, but not overly wet? How is your soil drainage? Do the tomatoes get full sun (6-8 hrs per day)? Where are the tomatoes planted? (The ground, pots, raised beds?) Do you fertilize the tomatoes? Especially if the tomatoes are in the ground or a bed filled with regular garden soil, you might want to consider getting a soil test to make sure the soil still has a sufficient amount of nutrients.

There are a few common fungal foliar diseases on tomatoes in MI. Early blight is a very common issue, late blight occurs and can be devastating when the conditions are right for it. A disease called Septoria leaf spot also can also cause spots on your tomato plants. Yes, some of the spores from these pathogens might overwinter on plant debris on or in the soil. Bacterial pathogens could also potentially be an issue. It is best to try to identify the specific disease you have before treatment.

If you’d like, during the season if the same issue starts to occur, you can send photos or a sample to MSU Plant and Pesticide Diagnostic Services. Photos are free, routine plant sample analysis is $20.

Some ideas about reducing transfer of inoculum from the soil to the tomato plant:

1. If possible, do not plant the tomatoes in the same spot that they were last year.

2. Consider planting in raised beds or LARGE pots with fresh soil/potting medium.

3. Adding mulch around the plants can help prevent some of the pathogen that may be harbored in the soil from splashing up onto the upper part of the plant.

4. In the MSU tip sheet “Organic Management of Early Blight on Tomato” it notes “The biopesticide Tricchoderma harzianum (PlantShield HC, RootShield G) has shown fair results when used as a drench at planting.” I’m not sure how easy these products are to find. Always read and carefully follow all pesticide instructions. The label is the law. Source: https://veggies.msu.edu/wp-content/uploads/2016/06/FS_Organic-Management-of-Early-Blight-on-Tomato.pdf

If you decide that a fungicide or bactericide is necessary to protect the upper parts of the plants this season, look for a product where the label mentions that it can be used for the specific crop you’d like to protect (tomatoes), and against the specific diseases/pathogens that you think you have in your garden. Please see the resources I listed below for fungicide options. Make sure to READ AND FOLLOW all label instructions when using a pesticide product. The label is the law.

I also recommend looking for disease-resistant tomato varieties. Cornell has nice tables of disease resistant vegetable varieties. They separate the tomatoes into cherry, grape, heirloom, plum, slicer. The couple tip sheets I’m going to list for resources on early blight and late blight on tomato also have a few variety suggestions each. You can also look for notes about disease resistance or tolerance when browsing transplant varieties in the garden center.

Plus, check the roots, soil line, and upper parts of the transplants in the store for potential disease symptoms before purchasing plants!

Some resources:

Organic Management of Early Blight on Tomato from MSU Extension

Organic Management of Late Blight on Tomato from MSU Extension

A Pocket Guide to Common Diseases in the Urban Garden from MSU

Homeowner’s Guide to Fungicides from the University of Kentucky, updated 2019

Home Garden Fungicides from the University of Wisconsin-Madison, revised 2012

Tables of Disease Resistant Varieties available for the following crops… from Cornell University – Vegetable MD Online

Tomato Disease Identification Key from Cornell University – Vegetable MD Online

Tomato diseases, insects and disorders webinar from MSU Extension

Also consider going to our MSU Extension Gardening in Michigan website and checking out our resources on growing vegetables, including our Smart Gardening vegetable tip sheets.

Please let me know if you have further questions,

Regards,

Irene