How to prevent tomato leaf spot

News Leader reader Rosalie writes with the following dilemma: “Last summer was a disaster for my tomatoes.  The leaves got spots on them and then they died. I spent the summer clipping off the leaves as they died, all the while hoping that eventually some tomatoes would form and ripen.  We ended up with about half of a crop.  What can I do this summer? There are no beds that aren't infected.  I feel discouraged before I even begin.”

First of all – don’t give up the ship. Preventing “leaf spot” in tomatoes is a challenge, but certainly worth pursuing.

What you describe as “leaf spot” may be one of several different fungal diseases in tomatoes. One of the most common in our region is Septoria Leaf Spot. The first leaves affected are typically toward the bottom of the plant. Leaves develop small, dark spots that rapidly enlarge to ¼ inch and have a gray or tan center.

My first question concerns how you water the plants – is it generally late in the evening? If so, be aware that moisture which stays on the leaves on warm nights can lead to numerous problems.

Watering tomatoes has a lot to do with basic physiology. Dew forms late at night and builds until the following morning. Most plant cells ‘open up’ at this time to accept some of this moisture through their leaves.  Research finds this is the basic principle behind foliar feeding (spraying liquid plant food directly on plants to feed them, which you should only do in the morning).

My dad always told me to “never put a tomato plant to bed wet.” It was his way of teaching me that disease-prone plants (including tomatoes) are susceptible to moist fungi if allowed to stay wet all night long. Watering in the morning, however, allows the rising sun to quickly dry foliage off before disease spores can multiply. If you must water at night, water only the root zone, keeping foliage dry.

Other techniques to prevent Septoria Leaf Spot include:
* Improve air circulation around the plants by spacing them farther apart. 
* Mulch the base of tomato plants. Mulching reduces splashing soil, which may contain fungal spores.
* Practice crop rotation and never plant tomatoes in the same bed where diseased tomatoes grew the year before. Wait 2-3 years before replanting tomatoes in these areas.
* As a last resort, use fungicidal sprays approved for vegetables.

Jeff Ishee writes about farm and rural issues and produces a daily farm report for radio stations in seven states. He is also host and producer of the nationally broadcast TV series “Virginia Farming.” Contact him at jeff@onthefarmradio.com.