(News article for May 22, 2020)
Besides diseases that kill tomato plants outright, as discussed in last week’s article, there are leaf spot diseases that can reduce plants’ productivity and sometimes infect fruit.
Almost all leaf spot diseases – on tomatoes and other plants – are favored by water on the leaves. We can’t control when it rains, but we can avoid watering plants from overhead in the late afternoon or evening.
If you use a sprinkler or spray water around plants with a hose, do this in the early morning so that the leaves dry quickly rather than staying wet for longer, as they typically will when watered late in the day. Using a soaker hose or drip irrigation is preferable with respect to disease management, since it avoids wetting leaves altogether.
Other ways to minimize leaf spot problems include not working in the garden when leaves are wet from rain, dew, or irrigation; rotating where in the garden you plant tomatoes; removing old plants at the end of the season; and doing a good job of managing weeds in the garden.
When leaf spot diseases are present, fungicides can be applied to minimize spread, or to protect not-yet-infected leaves. Home garden fungicides containing chlorothalonil are common. The fungicide mancozeb is another option but cannot be applied within 5 days of harvest. These help protect plants from most fungal leaf spots.
Copper-containing fungicides (active ingredients like copper octanoate, basic copper sulfate, or copper diammonium diacetate complex) have the benefit of protecting plants from leaf spots caused by bacteria as well as fungi. When bacterial leaf spot is known to be the problem, a combination of copper and mancozeb is suggested.
If you’re spraying plants to protect them from leaf spots, they should be sprayed regularly to maintain protection. Fungicides vary with respect to how frequently they can be applied, but most are allowed once every 7 days.
Be sure to read and follow label instructions when using any fungicide or other pesticide.
Let me know if you have questions.
Contact Mary Helen Ferguson.
Early blight on tomato. (Photo source: Rebecca A. Melanson, Mississippi State University Extension, Bugwood.org)
Phoma blight on tomato. (Photo source: Don Ferrin, Louisiana State University Agricultural Center, Bugwood.org)
Bacterial spot on tomato. (Photo source: Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org)
The LSU AgCenter and the LSU College of Agriculture