Tomato Leaf Spot Diseases

(News article for June 17, 2019)

I mentioned last week that the causes of tomato wilt I was talking about were different from the leaf spot diseases that we often see on tomato plants. I’ll address leaf spot diseases this week.

The best known of the tomato leaf spot diseases may be early blight, but there are a number of others, including target spot, Phoma, leaf mold, Septoria leaf spot, gray leaf spot, bacterial spot, and bacterial speck. Late blight – or the disease behind the Irish potato famine – is occasionally seen, but our temperatures during tomato-growing season are often warmer than ideal for late blight.

There are several simple practices that can help you avoid leaf spot diseases.

One mistake that gardeners sometimes make is to water plants from overhead – whether with a sprinkler or hose – in the late afternoon or evening. In terms of water conservation, this is a good practice, since not as much water is likely to be lost to evaporation than at mid-day. However, in terms of disease management, this is not a good idea.

Watering from overhead in the evening means that water that gets on plant leaves is likely to stay there for quite a while. In general, the longer water remains on plant leaves, the more likely a fungal or bacterial pathogen is to infect the plant. A better practice that still conserves water is to water in the early morning. In this case, water evaporates from plant leaves as the sun comes out and temperatures increase.

Another way to avoid problems associated with overhead watering is to apply water directly to the soil, since the point of watering is to get water to the roots. In a home garden, this can be done by hand (with a hose or watering can) or by using a soaker hose.

Our commercial vegetable and strawberry growers typically use drip irrigation, which also accomplishes the objective of getting water directly to the soil and not on the leaves. There are drip irrigation kits that are available to home gardeners, as well.

Of course, when we have rain, leaves are going to get wet regardless of how you irrigate, and the large amount of rainfall we get is one reason that leaf disease management is often more of a challenge here in the southeastern United States than in other parts of the country.

Another thing you can do to prevent spread of leaf diseases is to avoid working in the garden when leaves are wet from rain, dew, or irrigation. Diseases are often spread by fungal spores or bacteria carried in water.

Last week, I mentioned crop rotation, or rotating where in the garden you put plants that are susceptible to the same diseases. This practice is helpful for managing some leaf spot diseases, too. Some leaf pathogens, like the one that causes early blight, can survive from year to year on plant debris in the soil. A rule of thumb is not to plant the same thing (or another plant susceptible to the same diseases) in the same place more than once every three to four years.

Along the same lines, remove your old plants after the season is over, to minimize the amount of plant debris left in the garden.

Something that we might not always think about is that weed management can help with leaf spot disease management. One reason for this is that weeds can interfere with air flow within the garden, and good air movement is important for helping leaves to dry more quickly. Another reason is that some of the leaf pathogens can survive on certain weedy plants and spread to tomatoes from those weeds.

Fungicides can be useful in preventing the spread of leaf diseases. For home gardens, fungicides with the ingredients chlorothalonil or mancozeb are available. “Fungicides” with copper-based ingredients can be used for bacterial leaf diseases, as well as for most fungal diseases. A combination of a copper-based product with mancozeb may improve bacterial leaf spot control over that provided by either product alone.

Unfortunately, a couple of studies from other states have found that 95% or more of isolates of the bacterial leaf spot pathogen were resistant to copper. (When we repeatedly use any one type of pesticide, the pest tends to become resistant to that pesticide.) I am not aware that any such studies of copper resistance have been done here in Louisiana and so am not sure how effective we can expect copper products to be against bacterial leaf spot.

To be effective, fungicides should be used preventatively, before disease is widespread. Read and follow label instructions. When using pesticides (fungicide, insecticide, herbicide, etc.), “The label is the law.”

This article addresses tomato leaf spot diseases in general, but the management practices that work best do vary among the different diseases. If you need help diagnosing vegetable problems, please contact me.

Contact Mary Helen at mhferguson@agcenter.lsu.edu.

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 Cooperativ Extension Slide Series, Bugwood.org)

7/2/2019 4:36:44 PM
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