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Spot check: look for tomato diseases now

Young tomato plant with lower leaves infected with a leaf spot disease.
M. Grabowski, UMN Extension

Tomato leaf spot diseases are just beginning to appear on Minnesota tomato plants. Haven’t seen them yet? Take a closer look.

Most leaf spot diseases in tomato overwinter in the soil and then splash on to the lower leaves of the plant. As a result the first leaf spots can be found on the lowest leaves. To find them you may have to push aside the upper leaves and peer down at the leaves closest to the ground.
 

What to look for?

Look for brown to black round spots that are the size of a pencil tip or larger. There are three leaf spot diseases commonly found on garden tomatoes in Minnesota; Septoria leaf spot, early blight and bacterialspot. At the earliest stages of disease, they are difficult to tell apart but the management practices listed below will work for all three disease problems.

 
Spots from Septoria leaf spot on a tomato leaf.
M. Grabowski, UMN Extension 

My tomato has leaf spots, what do I do?

Pinch off leaves with leaf spots and bury them in the compost pile. It is ok to remove up to 1/3rd of the plants leaves. This should not be a problem if you catch the disease early but if the leaf spot disease has spread throughout the plant, remove only the worst infected leaves. Do not remove more than 1/3rd of the plants leaves.

Many gardeners will remove the lower third of the leaves on every plant regardless of if they have leaf spots or not. This makes it harder for plant pathogens in the soil to get splashed on to the lower leaves. This practice also improves air circulation around the plant and allows the leaves to dry quickly after rain or irrigation. Fungi and bacteria need moisture on the leaf surface to start a new infection so keeping leaves dry is important.

 Why should you remove leaves with leaf spots?

Each leaf spot produces hundreds of fungal spores or bacteria that can be splashed or blown onto other leaves resulting in even more leaf spots. Under the right weather conditions, these new leaf spots produce more spores or bacteria in as little as 2 weeks.

This cycle can repeat throughout the summer, resulting in brown blighted plants. By removing leaf spots early, you slow the spread of the disease through the plant.
 
Staked tomatoes with the lower leaves pinched off to prevent disease.
M. Grabowski, UMN Extenssion

What else can I do?

Cover the soil below the tomato plants with mulch. This will reduce the ability of pathogens in the soil to splash onto the lower leaves. Landscape fabric, straw, plastic mulch, or dried leaves are all acceptable mulches. Just remember that tomatoes are sensitive to many herbicides used in lawns so do not use grass clippings from a lawn that has been treated with an herbicide.

Water the soil not the leaves. The fungi and bacteria that cause leaf blight need moisture on the leaves to start infections but tomatoes only take up water through their roots. So keep the water in the soil where the plant can get it and keep the leaves dry to reduce problems with plant disease. This can be done by using drip irrigation or a soaker hose or just by directing the garden hose at the base of the plant.

Stake or trellis your tomatoes. This will increase air circulation around your plants and help the leaves dry quickly after rain or irrigation. Staking and trellising has also been shown to reduce the amount of fruit rot on tomatoes.

Should I spray a fungicide?

Most home garden tomatoes do not need to be treated with a fungicide. Tomato plants can tolerate high levels of leaf loss from leaf spot diseases without any effect on the number of juicy tasty tomatoes produced by the plant. In one study, researchers had to remove 75% of a tomato plant’s leaves before they saw a reduction in the number of tomatoes produced by the plant.

Gardeners should use cultural control practices like staking, mulching and pinching off infected leaves to keep leaf spot diseases in check. But it is important to remember that it is not necessary to have leaf spot free tomato plants to have as many homegrown tomatoes as your plant can provide.

Author: M. Grabowski, UMN Extension Educator


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