Tomato leaf spot diseases
- Staking and mulching plants and pinching off infected leaves help keep leaf spot diseases in check.
- Improve air circulation around plants.
- Keep leaves dry by watering at the base of the plant.
- Do not save seeds from infected plants.
- Leaf spots should not affect the amount of fruit your plants produce.
What to look for?
There are three leaf spot diseases commonly found on garden tomatoes in Minnesota: Septoria leaf spot, early blight and bacterial spot.
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.
Most tomato leaf spot diseases 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 closest to the ground.
Look for brown to black round spots that are the size of a pencil tip or larger.
Bacterial speck and spot
Bacterial speck is a disease that is similar to bacterial spot. Bacterial speck and spot can cause spots to form on the leaves, stems and fruit of tomato plants.
The leaf spots caused by bacterial speck and spot look identical but the two pathogens can be distinguished by differing types of fruit spots that form later in the season. Speck infections on fruit are small (the size of a pencil tip) raised black spots. Bacterial spots on fruit are larger (the size of a pencil eraser end).
Do not can tomatoes infected with bacterial speck or spot as these diseases can change the pH of the fruit.
What to do about tomato leaf spots
- Pinch off leaves with leaf spots and bury them in the compost pile.
- It is okay to remove up to a third of the plants leaves if you catch the disease early.
- Do not remove more than a third of the plant's leaves.
- Keep leaves dry to reduce spreading the disease.
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.
Many gardeners will remove the lower third of the leaves on every plant whether they have leaf spots or not. This makes it harder for plant diseases in the soil to get splashed on to the lower leaves.
Removing lower leaves 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.
What else can you 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.
Avoid working in tomato plants when the leaves are wet to reduce the spread of the disease.
Under moist conditions the bacteria reproduce and easily stick to hands and tools. Wait until plants are dry for chores like staking, pruning and weeding to reduce the spread of the bacteria.
Make sure air circulates well around plants.
Providing good air movement around the plants by staking or caging tomatoes, pulling weeds, and spacing plants far apart will allow leaves to dry quickly.
Allow 2 years to pass before planting tomatoes or peppers in the same location. Do not save seed from infected plants.
Should you use 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 affecting 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.
Use cultural control practices like staking and mulching plants, and pinching off infected leaves to keep leaf spot diseases in check.
Remember that it is not necessary to have leaf spot free tomato plants to have as many homegrown tomatoes as your plant can provide.
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.
When treating fruits or vegetables, make sure the plant you wish to treat is listed on the label of the pesticide you intend to use. Also be sure to observe the number of days between pesticide application and when you can harvest your crop.
Reviewed in 2018