Hot Iowa summers offer a promise: get through a few weeks of heat and humidity and you are rewarded with fresh tomatoes. However, if you are growing your own, there is a lot to watch out for. Linn County Master Gardener Lisa Slattery offers some advice on how to spot danger among the tomato vines.
Q: What are the black spots on my tomato plant leaves?
A: Tomatoes have a lot of enemies in the garden, even deer will eat tomato plants, but more common are fungus and bacterial diseases.
Tomato blight is common and attacks foliage, particularly older (lower) leaves. Early blight causes brown spots of varying size that typically contain concentric rings of darker brown. Septaria blight causes numerous small brown spots that develop light tan to white centers as they age. Leaves turn yellow, brown, then wither and die. These diseases are caused by a fungus that overwinters on plant debris in the soil. Fungal spores are splashed onto the plant’s leaves by raindrops or splashing water. A wet leaf surface is required for the spores to invade the plant tissue. Wet spring and wet early summer weather favors development of foliar diseases. There also are bacterial spot diseases that behave the same way, which can affect foliage and fruit.
Another tomato foe is fungus that attacks the water-conducting tissues in the stem so an entire plant wilts (Fusarium Wilt and Verticillium Wilt).
If tomatoes receive inconsistent watering, you can end up with “Leaf-end toll,” “Blossom-end rot,” and “Cat-facing of fruit.”
Good cultural practices can help you successfully grow lots of tomatoes. Iowa State Extension recommends rotating tomatoes to different locations in the garden each year. It’s best to rotate all “solanaceous” crops (potatoes, peppers and eggplants) and not grow them in the same area for at least three years, which helps destroy the pathogens that live in the soil. If you can’t rotate every three years, try every other year. Other tips to keep in mind:
Space plants at least 3 feet apart, giving plants better air circulation for faster foliage drying.
Stake tomatoes. Don’t let them sprawl along the ground.
Mulch plants with at least a 2- to 3-inch layer of mulch around each plant, which helps prevent the soil from splashing onto the plant during rain or watering.
Avoid wetting tomato foliage when watering. Apply water directly to the ground and water only in the morning.
Don’t work with wet plants.
Remove diseased leaves and all plant debris in the fall. If you have diseased plant debris don’t compost it.
Use of a fungicide can help but you’ll need to apply it at the first sign of disease.
Plant disease free and disease resistant varieties.