Summer Pests & Diseases to Watch For

The garden to-do list can get a little shorter in the heat of the summer. Lawns slow, planting is put on hold until fall and, apart from watering and weed-pulling, the tasks are not as demanding in July and August. This gives the gardener more opportunity to watch for common pests and diseases and address them before they get bad. Here are a few of the common issues that pop up in the garden in the heat of the summer.

Blight on Tomato – Yellowing leaves with black spots infecting leaves at the base of the plant and moving upward is the sign of a common fungal disease on tomato often referred to broadly as “the blight.” While several different species of fungus cause these symptoms, they are often controlled in a similar fashion. Reduce overhead watering, apply mulch, and rotate crops every 3 years to keep this foliar disease at bay. Learn more about control of early and Septoria blight.

Squash Vine Borer – Impacting many squash vegetables and their close relatives like zucchini, winter squash, and pumpkins, infested plants wilt and eventually die mid-summer. This is due to the large larva feeding inside the stem. Treatment with insecticides at the base of the stems can prevent infection. Good garden sanitation goes a long way to preventing future problems. Be sure to remove infected vines at the end of the season and turn the soil to expose and kill pupae. More information about this common pest of squash and pumpkins can be found here.

Powdery Mildew – This white fungus that forms on the leaves looks like a white dusting or coating. There are several types of fungus that cause powdery mildew and most are species specific. For example, the fungus causing powdery mildew on lilac would not infect turf grass, but turf does have another powdery mildew fungus specific to it. Improve air circulation, reduce overhead watering, and increase light levels to help control for this unsightly, but rarely fatal disease. Learn more about powdery mildew.

Crabgrass in the Lawn – Once you see this common lawn weed in summer, it’s often too late to control it efficiently. The coarse leaves of this warm-season grass easily overrun a stressed out cool-season Kentucky blue grass lawn during summer months. While many gardeners see it every year, crabgrass is actually an annual, growing from seed each summer. Applications of a pre-emergent herbicide in late April to prevent germination is the best way to control crabgrass. If that date has past, you can pull grass by hand or treat with a non-selective herbicide. Then reseed those areas in fall. A strong healthy stand of turf is the best defense against any weed, including crabgrass. Learn more about this difficult to control weed.

Prepared by Aaron Steil, Assistant Director

Photo caption: powdery mildew on a lilac – photo by the ISU Plant Disease Clinic