Powdery mildew is easy to spot, yet tougher to control. Here’s how.

There are a bunch of white spots on the leaves of my phlox. A few other plants in my garden have the same white spots. Is there something I should be doing now to help the plants?

— Andrew Granada, Libertyville

Your description sounds a lot like powdery mildew, which is an easy disease to recognize, and it is common in the landscape at this time of year. It can be a problem that shows up on annuals, perennials, shrubs and even some trees, such as maples. Powdery mildews are caused by fungal pathogens that are specific to a host and thrive when the days are warm to hot with cool nights and high humidity. Just because one type of plant in your garden has powdery mildew does not mean it will spread to other types of plants in your garden. Generally, powdery mildews are more severe on plants that are crowded or growing in a shady site with poor air circulation. It is best not to use mildew-prone cultivars, especially when a site is shady.

The first disease symptoms are distinctive white or gray powdery patches on leaves, buds, flowers, fruits or stems, which you may not notice. These patches can become enlarged until they cover the entire leaf on one or both sides with a felt-like white mildew growth. Leaves are the most affected part of a plant. Generally, just the tops of leaves are affected. Small, spherical, dark-brown to black fungal structures may be seen mixed in with the mildew. You can also find infected leaves that are stunted, curled and chlorotic in more severe cases of powdery mildew. Leaves can turn brown and drop off plants early, hastening fall dormancy while leaving plants that look dead late in the season. Look for buds on these plants to help ascertain whether the plant is dead.

Damage from powdery mildew is mostly aesthetic and, in most situations, will not harm the long-term health of the plant unless the infections are severe year after year. If any of your plants are completely brown, go ahead and cut them back if you do not like their appearance. Death of infected plants is rare. The flowers may be reduced in number and quality due to this disease and plants may look unsightly. The powdery mildew covering the leaves may also lower the plant’s photosynthetic efficiency, resulting in reduced growth. Infections have also been associated with increased winter injury to perennials. Severely affected perennials and annuals can be cut back to “restart” them in the summer. Fungicide treatments are an option for plants that consistently display significant damage. My preferred options are to either replace mildew-prone plants with more resistant cultivars or use different plants altogether. Also, consider putting the plants in an area of the garden that has better air circulation. Powdery mildew is common on lilac, bee balm, phlox, squash, cucumbers, and zinnia. You have not listed all the plants that are infected in your garden, but in my home garden, I use David phlox and raspberry wine bee balm, both of which have some resistance to powdery mildew. As the cucumbers and squash with powdery mildew stop production, the vines should be removed. It is OK to plant them again next year. Most years in my garden there is powdery mildew on these plants in the late season.

For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.