How to Treat Powdery Mildew on Honeysuckle

Honeysuckle (Lonicera spp.) is an eye-catching ornamental plant which offers a pleasing scent. With native varieties that grow throughout North America, honeysuckle grows in U.S. Department of Agriculture plant hardiness zones 3 to 11 depending on species. Honeysuckle comes in bush and vining varieties, with blooms in pink, orange, yellow or red. Dropmore Scarlet honeysuckle (Lonicera brownii, USDA zones 4 to 7) attracts hummingbirds to the red flowers on its 12- to 20-foot vines. The Trumpet honeysuckle (Lonicera sempervirens, USDA zones 4 to 9) is a slightly smaller variety with red-orange blooms.

When selecting honeysuckle varieties for the home garden, avoid Japanese honeysuckle (Lonicera japonica, USDA zones 4 to 11), which is an invasive species.

Powdery mildew is a common problem that occurs on a variety of plants including vegetables, herbs and ornamental plants. Caused by fungi, powdery mildew appears as the name suggests: white, powdery spots on leaves that grow over the surface of the plant. Left untreated, powdery mildew will spread throughout the plant, robbing it of nutrients.

Powdery mildew affects plants when conditions are warm, 60 to 80 degrees Fahrenheit during the day, and when humidity is between 50 and 90 percent. Plants in shady areas are most affected by powdery mildew because these areas are more likely to retain humidity, offering fungi spores an ideal environment to grow and spread.

Powdery mildew is often mistaken for downy mildew, although the two are different diseases. Powdery mildew mostly appears on the top of leaves, while downy mildew grows primarily underneath leaves.

The key to getting rid of powdery mildew is to catch it early. Once mildew has affected a plant systematically, it is not only difficult to eradicate, it has also severely damaged the plant. When powdery mildew is allowed to take over a perennial like honeysuckle, it can rob it of nutrients so that the plant is not able to store enough energy to return the next year. Inspect plants daily for signs of powdery mildew when conditions are appropriate. Start treatment when the first signs of mildew appear.

When caught early, powdery mildew is relatively easy to remove.

Combine 2 1/2 tablespoons of neem oil -- available at most garden centers -- with 1 gallon of water.

Pour the mixture into a clean hand-held spray bottle. If the bottle has been used before, rinse it with a solution of 1 part bleach to 10 parts water to prevent cross-contamination.

Spray the leaves on the entire plant with the mixture once every seven days to eradicate the existing powdery mildew and to prevent the spread of mildew spores to the rest of the plant. 

Once the mildew is removed, continue to spray the plant once every 14 days to prevent the mildew from returning. 

Neem is a naturally occurring oil derived from the fruits and seeds of the neem tree (Azadirachta indica). This control method is approved for organic gardening. When shopping for neem products, read the label carefully to make sure no other chemical products have been added.

Another solution for getting rid of powdery mildew is a mixture of baking soda and horticultural oil, which can be found at plant nurseries. Mix 2 1/2 tablespoons of horticultural oil with 1 gallon of water and add 1 tablespoon of baking soda. Pour the mixture in a garden sprayer and spray the entire plant once a week. Test in a small area first to make sure the mixture doesn't harm the plant.

Plants that have been severely affected by powdery mildew -- more than 50 percent of the leaves are affected -- must be removed and the plant burned or put in the garbage. Never compost plants with powdery mildew or other diseases.