How to Treat Powdery Mildew on Rosemary Leaves

A Mediterranean native, a rosemary plant (Rosmarinus officinalis) will grace an outdoor herb garden in U.S. Department of Agriculture plant hardiness zones 7 through 9, but you can also grow it in a pot and bring it indoors for winter in cooler climates. Although easy to grow, this fragrant herb is susceptible to several plant diseases, including powdery mildew. Several natural, homemade spray solutions help get rid of powdery mildew disease on a rosemary.

Rosemary
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Rosemary foliage has an aromatic, piney scent.

About Powdery Mildew

Powdery mildew infections typically start out as small, dusty, white spots that enlarge to cover entire plant surfaces with a white to light gray, powdery coating. Once covered, the affected leaves can't photosynthesize efficiently and the plant may experience stunted growth and reduced vigor. Affected foliage sometimes turns yellow, curls, dies and falls from the plant prematurely.

The white powder is a mass of tiny fungal spores, which spread via wind gusts to other plants. Unlike other fungal pathogens, powdery mildew spores require dry plant surfaces to germinate and grow -- standing water kills the spores. Powdery mildew disease thrives during humid weather when cool nights follow warm days. Temperatures falling between 60 and 80 degrees Fahrenheit create the ideal environment for the fungus to infect.

Cultural Corrections

One of the best ways to treat rosemary powdery mildew is to prevent it from taking hold in the first place by giving the plant the care it needs. Powdery mildew fungi thrive in shady areas with moderate temperatures, while extreme heat and sunshine can kill them. A rosemary grows best in a warm area that receives at least six hours of sunlight every day. If you overwinter a rosemary indoors, find a drafty, sunny spot in a room with temperatures that consistently hover around 60 F.

Crowded, humid conditions promote powdery mildew development. Increase air circulation around plants by leaving enough space around them that they won't touch when mature, generally about 2 to 3 feet apart. For indoor plants, set a small fan to blow gently on the foliage for three or four hours every day to increase airflow. When watering rosemary, wet the foliage to help rinse the fungal spores from the leaves.

Compost Tea

Compost tea solutions have antifungal properties that will kick powdery mildew fungi to the curb. Commercial compost tea kits exist, but you can easily make your own. Combine 1 part mature compost with 5 or 6 parts water. Finished compost containing a little manure seems to work most effectively. Let the solution sit for seven to 14 days before you strain it and slowly add water until the solution reaches the color of brewed tea. Spray the final solution on your plants every 14 days throughout the growing season.

Baking Soda

A baking soda solution can help protect plants from powdery mildew pathogens as well as get rid of existing fungi. Baking soda works by changing the pH levels of leaf surfaces so they are inhospitable to the fungi. Mix 1 1/2 tablespoons of baking soda and 3 tablespoons of lightweight horticultural oil in 1 gallon of water. Treat plants once a week, taking care to thoroughly coat the tops and undersides of leaves. Although effective, the solution can leave behind a powdery, white film that looks a lot like the disease itself. In addition, some plants are sensitive to the solution, so test it on a small section of leaves first. If the solution doesn't cause any damage after 24 hours, treat the entire rosemary plant.

Neem Oil

Spraying neem oil solutions on a rosemary plant with a moderate to heavy infection helps eradicate powdery mildew and prevent fungal development. Mix 2 1/2 tablespoon of neem oil and 1 gallon of water in a small spray bottle. Coat the tops and undersides of leaves until they glisten with moisture. Repeat applications every seven to 14 days until the disease symptoms disappear. Avoid spraying plants with neem oil when temperatures reach higher than 90 F or to rosemary plants suffering from water stress.

Milk Spray

As soon as you spot patches of powdery mildew on a rosemary plant, spray the affected foliage with a milk solution. Mix 1 part milk with 5 parts water, pour it into a clean spray bottle and squirt the foliage once a week until the disease disappears. The amino acids or the salts in the milk act as a fungicide that wipes out powdery mildew pathogens, but it works best if your plants is placed in bright sun for the treatment. You can use any type of milk, but skim milk is less likely to cause a sour-milk odor.

Staying Safe

Although natural powdery mildew treatments aren't generally toxic to people or animals, any solution can irritate your skin or eyes with direct contact. Reduce your risk of exposure by wearing protective eyewear, waterproof gloves, a face mask, long sleeves, pants and shoes with socks when mixing and spraying fungicidal solutions.