How to Make a Homemade Fungicide for Mildew on Begonias

Unfortunately for begonias (Begonia ssp.), their love for damp shade is also what the plant fungus powdery mildew craves most: low light, mild temperatures and high humidity. You can't change the weather, but you can battle the disease's unsightly, white leaf blotches with effective, inexpensive homemade fungicides or cultural changes.

Powdery mildew doesn't discriminate among shade-loving begonias, whether they grow as perennials in U.S. Department of Agriculture plant hardiness zones 6 through 11 or as annuals lifted for overwintering elsewhere.

The answer to your powdery mildew problem may be as close as the kitchen cabinets or refrigerator.

Powdery mildew spores can't germinate on plant tissues with pH readings below 7.0, which is classified as acidic. Baking soda is naturally alkaline, and spraying the begonias raises their pH enough to prevent infection. Use it when conditions favor the disease -- evenings with humidity between 50 and 90 percent and temperatures in the 60- to 80-degree Fahrenheit range.

Clean 1-gallon container

Measuring spoon

Baking soda

Light, plant-based oil such as canola or safflower

Liquid non-ammonia dish soap

Clean funnel

Clean spray bottle

Measure 1 tablespoon of baking soda into a clean 1-gallon container.

Add 1 tablespoon each of a light plant oil and a mild, ammonia-free liquid dish soap.

Fill the container with water.

Funnel enough of the solution to treat the begonias into a clean spray bottle. Store what's left in the refrigerator.

Spray until the solution coats both sides of all the leaves. Shake the bottle frequently to keep the oil mixed into the water.

Repeat every one to two weeks while mildew is likely to strike.

To speed drying and discourage other fungal infections, spray in early morning.

The soap and oil help the baking soda stick to the leaves, prolonging its pH-raising effect. 

Overusing the baking soda spray may raise the soil's alkalinity and salt levels and cause problems for the plants.

Researcher Peter Crisp at Australia's University of Adelaide Department of Applied and Molecular Ecology tested many alternative treatments for powdery mildew on different types of plarns. He found 10 percent solution of low-fat or whole milk and water reduced powdery mildew symptoms.

This research confirms earlier findings by Brazilian researchers that spraying a raw milk and water solution on zucchini (Cucurbita pepo) and melon (Cucumis melo) leaves reduced the symptoms by 90 percent.

To make the solution, add 1/3 cup of the milk to 3 cups of water. Spray the begonias weekly while the disease is active.

A few cultural practices may stop powdery mildew before it starts, or at least confine it to a few plants.

Consider replacing shade-loving begonias with resistant, sun-loving varieties. Bronze-leaved wax begonias (Begonia x semperflorens-cultorum) grow as perennails in USDA zones 10 through 11 and bloom as annuals from spring to fall elsewhere.

Lower humidity around the plants by spacing them with plenty of room for air circulation. 

Fertilize sparingly, or with a slow-release fertilize that won't stimulate rapid new leaf growth.

Prune and dispose of infected leaves at the first sign of disease. To avoid spreading the spores, disinfect your pruning tools by dipping them in rubbing alcohol between cuts.

Water from above when other conditions favor mildew. Its spores can't germinate on wet leaves.

Once the chances of a mildew attack subside, resume watering the begonias at soil level. Otherwise, the wet leaves may attract a host of other fungi.