Garden Docs: What's eating my begonia?

Phyllis T. of Santa Rosa asks: I was at the Sonoma County Fair in August and in one of the Hall of Flowers displays, there were some beautiful hanging baskets of orange begonias. I have a few begonias and they were doing well up until last month. Now they are covered with a white powder. Can you tell me what is causing this? I would also like to know the name of that begonia!

Those beautiful orange hanging begonias were seen in ‘Best of Show’ winner, Daniel Gibbs’ display, and owner of A Better Way Produce and Nursery Garden Center at 3615 Stony Point Road.

So, we went to Daniel for his advice on successfully growing begonias and how to control powdery mildew disease, the white powder developing on your begonias.

Daniel said the attention-getting variety of hanging begonia in his show garden is called Illumination, (Begonia tuberhybrida). Tuberous begonias are quite susceptible to this powdery mildew disease. An infected plant develops spots or patches of white to grayish talcum-powder-like growth.

The disease is most commonly seen on the upper sides of the leaves, but also affects the bottom sides of leaves, young stems, buds, and the flowers. The infected leaves may become distorted, turn yellow with small patches of green, and fall prematurely. Infected buds may fail to open altogether. The severity of the disease depends on the variety of the begonia, age and condition of the plant, and weather conditions during the growing season.

Powdery mildew loves warm, dry climates. This is because the fungus doesn’t need the presence of water on the leaf surface for infection. But, the relative humidity of the air does need to be high for spores to germinate. The disease is common in crowded plantings where air circulation is poor.

Powdery mildews are host specific, which means they cannot survive without the necessary host plant. Powdery mildew produces mycelium (fungal threads) that grow only on the surface of the plant. They never get into the plant tissue, but the fungi feed by sending rootlike structures into the top cells of the plant. The fungi overwinter on plant debris. In the spring, they produce spores that are moved to susceptible host plants by splashing raindrops, wind or insects.

There are a few things you can do to reduce or prevent powdery mildew. Look for resistant varieties. If resistant varieties are unavailable, do not grow them in low, shady locations.

Avoid late-summer applications of nitrogen fertilizer to limit the production of succulent new growth, which is more susceptible to infection. Do not overhead water. Remove any part of the plant that is infected.

Do not compost infected plant debris. Temperatures in most compost piles don’t get hot enough to kill the fungus. Selectively prune overcrowded plant material to help increase air circulation. This helps reduce relative humidity and infection.

If cultural controls fail to prevent disease buildup or if the disease pressure is too great, an application of neem oil may be necessary. Follow the label instructions.

An alternative nontoxic control for mildew is baking soda combined with a lightweight horticultural oil. Applications of 1 tablespoon of baking soda and 2½ tablespoons of horticultural oil in a gallon of water, applied to the plants every 14 days, may be helpful.

Tuberous begonias form tubers and these tubers would prefer to be removed from the soil, and stored in a cool, dark place, rather than leaving them in the pots over winter. Before the frost kills the top of the plant, cut back the stems and carefully dig the tubers out from their pots. Wash off any soil and then allow the tubers to dry on newspaper or shavings.

Don’t forget to label each begonia if you want to remember who’s who. It’s time to replant them when you start to see signs of new growth.

Dana Lozano and Gwen Kilchherr are garden consultants. Contact them at pdgardendoctor@gmail.com.