Powdery Mildew Control

By Richard Shiell

Many of my favorite roses are susceptible to powdery mildew. Souvenir de la Malmaison, Madame Alfred Carriere, and Papa Meilland are among those that develop white smudges and leaf distortions symptomatic of the fungus. In Bakersfield this occurs in early springtime and late in fall.

The Powdery Mildew fungus, Latin name Erysiphe graminis, occurs worldwide and effects such diverse flora as roses, grapes, and wheat. It spreads by microscopic spores carried on the wind. Powdery mildew thrives in temperatures in the upper 60ís to 70ís, high humidity, and stagnant air. It will grow on any susceptible foliage, stems, or buds having an acidic surface pH.

Therein lies the Achilles heel of Powdery Mildew; spray susceptible plants with an alkaline solution and the fungus is deterred. This applies to many other fungi as well.

Researchers at Cornell University came up with a very simple spray for combating foliar fungi; they added baking soda (sodium bicarbonate) and horticultural oil to water. The mix required constant agitation or it would separate, but it proved surprisingly effective against black spot and powdery mildew fungi. Since its first introduction many modifications have been made by rosarians and garden writers to the "Cornell Formula", adding everything from detergent to kelp extract.

There are two problems with using a baking soda spray. First, there are more effective materials available. Second, in the Bakersfield area alkaline soils are the norm, and most all of them are high in sodium. Adding more sodium encourages chlorosis (yellowing of plants) and soil problems.

Potassium bicarbonate is the preferred alternative to baking soda. Potassium is the third macronutrient in the N-P-K fertilizer coding. Adding some potassium to the garden is quite a good idea in our region. Laboratory tests have also shown that potassium bicarbonate is much more effective than sodium bicarbonate at deterring fungi. In fact, it has been shown in photos from electron microscopes to actually burst and destroy the growing fungus.

Several potassium bicarbonate fungicides are available, such as Kaligreen, Armicarb, and Remedy.

I purchased about a half-pound of Kaligreen from Britz Fertilizer last year for eighteen dollars and have been quite pleased with the results. To a gallon of water I add a tablespoon of horticultural oil, two teaspoons of Kaligreen, and about a teaspoon of Dr. Bonnerís liquid Castile Soap (from Trader Joeís). The spray needs to be reapplied if rain washes it off.

One caution about applying any spray containing oil; donít do it if daytime highs will go above 80įF or you risk burning foliage. Besides, when the temperatures start climbing the powdery mildew goes away on its own.

Britz Fertilizer is located at 19421 Creek Rd, half a block north of 7th Standard Road just west of HWY 43 (i.e. out in farm country). I had to order the Kaligreen in advance, and they got it in a couple days later from their main branch in Fresno. Their phone number is (661) 746-9701.