10 June 2009
In The Garden With the Fluvanna Master Gardeners
By Irene C. Burke - Fluvanna Master Gardener
Fluvanna County Extension Office: 434.591.1950
Black spot and powdery mildew are such important considerations when selecting and growing roses, that breeders have spent decades propagating resistant cultivars like the Knock Out, and gardeners cultivating their home remedies.
This is not unlike those special solutions we all have for the common cold; everyone has a theory and a favorite "cure", or at least a way to shorten the progress of the disease.
One common practice, among those of us who try to grow roses, has made the front page of backyard talk recently---sodium bicarbonate (SBC) spray.
In the January 27, 2009 column I advised, "If you wait until the black spot or powdery mildew appears then youre too late," followed by a homemade preparation based on SBC.
"Apply frequently in the early morning, above and below the leaves: one gallon non-chlorinated water, one tablespoon baking soda, and one tablespoon dishwashing liquid. The alkaline baking soda makes the leaf surfaces inhospitable to fungus spores while the dishwashing liquid suspends the soda in the water for even distribution."
This formula, reported in a June 3, 1990 N.Y. Times article, was based on research by R. Kenneth Horst, plant pathologist at Cornell University.
Research continues and Linda Chalker-Scott at Washington State U. has completed a comprehensive review of the literature to sort out what is worth your time, effort and money for combating powdery mildew and black spot.
The Cornell formula works well as preventative maintenance, keeping fungal diseases from growing or from getting started, by keeping the leaf surface alkaline because the fungus prefers an acidic environment.
From her readings, however, Chalker-Scott observed that adding horticultural oil to a dilute (1%) SCB solution powers-up the anti-fungal spray.
At the UC-Davis web site http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7463.html there is a horticultural oil and SCB recipe reflecting Chalker-Scotts findings: "4 teaspoons of baking soda per gallon of water with a 1% solution (or about 1 oz) of narrow range oil."
Found at any garden or farm center, narrow range or superior oil should say on the label that it contains at least 92% unsulfonated residue, making it lighter and purer than other oils.
To avoid damaging the leaves, spray during cool weather or early morning. More isnt better; so dont drench the soil; drenching has the potential of changing the soil pH, with a negative effect.
There you have it, the latest research-based, homemade, spray formula for suppressing powdery mildew and black spot. But theres more to tweaking the gardens health than sprays.
Just as you fight a cold better, when youre strong and healthy, so too, your roses combat disease better, when theyre healthy.
Make this happen with three good gardening habits.
Keep your roses hydrated.
Remember---location, location, location. Provide at least six hours of direct sun, preferably more, with lots of elbowroom for air circulation.
Blanket the ground with a three to four-inch deep, coarse mulch, and not finely shredded bark. Wood chips work best.
Tip of the Week
Repot houseplants in refreshed potting soil; use bark chips for orchids. To increase the number of plants, trim the leggy ones. Put each cutting into a three-inch pot of moistened media: one part perlite, one part vermiculite and one part sphagnum peat moss, or gently insert the cutting in a one-inch cube of floral foam covered with new potting soil.