Know Your Spots! by Gary Burst		Comparison, Identification and Control of Rose Diseases.
Anthracnose, blackspot and downey mildew all first appear as spots on rose leaves, and will weaken or totally defoliate a bush. I have condensed Paul Desmet's experiences published in the 1993 rose annual. Mr. Desmet is a chemist assisting the American Rose Society. I have also added my own experiences growing roses in Victoria. Control: MOST IMPORTANT, Continually hand pick any infected leaves and fallen leaves, then burn or dispose in garbage - do not compost. In the dormant season, strip all remaining leaves from the plant, prune off any diseased canes, clean up all leaves, trimmings and litter from the ground, and spray the ground with dormant oil. Then apply a mulch over the soil to prevent overwintering spores from being blown or splashed onto new leaves. Anthracnose, Blackspot, Rust and Downey Mildew fungi rely on a damp leaf surface or high humidity to get established on the plant. Keep leaves dry - don’t overhead water* as water splashing on the leaves and the ground spreads the spores, and creates the humid environment needed for invasion of the leaf surface by the developing fungus. Prune to encourage good air flow between bushes and into the centre of the bush. Sprays: Handle all sprays with care, and follow the label directions. Every two weeks (May to September) spray with chlorthalonil (Daconil, Bravo, C-I-L Lawn fungicide: can leave a white residue on leaves) triforine (Funginex: systemic) and/or mancozeb (Dithane, Manzate: contact only, not systemic) myclobutanil, (Nova (Can) Sustain (UK), Immunox (US)) at label rates, alternating treatments between different formulations, so as not to allow the fungus to develop resistance to one chemical. I have found Dithane to be very effective against blackspot in my garden.		Catch it early - most sprays are effective as preventatives, and are not as effective once the disease has become established. Alternatives: Bordeaux mixture (copper sulfate / hydrated lime powder - will stain trellises and walls), sulfur powder (either dusted or sprayed as a wettable powder solution ) neem oil solutions (may not be available in Canada, kills beneficial insects such as ladybug larvae) jojoba wax solution, (E-Rase, not available in Canada) Baking soda - the Cornell Formula: 2 Tbsp Safers Sunspray horticultural oil and 1 Tbsp baking soda mixed in 1.5 quarts of water, sprayed on the foliage weekly. Later research indicates that Potassium Bicarbonate is more effective than Sodium Bicarbonate, and that the mixture with the horticultural oil is more effective than bicarbonate alone. Some studies report that bicarbonate is effective in curing an established powdery mildew infection. www.attra.org/attra-pub/PDF/bakingsoda.pdf Milk sprays (milk diluted 1:9 to 1:7 with water) are reported to discourage the fungus, whether through a germicidal action, increasing the plant's resistance or a pH change. Skim milk has less fat and therefore less odour. www.pioneerthinking.com/tv-mildew.html Anti-transpirants (Wilt-Pruf, jojoba oil, silica sprays) aim to create a barrier on the leaf surface that the fungus cannot penetrate. Baldo Villegas’ Bugs and Roses site is an excellent Web resource for photos and descriptions of diseases and pests. http://members.tripod.com/buggyrose/irosepests.html (Edited and additional information May 2002, by Trevor Inkpen)   * There is a school of thought that promotes overhead watering as a method of washing disease spores off of the foliage. This is thought to be effective against powdery mildew. If you try this, water early in the morning - the timing of the watering is critical - so that the leaves have a chance to dry rapidly in the sun.	Blackspot Black or dark purple spots with blurry edges, dead tissue starts appearing in the middle, leaves will turn yellow and eventually drop. Infection starts on the leaves in the lower portion of the bush.   Powdery Mildew Looks like a dusting of flour on both the leaves and buds. Most prominent on new leaves and at the tips of the stems.   Rust Appears on the underside of the leaf. Looks like small nodules of bright orange rust. You have to turn over the roses' leaves regularly to inspect; by the time spots appear on the top, the rust is well-established.

	Athracnose	Blackspot	Downey Mildew
Time of Year	Late Spring	All year	All year
How Spread	Spores spread by air currents and splashing water	Leaf surface must be wet to germinate	Air and splashing water
Defoliation	Little to none	Near total on lower half of the bush	Entire bush
Effect on Plant	Weakens plant somewhat	Weakens plant, poor flowers	No leaves, bush will die
Areas Affected	Any leaves, lesions on canes	Starts at bottom, moves upward. Canes have small dark blotches	Starts at top 1/3 and moves down, canes have purple blotches
Spot Progression on Leaves	Circular 1/16" to 1/8" dark brown spots enlarge in size and turn white in the centre	Black spots grow up to 1/2" or more in diameter	Purple-red blotches poorly defined and up to 1/2" in diameter. Do not seem to grow in size or number
Unique Identifier	Most of the leaf will remain dark green. Spots have well defined edges.	Spots have feathery edges and surrounding leaf tissue turns yellow	Continual yellowing of the leaf until only islands of green remain on a yellow leaf, at this stage the leaf falls from the plant
Prevention	Cut out any canes with purple blotches on normal green canes. Don't wait for spring, remove diseased leaves	Plant bushes in open areas, continually prune to open centres. Total leaf removal in the fall is desirable Cover ground around plants with new mulch	None. Downey mildew differs from powdery mildew in that it invades and kills all plant tissue, while powdery mildew is superficial