Can it be controlled?
Blackspot on roses is a very common occurrence for me, living in the damp Pacific North-West as I do. I continue to shake my head every time a new list of “Roses resistant to Blackspot” is produced. Not in my garden: EVERYTHING gets blackspot! If you don’t spray for blackspot you don’t grow roses, and I would like to eliminate chemicals but if I did there would be few rose bushes with leaves still on them by the late spring.
In the Pacific North-West we have had to become experts at dealing with blackspot on roses and as such, the topic is usually the major event for most Rose Society meetings.
Couple this with the fact that many areas are starting to ban chemical sprays (new local bye-laws) and we have a major problem for Rosarians.
So what exactly is blackspot, what can be done to control it and what does the research show?
[And, should we add: is there an organically safe method of controlling it!]
During spring, as the weather warms (over 70 degrees) and spring rains descend, fungal spores are windblown or splashed up from the ground to the lower, newly formed leaves as they open. Spores spread upwards. The fungus itself does not survive in soil, only in water droplets and that is how it spreads.
Leaf spot lesions are roundish with feathery edges, about 1/8 to 1/2 inch across. Leaves develop multiple spots, which eventually turn yellow and fall off. Canes can also be seriously affected, with purplish spots that become black. As a result not only can blackspot on roses seriously defoliate a complete bush (I have had many!!) but seriously weaken the canes.
Before considering a spraying program the following plan should be followed:
- Plant roses with resistance to Blackspot
- Plant in an area with good sunlight
- Plant with good space for air movement
- Don’t water at night or in the evening
- Don’t water from above: keep the water from the leaves
- Remove all leaf litter and deadheaded blooms
- Pick off infected leaves and discard them early on.
In general, Hybrid Teas, Grandifloras, Floribundas and many climbers are usually susceptible. Rugosa, some Polyanthas, Moss roses and Wichurianas show some resistance.
Spraying For Blackspot
Unfortunately for most of us, even though we follow the list above, some form of spraying will be necessary if we are to prevent blackspot on roses. Start your spray program as soon as you see new leaves appear in the spring and the temperatures begin to warm up. Spay every 10 days, drenching the plant from bottom up. When the bush is fully leafed out I usually stop spraying for awhile and begin again before the second blooming.
What to spray
This will depend on what’s allowed and available in your area. The only two that seem to have worked for me were RoseClear from the UK and Funginex from Canada, although there is a growing interest in the use of Neem oil. The following is a list of Fungicides, usually associated with blackspot control. You need to check if they are permitted or available in your area.
- Neem Oil
- Thiophanate Methyl
What works for you needs to be tested locally. The best way is to ask your local Rose Society: it will be a major topic…”What do I do for blackspot on roses”…always is!
So what have tests shown?
Agricultural research has tested Fungicide against Baking Soda with Oil, against Oil only, and used untreated plants as a control group. Fungicide produced the least disease and the most flowers. Baking soda had little effect!
They also tested the use of calcium, magnesium, Epsom salts and sulphur. All these nutrient treatments had little effect on blackspot although they did adversely affect the bloom production! The conclusion from this bank of agricultural test was that b>complete control of blackspot could not be achieved without fungicide!
And what about the so called “Organic Controls” ?
Bordeaux mix (copper sulphate) is toxic to animals. Sulphur powder has some effect. Essential oils such as White Thyme Oil and Cedarwood Oil have anti-fungal properties but are unproven for blackspot. Baking Soda burns leaves in hot weather but Potassium Bicarbonate does not. This has some merit but apparently only as a preventative measure…nothing works once the blackspot has made a start! Neem oil looks promising but is having trouble getting approval for sale.
What are we to do?
Diplocarpon rosae, the fungus that causes blackspot results in considerable damage to the rose trade and to most of us rosarians. It has been said that commercial producers must spray up to 40 times a year! Pesticides have been banned in Europe and parts of Canada, and resistance to fungicides have been reported.
It is an epidemic that needs to be researched thoroughly. The University of Hertfordshire in the UK, together with other environmental agencies, is undertaking a three year study. Hopefully something can be found to help control blackspot on roses.
During Victorian times in London, England, it is said that blackspot was unknown. Then, coal fires, the ones that produced the sooty atmospheres and the famous London fogs, were banned and blackspot appeared. Evidently, leaves coated with sooty residue didn’t get blackspot…. there must be a moral there somewhere!