***
Organic Control of Black Spot
You know you have it by the light spot it creates on the leaf an initial yellowing of the leaf in a spot approximately ½ inch across. This initial yellow spot can occur on either side of the leaf and distinguishes itself from other leaf spot fungi problems by having a slightly fringed edge to the spot. Youll likely need a small lens or a magnifying glass to see the fringes so most gardeners assume black spot when they see the yellowing spots on the leaves. With the lens, youll also be able to see the black spore producing bodies on the spot. The interesting thing about this problem is that the fungus roots (theyre really mycellium but what the heck) penetrate down between the layers of the leaf so they are well protected and invulnerable to anything happening on the surface of the leaf. You cant destroy the fungus without destroying the leaf. That is one smart fungus.
Ethylene
The fungus is reputed to produce ethylene which causes roses to drop their leaves. Some roses hang onto the leaves for a longer time than others and on these varieties, the black spot fungus can really develop. It produces larger yellowing circles (sometimes edged with black dying tissue) and often several different circles on a leaf will meet, causing almost the entire leaf to go yellow. Naturally as the fungus works away, the leaf will become paler anyway as the nutrients arent getting to all parts of the leaf so the leaf just gets pale and yellow. Then it drops.
Control Black Spot
Gardeners have tried to control this black spot problem over the years by drenching their roses in all manner of fungicides. Invariably, the fungicide works for a while and then after a few years, it loses its effectiveness as the fungus become immune to that particular compound. In this way, the chemical industry has convinced rose growers to use more and more powerful compounds to protect their roses from the effects of the blackspot fungus.
Let me suggest an alternative system of control for you.
Clean Up
To begin with, once the fungus is well established in your garden, it is necessary to clean up the garden. Collect and remove all diseased leaves to the dump. Do not compost them. By collecting all the leaves, and I start with those that have symptoms on the plant, and removing them from the garden area, you are denying the fungus the ability to breed and overwinter on plant debris. Preventing successful overwintering is the first step in a control programme and a clean garden is the first step in this direction.
Overwintering Spores
The second step is to understand that many fungus spores overwinter on the canes. In my garden, I cut down the canes to the ground and remove them from the garden each year. I can do this because I plant my bud unions at least six inches deep (I describe this in my book Tender Roses for Tough Climates) and the roses survive quite nicely to resprout new and clean canes the following spring. If you garden in a more traditional manner, I suggest you spray your roses with a dormant oil and sulfur spray in the fall before hilling as well as first thing in the spring after you remove the soil covering. The dormant oil will suffocate the overwintering fungal spores. As a side benefit, it also suffocates the eggs and overwintering adults of several pests that can bother roses.
Rotation Spray
The third thing a gardener has to do is develop a rotation spray in the garden as a preventative measure. Remember, once the fungus is established, the mycellium are protected between the leaves so spraying will not kill them. Establishing a preventative spray programme is easy and several things work very well.
Baking Soda
The first is household baking soda. Mix two tablespoons of baking soda into a gallon of water and add one squirt of detergent or soap to the mix. The soap acts as a spreader-sticker to help keep the baking soda on the leaf. The mix should be sprayed onto the rose leaves both top and bottom to establish an alkaline leaf surface that will prevent the fungus from establishing itself. This will have to be repeated after a rain as the rain will clean the leaves allowing the fungus a clean leaf to colonize. This mixture will last about a week in normal practice the dew will wash it off and wind action will abrade it.
Lime-Sulfur
Go to your favourite garden centre to purchase lime-sulfur and use this the second week. Spray this mix onto both leaf surfaces as it creates a very acidic leaf surface that will prevent fungus from developing as well as killing off any immature fungal bodies.
Alternate
Each week you alternate sprays one week the alkaline baking soda and the next, the acidic lime sulfur. The poor black spot fungus will have to work very hard to establish itself if you are diligent.
Your roses are still going to get black spot, but they will get it later in the season and not as badly as under chemical spray programmes. This means youll get more blooms and your plants will be healthier. Youve done two things youve eliminated noxious chemicals from your garden and youve got better roses. Hows that for good gardening?
This article on black spot was written from a traditional point of view. In current practice, I find I dont have to spray my roses at all for several reasons. The first is my unusual planting method the bud union goes six inches under the soil. The second is that I spray my roses regularly with compost tea and the beneficial bacteria and fungi in the compost tea fight off most of the black spot spores before they can become established.
Do you have a question about Organic Control of Black Spot?
Custom Search