By: Ted W. Mills, ARS Consulting Rosarian and Judge
If most rosarians would confess, they surely have experienced a siege of blackspot. How can so many be affected with this malady? Isn’t there someone somewhere who can furnish an answer to our universal problem? And blackspot invasion is one problem that raises its ugly head at the slightest opportunity. There is no plant disease that discourages rosarians more than this scourge. Multitudes have cancelled the hobby just because there seems to be no end to the plague. But hearken your ears -- there are ways to minimize and probably eliminate this plant sickness. Listen closely as I offer 25 years of experience dealing with blackspot.
We need not doubt that professional rose growers have made strong attempts to eradicate blackspot. After all, their livelihood depends on the continuance of the rose-growing hobby. Of course there are other diseases that attack our precious rose bushes, but none with the frequency of blackspot. Plant pathologists refer to it as Diplocarpon rosae. Don’t you just love those high brow words? But only plant laboratory technicians would understand that lingo. I prefer to speak in down-to-earth terms and that’s what I will use in this article.
Of all methods thus far advanced in the fight against blackspot, the best thus far is that which includes plant genetics. In other words, the action calls for development of plant tissue that will become immune to blackspot invasion. Already we see progress in this light with the introduction of the now popular variety, ‘Knock Out‘. Hardly any bush of this species shows the slightest bit of fungus attack. Sales have skyrocketed as rosarians clamor for bushes that ward off disease. Wouldn’t it be ideal if all varieties could enjoy such immunity to disease? So I would say to the laboratory people: “get busy and furnish more bushes that possess this virtue.”
But until the rose world is blanketed with ‘Knock Out‘-type varieties, it behooves us to limit the bad effects of blackspot and render it powerless as much as possible. We all know that blackspot is incurable; however by practicing some basic rules we can manage to cope with it. We may even dismiss it from our rose gardens.
Cleanliness is the first rule of good health whether it involves humans or plants. This calls for the dismissal of all debris that might harbor fungus spores. And it is these spores that cause widespread affliction in roses. A blackspot of 1/4 inch in diameter expels thousands of disease spores that are microscopic, but very contagious to rose tissue, both leaves and canes. It is this reason the malady races through our gardens like wildfire. As soon as the spore reaches a healthy leaf it begins its damaging work. Leaves turn yellow, and if left on the plant, their spores will invade the tissue of neighboring rose plants. Soon the yellowing maximizes and the leaves fall to the ground. If the plant becomes defoliated it suffers immeasurably and sometimes dies. Removal of infected leaves is a must. These must be moved from the site and destroyed -- not in a compost pile but tranferred far from the rose garden premises.
Since blackspot cannot be cured, it calls for the rosarian to develop a defensive program that will prevent blackspot invasion. The most effective method deals with spraying. I am conscious of the fact that most rosarians despise the chore of spraying. However, the successful rose grower adheres to a spray program that is both regular and effective. If by chance the hated blackspot does appear, hurriedly remove the infection and accelerate the spray program. Never increase the dosage of fungicides, rather increase the frequency of chemical application. When spraying, always follow the instructions listed on the chemical’s container and practice safety rules during the actual spraying. It is very important to do so.
Now what should the spray program include? Contact a Consulting Rosarian for advice on effective chemicals designed to counteract blackspot and determine the most effective time frame the roses require spraying. Stick to that regimen with diligence. If it rains immediately after spraying, repeat the spraying just as soon as the weather permits. Always combine a systemic chemical with a contact chemical. It is a good policy for rosarians to look for products that carry the American Rose Society’s seal of endorsement. This insures that the product has been garden-tested by ARS’s approved testers throughout the USA. Remember that contact or surface chemicals provide immediate attention; whereas, the systemic-type chemical’s action is effective but delayed. Become familiar with the products that are best suited for blackspot prevention by accessing information on the ARS website www.ars.org Also, my own local rose society maintains an excellent website that offers advice on various spray formulations. Its site is www.chattanoogarose.org Click on Spray Formulations for the data. A helpful toll free number of 1-800-858-7378 is the National Pesticide Telecommunications Network. This is a 24-hour service. These sources provide a wealth of free advice on plant-disease and insect problems facing rosarians.
Since blackspot develops on wet tissue, it behooves the rosarian to maintain rose beds that are not constantly saturated with water. Never expose bushes to sprinkling systems if failure to spray has entered the scenario. If the temperature is warm and damp conditions prevail, the untreated area is ripe for blackspot invasion. I remember well trying to help a greenskeeper cope with blackspot on roses planted near a golf course sprinkler. It was a losing battle and the rose bed had to be re-located. Be that as it may, remember this fact: water does not cause blackspot, failure to spray on a regular basis does allow its invasion.
I know I have preached that the rosarian should never use a power blower to clean rose beds of debris. I still concur with that thesis. Since the wind is the force that helps spread spores from bush to bush, doesn’t it make good sense to not provide the air current that carries the devilish blackspot spores? Use a rake to clean rose beds. It’s a bit more work but its use will never spread the spores that lurk on diseased leaves.
We all know that some rose varieties are more hardy than others. Hybridizing often weakens plants and strains of this weakness make certain bushes more prone to disease. For instance, the well known variety, ‘Garden Party‘, has served as a barometer for me to detect powdery mildew. Some varieties also acquire blackspot easier than others. Become acquainted with those bushes that seemingly allow blackspot invasion to enter the garden repeatedly. Make notes of this fact and be sure to spray these varieties with extra diligence.
Yes, we who are truly dedicated to rose growing can help ward off dropouts in our hobby simply by dispelling the fear of blackspot invasion. Encourage the struggling rosarian to practice some of the actions listed above. To do this you will be serving the band of rose brothers and sisters who want roses to continue to thrive and the participants in the hobby to increase immeasurably.