Q: I put in a dozen hybrid teas five years ago, and they looked awesome for three years. But two years ago they began having trouble. There is severe black spotting on the leaves, which eventually fall off; that leads to weak cane growth and poor roses in general. I have sprayed religiously with a rose fungicide, but it doesn’t help. Am I spraying for the wrong thing? I’ve heard that growing chives as ground cover would kill the problem. I’m ready to try anything. — Eric Welles, Chicago, Ill.
A: If this problem — the fungal disease called black spot — somehow ceased to exist, then perfect hybrid tea roses would lose their cachet as gardening’s ultimate trophy. In wet seasons black spot can be devastating, as you have discovered, and it is rampant in the humid eastern half of the continent. The difficulty with using sprays, organic or not, is that they are all strictly preventive. The material must be on the leaves and canes before the disease has a chance to get started. Once black-spot spores germinate and start growing into a leaf, no amount or kind of medicine will reverse the disease.
If by spraying religiously you mean every Saturday or Sunday, that won’t be enough in some years. Other times it could be too much. If it rains hard a day or two after you spray, you need to put the stuff on again right away. If you get two weeks of bone-dry weather, on the other hand, you might think you could safely skip a spray. But all the new leaves and stems that have formed during that time are completely vulnerable. Achieving consistent protection is tricky and a chore, which is why only the most devoted rose fanatics get it right. For us ordinary types, all this spraying takes the fun out of growing roses. The good news is that switching varieties will help enormously. Many roses can tolerate black spot; they get the disease but bloom well despite it. These include several shrub roses (any rugosa variety, for example) and climbers such as City of York, Dortmund, Don Juan, and Red Fountain, to name a few.
But you, like many others, want hybrid teas, which are, of course, the roses most susceptible to black spot. Even within this group, though, are some you can grow successfully without spraying. Your best course is to replace the most sickly specimens with varieties like Electron (pink), Elina (light yellow), Folklore (orange), Pascali (white), ‘Precious Platinum’ (red), and Silver Jubilee (coral/cream). There are at least 18 black-spot-resistant, though not immune, hybrid teas available — a huge improvement that has occurred mostly over the past 15 years. The healthier roses are, the better they will resist the disease, which in some years may be quite severe. Plant in full sun, and space them far enough apart to ensure good air circulation. Fertilize with a top dressing of manure in the fall or a high-quality compost in the spring. When warm weather arrives, cover the ground with an organic mulch. In the spring, dispose of all the prunings, and in the fall do the same for the leaves, since black-spot spores usually survive home composting. A ground cover of chives will have no effect on black spot and will only make cleanup more difficult.
Farther south, where warm, wet summers are longer, the disease will be even more tenacious. There, even resistant roses can have significant black-spot problems. So wherever you live, when you see the weather turning in favor of black spot, you may want to consider spraying. Garden centers offer an arsenal of products, but there are also a couple of low-tech alternatives. A Cornell University researcher demonstrated that a mixture developed for powdery mildew — 1 tablespoon of baking soda plus 2 tablespoons of summer-weight horticultural oil mixed in a gallon of water — also helps with black spot as long as a covering is maintained on the leaf surfaces. This means spraying at least once a week. Many gardeners don’t like the way this makes their plants look, but it works.