Source: JARS V53:No.1:p5:y1999
Tips for Beginners: Controlling Petal Blight
Dr. G. David Lewis
Petal blight is an annual problem in many of our gardens. Elepidotes, lepidotes and azaleas are all susceptible - so is mountain laurel. The early bloomers escape infection (a good reason to include lepidotes in your garden) because petal blight is a warm weather disease and normally does not become severe until May. It's those muggy, misty, rainy days that all the petal blight fungus go berserk and rot all those flowers that you waited a whole year to see.
So what can you do about this disgusting scourge of our beloved rhodies? You can spray! If you do it right you will have difficulty in finding blight in your garden. If you don't do it right, you are wasting your time and a lot of money. Any of the fungicides labeled for petal blight control will do the job. The trick is in the proper timing and the proper application.
Timing is critical. If you wait until you see petal blight, you are too late for a good job of control. Your garden will be loaded with blight spores and complete control will be very difficult. Your first spray should be applied before blight is seen. I put my first spray on when the early evergreen azaleas ('Hino-crimson', 'Delaware Valley White', etc.) are showing color - usually in the first week of May in our location. I then spray once a week for four weeks unless it is especially rainy when I close up the spray schedule to every five days. We have a number of Satsuki azaleas and late blooming elepidotes so I usually put on an additional spray in early June.
When you spray, you must get the fungicide on the flowers and buds that are showing color. Spraying leaves won't hurt but will have no effect in controlling a fungus that only attacks the flowers. Any kind of sprayer will work if you cover the flowers. Good equipment will do the right job using the least amount of fungicide, water, time and effort. You can also do the job with a stepladder and a Windex spray bottle, but I don't recommend it.
I use a powered sprayer with a high pressure pump. It has a 22-gallon (100 liter) spray tank, 50 feet (15m) of spray hose and a single nozzle spray gun. The spray pressure is set at 150 psi (I would go higher but that can damage the flowers). Low pressure sprayers are less efficient (spray droplet size is too large resulting in a need for much higher amounts of spray solution to get adequate coverage) and hose end sprayers are a last resort. My sprayer is on wheels and I tow it around our steeply sloped garden with my lawn tractor. Using my sprayer, I can spray our entire garden of approximately 1,200 rhodies and azaleas (they don't all bloom in the same week), rinse out the sprayer (be sure you rinse well if you want the machine to last) and put it away in about three hours. Depending on how much is in bloom, I put out anywhere from 50 to 125 gallons (567 liters) per application.
We have had petal blight in our garden for many years, but by using my sprayer in the manner described above I have been able to control petal blight well enough that most of you would not be able to find the occasional infected flower that does occur. You have to wait 51 weeks for those flowers. Don't let them rot on the fifty-second!