Treating Black Spot

What are these spots on my leaves and how do I get rid of them? This is a commonly asked question. More than likely it is blackspot. Blackspot is a common fungus in roses (Diplocarpon rosea). This fungus travels by moisture onto the leaves and infects the plant. It causes black spots on the leaves which will eventually turn yellow and fall off. Water or moisture on the leaves for longer than 7 hours continuously, will cause this condition. When a plant becomes infected and it is watered overhead or rains, the spores are spread to the rest of the plant by splashing.

 Almost all roses are susceptible to black spot in the humid south. I think the reason many gardeners don't grow roses anymore is because of this fungus. In our garden very few roses are blackspot free. Darlows Enigma, Rugosas, some of the antiques and some of the modern varieties. Roses that have healthy leaves in abundance always have better and bigger bloom performance. The more leaves, the more it can absorb the sun. 

 In our Oklahoma springs, it can get really bad after the new growth starts to harden off. In the summer it won't be as bad, but we rarely get rain from the end of July to September. Blackspot returns in the fall when the rains start again. We have tried all the homemade remedies and none of them work. If you want blackspot free roses in the South, you have two choices. Grow resistant varieties or spray.

 There is only one way we have found to keep the roses clear. A combination spray of fungicides propiconazole and ethylenbisdithicocarbamate. Most of the fungicides can only be purchased in larger quantities and will last for several years. Propiconazole 14.3% is a  systemic fungicide and can be purchased in a concentrate quart. Ethylenbisdithicocarbamate is a contact fungicide and goes by Diathine M-45. It is a powder and can be purchased as 6 oz or 2 lbs. Both of these fungicides are readily available on the internet. You will also need a surfactant. (sticking agent) We use a couple of drops of dishwashing detergent or you can use a professional product. 

 We mix both fungicides and the surfactant together. Measurements for 1 gallon;  Propiconazole is 1/2 teaspoon and for the Diathine M-45 is 1 tablespoon. Surfactant is 1 or 2 drops. Mix well. Spray every 10 to 14 days throughout the growing season. Every 4th spray omit the propiconazole. This keeps the plant from building up the growth inhibitor that is in the propiconazole. Spray as normal the following period. A note on the Diatathine M-45. At 1 tablespoon we see residue streaks on the plants that is sometimes undesirable. Cut back to 3/4 tablespoon if you don't like the streaks. For maximum effectiveness, use 1 tablespoon. Also use a sprayer capable of a fine mist. Coat the entire plant and undersides of the leaves thoroughly. Battery powered sprayers are wonderful. A 1 gallon mix goes a long way.

 Please read the label and follow all protective equipment reccomendations and other requirements.