Black Spot and How to Treat It

Where we live in the deep south, the reason most people don’t grow these beautiful roses is that they can’t keep the blackspot off the rose bushes. Blackspot is a fungus called Diplocarpon rosea, and travels via water to infect plants. It our wet and humid environment, blackspot is a given. It will cause leaves to turn yellow with black spots. These leaves will eventually die and fall out, leaving your plant weakened.

 I have found that the only fungicides that keeps blackspot off of the roses are propiconazole, which is found in Banner Max, or in generics such as Honor Guard and others. Most of this fungicide is only offered in large quantities. The other fungicide is Ethylenbisdithicocarbamate which also includes manganese and zinc. This product comes under a number of names, such as Manzate, Diathine M-45, and others.

The two products we use are Honor Guard (a systemic fungicide) and Diathine M-45 (a contact fungicide). These are available for sale. Honor Guard is in a 1 pint container for $49.95. Diathine M-45 is in a 2 lb container for $34.95. The recommended use for Honor Guard is ½ teaspoon to a gallon of water. The recommended use for Diathine M-45 is a tablespoon to a gallon of water. These will last you for many sprays.

We spray our roses every 7 to 10 days, and this controls the blackspot. We use both products in the same spray as one is a systemic and the other is a curative or contact. We also add a couple drops of dishwater detergent to a gallon of water as a sticking agent for the spray. We spray early in the morning or late in the afternoon so as not to spray during the hot hours of the day ( this prevents the leaves from burning).

We also recommend protective gear that in on the label of the product as well as following all the other directions on the label.

Every 4 times we spray our roses we omit the Honor Guard and spray only Diathine M-45. This keeps the roses from building up the growth inhibitor that is in the Honor Guard. Then the next time we spray we use the Honor Guard as usual.

In all of the years we have grown roses, this is the most effective way we have ever used. You should also use a sprayer that will spray a fine mist so as to really cover the leaves. There are many types available on the market such a backpack sprayers or the easy rechargeable 12 volt sprayers.