Roses Suffer in Summer, Thrive in Winter in Vegas

Q. My roses did not do well this summer after a spectacular spring. Now there are black spots on the plant's leaves.  I was told this is "black leaf" disease. What should I do now? They also got that white fungus when we had all that rain.  

(Pictures were not submitted)

A. Summers are our “winters” for roses here in Las Vegas. They don’t grow well during our very hot summer temperatures. They like the cooler spring and fall months and even the winter months if planted in warm, protected locations. They do wonderfully here for about 8 months of the year.

Rose plant growing in rock mulch. Sooner or later the rock mulch will ruin it.

            We don’t usually get black spot disease on roses here because we are so dry. I sent you a picture of this disease on roses and I will post it on my blog as well. This disease is more common in humid climates. 

Read about black spot disease of roses by Rosarians 

            This is a fungal disease and so watering roses with drip irrigation, not overhead spraying, usually corrects the problem as well as eliminating powdery mildew disease (that white fungus you mentioned on the leaves). It also helps if roses are grown in the sun, not shade, for 6 to 8 hours every day. Morning sun is best.

            Keeping roses healthy helps fight disease and damage from heat and sunlight. It also helps if air can move easily through their canopy which helps control leaf diseases like black spot and powdery mildew. Pruning roses lightly during summer months helps keep their canopy open.

            Roses growing in the desert like compost applied beneath them to improve the soil and wood chips applied after that as a mulch. Apply compost around the base of roses in January and cover the soil with 3 to 4 inches of wood chips instead of bare soil or rock. This makes a huge difference in the performance of roses.

This is a fruit tree but fertilizing roses are done the same way. Keep it away from the trunk when applying compost because some composts are "hot"

            Compost is also great fertilizer. This same 1-inch layer of compost applied in January will also fertilize them until about April. Use a follow up rose food in late spring and again in the fall months.

            If you are convinced this problem is black spot disease, then pick up a rose fungicide that states it controls black spot on roses and apply it according to the label. Repeat applications may be necessary to protect any new growth from getting this disease.

Powdery mildew disease on rose

            The white powdery fungus you saw was probably powdery mildew. This tells me your roses may be growing in some shade. Shade, and watering with overhead sprinklers, keeps the leaves wet, a perfect environment for powdery mildew. Splashing water from overhead irrigation sprinklers spreads the disease from leaf to leaf and plant to plant.

Read about powdery mildew on roses by Rosarians

            Prune out unhealthy growth now, getting rid of diseased plant parts, and leave 4 to 8 healthy canes for further pruning this winter.