Black spots on leaves on Plants

Symptoms

Start with the upper surface of the leaves, because that is where the black spots show. The stems can carry black spots as well. The spots are round and range from brown to black, and their edges look feathery.

Look closely at a spot and you may see tiny black fruiting bodies of the fungus within it. Yellow areas form around the black spots. A leaf that shows black spots, feathery edges and a yellow area around the spots fits this description.

Leaves that fall early are one of the signs of this problem. Small purplish spots on the canes are another sign. Check the canes as well as the leaves, since small purplish spots can appear there.

These signs are described for roses, so compare them with what you see on your own plant. Spots on the leaves and spots on the stems both count, because the fungus produces black spots on both. Take note of which parts of the plant show spots, the leaves, the stems or the canes.

Causes

The disease is caused by the fungus Diplocarpon rosae, which produces the black spots on the upper surface of rose leaves and stems. Its spores spread by splashing water. The leaves must remain wet for more than about 7 hours.

Foggy or humid conditions are named among the causes. So is sprinkler irrigation in the evening or at night that keeps foliage wet. In California, this fungal disease of roses is usually a problem only in foggy or humid coastal areas.

Splashing water, long leaf wetness, humid or foggy air and evening sprinklers are the four causes listed for this disease. In that list, the fungus is the one biological factor, and the other three are about water and humidity. Roses are the host plant named for this disease. If you water your roses with a sprinkler in the evening or at night, that is one of the listed causes.

Diagnosis

Begin by checking the plant type, since the host named for this disease is roses. Then look at the spots themselves: black on the upper surface of the leaf, round, with feathery edges. Tiny black fruiting bodies within the spots are another detail to check for. Yellow areas around the spots and purplish spots on the canes add to the match.

Record where the spots are, on the leaves, the stems or the canes. Next, compare the conditions around the plant with the listed causes, which include wet foliage, humid weather and splashing water. Leaves that stay wet for more than about 7 hours fit the cause described. So do foggy or humid weather and sprinkler watering in the evening or at night.

What sets this disease apart is the pairing of a fungus with water, meaning spores moved by splashing water and leaves that stay wet. No look-alike problems are listed in the facts for this page, so this comparison rests only on the signs and causes above.

Treatments

Irrigate in the morning rather than in the evening or at night, so the foliage dries quickly. Water in a manner that keeps the foliage surfaces dry. Both watering steps are about the leaves.

Do not plant roses too close together. Prune canopies to increase air circulation. Prune off infected stems or canes, especially during the dormant season. Pruning appears twice in the cultural steps, once for the canopy and once for infected canes.

Remove fallen leaves and stems and dispose of them away from the plants. When you plant, choose disease-resistant varieties. Choosing a resistant variety is a step you take at planting time.

The organic options listed are neem oil, potassium bicarbonate and sulfur. Each is applied as a preventive fungicide during weather that favors severe disease.

The chemical options listed are the synthetic fungicides triforine and chlorothalonil. These are also applied as preventive fungicides during weather that favors severe disease. A fungicide can also be applied to protect new foliage. The two chemical active ingredients are described as synthetic. The organic and chemical methods share the same timing, which is weather that favors severe disease.

The product label is the legal instruction, so follow it for the crop and site you are treating. This page gives no dose, rate, dilution or spray interval.

Prevention

Prevention is mostly about the conditions around the plant. Water in the morning so the foliage dries quickly, and avoid evening or nighttime sprinkler irrigation that keeps foliage wet. Splashing water spreads the spores.

Do not plant roses too close together, and prune canopies to increase air circulation. Remove and dispose of fallen leaves and stems away from the plants. Prune off infected stems or canes, especially during the dormant season. Choose disease-resistant varieties when you plant.

Foggy or humid conditions are a listed cause of the disease. In California, the disease is usually a problem on roses only in foggy or humid coastal areas. Roses are the host plant named for this disease, so these steps are written for roses.

Plants most often affected

The plant named for this problem is the rose. This fungal disease of roses is usually a problem in California only in foggy or humid coastal areas. The fungus behind it is Diplocarpon rosae. If your plant is a rose with black spots on the leaves, the rest of this page applies to you.

No plant page in this build records this problem yet.