Black spot on Plants

Symptoms

The fungus produces black spots on the upper side of rose leaves and stems. On leaves, the spots start brown to black and round, with feathery rather than smooth edges. After the spots form, the leaves around them turn yellow and then fall from the plant. Canes show a different symptom: small purplish spots on the cane surface. The fungus behind these symptoms is Diplocarpon rosae, and its spores travel in splashing water. For the spores to infect a leaf, that leaf must stay wet for more than about seven hours. These symptoms are tied to foggy or humid coastal conditions, and to any situation that keeps foliage wet, such as evening or night sprinkler irrigation. Roses are the host these symptoms affect. You can check both leaves and canes for signs, since the fungus marks each part differently. Removing fallen leaves and infected canes is a cultural step growers use once these symptoms appear. Pruning for better air circulation is another step that can follow once spots are spotted.

Causes

A fungus called Diplocarpon rosae causes black spot. Its spores spread through splashing water. Both infection and spore production require about seven hours or more of continuous leaf wetness. That long wet period is more likely in foggy or humid coastal areas. It is also more likely anywhere foliage stays wet for other reasons, such as sprinklers that run in the evening or at night. Roses are the host plant this fungus infects. Once established, the fungus produces the black spots on leaves and stems, and the purplish spots on canes, that give the disease its name. One cultural step is to water in the morning rather than in the evening or at night. Another cultural step is to avoid planting roses too close together. Pruning the canopy to increase air circulation is another step. Rose growers also remove infected canes, and remove and destroy fallen leaves, as cultural steps. They also choose disease-resistant rose varieties when planting. The symptoms that result are black spots on leaves and stems, brown to black round spots with feathery edges, and small purplish spots on canes.

Diagnosis

Look at the upper side of rose leaves and the stems, since that is where the fungus produces its black spots. A true black spot lesion is brown to black, round, and has a feathery rather than smooth edge. If the leaf around the spot has turned yellow or has already fallen, that fits the disease. Check the canes as well, since infected canes show small purplish spots of their own. Black spot is a disease of roses, so the plant you are checking should be a rose. If nearby leaves have stayed wet for more than about seven hours at a time, conditions were right for the fungus to infect them. Humid or foggy weather near the coast, or sprinklers that run in the evening or at night, point to the kind of wetness that lets this fungus take hold. Also check whether roses nearby were watered in the evening or at night, since that habit keeps foliage wet and favors the fungus. If you see spots only on roses and not on nearby plants, that also fits, since roses are the host for this disease. The fungus responsible is Diplocarpon rosae, which spreads through splashing water onto wet leaves.

Treatments

Water roses, and hose off aphids, in the morning instead of in the evening or at night. Do not plant roses too close together. Prune the canopy to increase air circulation. Plant roses where they will get at least 6 to 8 hours of full sun. Choose disease-resistant rose varieties when you plant. Remove infected canes, and remove and destroy fallen leaves. Water in a way that keeps the leaf surfaces dry. Where the weather favors severe disease, you can apply neem oil, potassium bicarbonate, or sulfur as preventive organic fungicides. Triforine and chlorothalonil are synthetic preventive fungicides for the same conditions. A fungicide applied to new foliage also protects it from the disease. The fungus behind black spot, Diplocarpon rosae, spreads through splashing water. Infection cannot happen until a leaf has been wet for roughly seven hours or longer. Roses are the host plant affected. Symptoms include black spots on leaves and stems, brown to black round leaf spots with feathery edges, and small purplish spots on canes. Humid or foggy coastal weather favors this fungus, as does any habit that leaves foliage wet, such as night-time sprinkler use. Its spores travel in splashing water. These treatments apply specifically to roses, the host plant for black spot.

Prevention

Switch any evening or night watering to the morning, and do the same when you hose off aphids. Give each rose room by not planting them too close together, and open up the canopy with pruning for better air flow. Pick a spot that gets 6 to 8 hours of full sun or more, and favor rose varieties bred to resist disease. Clear away infected canes and fallen leaves rather than leaving them on the ground. Choose a watering method that keeps the leaf surfaces dry. In weather that favors severe disease, a preventive fungicide applied ahead of time, and again on new growth, is part of this routine. All of this matters most in foggy or humid coastal conditions, and anywhere foliage stays wet, since that wetness is what the fungus needs to infect leaves.

Plants most often affected

Roses are the host plant for black spot, and some sources name the disease specifically as black spot of roses. The disease shows up most in foggy or humid coastal areas, and also wherever foliage stays wet, such as under evening or night sprinkler irrigation. The fungus behind it, Diplocarpon rosae, spreads through splashing water. Rose growers in these conditions can expect spots on leaves, stems, and canes.

• Dracaena masoniana
• Rugosa Rose