Cause: Black Spot is a fungal disease, caused by Diplocarpon rosae. It is spread by windborne spores and develops when conditions are right - temperature, moisture, and a susceptible host. Overhead watering will not cause the disease unless the plant stays wet well into the night. Black spot is the least prevalent of the fungal diseases in our climate due to our warm, dry summers. It can be a problem at the beach.
Symptoms and Effects: Roundish black to purple black spots on leaves. Usually several spots on a leaf that grow together will cause a necrotic (dead) area. Advanced infection will cause the plant to die due to lack of foliage.
Remedy and Avoidance: As with most problems, contact your local county agricultural agent or a dependable nurseryman. At The Huntington, we don't spray. You can pick off affected foliage. You can also use a home remedy of 1 Gallon water, 1Tablespoon baking soda, 1 Tablespoon vinegar, and 1 Tablespoon of canola oil or superfine horticultural oil. (Do not use any other household cooking oil because it will not mix well with water.) Spray this mixture on the leaves any time except if the daytime temperature is going to be above 80 degrees. If it's too hot when this mixture is applied, the leaves will suffocate. Usually, when itıs above 80 degrees in our climate, Black Spot is not a problem.
Cause: Powdery mildew is caused by a fungus, Sphaerotheca pannosa var. rosae. Powdery mildew is very temperature dependant. It is only a problem in cool, moist weather, such as when we have warm days and cool nights. We see it from March until the 4th of July, then again in mid to late September, October, and November.
Symptoms and Effects: A white, downy growth on foliage and/or stems. Powdery mildew will not kill the plant. It will deform the plant, but the plant will come back.
Remedy and Avoidance: Keeping foliage clean by showering in the warm part of the day will help. You can also use a home remedy of 1 Gallon water, 1Tablespoon baking soda, 1 Tablespoon vinegar, and 1 Tablespoon of canola oil or superfine horticultural oil. (Do not use any other household cooking oil because it will not mix well with water.) Spray this mixture on the leave any time except if the daytime temperature is going to be above 80 degrees. If it's too hot when this mixture is applied, the leaves will suffocate.
To avoid getting powdery mildew, select varieties with more resistance to the problem, keep foliage as dry and clean as possible, and use the home remedy mentioned above.
ROSE MOSAIC VIRUS
Cause: This disease is caused by a virus. It is not transmitted from plant to plant or by pruners or shovels. Current science says that it only spreads through propagation using infected cuttings or rootstock.
Symptoms and Effects: Yellow patterns on leaves, often in a flame-like formation. Rose mosaic virus will not kill the plant, but it does cause the foliage to look strange. Over many years, it will weaken the plant. Flower production typically goes down in 3 to 5 years.
Remedy and Avoidance: There is no cure for this virus, so the only way to get rid of it is to remove the plant. The best prevention is to buy from reputable nurseries and growers.
Cause: Salt damage is a naturally occurring activity. Plants can't excrete the poisonous byproducts of life, so one of the common things a plant does is to store these poisons in the outer edge of foliage. Since the leaf is going to fall off at some point anyway, it is an effective way to rid the plant of poisons. In our alkaline soil, we have many salts. This can also be seen after an application of fertilizer to a dry plant.
Symptoms and Effects: Discolored leaf edges. If it gets very bad, the leaves fall off and fresh leaves develop. We usually see this more at the end of the year, but salts also build up during periods of long drought.
Remedy and Avoidance: There is no way to fix the affected leaves. To prevent future occurrences, deep, but less frequent watering will help dilute the salts in the soil. To avoid causing this with fertilizer, water the plant the day before you fertilize. With potted plants, you can flush the salts out once per month by heavy watering and draining. Don't let the plant sit in the drained water.
(Note: Photo shows both nitrogen deficiency and black spot.)
Cause: On lower leaves, this is a natural occurrence that is a physiological response to growth. Plants transfer nutrients to the parts of the plant that need them most - the tips, where growth takes place. Lower foliage gets bypassed and becomes nitrogen deficient.
Symptoms and Effects: Lower leaves on the plant turn yellow and fall off. This is common in the spring, as the first flowers are growing and the base of the plant gets shaded. If the upper leaves of the plant exhibit these symptoms, then it's real nitrogen deficiency and treatment is needed.
Remedy and Avoidance: If only lower leaves are affected, do nothing - this is a natural occurrence. If upper leaves are affected as well, use a balanced nitrogen fertilizer that also contains iron (to prevent chlorosis.) Be sure to water the plant the day before fertilizing.
Cause: Also known as green fly, aphids are soft-bodied insects with a sucking mouth. Instead of chewing foliage like grasshoppers, they insert their mouths into the cell of the plant and the water pressure inside the plant pushes the sugars into the gut of the aphids. Mealy bugs and scale are similar in the way they attack a plant.
Symptoms and Effects: Small green bugs and their white castoff skins on buds, leaves, and stems. They weaken the plant by sucking out juices. A large infestation will deform the flowers. Aphids are only a problem when we have cool days. Once the temperature gets into the 80's, they lay eggs and go away. The eggs remain dormant until ideal conditions return.
Remedy and Avoidance: You can hose them off, tap them off, squash them, or use a predator insect like ladybugs. Anything with oil will suffocate them, so you can use a home remedy of 1 Gallon water, 1Tablespoon baking soda, 1 Tablespoon vinegar, and 1 Tablespoon of canola oil or superfine horticultural oil. (Do not use any other household cooking oil because it will not mix well with water.) Spray this mixture on the leave any time except if the daytime temperature is going to be above 80 degrees. If it's too hot when this mixture is applied, the leaves will suffocate.