Penstemons are short-lived perennials that are often grown as an annual. Over 200 species of penstemons exist, each with its own habitat requirements, making them somewhat difficult plants to cultivate. Most penstemon species grow best where summers are cool. Penstemons are susceptible to several foliar diseases, so select disease-resistant varieties to help ensure that your plants are vigorous and less vulnerable to potential leaf disease.
Botrytis Leaf Mold
This fungal disease is spread by wind and water. Initially, brown spots appear on infected leaves. As the diseases progresses, flowers appear damaged and distorted. Remove all infected buds, blossoms and leaves. Destroy badly infected plants. Severe infestations may require the application of an organic fungicide.
This fungal disease causes the formation of small yellow spots on top of leaves. Powdery orange to brown spores develop on the underside of leaves. Severe infestation causes the entire leaf to turn yellow and shrivel. Rust is especially active during humid weather. Apply fungicidal soap early in the growing season as a preventative measure. Remove infected leaves and stems as they appear to control the spread of the disease. Destroy badly infected plants.
Leaves infested with this fungus appear dusted with white powder. Powdery mildew primarily attacks new leaves, causing distorted appearance and spindly growth. Spores appear as tiny black dots and are spread by the wind. Remove and destroy infected plants before the powdery mildew spores form. To help control the disease, mix together one teaspoon baking soda in one quart of water and use as a spray. Unlike other fungal diseases, powdery mildew is less of a problem in damp weather, thriving during hot days with accompanying cool nights.
Nematodes are microscopic, wormlike parasites that attack and kill the leaves of the host plant. Penstemon plants attacked by nematodes appear limp and wilted. Infected leaves display yellow patches that gradually turn black or brown. Lesions may eventually cover the entire leaf, causing leaf drop. Plant nematode-resistant cultivars. Interspersing marigolds among the penstemons may repel the parasites.
Prevention of Leaf Diseases
You can take some preventative measures to decrease penstemons' susceptibility to attack from leaf diseases. Purchase disease -resistant cultivars. Provide proper spacing for your plants to give them good light and air circulation. Many foliar diseases are spread by wind and rain. Use mulch to help avoid rain splashing up against the plants. Water the ground around your flowers carefully to help keep foliage dry. Don't work in the garden when plants are wet. Many fungal spores and bacterial diseases overwinter in infected leaves and plant material. Clean up garden debris in the fall. Finally, don't dispose of any infected leaves or plant material in your home compost pile or bin.