NCSU PDIC: April Showers Bring May Flowers... and DISEASES!

Last week, we told you about diseases to watch for in your home vegetable garden. In this post, we will tell you about some diseases to watch for in the home landscape. We have already seen these diseases active around campus and you can probably find them hanging out in your own back yards.

Septoria Leaf Spot on Black Eyed Susan

Last week we wrote about Septoria leaf spot on tomato. A different species of Septoria, Septoria rudbeckiae, attacks black eyed Susan and other members of the genera Rudbeckia and Ratibida. This species of Septoria will not cause disease on tomatoes! Symptoms on Rudbeckia begin as small, dark-brown lesions that enlarge as the disease progresses. As the lesions age, small black dots (fruiting structure of the fungus) can be seen in the lesions. Spores are released in late spring and early summer and are spread by splashing water. Lower leaves are usually infected first. To manage this disease, avoid the use of overhead watering and practice good sanitation. Remove fallen leaves at the end of the season to reduce inoculum levels for the following year. Improve air circulation in your beds by spacing plants out. Chemical control can be achieved through the use of preventative fungicides. Products containing chlorothalonil or mancozeb are recommended.

Septoria Leaf Spot on Rudbeckia (Photo: M.J. Munster)

Hollyhock Rust

Hollyhock rust is caused by the fungus Puccinia malvacearum. This very destructive disease often limits the use of hollyhocks in the Southern landscape. Symptoms of rust infection begin as light yellow to orange spots on the upper leaf surface. Brown pustules develop on the underside of the leaves and erupt to reveal bright orange rust spores. These spores are windblown to healthy leaves and infection begins again. Disease increases rapidly because hollyhock rust can infect over and over again. The plants become very unsightly as the infected leaves start to die and fall off. Typically, rusts are highly host-specific. This particular rust infects hollyhocks and other members of the mallow family. Control rust by limiting leaf wetness. Avoid overhead watering and space plants to improve air circulation and promote leaf drying. Rust can overwinter in plant debris so sanitation is important. Remove infected leaves in early spring and remove infected stalks and lower leaves at the end of the season. Plant debris should be buried or burned. Removing nearby susceptible plants, like rose of Sharon and mallow weeds, is important to prevent new sources of inoculum. Chemical control can be achieved through the use of products containing chlorothalonil or myclobutanil. Some seed catalogues list rust-resistant hollyhocks but we do not know of any trial results in this area. Ultimately, you may be forced to forgo planting hollyhocks.

Hollyhock Rust (Photo: M.J. Munster)

Powdery Mildews on Euonymous, Gerbera Daisy, Dogwood and Rose

Right now, several powdery mildews are active in the landscape. Be on the look-out for powdery mildew on euonymous, gerbera daisies, dogwoods, and roses. Control of powdery mildews can be achieved through sanitation and pruning during dormancy. Take care not to over-fertilize since succulent leaves are more susceptible to infection. The best control is host resistance so try to plant resistant varieties whenever possible. Several active ingredients provide effective chemical control of powdery mildews, but control requires applications on continuous 7-14 day intervals. Repeated sprays with the same fungicide can select for fungicide-resistant strains of powdery mildew. For more detailed information about powdery mildews, click here to read our earlier blog post.

Powdery Mildew on Euonymous (Photo: M.J. Munster)

Powdery Mildew on Gerbera Daisy (Photo: M.J. Munster)

Powdery Mildew on Rose (Photo: M.J. Munster)

Powdery Mildew on Dogwood (Photo: B.B. Shew)

Azalea and Camellia Leaf Gall

Azalea leaf gall is caused by the fungus Exobasidium vacinii. This disease is common and widespread in the spring and often attacks new leaves and flower buds. On azalea, infected leaves become thickened, curled, fleshy and pale green to white, forming galls. Infected azalea flowers are usually pale pink. In later stages of disease, white powdery spores cover the surface of infected tissue. A different species of Exobasidium is very common on new growth of sasanqua camellia. On camellia, entire leaves are swollen and distorted and extensive swelling causes the leaf epidermis to split, revealing the white spores beneath. Spores of Exobasidium are windblown or splashed to healthy leaves or flower buds. New infections are not evident until gall-like swellings form the following year. Eventually affected leaves and flowers turn brown, become hard, and fall to the ground. This disease is favored by cool, wet weather. Although unsightly, infected plants are not usually seriously damaged. This disease can be problematic in a greenhouse setting where humid conditions promote sporulation and disease spread. The best control is sanitation. Pick or prune off diseased tissue as soon as swelling starts. Be sure to remove infected tissue before the fungus starts producing spores to prevent new infections next year. Burn or bury the swollen tissue. Often disease is not severe enough to warrant chemical control. When possible, try planting resistant varieties. Susceptible varieties of azalea include White Gumpo, Rosebud, Mother’s Day, and China Seas. Resistant varieties include Amonena, Gloria, Coral Bells, Glacier, Formosa, and Aphrodite. Exobasidium vacinii can also infect blueberries and is responsible for the “green spot” symptom on fruit and leaves. See Bill Cline’s post on Exobasidium vacinii infections of blueberry for more information.