Forsythia Canker

Canker Fungus

• Mature, environmentally stressed forsythias develop cankered canes following infection by the Botryosphaeria ribis fungus. Forsythia are among more than 170 widely grown ornamentals susceptible to this and several other Botryosphaeria strains, notes the Purdue University Extension.

Fungal Life Cycle

• After overwintering on the foliage and bark of previously infected plants, dormant Botryosphaeria fungi begin releasing spores from raised, pimplelike structures called pycnidia. Spring rains, wind, insects and contaminated garden tools disperse the spores to infect forsythias through natural openings, such as the scars left where leaves have fallen, and through insect damage or mechanical wounds. Forsythias experiencing drought stress are highly susceptible to canker infection.

Effects

• Botryosphaeria-infected forsythias start wilting, even though the plants may otherwise appear vigorous. Closer examination reveals sunken lesions, or cankers, along their canes. Peeling back the cankered bark reveals brown or black diseased wood, sometimes riddled with the spore-producing pycnidia. Leaves on canker-girdled canes become brown and die as the lesions cut off their moisture supply.

Protecting Your Forsythias

• Proper watering, preemptive pruning, and care when mowing or trimming near your forsythia protect the shrub against canker disease. To prevent drought stress, soak the entire root zone slowly with a drip system or sprinkler every 10 to 14 days during hot, dry spells. Prune dead or damaged canes before the fungus finds them. Avoid mower- or trimmer-related bark damage to the plant, so you don't inadvertently expose it to infection.

Fertilizing

• In some instances, fertilizer may raise a forsythia's resistance to canker disease. Before fertilizing, however, have a professional pH test performed to determine if your soil falls within forsythia's 6.0- to 7.0-pH comfort zone. Home pH tests don't give reliable results for alkaline soils with pH readings above 7.0, and your soil's pH determines how many soil nutrients are available to your plants. If the pH is suitable and a soil test kit indicates nutrient deficiencies, feed your forsythia in March or April and again in September. Scatter a granular fertilizer recommended according to the soil test results evenly from the trunk to where the outermost canes touch the ground. Mix it lightly into the soil, and water well.

Treating Forsythia Canker

• No fungicides will treat forsythia canker. To control the disease, prune the cankered canes in late winter, before their flower buds break. Cut them back to healthy, white wood, usually 6 to 8 inches below the lowest cankers. Schedule this pruning separately from pruning your healthy forsythias. Wipe down your pruning tools with a rag dipped in rubbing alcohol between pruning cuts, and dispose of the canes where they can't spread the infection.