The leaf blight fungus infects all green parts of the plant and rarely causes a soft rot on ripening and ripe fruit. In Minnesota, leaves are most severely infected in shaded patches that have heavy dew or in years with frequent rain events.
T. McCamant, Northland Community College
Fruit infection by Phomopsis obscurans, leaf blight pathogen
Symptoms on leaves begin with solid reddish-purple spots that develop a tan center as they grow. As the disease progresses leaf spots enlarge to V-shaped lesions with dry brown centers with reddish purple boarders. Severe infections can turn a whole leaflet brown. Fungal spore producing structures appear as black specks that dot the central area of the older lesions. These can be seen with a 10x hand lens.
Stem infections appear as reddish purple round to oval spots that never develop a tan center. Soft, mushy, pink spots form on pink to red fruit. As fruit infections age, the fruit spots become dark brown and dry.
The fungus that causes leaf blight overwinters in infected leaves of the strawberry plant or in infected leaf debris in the strawberry patch. Spores are released in response to moist conditions, and are transported to new leaf tissue by splashing water. Fungal spores of leaf blight require periods of extended leaf wetness (> 15 hours) to germinate. The duration of leaf wetness is the critical factor in disease development because spore germination can occur over a wide range of temperatures. Frequent rains, overhead irrigation, and heavy dews favor disease development.
- There are no strawberry cultivars that are resistant to leaf blight.
- Choose sites with full sun, good soil drainage and air circulation. This fungus requires long periods of continuous wetness to infect plants, thus any practice that promotes quick drying of leaves and fruit will reduce disease.
- Remove weeds to improve air circulation around plants.
- Plant in rows or narrow beds, no wider than 12-18 inches, to promote good air movement in and around plants. Patches grow with time as new runners are produced. Use renovation to maintain narrow beds.
- Renovate strawberry beds every year after harvest. The process is described in Strawberries for the Home Garden.
- Following renovation, rake and remove old leaves.
- Irrigate with drip irrigation or a soaker hose. If overhead sprinkling is your only option, water early in the morning on a sunny day so leaves dry quickly after irrigation.
- Apply nitrogen fertilizers after renovation. Avoid early spring applications of nitrogen which encourage overly lush growth which reduces airflow and promotes a moist microclimate in the canopy optimal for the growth of leaf blight.
Fungicides are not considered necessary to control leaf blight in home strawberry patches. Cultural practices usually keep this disease from spreading beyond a tolerable level. Although fungicides are available that will reduce leaf blight infection, they would need to be applied regularly throughout the growing season to maintain continual protection of young leaves. This results in a significant cost, in time and resources, to the gardener. In addition, there is little evidence that reducing leaf blight infection through fungicide sprays will increase yield in following years.