Leaf spot was once one of the most common and destructive fungal diseases of strawberry. Severe infection on susceptible cultivars can result in death of leaflets and defoliation of plants. Many new strawberry cultivars, however, have resistance to leaf spot and the disease is no longer as common or as problematic as it once was.
M.Grabowski, UMN Extension
In young infections of leaf spot, the white center may be difficult to see
The leaf spot fungus can infect leaves, petioles, runners, fruit stalks, berry caps, and fruit. The symptoms of the disease begin with small purple spots on leaves or stems. The centers of leaf spots turn gray and then white with age. As the disease progresses multiple leaf spots merge together creating a reddish purple area with multiple round white centers. In severe cases, the leaves turn brown and die. Fruit infections are not common, but appear as small, sunken, leathery, black spots on unripe and ripe fruit. Seeds within the infected area of the fruit turn black.
The leaf spot fungus overwinters on infected living leaves and in leaf debris. Spores are produced from both of these sources in the spring, and are spread to healthy tissue by splashing rain or irrigation. Cool temperatures (68 to 77o F) and long periods of leaf wetness (>12 hours) are required for new infections to develop. Consecutive wet days with temperatures between 50- 86o F favor disease development. The disease will progress as long as temperature and moisture are in acceptable ranges.
Many strawberry cultivars commonly grown in Minnesota have some ability to tolerate leaf spot infection. Although leaf spots may be observed on foliage, the damage is typically not severe enough to reduce yield.
- 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 through 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 promoting a moist microclimate in the canopy optimal for the growth of leaf spot.
Fungicides are rarely necessary to control strawberry leaf spot. The cultural control practices listed above typically reduce disease to a manageable level.