In addition to leaves, leaf scorch can infect petioles, runners, fruit stalks and berry caps. If unchecked, plants can be significantly weakened reducing the growth of all plant parts. Severely infected plants have little capacity to cope with other stresses and can die from drought or extreme temperatures.
Numerous dark-purple, angular to round spots appear of the upper surface of the leaf. As the disease progresses the tissues around these spots turn reddish or purple. In severe cases, the infected area dries to a tan color and the leaf margin curls upward looking scorched. In contrast to spots caused by leaf blight (caused by Phomopsis obscurans) and leaf spot (caused by Mycosphaerella fragariae), leaf scorch lesions will remain completely reddish purple and will not turn tan or gray in the center.
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.
The plants energy resources are depleted as the number of leaf spots increases, reducing the plants ability to do photosynthesis and store energy. As a result, severe infection by leaf scorch in summer and fall often results in reduced yield the following year. Leaf scorch infections that form on fruit and flower stalks can girdle the stalk which will kill the fruit and flower.
Although some cultivars have been reported to show resistance to leaf scorch, these have not proven to be reliably disease free in Minnesota.
- 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 air flow promoting a moist microclimate in the canopy that is optimal for the growth of leaf scorch.
There are no fungicides available to home gardeners that effectively control strawberry leaf scorch. Cultural practices usually keep this disease from spreading beyond a tolerable level.