Disease Problems of Strawberry

Each year, the diagnostic clinic receives several samples of strawberries with disease problems. The common problems are discussed in this article.

Several leaf diseases will affect the overall vigor of the plant. If the plant is growing poorly, has reddish coloration to the leaves, or is producing less fruit than usual, consider the following problems: leaf spot (Mycosphaerella fragariae), leaf blight (Dendrophoma obscurans), or leaf scorch (Diplocarpon earliana). Leaf spot, leaf blight, and leaf scorch all have very similar symptoms with slight variations. It may be difficult to diagnose these problems without the aid of a microscope or laboratory evaluation.

Leaf spot appears after fruiting and as 1/8 to 1/4 inch red-violet spots on the leaves. These spots gradually enlarge, and the center turns brown, then gray, and finally white on the older leaves. The spots generally become worse with each successive year. Leaf spot gradually reduces plant vigor and increases the plant’s susceptibility to winter kill.

Leaf blight also appears after fruiting, but instead of small red- violet spots, this disease appears as round to wedge-shaped tan areas on the leaves, surrounded by a dark red margin. This disease eventually will kill entire leaves. It also reduces the vigor of the plant and increases the plants susceptibility to winter injury.

Leaf scorch is similar to leaf spot and leaf blight in that it reduces vigor and winter survival. Leaf scorch appears as violet to red spots that may coalesce to affect the leaf margins. Entire leaves may turn a bright red or violet color. As the disease progresses, the margins of the leaves will curl and develop scorch.

Several cultural practices will help minimize problems from leaf spot, leaf blight, and leaf scorch. When starting a new strawberry bed, choose varieties with good disease resistance. Many varieties have a tolerance to leaf spot and leaf scorch. Choose a site that is well drained with good sun exposure and air circulation. Keep plantings well weeded to promote rapid drying of foliage after heavy dews or heavy rains. The fungi of these diseases overwinter on infected plant material and are introduced to the new growth each spring by splashing rain or irrigation water. Picking off diseased leaves and mowing the foliage just after harvest will help minimize infections. In addition to these cultural practices, fungicide applications may provide added protection. See MU Guidesheet G6010, Fruit Spray Schedules for the Homeowner, for timing and product information.

If the plant displays poor vigor and poor fruit production, there are several other disease problems to consider such as Anthracnose crown and root rot (Colletotrichum fragariae), black root rot (caused by a complex of fungi and environmental conditions) or red steele (Phytophthora fragariae). Plants affected by anthracnose crown and root rot will have reddish brown streaking in the crown of the plant and can be observed by slicing the crown in half lengthwise. Plants infected by black root rot are typically stunted, produce few runners, and sometimes die. The root system is brown and diminished. Plants infected with red steele are also stunted and often die. The cortexes of the major roots are decayed and may have a "rattail" appearance. If the roots are split lengthwise, the centers of infected roots are dark instead of the normal creamy coloration. If such disease problems are observed in the home strawberry patch, the best tactic is to start a new bed in another area. These diseases are generally most severe in heavy, poorly drained soils. If well-drained soils are not available, consider planting in raised beds. Choose healthy, certified disease-free plants. Mulch plants in the fall to protect crowns from freeze injury to reduce the risk of infection of black root rot.

Two diseases affect the quality of fruit: gray mold (Botrytis cinerea) and leather rot (P. cactorum). Gray mold appears on the fruit as light brown spots. Infected berries turn light brown, but remain firm. The fungus will quickly produce a light gray, fluffy looking growth, which produces the spores. These spores are spread to neighboring berries and plants by wind during wet conditions. Gray mold is most severe in situations where there is poor air circulation and high humidity. Berries touching the soils or other infected berries are likely to become infected themselves. Mulching strawberries with straw, avoiding overhead watering, and removing infected berries and plant parts will help minimize the infections. Fungicide applications may also suppress gray mold. See MU Guidesheet G6080 for management recommendations.

Leather rot causes brown, firm spots on green berries and light to dark purple or brown, soft spots on ripe fruit. Ripe berries may smell rotted and have a bitter taste. The infected fruit eventually shrivels and becomes leathery. The fungus that causes leather rot is favored by wet and poorly drained soil conditions. It can be serious during cool wet springs. Control measures include mulching plants to minimize fruit contact with soil, minimizing overhead irrigation, planting on well-drained beds, and removing infected fruit.

Laura Kabrick, Extension Plant Diagnostic Clinic, UMC (573) 882-3019