Brown spots on strawberry leaves are a common problem, often signaling fungal or bacterial diseases that thrive in moist conditions. These blemishes weaken the plant by reducing the leaf surface area available for photosynthesis. Diagnosing the exact cause is the first step, as treatment depends entirely on whether the pathogen is a fungus or a bacterium.
Identifying the Common Causes of Brown Spots
The appearance of the spots provides the most reliable clue for differentiating between common strawberry leaf diseases.
Leaf Spot, caused by the fungus Mycosphaerella fragariae, initially appears as small, deep purple, circular spots on the upper leaf surface. As these spots grow, their centers turn grayish-white or light brown, maintaining a distinct reddish-purple border. This often gives them a characteristic “bird’s-eye” look.
Another fungal issue is Leaf Scorch, caused by Diplocarpon earliana. It presents as numerous, irregular, reddish-purple blotches instead of circular spots with defined centers. These blotches can merge, turning large sections of the leaf tissue brown or reddish-brown. The leaf margins often curl inward, making the foliage look dried out or “scorched.”
The third common cause is Angular Leaf Spot, a bacterial infection from Xanthomonas fragariae. This disease is distinguishable because the spots are angular, or straight-edged, restricted by the leaf veins. When held up to the light, young, water-soaked lesions often appear translucent, creating a “windowpane” effect.
Immediate Treatment Methods
Once the specific disease is identified, immediate action must be taken to halt its spread. For all leaf diseases, the first step is to remove and destroy all heavily infected leaves and plant debris to reduce the pathogen load. Infected materials should be placed in the trash, not added to a compost pile, where pathogens can survive and reinfect the garden.
If the issue is fungal, such as Leaf Spot or Leaf Scorch, chemical treatments are effective for severe infestations. Broad-spectrum fungicides like captan or myclobutanil are often used. Organic options, including copper-based sprays or neem oil, can also be applied to disrupt the fungal lifecycle.
Applications should follow the manufacturer’s instructions closely, often requiring repeat treatments every seven to fourteen days until conditions are no longer favorable for the fungus. When using copper-based products, apply them before flowering to avoid potential phytotoxicity on blooms or developing fruit.
For Angular Leaf Spot, chemical fungicides are ineffective because the cause is bacterial. While copper compounds are registered, their effectiveness is highly variable, and repeated use can harm the plants. The primary control for this bacterial disease is cultural, requiring immediate removal of infected material and a significant reduction in leaf wetness.
Long-Term Management and Prevention
Preventing the recurrence of brown spots requires adjusting the growing environment to be less hospitable to pathogens. Improving air circulation helps foliage dry quickly after rain or irrigation. This is achieved by ensuring proper plant spacing, typically 18 to 24 inches apart, and by regularly removing old, dense foliage.
Changing how the plants are watered is crucial, as prolonged leaf wetness drives disease development. Switching from overhead sprinklers to drip irrigation delivers water directly to the soil, keeping the leaves dry. If overhead watering is necessary, do it early in the morning so the sun can dry the foliage completely before evening.
Sanitation practices are important because many pathogens survive the winter in infected plant debris. A thorough cleanup of the patch at the end of the season, including removing all old leaves and matted material, significantly reduces the source of infection for the following spring. Crop rotation also helps by not replanting strawberries in the same location for several years, starving soil-borne pathogens of their host.
Selecting strawberry cultivars known for genetic resistance can reduce future disease issues. Varieties like ‘Hood’ and ‘Rainier’ show resistance to Leaf Scorch, offering a proactive defense. Choosing certified, disease-free plants at the time of planting provides the healthiest possible start.