Fungal Foliar Diseases of Strawberry
Whether you grow June-bearing, everbearing, or day-neutral strawberries, you will likely encounter at least one fungal foliar disease. These diseases first appear before fruits develop but can persist throughout the growing season, and if not managed, can weaken the plant. In general, these fungal foliar pathogens thrive in wet conditions, so cultural practices that limit water on leaf surfaces will help control these diseases.
Leaf spot, leaf scorch, and leaf blight are three major foliar diseases of strawberry. These pathogens can occur individually or together on the same plant or even on the same leaf if the cultivar and climate are conducive. A few spots from these diseases are relatively harmless if spread is controlled. However, when these fungal diseases are severe enough, they can cause economic damage due to reduced photosynthesis and plant vigor. Further, severe infections weaken plants, making them more susceptible to winter injury and root diseases.
Leaf spot, scorch, and blight share similar disease cycles and management strategies. The fungi responsible for these diseases may become active as the first strawberry leaves unfurl in early spring and continue spreading until they become dormant in the late fall with cooler temperatures. The fungi produce small, black fruiting bodies on leaves, which produce spores. These spores spread in moist, moderate conditions, especially by splashing water. All three of these fungal pathogens overwinter on infected tissue and crop debris. Despite their many similarities, these foliar diseases can be distinguished from each other according to the symptoms visible on the leaflets (see individual descriptions and images below).
Common leaf spot is a disease caused by the fungus Mycosphaerella fragariae. Symptoms of leaf spot begin as small, circular purple or red spots on the upper leaf surface, later developing into tan, gray, or white spots with reddish-purple borders. As infection progresses, these blotches may coalesce, and the lower surface may also establish spots. Fruit may develop one or two superficial dark spots per fruit around groups of seeds. Fruit infection is not common but may occur under prolonged wet conditions. It is possible for lesions to form on stems, petioles, and runners as well. Severity of infection depends on the strawberry cultivar, strain of fungal pathogen, and weather conditions. Middle-aged leaves tend to be the most susceptible to common leaf spot. Today, this disease is often a cosmetic problem, not usually hindering crop yield or fruit quality.
Leaf scorch is caused by the fungus Diplocarpon earliana. Symptoms of infection include numerous small, irregular-shaped purplish-brown spots on the upper leaf surface. Unlike those of leaf spot disease, leaf scorch spots stay dark in the center. Between these lesions, the leaf area may turn somewhere between yellow to bright red in color. Eventually, the spots may merge to cover the leaflet, making it a purple, red, or brown color, appearing scorched. Leaves can be infected at any stage of development, but older and middle-aged leaves tend to be more susceptible than younger leaves. Leaf scorch can weaken plants significantly, and increase susceptibility to drought stress and winter damage.
Leaf blight of strawberryis caused by thefungus Phomopsis obscurans. Symptoms first appear on a leaflet as reddish-purple circular or lens-shaped spots. These gradually enlarge to V-shaped lesions with a light brown inner zone and a dark brown or purple outer zone. These areas of necrosis (dead tissue) are found near leaf veins and spread along the leaf margin. On ripening fruit, lesions may form as soft pink spots which later turn brown and dry. When weather conditions are favorable, new lesions appear throughout the summer and fall. Leaf blight can be devastating to older foliage, weaken the plants, and cause reduced yields the following year.
An emerging fungal foliar disease of strawberry is Neopestalotiopsis leaf spot and fruit rot. This disease has been reported in several states in the U.S. and in Canada; it has not been reported in Wisconsin. More information on this emerging concern in strawberry production can be found here.
Management
The use of resistant varieties is the most effective way to control strawberry foliar diseases. June-bearing varieties that are resistant to leaf spot and leaf scorch include Allstar, Canoga, Cardinal, Delite, Earliglow, Honeoye, Jewell, Lester, Midway, and Redchief. Tribute and Tristar are everbearing varieties that are tolerant to these diseases as well. There are no reported varieties resistant to leaf blight.
In addition to planting resistant varieties, there are other cultural practices that are effective in minimizing pathogen spread. These include:
- Removing old and infected leaves and mummies from the field to reduce fungal inoculum
- Ensuring transplants are certified disease-free
- Avoiding dense canopies and overcrowding to allow plenty of air flow by:
- Spacing runner plants in matted-row culture
- Planting in sunny areas with well-draining soil
- Controlling weeds and overgrowth to promote air circulation
- Avoiding over-application of nitrogen fertilizer, which can result in excess leaf tissue
- Using drip irrigation systems (as opposed to overhead watering systems) to minimize water on leaves and reduce the spread of fungal spores
If a field is severely infected, mow, rake, and destroy diseased strawberry debris after harvest, during renovation. Chemical control can be achieved using fungicides. Thorough coverage on both the surface and underside of leaves is important since infection can occur from either side. Consult the 2021-2022 Midwest Fruit Pest Management Guide for fungicide recommendations. For organic strawberry growers, OMRI-approved fungicides are provided in the Cornell Organic Production and IPM Guide for Strawberries. Remember, the label is the law. Additionally, the UW BioIPM Strawberry Workbook was published in 2016 and provides a year round, self-assessment tool and reference of pest management and cultural practices in perennial strawberry production.
If you suspect you are dealing with a disease in your field, photos and/or physical samples can be submitted to the UW Plant Disease Diagnostic Clinic for diagnosis.
References & Additional Information:
- Ohio State University Extension – Strawberry Leaf Diseases
- Michigan State University – Protect strawberries from foliar diseases after renovation
- Cornell University – Strawberry Leaf Diseases: Identification and Management
- UMass – Strawberry IPM
- NC State – Phomopsis Leaf Blight of Strawberry