Common Strawberry Diseases

Anthracnose¹

Anthracnose is caused by Colletotrichum acutatum, C. fragariae, and C. gloesporoides fungi, which all cause similar symptoms on strawberries, although C. acutatum is usually associated fruit rot (Figure 1). Anthracnose can infect foliage, runners, crowns, and fruit. Berries can be infected at any stage of development, and while immature and mature berries are both susceptible to infection, the disease is most common on ripening or mature fruit. Whitish or tan water-soaked lesions develop on berries, and then turn brown, become sunken, and enlarge within two to three days to cover most of the berry. Infected berries eventually desiccate, turning hard and black. The optimum temperature for infection is 77 to 86 °F. Up to 90% of the berries can be infected in under a week.

Management

1. Use disease-free plants. The disease is introduced to the field with infected plant material. Inspect plants before planting.
2. Proper irrigation. If the field was previously infested with the pathogen, or the disease is present in the field, minimize the amount of overhead irrigation used as spores are spread by splashing water. Use of drip irrigation rather than overhead irrigation is recommended.
3. Mulching. Mulching with straw is recommended in perennial matted row plantings to reduce water splash.
4. Sanitation. Remove and discard infected berries during harvest.
5. Fungicide use. Fungicides for control of anthracnose fruit rot should be used in a protectant or preventative program. In order to obtain effective disease control, fungicides should be applied before the disease develops. Luna^® Flex fungicide contains both fluopyram and difenoconazole, and more information can be found at https://cs-assets.bayer.com/is/content/bayer/2023-Luna-Flex-Strawberrypdf.

Gray Mold²

Gray mold can affect the leaves, stems, flowers, crowns, and fruit. This disease is hard to manage, as the gray mold pathogen has a wide host range and overwinters in infested fields. The most common form of gray mold is fruit rot, and the first symptom is a light gray or brownish lesion near the calyx. Later, a grayish, fuzzy mass of spores will spread over the lesion to eventually cover the entire fruit. Lesions can be found on green fruit as well as ripening fruit (Figure 2). Gray mold can also cause a post-harvest rot, developing on the fruit after harvest. Gray mold spores can be spread by wind and rain, as well as by workers and pickers via their hands, clothes, or shoes. The pathogen thrives in cool, humid conditions and temperatures in the range of 65 to 75 °F.

Management

• Use disease-free plants. The disease is introduced to the field with infected plant material. Ensure that plants are certified disease free.
• Sanitation. Remove dead and infected plant material to reduce the pathogen inoculum level. Harvest all known areas of infection last to minimize spread of the disease.
• Allow sufficient spacing between plants. Allowing for airflow through the canopy hastens drying.
• Avoid over-fertilization. Excess nitrogen is associated with an increase in fruit rot when weather conditions are favorable for infection. Schedule fertilizer programs based on tissue sampling.
• Fungicide use. Initiate a fungicide program early, following the fungicide label.
• Crop rotation. Rotation away from strawberries for two to three years may help in reducing pathogen inoculum.
• Consider harvesting when conditions are dry. Harvest berries under dry conditions to limit spread.

Leaf Spot³

Common leaf spot of strawberry is a fungal disease with several names, such as Mycosphaerella leaf spot, Ramularia leaf spot, strawberry leaf spot, bird’s-eye spot, gray spotness, and white spot. Resistant strawberry varieties and production improvements have been effective in helping to manage the disease and reducing its impact, to the extent that this disease is now mostly a cosmetic concern. The common leaf spot symptom is small, round, necrotic spots on strawberry leaves, though lesions can occur on berries, berry caps, and petioles. Initially, the spots develop on the upper leaf surface and are deep purple to red in color. The spots develop tan, gray, or almost white centers with distinct reddish-purple to brown borders (Figure 3). As the disease progresses, spots enlarge and may merge, leading to leaf death in extreme cases. Shallow black spots (¼ inch in diameter) may develop on infected fruits and are often surrounded by brown or black leathery tissue. The disease is most active when temperatures range from 65 to 75 °F, with periods of high rainfall and humidity. The leaf spot pathogen survives the winter on dead plant material, and it is moved to new foliage in the spring by early season rains and overhead irrigation.

Management

• Use disease-free plants. Select a resistant variety and use certified disease-free stock.
• Establish beds with proper spacing. This optimizes air movement and creates a less conducive environment for disease.
• Avoid overhead irrigation. Use drip or soaker systems instead.
• Do not work beds under wet conditions. Doing so may hasten the spread of the disease.
• Fungicide use. Select a fungicide labeled for use against leaf spot in strawberries, such as Luna^® Flex fungicide.

Powdery Mildew⁴

Symptoms of powdery mildew initially appear as small white colonies on the undersides of leaves. The colonies can merge and cover the entire lower leaf surface (Figure 4), resulting in leaf curl. As symptoms progress, reddish-purple lesions appear on the upper and lower surfaces of leaves. Infected flowers produce deformed fruit or no fruit. Infected immature berries become hardened and desiccated. Infected mature berries become seedy in appearance and support spore-producing colonies that look powdery and white (Figure 5). In some locations (e.g. California and Florida) the powdery mildew pathogen overwinters on leaves as mycelium. Powdery mildew is introduced into the field through infected planting material or as spores from neighboring fields. Infected nursery plants may have closed, spore-bearing structures called chasmothecia. Infection occurs when leaf surfaces are dry, relative humidity is high, and temperatures are cool to warm. Powdery mildew is particularly severe in greenhouse and plastic tunnel production systems.

Management

• Limit nitrogen and overhead irrigation. Avoid overhead irrigation and excess use of nitrogen.
• Plant selection. Use less-susceptible cultivars, such as Albion, San Andreas, and Fronteras, if doing so is possible and practical.
• Fungicide use. Apply fungicides at the first signs of powdery mildew, especially in the fall and spring. Managing powdery mildew in the fall can reduce disease development the following spring. Controlling foliar disease may help limit fruit infections. For new plantings, apply fungicides about one month after planting and a month later. Make additional fungicide applications when plants begin to bloom according to the fungicide label.

Rhizopus Fruit Rot⁵

The fungus that causes Rhizopus fruit rot infects strawberries through open wounds on the fruit. After infection and before fungal growth, the fungus secretes a chemical that begins breaking down and destroying the fruit tissue. The first symptom of Rhizopus fruit rot is the appearance of discolored, water-soaked lesions on the fruit, which progress to limp, brown berries that leak their contents onto the bed. At high relative humidity, an infected berry is rapidly covered with white mycelium and sporangiophores. The sporangiophores (spore producing stalks) develop black, spherical sporangia that can spread the infectious spores (Figure 6). Rhizopus fruit rot can be confused with Mucor fruit rots. To tell the difference, use a hand lens to examine the fungal strands. With a Rhizopus fruit rot infection, the sporangia will appear to be dry. Mucor sporangia will appear to be covered in a liquid film and will look wet. The fungus is active in warmer months and overwinters in the field on organic material. It has a very wide host range and is common worldwide.

Management

• Sanitation. Carefully handle fruit and remove all ripe berries at harvest.
• Pick the entire fruit. Any injury to the fruit will provide a point of entry for the pathogen.
• Quickly cool harvested fruit. The pathogen stops growing at temperatures below 46 °F, so cooling the fruit quickly will help control fungal growth. Rapid postharvest cooling of fruit is essential for disease control.

Sources

¹Ellis, M.A. and Erincik, O. 2016. Anthracnose of strawberry. Ohio State University Extension. PLPATH-FRU-16. https://ohioline.osu.edu/factsheet/plpath-fru-16

²Vinson, E., Conner, K., and Sikora, E. 2021. Management of gray mold in strawberries. Alabama Cooperative Extension System. https://www.aces.edu/blog/topics/crop-production/management-gray-mold-strawberries/

³Hammel, J. 2024. Common leaf spot of strawberry. University of Wisconsin–Madison Plant Disease Diagnostics Clinic. https://hort.extension.wisc.edu/articles/common-leaf-spot-of-strawberry/

⁴Koike, S.T., Browne, G.T., Gordon, T.R., and Bolda, M.P. 2018. Powdery mildew. University of California Integrated Pest Management. https://ipm.ucanr.edu/agriculture/strawberry/powdery-mildew/#gsc.tab=0

⁵Koike, S., Browne, G., Gordon, T. and Bolda, M. 2018. Rhizopus fruit rot (LEAK). University of California Integrated Pest Management. https://ipm.ucanr.edu/agriculture/strawberry/rhizopus-fruit-rot/#gsc.tab=0

Web sources verified 01/03/25. 9211_482612