Cooperative Extension Publications

By Seanna Annis, Ph.D., Extension Professor and Professor of Mycology, University of Maine; and Leala Machesney, Wild Blueberry Extension and Research Professional, University of Maine

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Introduction

Valdensia leaf spot (VLS; previously called Valdensinia leaf spot) is produced by the pathogenic fungus Valdensia heterodoxa. VLS can cause leaf drop early in the season in lowbush blueberries, so that the infected stems produce few or no flower buds or fruit. An outbreak of VLS in 2009 was first recognized in the Canadian provinces of Nova Scotia and Quebec, and then in Maine wild blueberry fields and garden plantings. The outbreak was likely due to wet weather from May to July that year. Since 2009, VLS has shown up in small areas in some fields, but may become more widespread again if there is a long period of wet weather in the spring. The Valdensia fungus infects lowbush blueberry leaves in both vegetative (prune) and crop years.

There may be variation in susceptibility among blueberry genets (clones), but no such genets have been observed to be resistant to this fungus. Symptoms occur on the leaves and are typically round, large, and brown circular spots that can have a “bull’s eye” appearance (Image 1) where the center of the spot is lighter brown than the outer margin. Leaves can have 1 to about 10 spots that can be from 1/8- to 1/2-inch and larger. These spots rapidly enlarge from a small infection to a larger spot in only a few days. Young leaves are especially vulnerable to leaf drop and usually fall off with only one or a few VLS lesions and while still green. In areas with high levels of VLS in prune fields, the majority of leaves can fall off by late June. These plants will produce some new leaves, but will not produce flower buds for the next year. Crop fields with early leaf drop will have decreased yields. Later in the growing season, leaves develop smaller VLS spots and will remain on the plant until leaf drop in the fall.

Valdensia Life Cycle

The Valdensia fungus can overwinter in infected dropped foliage. About the time of early bloom, after about 3–4 days of wet weather, new spores (conidia) will be produced on overwintered, infected leaves and new leaf infections will occur. New Valdensia spores, which are large, clear, and star-shaped, can be produced in about 2–3 days on damp, overwintered, infected leaves. Mature spores are propelled up to 12 inches away from their source and will stick to leaves, typically the underside, of most plants. These large spores look like thorns sticking out from the VLS lesions. Valdensia spores are too heavy to be dispersed by wind, but once mature may also be moved by water splashes. On nonhost plants, such as bunchberry, birch, and ferns, the spores will attempt to infect the plants but produce only a small dead area (1/16–1/8 inch) under the spore, and the lesion will not expand or produce new spores. Spores that land on blueberry leaves of any age can infect and rapidly grow in the leaf tissue to produce a characteristic VLS lesion, and these lesions can produce new spores (Image 2).

The fungus requires about 6–8 hours of wet weather (rain or fog) for the spores to infect new leaves, and can rapidly spread from infected plants to adjacent plants in wet weather. Infected leaves, which remain attached or fall and remain damp, can produce new spores in a few days. Large areas of lowbush blueberry fields can be affected by VLS. Plants with high numbers of lesions will be surrounded by plants with lower levels of infection as the fungus spreads with new spore production (Image 3). Infected leaves that fall early in the growing season can develop thickened, dark brown or black pigmented veins, which are a fungal overwintering structure called sclerotia. The fungus can survive in sclerotia for a few years.

Spread of Valdensia in Fields

Dead Valdensia-infected leaves will cling to shoes, tires (Image 4), harvest totes, and other equipment when the leaves are wet, and VLS infection can spread within a field or to new fields. The fungal spores themselves can also be moved between fields by contaminated equipment such as blueberry totes, mowers, harvesters, and other vehicles. Often the first patch of VLS is at the entrance to a field, presumably introduced by contaminated equipment that was not cleaned before being moved to a new field.

Prevention of Valdensia Spread

Remove all dead blueberry leaves from shoes, clothing, equipment, and vehicles before moving between fields. Spraying disinfecting solutions on leaf-contaminated areas of equipment will not kill the fungus; the leaves must be removed, which can be done with water. Clean blueberry boxes thoroughly, and in particular remove any dead leaves stuck to the boxes before bringing them into a field. Dispose of infected leaf litter in a location away from the blueberry field, and preferably burn it to eliminate any potential sources of infection.

Scout for VLS in early June, looking particularly at bare areas with stems and lower leaves on stems. Infections will usually occur on lower leaves first and may be difficult to find in dense canopies. Infections also often occur in shaded edges of fields.

Do not walk or drive through infected areas when the plants are wet. Avoid driving through plants with disease symptoms or dropped leaves at all times. 

If you find an area with VLS, flag off the area so people or vehicles will not go through it. Inspect and thoroughly wash all vehicles and equipment that may have come into contact with the infected area.

Treatment

Treatment of VLS may differ depending on whether the disease is identified in a prune field or a crop field, and the time of year. Regardless of field cycle, it is critical you do not move infected dead leaves around the field or between fields during and after treatment.

Cultural Controls

As soon as possible, use a handheld propane burner or a controlled straw burn to burn the plants and leaf litter in the diseased area and in an approximately 10-foot border of healthy-looking plants around the visibly VLS-infected plants. Burn around edges of the area first, and then move into the center of the diseased area so you do not accidentally spread leaves outside of the infected area. You must burn all upright plant material down to the ground and all leaf litter to effectively remove the possibility of infection. You do not need to burn the whole field unless the whole field is affected by the disease. If you notice VLS in a crop field, burn if possible, or mark off the diseased areas and do not harvest that area or move equipment through it. Burn as soon as possible. Check the treated area later in the growing season and the next spring for any remaining VLS symptoms. Burning has eliminated VLS from many fields.

Fungicide Control

Apply recommended fungicides following their label. The applications should occur when leaves have fully expanded, but before bloom, to decrease any early infections, and then 7–10 days later to decrease any new infections (or after bloom in crop fields). Fungicides will suppress infection but not eliminate the fungus from the field.

Conclusion

Valdensia leaf spot can pose an economic threat to lowbush blueberry production because it can cause rapid defoliation. Left unmanaged, VLS can result in long-term impacts on yield and plant health. Early identification, strict sanitation practices, and prompt treatment are critical to managing and containing this fungal disease. By following recommended prevention and treatment strategies, you can reduce the risk of widespread infection and protect the health and productivity of your blueberry fields.

For More Information

For questions or more information about Maine wild blueberry diseases, contact Dr. Seanna Annis, UMaine Extension Professor and Professor of Mycology, via email at sannis@umaine.edu or visit the UMaine Extension Wild Blueberry website for wild blueberry bulletins, videos, tools, and more.

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