Bulletin #2381, Management of Sphaerulina Leaf Spot, Leaf Rust, and Powdery Mildew

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By Seanna Annis, PhD., Extension professor and professor of mycology, University of Maine and University of Maine Cooperative Extension and Leala Machesney, wild blueberry Extension and research professional, University of Maine Cooperative Extension

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Introduction

Sphaerulina leaf spot, leaf rust, and powdery mildew are the three most common leaf spot diseases found in wild blueberry fields. All three can reduce the amount of leaf area that plants can use for photosynthesis by causing plants to shed, or “drop,” leaves. We sometimes see early leaf drop in July, but not every year or in every field. Whether a leaf will be dropped depends primarily on how old it is and how much light it is getting. Older leaves at the bottom of the stem are likely to be lost due to normal processes, because these leaves will be the least productive in photosynthesis. Other factors impacting leaf loss are the amount of leaf tissue affected by leaf spots (more heavily infected leaves are less productive) and drought, which puts the plants under more stress, so they may lose the least productive leaves to save water.

Sphaerulina Leaf Spot

Blueberry leaves and stems with lower, older leaves showing dark brown to black leaf spots from infection with Sphaerulina leaf spot.
Image 1. Blueberry leaves and stems with lower, older leaves showing dark brown to black leaf spots from infection with Sphaerulina leaf spot.

Sphaerulina leaf spot is caused by Sphaerulina vaccinii (formerly known as Septoria leaf spot). It causes leaf spots on leaves and stems of wild blueberry. Colwell (2023) found that spores were produced, on dead leaves or infected stems, from May 10 to June 20 in 2022 and from May 10 to June 6 in 2023 in Downeast Maine fields. In both years, spores were found in the week before bloom and then throughout bloom. Some spores may have been produced a bit earlier or later, but most spores were produced during bloom. This fungus is thought to have only one infection stage: when the spores are produced during bloom. The lesions produced by early infection (Image 1) probably do not produce new spores that can reinfect the plants, but this is still unproven. The timing of spore production means that to decrease this disease’s symptoms, fungicides must be applied before bloom. Applying fungicides during bloom is not recommended due to possible effects on pollination, and applying fungicides after bloom will not prevent most infections.

Key Sphaerulina Leaf Spot Takeaways

  • One cycle of infection during bloom, with some spore production occurring a bit earlier and/or later.
  • Apply fungicides before bloom. Caution must be taken in applying fungicides to vegetative fields beside crop fields.
  • Applying fungicides in late June to early July is too late; infection has already occurred if spores are present.
  • Can cause leaf loss in July, particularly during drought.
  • Can be a problem in both crop and vegetative years.
  • Overwinters in blueberry fields in stem lesions and on dead leaves.

Leaf Rust

Leaf rust is caused by Thekopsora minima. It needs both hemlock and blueberries to complete its life cycle. We have evidence that the leaf rust spores that infect blueberry leaves are produced in late June and maybe even earlier in some years (Nguyen, 2019). The resulting leaf lesions produce new infective spores on the underside of the leaves (Image 2) that can reinfect blueberry leaves. Rust lesions increase over the summer and into the fall. Usually, maximum symptoms are seen from the middle of August to September, with most leaf loss occurring in September. In crop years, this buildup of symptoms and leaf loss occurs after harvest and so has not been found to significantly affect yield. In vegetative years, leaf rust lesions can decrease photosynthesis, and some genets can lose most of their leaves by late August. This could affect the production of flower buds and storage of nutrients over winter.

Orange-colored spores of leaf rust can be found on the underside of wild blueberry leaves.
Image 2. The orange-colored spores of leaf rust can be found on the underside of wild blueberry leaves.

Key Leaf Rust Takeaways

  • First infection probably after bloom in late June.
  • Produces spores that reinfect blueberry leaves and causes an increase in symptoms over the summer and into the fall.
  • Can grow only on living plant tissue.
  • Most leaf loss occurs in late August through September.
  • Probably not a large problem in crop years, but may affect vegetative years.
  • Overwinters in fields on dead leaves.

Powdery Mildew

Powdery mildew overwinters on dead leaves and likely starts producing spores early in the spring. Its symptoms are reddening of the top surface of the leaf and sometimes white powdery fungus and spores on top of the leaf. These spores can reinfect the leaves and symptoms can increase over the summer. Often, the symptoms appear on the uppermost leaves due to wind currents that carry the infective spores. This disease usually infects leaves higher on stems, which often remain on the stems the longest in the fall, and is not believed to be a cause of leaf loss. Although this disease will decrease photosynthesis, it may not cause significant yield loss. Because of this, applying fungicides to control only this disease is unlikely to be cost-effective and is not recommended.

Signs of powdery mildew on the upper leaves of wild blueberry.
Image 3. Signs of powdery mildew on the upper leaves of wild blueberry.

Key Powdery Mildew Takeaways

  • Likely first infects leaves in June.
  • Produces spores that can reinfect blueberry leaves and cause an increase in symptoms over a growing season.
  • Can grow only on living plant tissue.
  • May not be a cause of leaf loss but will decrease photosynthesis in infected leaves.
  • Overwinters in the field on blueberry stems and dead leaves.
  • Fungicide applications to control only this disease are not recommended.

Conclusion

Leaf spot diseases in wild blueberry vary widely in their timing, disease cycle, and potential impacts on plant health. Effective management of any leaf spot disease relies on understanding species biology, infection timing, and interactions with environmental conditions. Applying fungicides outside of the recommended application windows is unlikely to provide benefit and may carry unnecessary economic and ecological costs. An integrated pest management approach that considers disease pressure and seasonal conditions will best support long-term productivity in wild blueberry fields.

References

Nguyen, N. “The Dispersal Pattern of Thekopsora minima in Wild Blueberry Determined by a Molecular Detection Method.” Master’s thesis, University of Maine, 2019. https://digitalcommons.library.umaine.edu/etd/3065

Colwell, Z. “The Molecular Detection of Sphaerulina vaccinii: A Leaf Spot Pathogen of Lowbush Blueberry.” Master’s thesis, University of Maine, 2024. https://digitalcommons.library.umaine.edu/etd/3925

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