Cold, rainy weather increases risk of blossom and twig blight in blueberries
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Cold, rainy conditions with freezes during bloom are conducive to the development of blossom and twig blight in blueberries. Frost can cause microscopic wounds on plant tissues, that allow invasion by fungi and bacteria. Extended duration of wetness of plant surfaces enhances fungal and bacterial growth and infection. In Michigan, at least five different pathogens can cause blossom blight: Phomopsis vaccinii (Phomopsis twig blight), Botrytis cinerea (Botrytis blossom blight), Colletotrichum acutatum (anthracnose blossom/twig blight), Monilinia vaccinii-corymbosi (mummy berry flower strikes), and Pseudomonas syringae (bacterial twig blight). In addition, blueberry scorch virus and blueberry shock virus can cause blossom blight that can resemble Phomopsis twig blight. Just by looking at a blighted blossom or twigs it is difficult to identify the causal agent unless fungal growth is present, so it is a good idea to inspect the blighted tissues with a hand lens or magnifying glass.
Botrytis blossom blight, caused by Botrytis cinerea, is characterized by fluffy, gray to tan spores that are present all over the surface of killed blossoms. In the case of mummy berry flower strikes, a dense layer of gray powdery spores will be restricted to the flower stem or cluster stem. In general, flower strikes are much less common than shoot strikes, so it is unlikely to see flower strikes without shoot strikes. Anthracnose blossom and twig blight does not have very diagnostic features to distinguish it from Phomopsis twig blight. Pseudomonas blight is characterized by dark brown to black necrosis on the twigs. Incubation in the laboratory is necessary to identify the causal agents. Samples can be sent for diagnosis to the MSU diagnostic lab (http://www.pestid.msu.edu/; phone 517-355-4536).
To scout for blossom blight, walk several rows in a blueberry field and scan the bushes for symptoms. When you find any, inspect the flower clusters for twig lesions and fungal sporulation. Also be alert to the presence of insects, webbing, and insect frass, e.g., caused by cranberry fruit worm infestation. To get a better handle on disease severity and changes over time, flag five random bushes and record the number of blighted blossoms per bush every week for the next three to four weeks.
At this time, it would be good to apply a protectant fungicide that provides broad-spectrum control of blossom and twig blight pathogens. A spray of Pristine works well against most causes of blossom blight. Other options are Indar + (Captan or Ziram or CaptEvate) if you have high mummy berry and Phomopis pressure. CaptEvate and Switch have good activity against Botrytis and anthracnose, and moderate activity against mummy berry and Phomopsis. None of the common fungicides control Pseudomonas bacterial blight, since only copper products are able to control bacterial diseases. No antibiotics are labeled for use in blueberries. Serenade (Bacillus subtilis) and Regalia (giant knotweed extract) may also have efficacy against bacterial blight, but have not been evaluated for that purpose in Michigan.