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Botrytis Management after the Cold Event

The following information is of value to commercial blueberry producers in the southern part of the state or anywhere that blueberries are in bloom.  We just had a couple of nights of cold temperatures (<25 F) in most parts of our blueberry belt, so this message is especially targeted for those producers.  Where cold-damaged blooms/shoots are observed, Botrytis might be a real issue, since damaged blossoms and buds will provide infection courts for the spores.  The optimum temperature for infection of Botrytis is 59-68 F, but the optimum for spore germination is actually 68 F and above.  That means we will have perfect temperatures for infection within the next day or two, as temperatures around 70 F should be optimal for Botrytis; weather predictions for Alma and Homerville are indicating that highs will be ~70 F on Saturday and Sunday. Rainfall is currently predicted as well, and if heavy and prolonged dews are associated with the warm temperatures, we may have Botrytis development.

Where freeze damage has occurred on southern highbush, I would recommend an immediate application of Pristine at the high rate. Unless resistance has developed in the Botrytis and Botryosphaeria populations, Pristine should control Botrytis and suppress Botryosphaeria.  However, if Exobasidium has been observed, Pristine does not generally have Exobasidium activity due to resistance development, and another fungicide should be considered. There are several excellent Botrytis materials on the market: Pristine, Switch, Elevate and CaptEvate.  Captan has Botrytis activity, but it is not as efficacious as these newer products.  Switch is also excellent, and Botrytis resistance is not likely with one of the components in this fungicide.  Apply no more than two applications of Pristine before switching to another product with a different mode of action — any of the others.  Again, Captan alone is the least efficacious relative Botrytis management, but it will not have developed resistance.

If field conditions are wet, aerial applications may be necessary to apply fungicides in a timely fashion. As always, follow all label directions.  I have not reviewed all labels, but if I interpret the labels correctly, aerial application is now allowed for Pristine, Switch, and even some Captan products (e.g. Captan 4L).

Also, there are always questions regarding the tremendous amount of bark/ground wetting that occurs with overhead freeze protection.  I am hopeful that this will not increase root rot diseases substantially at this time, but the root zones are likely saturated.  Ridomil application might be warranted, but there is no guarantee it would be needed. However, these products will not resurrect dead plants, so it is a judgment call as to whether one applies the product now or later as the plants start to push more. There may not be enough foliage for good uptake and activity of Aliette or other phosphonate-type products (ProPhyt or Agri-Fos or Kphite for example), but in 3-4 weeks (after bloom but with good new expanded leaf flush), I would consider foliar application of one of these materials to stave off root rots during the early spring; follow label directions and do not over-concentrate these materials in the final spray volume, as damage can occur with their use if label directions are not carefully observed.

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About Phil Brannen

Phil Brannen is a Professor in the Plant Pathology Department at the University of Georgia. He attended the University of Georgia for his undergraduate degree in Plant Protection and Pest Management, where he also received an M.S. in Plant Pathology, followed by a Ph.D. in Plant Pathology from Auburn University. He has extensive experience with disease management programs in numerous cropping systems. He serves as the extension fruit pathologist for Georgia – conducting research and technology transfer for multiple fruit commodities. His blueberry efforts are directed towards developing IPM practices to solve disease issues and technology transfer of disease-management methods to commercial blueberry producers. He also teaches the graduate level Field Pathology Course, team-teaches the IPM Course, coordinates the Viticulture and Enology in the Mediterranean Region Course (Cortona, Italy), and guest lectures in numerous other courses throughout the year.