Apple Disease Update: Week of June 2, 2019

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Developing apple scab lesions on immature fruitThe theme of this week’s post is to PRACTICE PROTECTION! According to my weather apps, a fair amount of rain is predicted in Western NC beginning Wednesday afternoon and continuing through Friday. If you have not applied a fungicide in the past 10 days, make sure PROTECT foliage and fruit prior to the rain. If it rains more than 1.5″ (which is predicted), you should consider having a working weekend, and reapply if ONLY captan or another contact fungicide was used. If Merivon, Pristine, Flint, Luna Sensation, or another locally systemic fungicide was applied recently (within past 7 to ten days) you should be able to continue on that schedule due to the physical mode of action of these products.

There are a few reasons why it would be advantageous for a fungicide application prior to the onset of the rain. First, fungi (and bacteria) love humid, wet conditions. Rain assists in pathogen dispersal, and usually reduces the required time for pathogen infection to occur and symptoms to show up. Secondly, during mid-cover applications, captan is primarily utilized for management of rot diseases such as bitter rot, as well as for Glomerella leaf spot. This fungicide is strictly protective, and thus must be applied before an infection event (e.g. the next few days) occurs. Lastly, we don’t have any efficacious fungicides registered on apples that will provide more than 24 hours kick-back activity against pathogens causing GLS/bitter rot. Strobilurins may provide 24-48 hours curative activity, but they provide much greater control when applied protectively.

As for what fungicides to apply? This might be a good time to apply a strobilurin as it has a longer breakdown-period and should continue to provide control, even after the rain. Make sure the product is applied prior to the rain and has time to dry, and tank mix with captan for resistance management. Captan plus a phosphorous acid can also be used prior to the rain, however, you may need to hop back on the tractor again on Saturday or Sunday.

Other Observations of Interest

  • I’ve had a few reports about apple scab outbreaks throughout the region. Continue to monitor for scab lesions on leaves and fruit (see above). Fungicides being applied for GLS and bitter rot should continue to provide control against this disease.
  • Despite the relatively dry May, I’m continuing to receive reports of Glomerella leaf spot (see image of developing lesion below). As noted in the previous post-do not let the lack of rain fool you. It’s been humid, hot, and there has been a considerable amount of leaf wetting hours throughout the past month (dew on leaves). Continue to PROTECT every 7 to 10 days, especially for apples headed to the fresh market.Young GLS lesion on Gala
  • In young, non-bearing orchards, protect trees with a low rate (e.g. not the green-tip rate) of a fixed copper. Monitor for phytotoxicity to leaves.