April 24, 2019

Apple Scab Has Arrived.

Based on our NEWA Apple Scab Model, we had our first apple scab infection period at just after 5:00 PM on Monday evening (see chart below). The scab season has officially begun!  With no leaf tissue showing prior to green tip, any previous infection period would have no immediate effect on the leaf tissue.  Since we had our copper spray put on just after green tip Tuesday evening, we were protected from the infection.  Copper has been shone to be a protectant against scab on the tissue that has been covered, but not on any new tissue that would have grown since that copper spray was put on.   If no copper spray has been put on your trees as of today, and you have not put any other protectant on like Captan or Mancozeb, you still have time to get an eradicant fungicide put on your trees in the form of Immunox, which has a 72 to 96 hour reach back ability.  That means that you have up to 72 to 96 hours to get a spray on which will still be effective against the scab infection from Monday evening.  In fact, you will have until 5:00 PM on Thursday or Friday evening to still protect that new tissue if using Immunox.  Immunox is a xylum mobile fungicide which means when applied to leaves it will move throughout the leaves it was deposited on but will not move out of that leaf.  

 



Tree PhenologySpray GuideFor the home grower who may not have a weather station or scab model to help predict the apple scab infection periods, monitoring for apple scab can be quite complicated.  But there is an alternative.  Unless wetness periods are being monitored as outlined in the section above, you can simply apply protective or eradicant fungicides at regular intervals beginning with green tip. Spraying should be done every 7 to 10 days, depending on the number of rain events between sprays.  If there are no rain events between sprays, a single protectant spray will last at least 10 days but not more than 14 days, based on the product's label directions.  You will need to make sure that your trees and fruit are protected prior to any rain event if you are going to use only a protectant.  A good protectant is Captan or Mancozeb.  But, a protectant can lose its effectivness after 2" of rain, so you also want to keep an eradicant on hand like a myclobutanil, which is available as Spectracide Immunox, which I mention above.  A protectant like Captan has to be applied prior to a rain event.  If no protection is available during the wetting event, then only an eradicant like Immunox can be applied that has a reach back of 72 to 96 hours as I have stated above.  That means that it can still have an effect on the scab pathogen for up to 96 hours after a wetting event, although it is a good practice to use the shorter 72 hours.. A good option is to actually use both a protectant and an erdicant at the same time, like Captan mixed with Immunox, which will give you both protection and eradicant action after a wetting event. Be sure to monitor wetness periods throughout the spring to insure that trees are always adequately protected. 
 
But, regardless of the type of mobility that a fungicide possesses, no fungicide is effective after the development of visible disease symptoms. For that reason, timely fungicide application before establishment of the disease is important for optimal disease management.

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