Before I left for vacation last week, the untreated trees in my "Glomerella Gala" research block at the station were relatively Glomerella-less. However, while applying fungicide treatments yesterday, a quick glance at our untreated trees yesterday confirmed that GLS was in full swing and apples had early symptoms of the disease. Trees in other fungicide programs where captan has been applied weekly since petal fall so far have no GLS symptoms. We will conduct our first official disease rating for the season on Monday and I'll update you with our observations. This week I want to keep it short and sweet with some basic GLS/bitter rot management strategies....

• In general applying mancozeb (within PHI restrictions) or captan every 7-10 days is necessary for GLS control. We've conducted trials over several year looking at 7 day vs. 14 day spray intervals regardless of rainfall and the longer spray interval fails to control the disease on leaves or fruit.
• In general, make sure to reapply captan if it rains 1.5 inches or more. If using Merivon, Cabrio, Luna Sensation, Pristine, Flint Extra, Topsin, or Aprovia for GLS or bitter rot this is not critical since the fungicide is locally systemic and enters the plant tissue. That said, these locally systemic fungicides need a few hours to dry before it rains.
• From our residue studies, we've observed that Captan 80WDG persists longer on fruit than Captan 4L. Also, while the highest rate of Captan 80WDG (5 lb/A) persists longer than lower rates, you still cannot expect control after 2 weeks following application. Furthermore, if using the high rate of Captan, you will only get a maximum of 8 applications through the season. I would suggest applying 2.5 to 3 lb/A every 7 to 10 days regardless of precipitation amount to stay within maximum seasonal limitations.
• We screen for resistance to pyraclostrobin in our lab. This is the active ingredient in Merivon, Pristine, and Cabrio that is active against Glomerella and bitter rot. If you have resistance to Merivon, Pristine, or Cabrio, you will also have resistance to Luna Sensation, Flint Extra, and Sovran. Thus, I would avoid using any of these products if you have confirmed resistance for GLS or bitter rot control. Other options if you're looking for some extra "oomph" or if a rain event is coming and you can't get right back out to apply captan include Topsin, Omega, or Aprovia. These are the only fungicides with the exception of captan, mancozeb, or ziram that are effective against GLS in NC. Again, captan should be the backbone of your fungicide program throughout the summer.
• Always apply captan, ziram, or mancozeb with the locally systemic fungicides - Merivon, Pristine, Cabrio, Luna Sensation, Flint Extra, Sovran, Topsin, Omega, or Aprovia - for resistance management or to cover yourselves if you don't know you have resistance to any of these fungicides. If applying with these locally systemic fungicides, a half rate of captan (2.5 lb/A), ziram (3 to 4 lb/A) or mancozeb (77 day PHI, 3 lb/A) is more than sufficient.
• When applying the locally systemic fungicides mentioned above apply the highest labeled rate.