How long between fungicide applications?
I have been receiving a few questions about fungicide sprays over the past few days. i thought the answer may be helpful for others.
"What fungi do I have to worry about this time of year?"
Shot-hole, Anthracnose, Brown Rot, and Jacket Rot. For the varieties in petal fall/post petal fall, Anthracnose and Jacket Rot are the big ones to keep in mind. Most fungicides provide control for shot-hole, but not all provide control for Jacket Rot and Anthracnose. Make sure the spray you are spraying targets those pathogens. Remember, DMIs (FRAC 3) do not provide sufficient control of Botrytis, one of the pathogens that cause Jacket Rot.
"How many days can I wait before I make another fungicide spray?"
For the most part, fungicide sprays will last between 10 and 14 days, depending upon weather. If rainy conditions occur, the residual activity will be on the shorter side, while dry, warm, sunny weather may even extend the window beyond 14 days.. If continual wet, mild conditions that favor fungal growth persist, spraying every 12-14 days will provide control of most problematic leaf and flower infecting fungi.
"What if it rains, then clears up?"
If post rain conditions are effective in reducing the duration of leaf wetness, a spray may not be needed after a single day rain event. Sunny, warm, and windy weather quickly dries the leaf surface, reducing the growth of shot hole, anthracnose, jacket rot, and brown rot. A single day of rain that falls on the later end of the spray window - lets say day 12 of the 14 day window - and has these post rain conditions probably will not warrant another spray. Several days of rain - 2-3days - falling at the end of this period would.
"When should I spray for Scab?"
Scab sprays will should begin no earlier than two weeks after petal fall. Within orchards that had significant scan problems last year, targeting the spray within two to three weeks post petal fall is recommended.
How effective is a fungal spray when trees are wet (ie: spraying during morning hours when there is dew on the trees or they are wet from rain)?
The fungicides are effective when sprayed on wet trees, but it the effectiveness depends on how much fungicide "sticks" to the tree's leaf or petal. If applications are made to trees that have wet leaves, and the spray does not run off the leaf/petal, it will provide protection. If it rains shortly after the spray, some product may be washed off, but effective control of most diseases will most likely occur. It is best to spray at a time where the sprays will have at least 30 minutes to dry on the tree before the next rain
It would be ideal to spray when the applied chemicals has a chance to dry on the tree, but this is not always possible. If sprays are made between rain storms, plan to use a systemic material which includes a strobilurin or DMI over a contact fungicide. Systemic fungicides are also best for aerial applications. If it is speculated that some of the fungicide has washed off, shorten the spray window of protection to the shorter end - seven to ten days - especially if more rain is predicted at that time.
What do I do when there are numerous rain events that extend beyond my spray coverage with little time to make the needed applications?
Keep in mind the fungal diseases that are attacking your trees. If you have Butte/Padre and they are in petal fall, Brown Rot is your major concern and I would recommend making another spray. If you have earlier varieties, the fungal problem may be jacket rot or anthracnose - especially if you have a history of the disease. Target the disease and varieties that have the biggest problem. In regards to jacket rot, trees that cluster tightly are more susceptible. It may be prudent to spray these trees over varieties less susceptible. Also keep in mind that you have a "reach back" of one or two days that can help control diseases even after diseases have initiated infection. This gives a few more days to make the spray without seeing damage.
Generally, when there are three to four days in which rain is expected, and the spray window has expired, another spray would e recommended to target the fungal pathogens present.