Timing Fungicide Sprays: Thoughts for a Wet Spring
Improper timing of fungicides wastes money and leads to croploss from disease. Over the years, I have seen several disease epidemics thatwere due to the improper timing of a fungicide spray. These were observed inboth wet and dry years, indicating the difficulty and general misunderstandingof disease and spray application timing.
Proper timing of fungicide applications is difficult.Operational constraints in applying fungicides leads to mis-timed (too early,too late) or poor applications (every other row, applying too fast). This isoften a function of equipment availability, whether by grower/operation orcustom applied spray. Add to this aspect the various layers of fungicideavailability and delivery timing, the changing pathogens and diseaseconditions, as well as the general misconceptions of various diseases, it isvery easy for problems to occur.
In order to control diseases, as advocated in earlier postings, it is best to design a fungicide plan prior to the start of the season. This provides the opportunity to review reference materials and determine which fungicides are best for differing stages of the crop. This plan also creates the opportunity to provide options for differing modes of actions at each timing to help with pricing, and reduces the burden of having to put the plan together when the season has started (and there are several other demands). Personally, I like to design the plan for a wet year and remove fungicide timings if dry conditions occur. An example of a plan with different modes of actions can be found here.
Diseases don’t just occur. They require a susceptible host,the presence of a pathogen and suitable environmental conditions. For mostalmond foliar or bloom diseases, the pathogen is throughout the environment andmost of our varieties are susceptible to one disease or another. The largestvariable on a year-to-year basis is the environmental condition, and favorableconditions for diseases tend to be periods of leaf wetness. Sprays should betimed to provide protection for susceptible plant tissues when multi-day rainevents or multiple rain events within a short period of time occur.
Ideally, fungicides should be applied prior to a rain event.Fungicides provide protection for 10-14 days, depending on the amount andfrequency of rain and changing crop stage. Re-applications after this periodshould occur if wet or foggy conditions persist. Applications may not be neededfor passing storms, but multiple days of passing storms would warrant treatment.Generally, applications take about 30 minutes to become rainfast.
There is a lot of emphasis placed on the “reach-backpotential” of modern fungicides. This is the ability to kill an infection thathas already occurred and is due to the ability for these chemistries to moveinto the laminar layer of the leaf. Chemistries include the FRAC classes of 3,7 and 11 (and others). It is thought that this would provide an extended periodof protection for 1-3 days post infection. This provides the opportunity to continuea spray application after a rain event if it wasn’t completed prior to thestorm. Applications made 3 days or later after an infection event will resultin poor control of the targeted disease.
Fungicide rotation is important to prevent resistance amongthe various diseases. This process of utilizing different FRAC groups insuccessive applications will help keep the fungicide chemistry effectivelonger, leading to lower cost products and more fungicide options. Keep in mindthat different trade names may not be different modes of action. Avoid applyingthe same FRAC number back-to-back even if the product contains multiplefungicide classes.
Rainy or wet years can be challenging when managing bloomand springtime diseases. Proper timing and coverage of the trees with aneffective fungicide will reduce but not eliminate disease. Be prepared to makemultiple applications if wet conditions persist and apply the materials priorto the rain event when possible.