It looks like there is rain predicted for next week – just in time for almond bloom. Considering that the forecast is calling for 2-3 days of rain, a fungicide spray may be warranted. In making this decision, keep in mind the following considerations:
1. Fungal diseases are spread by wind blown and rain splash spores and require leaf or petal wetness to begin the infection process. Therefore, if the rain event is brief and followed by wind, a spray may not be needed.
2. If a fungicide application has already been made, it will provide 10-14 days of protection depending upon weather (No rain, longer persistence), application speed (slower, the longer) and product used.
3. A spray application needs to dry in order to be effective. Making an application while it is raining will lead to wash off of the product and loss of protection. It may take between 1-2 hours for a product to dry if the humidity is high.
4. A spray application made after the rain event onto susceptible, non-covered trees is effective in preventing disease. If treating blocks for the first time after the rain, it is important to use a stronger chemistry such as a strobilurin (FRAC 11) or DMI (FRAC 3). This application should occur no later than 2 days after the rain event.
5. Consider the ability to access different blocks. If a lot of rain is forecasted and falls, it will be difficult to get across blocks with clay to loam soils. If running behind schedule, it may be prudent to spray these blocks prior to the rain and spray blocks planted in sandier soils after the rain.
6. Spraying every other row and at high speeds does not provide adequate coverage and will reduce the effectiveness of the fungicide efficacy. It will also lead to the formation of fungicide resistant fungal populations.
7. Rotate chemistries or FRAC groups between sprays. There are many products that have been determined to be effective against bloom time diseases. They can be found here. It is important to realize that products with different trade names may have the same mode of action – so use the FRAC numbers to ensure a rotation is made.
9. Although it may be tempting to not spray an orchard due to various drought scenarios, double check with your crop insurance agent as not spraying during bloom for a rain event may be considered bad orchard management and may affect insurance coverage.