There have been a lot of inquiries in regards to the decision of making a fungicide application for almond rust. Rightfully so, as last year was a bad Rust year, and this year’s weather conditions are very similar. With that in mind, I would encourage people to consider a rust spray this year, especially since:
1. The severity of the rust problem last year was widespread, therefore inoculum is abundant;
2. Weather conditions have been mild and humid, conditions that favor fungal foliar pathogens;
3. Most people have not applied a fungicide in several weeks.
Rust timings for almonds begins at five weeks post petal fall and carries into the summer. Most fungicides are effective on rust. Of note are the strobilurins and DMIs. Avoid Copper, Iprodione, Topsin-M, Rally, and Laredo as they shown the least amount of control in the last round of efficacy trials conducted by Dr. Jim Adaskaveg. Sulfur and Microthiol can be used, but multiple applications are recommended (I.e. Now, Late May/early June).
Common missteps I see in regards to rust control using fungicides:
1. Trying to split the scab and rust treatment – applying a fungicide at one timing for control of both diseases. This spray is usually applied at five weeks post petal fall and usually misses one of the foliar pathogens;
2. Wanting a free ride for the fungicide. This means that they wait until the spray for mites. In cool years, mite sprays are delayed, and therefore the rust application is delayed as well.
3. Not understanding that sprays need to be made before symptoms are observed. Upon orchard infection, it is very difficult, if not impossible, to control a rust epidemic;
4. Expectations of no disease. In bad years, the goal should be to keep the leaves on the trees until late October/early November. Having some rust present in the orchard is okay, but the pressure should not be high enough to cause defoliation.
Hope that helps!