Why was rust an issue in 2016?

High levels of rust infection can be found in some orchards around the region.  This is not a small matter.  Severe rust infections can cause premature defoliation, reducing almond or peach yield next year and increasing the risk of scaffold and branch bark sunburn in prunes in the late summer and fall.

Why was 2016 a bad rust year and not 2015?  There are at least a couple of factors.  First, late spring and summer rains favor rust infection, and 2016 had a wet spring compared to 2015, with “better” rust infection conditions (see table below).  In addition, the May rains this year came in extended events, with rain falling for four straight days (May 5-8) in both Arbuckle and Durham.  There were also two consecutive days of rain in Durham this June (17-18).  Secondly, with virtually no crop in many prune orchards, growers may have skipped protective fungicide sprays ahead of forecast rain that they would have applied under better economic conditions.  The roughly 40% drop in nut prices since 2015 may also have influenced grower decisions regarding late season fungicide sprays in almonds.

Total Monthly Rainfall (Inches)
Month Durham - 2015 Durham - 2016 Arbuckle - 2015 Arbuckle - 2016
March 0.32 6.98 0.15 4.27
April 2.03 0.74 0.79 0.33
May 0.00 0.84 0.01 0.84
June 0.02 0.32 0.02 0.01
July 0.00 0.00 0.00 0.00
August 0.00 0.00 0.00 0.00

Rust is a fungal disease that infects almonds, prunes and peaches.  Infected leaves show yellow spots and premature defoliation can result.  (Peach fruit and shoots can also be infected with rust.) Heavy leaf loss is a major hassle at harvest, slowing nut drying and filling bins with trash.  Late summer defoliation can significantly reduce almond yield the following year.  The best rust control is delivered by effective fungicides, properly applied (good coverage throughout the tree) sprayed ahead of rain.  Once high levels of infection exist in the orchard, do not use single site materials (for example, FRAC Group 3 fungicides) to reduce the potential of fungicide resistance.  For detailed information on fungicide efficacy and timing for rust (and other disease) control, click HERE to download the most recent UC IPM Fungicide Efficacy and Timing publication.

Orchards with leaf loss from rust this fall should benefit from a zinc sulfate spray in late fall to provide zinc to the trees ahead of next season demand and remove diseased leaves.  Consult with your PCA about rates and timing.


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