The history of fungicide loss in almonds as a result of disease resistance goes back to the 1980s, when brown rot blossom blight resistance to the benzimidazole class of fungicides was recognized. More recently, scab has developed resistance to the strobilurin as well as the benzimidazole classes, and now Alternaria leaf spot resistance to the strobilurin and carboxyanilide (boscalid) chemistries has appeared.
These and other newer fungicides carry a high risk for losing efficacy as a result of disease resistance because they have single-site modes of action, which disease organisms can more easily overcome. When these fungicides are misused by making back-to-back sprays of fungicides in the same class, or applied at low rates or in an alternate-row pattern, fungicide resistance is likely to develop. Older fungicides, by contrast, more often had multiple-site modes of action and the risk of resistance development was low.
Over the years, the Almond Board of California has funded research led by plant pathologists Jim Adaskaveg, UC Riverside, and more recently Brent Holtz, Cooperative Extension, San Joaquin County, to develop new fungicides with different modes of action and use patterns to fill the gaps where the loss of fungicide efficacy threatened the health of the almond crop. Relying solely on new chemistries to replace old ones is only a short-tem solution, however. A better approach is to also adopt resistance management practices that will help retain a broader selection of effective fungicides with different modes of action.
One of the tools that have been developed to help prevent disease resistance is the grouping of fungicides by chemistries and mode of action. Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC, www.frac.info). The purpose of FRAC, which is a service of CropLife International, is to provide fungicide resistance management guidelines to prolong the effectiveness of “at risk” fungicides and to limit crop losses should resistance occur. Fungicides with different group numbers assigned by FRAC may be used in rotation in a resistance management program.
The Almond Fungicide Resistance Management section of the UC IPM Web site has been updated recently and contains several useful charts with critical information for planning a resistance management program, including the FRAC groupings for fungicides registered for use on California almonds (www.ipm.ucdavis.edu > Agriculture and floriculture > Almond > Fungicide Resistance Management). This information is also provided in the newly revised publication, “Fungicide, Bactericide, and Biological Tables for Fruit, Nut, Strawberry and Vine crops — 2009” available from your farm advisor or at the UC Kearney Ag Center Web site, www.uckac.edu/plantpath/.
This document contains a list of fungicides registered for use on almonds in California, including their FRAC code and efficacy on various diseases. It also has a treatment timing table for the most troublesome almond diseases, and a section titled “Suggested Disease Management Programs with Fungicide FRAC Groups.” Armed with this information, growers and PCAs have a powerful, up-to-date tool to protect California almonds from crop loss due to diseases while maintaining the usefulness of fungicides over time.
In treating any disease or disease complex, follow these guidelines for an effective resistance management program:
• Choose fungicides that are effective for the target disease(s), using the “Fungicide Efficacy” table.
• Time treatments for the greatest efficacy at the appropriate stage of almond development, and initiate treatment at initial stages of disease development (see “Treatment Timing” table).
• Do not apply single-site mode of action fungicides when high populations exist (i.e., when there are high levels of disease at the time of application).
• Use full label rates.
• In most instances, tart your annual fungicide spray program with multi-site (low resistance potential) fungicides, as identified by the table.
• Rotate from one chemical class to another within the same season. Except as noted in the table, do not apply single-site fungicides alone; use either fungicides in combinations or premixes that provide multiple modes of action.
• If possible, use fungicides within any given high resistance potential class only once per season and no more than twice per season.
• Do not use an alternate row application strategy. It is one of the fastest ways to lose the efficacy of an entire class of materials.
• Do not carry out an application of fungicides by air at full canopy.
For more on disease management in almonds and the opportunity to earn CE credit, go to almondboard.com/farmpress3.