Several important diseases of almond and other stone fruits are developing resistance to our fungicides.

Almond scab began showing widespread resistance to strobilurin fungicides (Abound, Flint, Gem) in 2006. Scab is also resistant to the strobilurin component of Pristine. The second component of Pristine, boscalid, was never very active against the scab pathogen.

Alternaria leaf spot, a real problem in certain areas of Kern, Butte, Glenn and Tehama counties, developed resistance to the strobilurin class of fungicides beginning in 2003.

This disease is now also showing resistance to carboxamide fungicides (boscalid) in many locations. Pristine is a mixture of pyraclostrobin (a strobilurin) and boscalid. Initially, both materials were active against the pathogen, but now the Alternaria pathogen is becoming resistant to Pristine.

In 2007 we had our first confirmation that brown rot is becoming resistant to anilinopyrimidine fungicides (Scala, Vangard) in prunes. Resistance most likely developed due to overuse and/or improper use of these materials.

How does resistance to a fungicide begin to develop?

Whenever you have a population of living organisms, there are always a few individuals within the population that are tolerant to a particular chemical. If a grower continually uses the same chemical or another chemical of the same class, he will kill most of the susceptible individuals within the population, but the resistant ones will survive and multiply. Pretty soon the whole population is resistant to this chemical class. Poor spray coverage also will increase the rate of resistance development.

Growers need to follow these important resistance management strategies:.

  • If possible, begin the season with a multi-site mode of action fungicide. Many popular fungicides control a fungus by acting only on one site of a particular biochemical pathway.

    The potential for resistance development to these types of fungicides is high. Multiple-site mode of action fungicides kill an organism in more than one way. Resistance potential is low for these fungicides.

  • Use fungicides from the same “class” only once per season if possible, especially fungicides with a single-site mode of action. Fungicides within the same class have the same mode of action.

    If an organism becomes resistant to a fungicide, it is also resistant to all other fungicides in the same class (example: Rally/Laredo and Elite). Fungicide classes are identified by a FRAC (Fungicide Resistance Action Committee) number. Do not use fungicides with the same FRAC number more than once in the same season.

  • Use label rates (not below label rates). Always use upper label rates for strobilurins (examples include Abound, Flint and Gem).

  • Make sure spray coverage is thorough. Do not use alternate row spray applications. Use enough spray volume to achieve good coverage. Drive slowly enough to achieve good spray coverage. Do not use airplane applications, especially at full canopy.

Almond growers may choose to hold off on using materials like Pristine and Abound at bloom so they can be used later in the season for diseases like scab, anthracnose, and Alternaria leaf spot if necessary. Also, note that rotating between Pristine and Abound is not a good resistance strategy (they both share FRAC number 11).

There are several very effective fungicides registered for use in almonds, other stone fruits, and grapes. There is no need to use the same fungicide more than once in the same season. If resistance develops against a fungicide class, this class may be lost indefinitely as a management tool.

Generally, it takes the chemical industry and researchers years to develop new products with unique modes of action. Following resistance management guidelines will maintain the effectiveness of the currently registered fungicides for many seasons to come.

(Roger Duncan, University of California Cooperative Extension farm advisor in Stanislaus County, as well as Jim Adaskaveg, University of California, Riverside plant pathologist prepared this report a growing fungicide resistance problem in almonds.)