Preventing scab fungicide resistance

Almond Board-funded research being conducted by UC plant pathologist Dr. Jim Adaskaveg since 2006 demonstrates that the scab pathogen Cladosporium carpophilum has become resistant to strobilurin fungicides in orchards in the northern San Joaquin and Sacramento valleys with a history of high disease pressure and multiple sprays.

Strobilurin fungicides, also known as quinone outside inhibitor or QoI, fungicides, are single-site mode of action fungicides that are prone to resistance development in some fungal pathogens, such as Cladosporium spp. For scab, these fungicides include tradenames such as Abound, Gem (previously called Flint on almonds), and Pristine.

Fortunately, recent research by Dr. Adaskaveg shows that there are a number of tactics and fungicides that can be used to combat scab and counter resistance. One very effective approach in orchards with a history of disease is to apply delayed-dormant applications of copper and oil or liquid lime sulfur. His findings show that a delayed dormant application in late January with copper and oil is the most effective in reducing the production of scab inoculum from overwintering twig lesions. Specific guidelines can be found at the UC IPM Web site. Dr. Adaskaveg advises that in orchards where scab is an issue, these sprays should be included to reduce the risk of fungicide resistance and improve control.

Dormant and spring-time sprays are directed at delaying sporulation of scab twig lesions and protecting leaves, fruit, and young twigs, respectively. Dormant applications delay and reduce the pathogen inoculum production in twig lesions, whereas spring sprays between March and May focus on protecting new tissue from infection. Pathogen sporulation and spread is favored by wetness and high humidity. Traditionally, spring treatments are most effectively done in the period of two to five weeks after petal fall. If rains persist, applications may have to be extended into May.

Fungicide resistance management is critical for scab and other almond diseases. To maintain effective use of fungicides under high disease pressure when multiple applications are required, UC experts currently advise the following for in-season applications against all pathogens:

• Use fungicides that are effective for the target diseases.

• Time treatments for greatest efficacy at the appropriate stage of almond development, and initiate treatment at initial stages of disease development.

• Do not apply single-site mode of action fungicides when high populations exist (i.e., high levels of disease at the time of application).

• Use full label rates.

• Start programs using a fungicide with a multiple-site mode of action (e.g. copper-oil).

• Rotate from one chemical class to another within the same season.

• Do not apply fungicides within any given high resistance potential class more than twice per season.

• Do not apply single-site fungicides alone: either use fungicides in combinations (tank mixes) or premixes that provide multiple modes of action.

• Do not use alternate row applications.

• Do not use air application at full canopy.

TAGS: Management
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