Tips To Manage Scab In Almonds

Scab is becoming increasingly prevalent in almond orchards.

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Jim Adaskaveg, professor of plant pathology and microbiology at the University of California-Riverside, says it’s hard to pinpoint the exact reason for increased scab pressure, but essentially, it comes down to the “disease triangle” – having a susceptible host (almond), the introduction of the pathogen (scab), and the right environment (high humidity or rainfall) for that pathogen to thrive.

“Over time, with a favorable environment, diseases get established. They sometimes tend to develop very slowly, so people don’t even recognize it until it’s actually become a major problem in the orchards,” Adaskaveg says.

A likely theory for the increase in scab incidence is that newer farming practices can lead to greater susceptibility. Higher-density plantings, plus micro-sprinkler irrigation that can increase humidity and create an ideal environment for spreading pathogens, could be contributing factors.

But that doesn’t mean these newer practices aren’t beneficial, Adaskaveg notes.

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“Obviously, growers are doing this because they get better tree growth, and you get higher production per smaller unit area, so there are trade-offs,” he says.

Properly Timing Sprays
The fungal scab pathogen Fusicladium carpophilum forms superficial twig lesions on green shoots, where it is able to overwinter. When new leaves start growing again after the dormant season, the lesions begin to sporulate. By looking at when lesions sporulate, growers can determine the best time to spray to protect new leaves, Adaskaveg explains.

“[If] those spores are wind-disseminated from the twigs to the leaves and fruit, that’s when new infections occur,” he says. “The pathogen is thriving again, and when the tree produces new shoots, these are also infected. The pathogen is able to survive for another year, and so the cycle is repeated.”

Adaskaveg recommends using dormant treatments to suppress and delay sporulation much later into the year.

“Instead of sporulating in late-March/April, they start sporulating in May or June,” Adaskaveg says. “Those extra weeks of suppression from the dormant treatment reduces the number of applications to only one or two sprays in late May or June. This way you can push that disease cycle back so that it doesn’t ever cause economic losses by defoliating the tree.”

He adds that defoliation in July or August is a major issue, because it interferes with bud formation for next year’s flowers, and it can cause sunburn.

“You’re going to lose the leaves in the fall anyway so, as a grower, you’re trying to prevent leaf loss until October or November,” he says. “If the disease builds at this late time of the year, it’s of no consequence.”

The dormant treatment helps save sprays, because growers don’t have to apply fungicides before sporulation.

“The in-season sprays don’t last more than a couple of weeks, so if you start spraying in February or March and the [lesions] don’t sporulate until May, you’re not really being very efficient with your scab management program,” he says.

Check out our tips on avoiding common mistakes in almond scab here.

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