Almonds: After Wet Spring, Alternaria And Rust Are Possibilities

Growers are encouraged to monitor for rust this season. Rust pustules are reddish-orange on the undersides of leaves (top) and appear as yellow spots on the upper leaf surface (bottom). Photo credit: Jack Kelly Clark, UC IPM Program.

A late, wet spring has increased disease pressure in many almond orchards across the state, and experts are warning growers to pay attention, particularly in the northern and southern extremes of California’s Central Valley.

“It’s probable this year is going to be a high-pressure disease year in places like Kern County,” said Mohammed Yaghmour, a plant pathologist and area orchards systems advisor based at the University of California Cooperative Extension (UCCE) office in Kern County. “We had rains all throughout May and we’ve seen 30 percent humidity in the last two days,” he said in an interview in early June.

Of particular concern are fungal diseases such as Alternaria leaf spot, which presents as large brown spots on leaves, turning black as the fungus produces spores. Also of concern is rust (officially Tranzchelia discolor), which causes small yellow spots on the upper surfaces of leaves, while the lower leaf surface takes on a rusty red appearance as spores are released.

When severe, both diseases can lead to early defoliation, weakening the tree and reducing the following year’s bloom, according to the UC Integrated Pest Management program.

“In the past, I’ve seen trees that have lost three-quarters of their foliage,” said Dr. Dani Lightle, a UCCE orchards systems advisor based in Glenn County.

“We saw a serious rust outbreak in 2016 in the Sacramento Valley,” Lightle said. “We certainly have a repeat of those conditions this year. It wouldn’t surprise me if we saw an outbreak again this year.”

Regarding Alternaria, she added: “We’re right in the peak period — another set of rainy conditions could really make things worse. Another front coming through would not be out of the ordinary considering the unpredictability we saw this spring.”

Varietal Differences: First Things First

Almond varieties that are most susceptible to Alternaria leaf spot include Carmel, Sonora, Monterey, Winters and Butte. Dr. Yaghmour suggested that growers monitor their most susceptible varieties first and start applications when symptoms appear in those varieties.

Growers are advised to monitor for symptoms and signs of the Alternaria leaf spot in April through June. They should also reference the UC Disease Severity Value (DSV) Model, which uses leaf wetness duration to assess when treatment against Alternaria is needed.

While there is no disease model for rust, Yaghmour said the disease is usually seen in second- and third-leaf orchards. Lightle said young trees are particularly susceptible to rust, as are trees where high amounts of nitrogen fertilizer have been applied. She said the most affected trees she saw during the 2016 outbreak were at the third-leaf stage.

In orchards with a history of the disease, two to three fungicide applications should be considered between mid-April and late June, depending on environmental conditions (e.g., leaf wetness for extended periods and warm temperatures) that are conducive to disease development.

In some orchards, it is important to note that Alternaria resistance to QoI (FRAC Group 11), a.k.a. strobilurins, and SDHI (FRAC Group 7), such as boscalid, fluxapyroxad fungicides, has been reported and growers should not apply these fungicides in these orchards. Take a look at the the UC publication by Adaskaveg et al, titled 2017 Efficacy and Timing of Fungicides, Bactericides and Biologicals for Deciduous Tree Fruit, Nut, Strawberry and Vine Crops, for more information on fungicide programs for resistance management.

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