The late spring and summer seasons bring warmth and humidity to California’s almond orchards. Unfortunately, these climate shifts can leave almond trees more susceptible to diseases such as Alternaria leaf spot, which favors dew and high humidity coupled with stagnant air. To avoid the short- and long-term damage caused by Alternaria and help drive optimal return on investment, it is important for almond growers to monitor their crops and implement prevention and treatment best practices.

Alternaria leaf spot is caused by Alternaria alternata and can have devastating consequences for almond growers during the summer months. Leaf spot infections are accelerated when leaf wetness occurs at average temperatures between 60 to 82 degrees. In severe cases, the disease can defoliate trees rapidly, sometimes leading to complete defoliation by early summer. Defoliated trees result in yield loss in the current year and potentially reduce yields through the following season.

In addition, the Alternaria alternata pathogen has developed resistance to several fungicides within a few years of their introduction.

“This resistance is thought to have developed in part because the pathogen generates high spore numbers and the season is so long, which results in long periods of minimal protection,” said George Musson, fungicide product development manager at Crop Science. “Currently, in tree nuts there is already resistance to the QoI, SDHI and Benzimidazole fungicides. To date researchers have not seen resistance to Luna® for this pathogen, but this could change over time, especially with incorrect use. There are procedures described on the Luna labels and on the University of California website one can use to reduce the risk of losing this effective tool.”

Since it can sometimes take several weeks for symptoms to occur, timely treatment for Alternaria based on weather conditions is crucial. Growers should monitor trees beginning in early April. The disease appears as fairly large brown spots on leaves, about 0.5 to 0.75 inches (12-18 mm) in diameter. As it progresses, the spots turn black as the fungus produces spores. 

According to Musson, growers should initiate their first fungicide application, such as Luna Sensation® or Luna Experience® fungicide from Crop Science, preventively in locations where the disease has historically been a concern.  Additionally, growers can use the University of California model as guidance for when to target the first applications, especially in lighter pressure areas.

“One highly effective way to delay resistance is to apply products before the pathogen gets a foothold,” said Musson. “For this reason, the first application may be the most important for setting up for a good season. Beginning a well-timed spray program before pathogens attack helps keep fungal populations low and reduces the likelihood of selecting resistant strains.”

To maximize return on investment from that first Alternaria application, Musson advises almond growers to choose a fungicide that not only provides control of Alternaria leaf spot, but also manages other diseases which may also be present such as Anthracnose, Scab, Shothole and Rust.

In five years of California trials, the Luna fungicides have demonstrated outstanding control of Alternaria leaf spot and other key diseases affecting almonds. Luna fungicides also offer excellent control of Alternaria where other products fail due to disease resistance to carboxamides and/or strobilurins.

Timing and thorough coverage are critical for optimum performance.  Always be sure to follow label directions to make the most of Luna in your spray program.

Learn more about Luna fungicides and managing diseases in almonds and other tree nuts in the interactive product learning module from Crop Science.

Protecting Almonds from Alternaria Leaf Spot

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