How We Grow

2020 July/Aug How We Grow

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A L M O N D O R C H A R D 2 0 2 5 G O A L S WATER USE 3 Ready for Combat: Preparing for Battle Against Hull Rot When it comes to combatting orchard threats, such as disease and various pests, growers may assume that providing more water to their orchards will help strengthen their trees, improve overall health and help fight off predators of various shapes and sizes. However, when it comes to fighting off hull rot, the reality is less is more. Sebastian Saa, Ph.D., senior manager of Agricultural Research at the Almond Board of California (ABC), explains, "Regulated deficit irrigation during hull split will not only reduce the likelihood of hull rot in your orchard but also prime your trees for harvest." In an effort to both reduce the amount of water it takes to grow a pound of almonds by an additional 1 20% and increase adoption of environmentally friendly pest management tools by 25% – two of the four California almond industry Almond Orchard 2025 Goals – growers are seeking more strategic practices that allow them to address multiple challenges at once and adopting a more holistic approach to orchard management. One of those strategic practices that may help growers address multiple orchard issues is regulated deficit irrigation, or RDI, which involves supplying less water to one's trees than what is required to meet an almond orchard's maximum water requirement, or evapotranspiration (ET) level. During the recent drought in California, many almond growers practiced RDI out of necessity. Today, even though the industry isn't facing drought conditions at quite the same intensity, there are still times during the crop year, mostly in summer months, when strategic RDI is beneficial in not only responsibly managing irrigation amounts but also addressing a common and serious orchard disease: hull rot. In fact, hull rot is one of the five key pests the industry will track its progress toward combatting as part of its 2025 Goal to increase environmentally friendly pest management, as fewer sprays to prevent hull rot will help the industry progress toward continually safer and more responsible growing practices. Below are four steps to guide you in the process of efficiently managing hull rot using RDI. 1 Understand your foe Hull rot is an infection of almond hulls that can be caused by either Rhizopus stolinifer, Monilinia fructicola or Aspergillus niger, pathogens that are especially threatening as they can occur year after year in an orchard. Once hull split occurs, hull rot fungi enter the hulls through their open sutures – "splits" – and target the spur on which the nut hangs. Upon infection, pathogens release toxins that are translocated into the spurs, killing the wood and causing potential crop loss in subsequent years. Mohammad Yaghmour, Ph.D., area orchard systems advisor for the University of California (UC) Kern County Cooperative Extension, explains, "If hull rot is very severe in an orchard, you can expect it to negatively affect next year's crop yield, too." Symptoms of hull rot can vary depending on the specific pathogen present, but Saa noted that once infection takes place the hulls begin to look rotten and the leaves near the infected hull become dry and brown, but don't fall off the tree limb. Almond hulls are of course susceptible to hull rot during hull split, a period that typically lasts from the end of June through July. However, Luke Milliron, Ph.D., UC Cooperative Extension orchard systems farm advisor for Butte, Glenn and Tehama counties, said hull split timing varies across multiple almond varieties. 1 Over the past two decades, almond growers have successfully reduced the amount of water needed to grow a pound of almonds by 33%. Source: University of California, 2010. Food and Agriculture Organization of the United Nations, 2012. Almond Board of California, 1990-94, 2000-14. Don't Over Fertilize Almond trees are more susceptible to hull rot when they are exposed to excessive nitrogen levels. That's why, according to Saa, "Applying the right rate and right amount of nitrogen will help growers reduce the risk of hull rot in their orchards." Visit to access ABC's free Nitrogen Calculator, provided through the California Almond Sustainability Program. This tool uses a research-based predictive model to advise how much and when to apply fertilizer nitrogen to effectively meet yield-based demand. Monilinia fructicola is one of the three main pathogens that can cause hull rot in almonds. Photo courtesy of the University of California Agriculture and Natural Resources A L M O N D O R C H A R D 2 0 2 5 G O A L S WATER USE

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