Keep an Eye Out for Phytophthora in Your Nut Orchard

After several years of drought, nut growers are now contending with a new issue — Phytophthora root rot. While this fungal disease affects fruit and nut trees across the country, a combination of factors in the West is allowing it to wreak havoc below the soil this year, says Garrett Gilcrease a Syngenta Agronomy Service Representative.Western Fruit Grower had a chance to talk to Gilcrease about the fundamentals of Phytophthora and what growers could and should be doing this growing season in light of added risk.

Garrett Gilcrease

Q: Why should California growers be concerned about Phytophthora this year?
A: We have had somewhat of a perfect storm for Phytophthora to become an issue: the droughts of the past few years, saltier water, irrigation changes, and the wet weather this winter. Ultimately, when root rot is present, there is no silver bullet. Even in drought conditions, infection can occur or even be magnified.

Q: How does Phytophthora affect nut and fruit trees?
A: Phytophthora infects the root tissue of plants and causes them to rot, leading to issues like lack of fruit set, poor vegetative growth, long-term orchard decline, and even death of trees within the orchard. When infection occurs, productive root tissue degrades and is no longer useful in pulling both water and nutrients out of the soil.

Q: How is Phytophthora detected in orchards? What can growers do to screen/detect Phytophthora?
A: Because Phytophthora is present in many of our soils in California, roots, rather than just soil, need to be tested. Root tissue is tested for the presence of a protein that is specific to the Phytophthora pathogen.
The best way to screen for Phytophthora is to be proactive and test fields. Dig up roots to see if there are any signs of a potential Phytophthora infection and if so, test them to identify the cause of the rotted root tissue. Not treating even the smallest infection can have significant impact on yield.

Q: How easy is it for growers to miss Phytophthora?
A: Very easy. The clue is when trees cannot cope with environmental stressors such as heat or drought. Because Phytophthora compromises the root system, foliar symptoms of root rot often mimic both drought stress and nutrient deficiencies in the leaf tissue. It’s only when problems persist that other issues such as Phytophthora are explored.

Q: What type of management practices can growers take to prevent Phytophthora at this point in the season (and into the next few months)?
A: Water management is key. Phytophthora is what we consider a water mold so it needs some amount of free water to live and grow. If the soil is saturated, manage water applications in such a way that they are only applied when needed.
Treatment with a fungicide that targets this pathogen would give long-term control, allowing the tree to divert all available energy into crop and shoot growth rather than developing new roots to replace those rotted by Phytophthora.

Q: What type of irrigation should growers consider in order to prevent the Phytophthora pathogen from taking hold?
A: For all types of irrigation methods (micro or flood), I would make sure that water penetration issues are solved to avoid standing water. I would also suggest that a system evaluation be done on micro-irrigation systems to ensure proper uniformity, lessening the potential for over- or under-watering areas of the field.
Lastly, as a good practice, subscribe to an evapotranspiration model of irrigation. We’ve seen this not only minimize the potential for both drought stress and overwatering but also reduce water costs by properly aligning water inputs with water usage.

Q: Should growers’ management strategies change throughout the season, knowing the risk of Phytophthora is high?
A: Very much so. Water management and water quality are key to minimizing this pest. This year, we should have more surface water than in years past, so this will help alleviate some salt issues, as well. Salt in soil and irrigation water puts additional stress on the tree and makes Phytophthora that much more devastating due to the cumulative effect of both.

What types of fungicides should growers apply to combat Phytophthora (certain class, AI, mode of action)?
A: This is one reason why the pest can be such a chronic problem. At the moment, we only have two available options for control. The first is phosphorus acid, commonly referred to as phosphite. This is a group 33 fungicide that is most commonly sold as a fertilizer and is widely over-used because of its low cost.

The second and final option for control is Ridomil Gold® SL, which is a group 4 soil-applied fungicide from Syngenta, making it a good fit in a resistance management program. When a root system is treated with Ridomil Gold SL, a reservoir of the product builds up within the root system to stop current and prevent future Phytophthora infection, thus letting the tree focus on growing a crop rather than fighting root rot.

Q: Is there anything a grower shouldn’t do when Phytophthora is suspected?
A: They shouldn’t wait to get it evaluated. This pest can be overlooked for many seasons before being identified. It takes years to establish a permanent crop, and by the time growers see the aboveground impact of the disease, it has been developing for some time and can take a while to overcome the damage that has already been done if it can be overcome at all.

Q: What else should tree fruit and nut growers know and understand about Phytophthora?
A: Phytophthora is a persistent pathogen, like any other fungal disease, as the inoculum is present, can hang around for a long time and is even being reintroduced year after year, so treatment should be considered as part of an annual management program.

Q: What are some resources for growers who might be interested in more information?
A: The University of California and University of California Cooperative Extension system has a great network of farm advisors, researchers, and industry professionals who are more than willing to make field visits to help identify issues.

Our Syngenta team is also a great resource. While we are a manufacturer of crop solutions, we are also very rooted in two principles: One, what is good for the industry is good for everyone in the industry. And two, by making sound recommendations for the best product for the job, even when it’s not our own, we earn respect and develop trust, which is much better than short-term gain.

Q: What is the Soil Pathogen Assessment and how can growers use it?
A: The Soil Pathogen Assessment program is a free program that Syngenta offers to assess Phytophthora infections on a field-by-field basis. We will take root samples from the field and send it to a third-party lab where it can take up to three weeks to return results. After that, we would discuss treatment options based on the results.

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