After accidentally discovering new water mold threatening Christmas trees, work continues

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by Courtney Llewellyn

As Christmas trees, Fraser firs are prized for their rich color, pleasant odor and their ability to hold onto their needles. However, they’re also highly susceptible to devastating root rot diseases caused by water molds in the genus Phytophthora. There are about 170 species of Phytopthora, which can affect everything from cucurbits and fruit trees to berries and potatoes, and even firs and pines.

Back in 2019, scientists in Connecticut were conducting experiments to test various methods of growing healthier Fraser trees when they accidentally discovered a new species of Phytophthora. After collecting diseased plants, the scientists isolated and grew the pathogen on artificial media, then inoculated it into healthy plants before re-isolating it to prove how harmful it could be.

“Once the organism was isolated, the presence of unusually thick spore walls alerted us that this may not be a commonly encountered species,” said Rich Cowles, a scientist at the Connecticut Agricultural Experiment Station involved with this study, “and so comparison of several genes’ sequences with known Phytophthora species was used to discover how our unknown was related to other, previously described species.” It turned out they had discovered an entirely new species – P. abietivora, named so because it affects fir trees (Abies fraseri).

The fact that these scientists so easily discovered a new species of Phytophthora that infects Christmas trees suggests there could be many more species waiting to be discovered. Noting the huge biodiversity among this genus infecting Christmas trees is important – it’s estimated that up to 500 Phytopthora species actually exist, according to a report from the USDA’s Forest Service. Infected nursery stock that’s transported from one place to another and chance encounters of different Phytophthora species in the field can lead to new hybrids arising, which can have different pathogenic characteristics than their parent species.

“Knowing how many and which species are present is important, not only for Christmas tree growers, but also for protecting our natural environment,” Cowles added.

Cowles owns his own Christmas tree farm and continues his research today. Some of his current research efforts are directed toward improved management of armored scales in Christmas tree farms using basal bark sprays of systemic insecticides and improving the genetics of Christmas fir trees. Last year, he published “Sulfur amendment of soil improves establishment and growth of firs in a field naturally infested with Phytopthora” (viewable at tinyurl.com/udm7jw9).

In continuing his work, Cowles shared a 45-minute video in May with updates on Phytopthora root rot experiments in Christmas trees. Find it on YouTube at youtu.be/xbEUAFxyNos.

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