Is habitat restoration actually killing plantsin the California wildlands?2 January 2019, by Kara Manke

The exotic soilborne Phytopthora cinnamomi wasintroduced in the Ione area of the Sierra Nevadafoothills, where it is literally wiping out two nativemanzanita species. This picture shows how vehiclesspread the pathogen along roads and tracks by kickingup infected soil, killing nearby manzanita. Credit: MatteoGarbelotto, U.C. Berkeley

In 2014, plant biologists with the CaliforniaDepartment of Agriculture reported an alarmingdiscovery: native wildflowers and herbs, grown innurseries and then planted in ecological restorationsites around California, were infected with Phytophthora tentaculata, a deadly exotic plantpathogen that causes root and stem rot.

While ecologists have long been wary of exoticplant pathogens borne on imported ornamentalplants, this was the first time in California thatthese microorganisms had been found in nativeplants used in restoration efforts. Their presence inrestoration sites raised the frightening possibilitythat ecological restoration, rather than returningdisturbed sites to their natural beauty, may actuallybe introducing deadly plant pathogens, such asthose related to Sudden Oak Death, into the wild.

New work by a UC Berkeley team in the College ofNatural Resources shows for the first time just howwidespread and deadly the threat of pathogensfrom restoration nurseries may be.

The team surveyed five native plant nurseries inNorthern California and found that four harboredexotic, or non-native, Phytophthora pathogens.Strains of the pathogens from native plant nurserieswere shown to be at times more aggressive thanstrains found in the wild, and some of them arerapidly developing resistance to the fungicides thatcan be used to control them, the researchers found.

Working with restoration nurseries around the state,the researchers showed that new managementtechniques, coupled with new methods fordetecting pathogens, can help these nurseries limitthe spread of exotic pathogens.

"Some of these restoration projects cost tens ofmillions of dollars, but of course their actual value ismuch higher, because of the wealth of serviceshealthy natural ecosystems provide, includingsupporting animal and plant biodiversity, providinggood water and air quality, and enjoyablerecreation sites," said Matteo Garbelotto,cooperative extension specialist and adjunctprofessor of environmental science, policy andmanagement at UC Berkeley.

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Sudden Oak Death is causing extensive tanoak and oakmortality in the Big Sur region. Credit: Matteo Garbelotto,U.C. Berkeley

"Such services are highly diminished in ecosystemsaffected by exotic plant diseases, while water runoffand erosion, the establishment of exotic plants andanimals, and even hotter wildfires may increase inconjunction with disease outbreaks in naturalecosystems," Garbelotto said.

Pathogens evolve to outwit fungicides

Bacteria that make humans sick are constantlyevolving to resist the antibiotics designed to fightthem, and resistance to fungicides has beendocumented in microbes causing diseases inagricultural plants. Garbelotto and his team wantedto know if the widespread use of fungicides in innative and ornamental plant nurseries could alsoaccelerate the development of fungicide-resistancein plant pathogens.

Their research was spurred in part by theirdiscovery of a new strain of the Sudden Oak Deathpathogen in Oregon forests that is highly tolerant ofa fungicide commonly known as phosphite, one ofthe main weapons used against plant parasites inthe wild because its application does not cause anyknown negative environmental side effects.

Together with a group of New Zealand researchers,they decided to study fungicide resistance ofPhytophthora—a genus of plant pathogens that can

case lethal cankers and root rot—to two importantfungicides, including phosphite.

The researchers gathered numerous samples of Phytophthora from 11 species present both inforests and plant nurseries. They then tested thesensitivity to phosphite of multiple individuals perspecies.

These Petri dishes show colonies of two strains of theexotic pathogen Phytophthora ramorum, the parasitebehind Sudden Oak Death, on a growth medium thatcontains a phosphite fungicide. The EU1 strain of P.ramorum (left) is phosphite-tolerant and grows well. TheNA1 strain (right) is still phosphite-sensitive and itsgrowth is inhibited. California forests have beencolonized by NA1 strains, but phosphite-tolerant EU1strains have recently been found in Southern Oregon.Credit: Matteo Garbelotto, U.C. Berkeley

While most of the species tested were overall stillsensitive to phosphite, strains of four species wereable to resist the effects of the chemical, theresearchers report in PLOS ONE. These include Phytophthora ramorum, the parasite behindSudden Oak Death in North America and SuddenLarch Death in Europe, and Phytophthoracrassamura, a species first discovered recently bythe same UC Berkeley researchers in native plantnurseries and restoration sites in California.

Some strains within each of these four species,although genetically almost identical to strains stillsusceptible to phosphite, were resistant to it. Thepresence of chemical tolerance or chemicalsensitivity when comparing nearly geneticallyidentical strains suggests that the development of

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resistance occurred relatively recently, perhaps inresponse to the widespread use of phosphites innative and ornamental nurseries, Garbelotto said.

"These pathogens can be literally flooded withthese chemicals in plant production facilities, and atthe beginning of the study, we hypothesized that insuch predicaments these pathogens would beforced to evolve resistance" Garbelotto said."Indeed, our hypothesis was correct, and we foundthat some of them evolved the ability to tolerateexposure to phosphite."

While phosphite can still help to spur a plant'simmune system, this may not be enough to quellthe spread of the disease, Garbelotto said.

"By pressuring these pathogens to evolveresistance to phosphites, we are effectively takingout phosphite as a potential tool to manage thesedisease outbreaks," Garbelotto said. "Furthermore,the ability to quickly develop tolerance to afungicide may be an indication these pathogenscan adapt quickly to new environments. Thus, theymay become formidable invasive organisms,infesting larger swaths of natural areas and causingsignificant disease and mortality of essential nativeflora."

Sudden Oak Death caused by phosphite-sensitive strainscan be controlled by preventive treatments usingphosphites. At left, a small lesion in an infected tree killsthe tree, while at right, treating the tree with phosphitekeeps it alive. Credit: UC Berkeley photo by MatteoGarbelotto

A widespread—but reparable—problem.

Since the first discovery of Phytophthora inCalifornia restoration sites, research by the UCBerkeley team and others have traced the deathsof wild trees and plants back to strains of thepathogen originating in native plant nurseries,rather than strains already found in the wild.However, few studies have documented just howprevalent the problem is.

In a recent study published in the journal PlantPathology, UC Berkeley researchers examined 203individual plants across five restoration nurseries inCalifornia and found that 55 of the plants wereinfected with Phytophthora.

"We were able to prove that this is a widespreadproblem in California," Garbelotto said. "Most of thestock that they used is infested, and the levels werevery high. For some species more than 50 percentof the plants we tested were infected."

The team then worked with the infected nurseriesto implement new best management practices to tryto limit the spread of disease without the use ofphosphite or of other fungicides. These simpleguidelines, which included more carefulmanagement of water runoff and soil to reducecross contamination, reduced the prevalence ofdisease to nearly zero a year after implementation.

"We were able to prove that after a year of followingthe guidelines, those facilities were clear ofpathogens, and other facilities that did not followthe guidelines still had the pathogens," Garbelottosaid. "As a result of these findings, people are nowputting a lot of money and effort into making surethat the plants are clean, by following similarguidelines and by making sure that no fungicidesare used to avoid the development of resistance."

More information: Shannon Hunter et al,Evidence for rapid adaptive evolution of toleranceto chemical treatments in Phytophthora speciesand its practical implications, PLOS ONE (2018). DOI: 10.1371/journal.pone.0208961

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Provided by University of California - BerkeleyAPA citation: Is habitat restoration actually killing plants in the California wildlands? (2019, January 2)retrieved 23 March 2022 from https://phys.org/news/2019-01-habitat-california-wildlands.html

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