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Common crop pathogens are finding new hosts

Canker is a plant disease characterized by a small area of dead tissue that spreads slowly, over years, and may eventually kill the plant. It is mostly found in stems, branches and twigs, and is caused by a variety of different pathogens. Canker in branches and stems of crop plants, such as gapes, tomatoes, citrus, nuts and stone fruits, can cause significant economic loss. 

Scientists from the University of California, Davis have now identified that pine trees may also host the fungus. In the journal Plant Disease, the experts report that during routine inspections of trees in Orange County between 2018 and 2022, they found more than 30 mature pines that had been infected within an area of 100 acres. The pines showed symptoms of severe dieback and death. 

The disease, identified as ghost canker, affected trees that belonged to three different pine species; Pinus eldaricaP. halepensis and P. radiata. Signs of the canker are first discernible in the lower parts of the canopy, and then they advance into the crown, causing rapid dieback and, in some cases, tree death. 

The symptoms start to manifest in the lower portion of the canopy, where areas of canker are visible and branches begin to die. Thereafter, the disease advances into the crown and trunk, leading to quick dieback and, in some cases, to tree death. Cross sections of affected branches revealed wedged-shaped areas of infection with indistinct margins, reminiscent of triangular ghosts.

Akif Eskalen, a professor of Cooperative Extension in the Department of Plant Pathology at UC Davis, suspects drought and other stress conditions brought on by climate change have weakened the trees, making them more susceptible to new threats.

“We have been seeing this on pine trees for the last several years,” he said. “Our common crop pathogens are finding new hosts.”

The researchers used genetic analysis to identify the pathogens causing pine ghost canker in the pine trees and found the fungal pathogens Neofusicoccum mediterraneum and Neofusicoccum parvum, both of which affect numerous other trees of economic importance, including walnut, chestnut and olive trees.

They tested whether these two species were indeed the causative agents by inoculating healthy trees with isolates of the two fungi. After three months, the newly infected trees showed lesions in their vascular tissue while the control group of trees remained symptomless.

The pathogens infect a tree by entering through wounds caused either by insects, such as red-haired pine bark beetles, or pruning. This means that trees in managed or landscaped areas could be at risk. Another route is via tiny natural openings in stems, known as lenticels, through which fungi can make their way, said Marcelo Bustamante, a Ph.D. candidate in Eskalen’s lab and first author on the paper. 

The danger is that these fungal infections can spread by means of spores. The higher the prevalence of diseased trees, the faster the rate of transmission to surrounding trees. In addition, rain, irrigation water and humidity by fog can trigger the right circumstances for the spores to spread, said Bustamante.

The authors encourage people to look out for signs of the disease among their own trees and those in the neighborhood. Dead branches can indicate the presence of a canker. Detecting the fungus is not an emergency but “people should keep an eye on their plants when they see abnormalities,” Eskalen said.

Other signs include localized areas on stems, branches and tree trunks that are discolored and sunken. On bark, the spores can look like strings of discolored dots.

The lab has posted a brochure about how best to manage wood canker diseases. Tips include:

  • Keep your trees healthy; proper irrigation and maintenance will keep trees strong.
  • Prune dead branches to reduce sources of infestation.
  • Avoid unnecessary pruning; perform structural pruning only.

“The detection of these pathogens in urban forests raises concerns of potential spillover events to other forest and agricultural hosts in Southern California,” wrote the researchers.

By Alison Bosman, Staff Writer

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