The wave of atmospheric rivers that swept the state this winter has created conditions for plant pathogens not seen in decades in California. Florent “Flo” Troyas, a plant pathologist at the University of California, Davis, is getting more and more calls from growers and farm advisors worried about potential damage to crops.
“Generally, whenever you have rain events, you’re going to have problems,” said Troyas, a cooperative extension specialist based at the Kearney Agricultural Research and Extension Center in Parlier. “In wet years, we get really caught up because most pathogens need and love water.”
Troila as the disease detective. Dividing his time between the field and the laboratory, he works on diagnosing pathogens, diseases, and other diseases affecting fruit and nut crops such as almonds, cherries, olives, and pistachios.
On a recent visit to an almond orchard near Fresno, Troyas joined forces with Mae Culumber, a nut farm consultant at California County Co-operative University of Fresno. A few weeks earlier, the two had walked through the grove, and noticed the base of some trees had mucilage—a thick, jelly-like substance that indicates that a pathogen has taken hold.
“A lot of what Florent does is try to assess patterns on landscapes,” said Colmbre. “Sometimes things may look like one thing, but it could be another problem.”
When the fortnight came back after that, the amber-colored gum had moved to the canopy, looking like gum stuck to the twigs, some already dead. “I’ve been out of control before,” Troyas says. kill this branch. This is widespread.
From the field to the lab
Lab tests confirmed what Troyas believed was the culprit: Phytophthora syringae, a pathogen that can affect almond crops but is rarely seen in California. If found, the infection site is usually pruning wounds, but this is not the case here, as the infection began in the canopy at twigs, or young branches.
It’s a threat to a major crop that, according to the California Department of Food and Agriculture, generates $5 billion annually. The last time Phytophthora syringae hit California was in the 1990s after a series of El Niño-influenced storms. Troyas, who has a photographic memory, remembered reading about it in an old brochure.
“It’s a rarity in California and one we see mostly following atmospheric rivers,” he says.
“The disease will not occur until after a very wet winter.”
Phytophthora plants migrate into the soil, are mostly found in the roots of trees, and generally do not spread to branches. Heavy storms created the right conditions for pathogens to “swim” up the trunks as winds blew spores into the air and rain fell back into the canopy, Troyas said.
Some trees will die in this grove. Others, he said, can be saved by pruning infected branches and applying a recommended fungicide.
Identification, diagnosis and education
Trouillas is one of more than 50 Cooperative Extension professionals at UC Davis who are each responsible for identifying problems and developing solutions to those problems in support of agriculture, ecosystems and communities across the state.
In his role, Troyas not only focuses on pathology and research, but also educates farmers, nursery staff, pest control consultants and others in agriculture on ways to manage potential threats and how to prevent crop damage.
“His role is absolutely critical,” said Muhammed Yaghmur, horticultural systems consultant at UCLA Kern County Cooperative Extension. “He is not only on this mission to educate farmers but he is also an educational resource for us.”
Trouillas typically make one or two site visits per week, usually after a farm advisor arrives about a problem they can’t solve on their own.
“This allows us to be at the forefront of disease detection in California,” he said.
He likens these visits to the doctor’s house calls, only with fields instead. One such call recently took him to a cherry orchard in Lodi.
“These guys are helping me out a bit,” said Andrew Vignolo, a pest control consultant with Wilbur-Ellis who asked to consult. “I annoy them a lot.”
The visit begins like any consultation at a doctor’s office, only questions come quickly as they wander through a lodi grove where branches die, there is gum and trees appear tense. Some appear to have sunburned from exposure. Old pruning wounds show cankers, indicating that previous disease treatments didn’t get rid of everything that was affecting the trees.
Trouillas asks about the tree’s variety because some varieties are more susceptible to pests or disease. Focus on stress because that opens the door to disease.
Do they fluctuate in the dormant winter months or in the summer when pathogens are most prevalent? Is soil tested? How old are the trees? What about nutrition?
“I’m trying to figure out how they got so badly infected,” Troyas said, walking through the grove. “Bacterial ulceration is a very mysterious disease.”
He thinks it may be the bacterial ulcer disease and shaves off some of the bark to take to the lab for testing. He wants to come back next winter to take some samples to see where the pathogen was in the winter.
“We will know in a few weeks if we have a fighting chance,” Vignolo said.
Whether it’s in Lodi, Fresno, or anywhere else in the state, Trouillas focuses on local conditions. But what is learned in one area can be transferred to others, providing early warnings or advice for those in similar situations. “All these efforts in collaboration, from the field, to the lab, through research projects, there’s only one goal here — to help farmers in California.”
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