August 2023 - 32

PLANT & SOIL HEALTH
A STRAWBERRY
THREAT
California researchers work to thwart
new fusarium wilt strain
USDA scientists are working to control a new " resistance-breaking " strain of fusarium wilt, a common cause of
strawberry crop loss and death. Photos courtesy of USDA.
BY DOUG OHLEMEIER
ASSISTANT EDITOR
U
SDA scientists are working on ways to control
fusarium wilt, one of the most common causes of
strawberry crop loss and death.
An estimated 55% to 59% of California strawberries
planted in the state since 2014 are not resistant to
fusarium wilt, according to the University of CaliforniaDavis.
The fusarium wilt pathogen is called Fusarium
oxysporum f. sp. fragariae race 1 (Fof race 1).
All fusarium wilt-resistant varieties possess the same
resistance gene, called FW1. Until this year, the gene
was effective against every known fusarium wilt strain
in California.
In late 2022, a new " resistance-breaking " strain, Fof
" race 2, " was discovered in a field of organic strawberries
planted that summer in Oxnard, California. Whole genome
sequencing indicates the Fof race 2 strain is new to
science and has never been detected worldwide. Scientists
believe the strain recently developed there. Research on
the prevalence, origin and management of the disease is
ongoing and likely to progress rapidly over the next year,
said Peter Henry, a USDA-Agricultural Research Service
plant pathologist in Salinas, California.
Warmer temperatures favor fusarium wilt and
32 | AUGUST 2023
Macrophomina crown rot, and an abnormally cool and wet
winter and spring delayed fruit production and development
of symptoms. The reduced incidence and severity of these
diseases has made it more challenging to identify other
fields that are affected by Fof race 2. Henry has visited or
tested samples from dozens of fields this year and expects
the work to accelerate over the next few months.
In collaboration with Driscoll's, Henry designed an assay,
a laboratory investigative procedure, that has passed initial
validation for specificity towards Fof race 2. The scientists
also designed another assay, which speeds identification of
infected samples. Multiple California research and industry
labs use the RPA (recombinase polymerase amplification)
as the most common method for identifying the causal
agent in wilted strawberry plants.
The scientists are in the process of collecting hundreds
of RPA extracts from previous seasons from the Cal Poly
Strawberry Center diagnostics lab, TriCal Diagnostics,
Driscoll's, and USDA's collection to test for CA Fof race
2. This will allow researchers to search more broadly
for infected strawberry fields than they could achieve
by sampling diseased fields in a single season. By the
end of the 2023 season, Henry said he should have a
comprehensive understanding of the distribution of CA Fof
race 2 in California strawberry fields.
The first of the three strains of Fof race 1 in California

August 2023

Table of Contents for the Digital Edition of August 2023