March 2022 - 22

Challenging analysis pursued to curb cranberry rot
By Lorry Erickson
FGN Correspondent
University of Wisconsin-Extension
fruit pathologist Leslie Holland and
others are striving to develop a holistic
approach for managing cranberry fruit
rot, a fungal disease caused by multiple
factors - many of which are not fully
understood.
Speaking at the 2022 Wisconsin State
Graduate student Evan Lozano. Photo: Leslie Holland
Cranberry Growers Association winter
meeting, Holland said cranberry fruit rot
is a complex of at least 12 different species
of fungi that causes fruit losses both in the
field and in storage. In Massachusetts and
New Jersey, cranberry fruit rot tends to
show up year after year, while the disease
occurs more sporadically on Wisconsin
cranberry marshes.
Holland is collaborating with plant
pathologists, plant physiologists, plant
breeders and others involved in cranberry
research in the major U.S. cranberry
growing regions.
" What we're trying to do is better
understand all the factors that lead to
the development of fruit rot to come up
with a holistic control strategy. We're
also lacking tools to predict epidemics,
especially in a place like Wisconsin where
we have more episodic fruit rot events.
Having a tool that can allow us to predict
and make decisions would be really
critical for these fruit rot epidemics, "
Holland said.
Current management practices of
well-timed fungicide applications during
blossom time are effective at reducing
disease incidence, but environmental and
cultural practices influence the incidence
and severity in the field. Due to the nature
of the fruit rot fungi, they may remain
in cranberry beds season after season as
well, Holland said.
Holland also noted that cranberry
growers are facing a diminishing number
of fungicides approved for use in
controlling cranberry fruit rot.
While there are some cranberry
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22 | FruitGrowersNews.com
varieties that show resistance to
cranberry fruit rot, the mechanism for
that resistance is unclear. There is also
limited knowledge of the impacts of
environmental factors and plant stress
on the incidence of cranberry fruit rot,
Holland said.
During the 2021 growing season,
Holland and graduate student Evan
Lozano placed 30 fungal spore traps on a
commercial Wisconsin cranberry marsh to
study fungal spore dispersal. Samples were
collected about every four days from mid
to late June. Holland noted that the bloom
period progressed quickly this past season,
so no spore trapping was done in July.
" For me as a plant pathologist, fungal
spore dispersal is interesting to study. But
why should you as a cranberry grower
care? I think it's meaningful because we
can try to understand and ensure that the
fungicide applications we are making are
on target with the dispersal patterns of
the fungi that we are trying to control, "
Holland said.
The primary fruit rot pathogens
Holland and Lozano found in their spore
traps were colletotrichum or bitter rot and
Allantophomopsis which is associated
with black rot.
" The black rot was an interesting result
as black rot is typically seen at harvest.
Finding it this early in bloom was of
interest to us, " Holland said.
Not found in the spore traps was
Coleophoma or ripe rot, which surprised
Holland. In previous assays done toward
the end of the bloom period and early
fruit set, coleophoma tended to show up
during this period. Holland surmised that
if sampling had been done into July, the
results might have been different.
Another part of the study assessed
latent infections throughout the season.
Plant samples were collected every one
to three weeks from May to September
to test for fruit rot before visible signs of
infections were observed.
The early findings of the 2021 study
showed colletotrichum was present
primarily in fruits and leaf tissue. " We
didn't really see this coming from runners
or the woody stems of uprights. We first
started to see this pop up in July and then
it ramped up a little bit more in August, "
Holland said.
A small area of fruit rot was missed when
fungicide was applied to a new planting of
Mullica Queen vines. Photo: Lorry Erickson
The study found allantophomopsis
present in all tissues for the entire season
in the sampled beds. Another fruit rot
pathogen - phomopsis - was found in
high levels in June but not detected after
that, Holland said.
Because so many factors influence
cranberry fruit rot development, Holland
said the best approach is to prevent
introduction of the pathogens into new
plantings. Since cranberries are propagated
vegetatively, Holland is researching heat
treatments of cranberry cuttings to reduce
or eliminate fruit rot fungi.
Holland used both molecular
diagnostic assays and fungal isolation
to identify the pathogens present in the
collected samples. Molecular diagnostic
assays allow for the detection of the 12
main fruit rot pathogens, while fungal
isolation aids in identifying secondary
pathogens that could also be of interest
that might otherwise be missed, she said.
Holland has received preliminary
approval for $8,500 in funding from the
Wisconsin Cranberry Board to continue
her research on cranberry fruit rot in
2022. FGN

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March 2022

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