November 2021 - 8

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Learning to grow broccoli with new challenges
NATALIE HOIDAL
University of Minnesota
Extension
Broccoli is a staple crop for vegetable
growers across the country, but
recent changes in disease evolution,
climate and insect behavior have made
broccoli increasingly difficult to grow.
This is especially true for organic
growers.
In our final episode of The Vegetable
Beet for 2021, host Natalie Hoidal
discussed these challenges with
Bhabesh Dutta, University of Georgia;
Thomas Björkmanfrom, Cornell
University and Susan Scheufelef,
University of Massachusetts.
If you asked a broccoli, kale or
cabbage grower in 2017 what their
biggest disease challenge was,
they likely would have said black
rot (Xanthamonas campestris pv.
campestris). In Minnesota, from 20172019
(three very wet years), organic
growers were routinely reporting 80100%
yield loss in broccoli. However,
in the last few years, there has been
a dramatic shift toward Alternaria
leaf spot and head rot becoming
the primary disease of plants in the
Brassica family.
Alternaria is caused by three
different species: A. brassisicola, A.
brassicae and A. raphani.
Dutta's lab in Georgia has been
studying the evolution of Alternaria to
better understand why this relatively
old pathogen has suddenly become
such a problem. So far, they've learned
from a survey conducted in 2020 and
2021 in commercial broccoli fields in
Broccoli screening trial in Minnesota. Photo: Charlie Rohwer
Georgia that the predominant species
associated is A. brassicicola.
Dutta's lab has developed a detached
leaf assay to assess aggressiveness
of Alternaria spp. in brassica
crops. One hundred percent of the
isolates collected from broccoli
were pathogenic on brassica hosts
(broccoli, kale, cabbage and collard)
but aggressiveness varied considerably.
Based on this assay, the majority
of the A. brassicicola isolates were
highly aggressive on broccoli and kale,
whereas the majority of isolates were
either moderately or less aggressive on
cabbage and collard.
Similar surveys were recently
conducted in Virginia (David Langston
and Steve Rideout) and New York
(Chris Smart and Christy Hoepting).
Field and laboratory evidence suggest
that Alternaria spp. has become
field-insensitive to the commonly
recommended fungicides.
The current recommendation is to
rotate fungicides from FRAC Group 7
(carboxamides), FRAC Group 3
(Demethylation-inhibiting) and FRAC
Group 9 (anilinopyrimidines), and
to use good cultural management
practices like crop rotation, drip
irrigation and weed management.
Dutta is leading a multi-state effort
to better understand the new strains
of Alternaria and develop updated
management recommendations. As
a part of a multi-state effort, we are
currently evaluating the importance of
different cultural practices (nitrogen
fertility, irrigation and varietal
selection) and inoculum sources
(seeds and weeds) for the Alternariaepidemic
in broccoli.
There is also a multi-state effort
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underway to screen broccoli for
tolerance to Alternaria and black rot,
as well as tolerance to changing climate
conditions. Dutta, Bjorkman, and
Hoidal (along with Charlie Rohwer
in Minnesota) are all conducting
screening trials for broccoli. Most of
the broccoli breeding in the U.S. is
concentrated within a few companies,
which can pose challenges for genetic
diversity. The multi-state group is
trialing common broccoli varieties
alongside landrace cultivars and
varieties from countries including
Italy,
Israel, Taiwan and the
Netherlands to screen for tolerance to
Alternaria and black rot.
A side benefit of this project
includes screening for varieties that
are tolerant to the various climates
across the country and to weather
extremes. This summer, the Eastern
U.S. experienced heavy rainfall
and flooding, whereas the Midwest
experienced the driest summer in
decades. These varied conditions
will allow us to learn about things
like bolting, brown bead and hollow
stem tolerance in addition to disease
tolerance. On the East Coast, Thomas
Björkman's team is breeding for
broccoli varieties that can withstand
warmer nighttime temperatures.
As if climate extremes and
increasingly pathogenic diseases were
not enough for growers, brassica
farmers across the country are also
dealing with insect pressure. Three
important insect problems include:
cabbage maggot, especially with the
banning of Lorsban, flea beetles, which
historically have caused damage in the
spring, but increasingly are damaging
plants throughout the summer (and
also can transmit black rot and
Alternaria), and the new invasive
Swede midge.
Susan Scheufele leads the Brassica
Pest Collaborative, a project funded by
Northeast-SARE that aims to develop
management strategies for these
insects, with a focus on strategies for
organic growers. The group is trialing
mulches, netting, conservation
biocontrol and insecticides, with
labor and time considerations. They
post regular updates about what
they're learning from these trials
on the UMASS Extension vegetable
program webpage.
Some highlights include: row covers
can provide 100% control of all three
pests, especially when used with plastic
mulch to keep weed pressure down,
and reflective silver mulch can reduce
flea beetle damage in some settings.
The OMRI-approved insecticide
Entrust SC was effective in reducing
flea beetle damage when repeated
applications were used, and also can
now be used to control cabbage root
maggot, with two applications made to
the soil coinciding with peak flight.
Harvanta, a new insecticide in
the diamide class (IRAC group 28),
effectively controlled flea beetles and
caterpillars for a period of about three
weeks. Insectary plantings with flowers
providing pollen and nectar to support
beneficial insects, can attract syrphid
flies and parasitic wasps to help reduce
populations of cabbage aphids in the
field. Of the flowers tested, Ammi
majus was most attractive to the
parasitic wasp Diaeretiella rapae while
sweet alyssum was most attractive to
syrphid flies whose larvae eat cabbage
aphids.
While broccoli production is facing
new challenges, researchers and
Extension programs across the country
are working hard to find solutions. As
we learn more, we'll continue to share
what we learn at grower conferences
and via our websites and newsletters.
Others contributing to this article
were Susan Scheufele, University of
Massachusetts Extension; Bhabesh Dutta,
University of Georgia; and Thomas
Björkman, Cornell University. VGN
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November 2021

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