Group Title: Berry/vegetable times.
Title: Berry/vegetable times. July 2002.
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Title: Berry/vegetable times. July 2002.
Uniform Title: Berry/vegetable times.
Physical Description: Newspaper
Creator: Gulf Coast Research and Education Center, Institute of Food and Agricultural Sciences, University of Florida
Publisher: Gulf Coast Research and Education Center, University of Florida
Gulf Coast Research and Education Center
Publication Date: July 2002
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Volume ID: VID00007
Source Institution: University of Florida
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Berry Times

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Fungicide resistance: How to prevent the
development of fungicide resistance in
strawberry pathogens Dan Legard

In the past, fungicides like Benlate lost their
effectiveness in controlling disease because the target
pathogens developed resistance to them. During the last two
seasons we have seen signs that resistance may be
developing towards some newer fungicides. In recent trials
at the University of Florida-GCREC we have observed an
apparent reduction in efficacy for Nova and Qtudlii in the
control of powdery mildew and anthracnose fruit rot,
respectively. If this reduction in control is due to the
development of resistant pathogens then this would seriously
limit the ability of growers to control these diseases.

application of
fungicides at

Fungicides are the most important tool growers
have to manage disease in annual strawberry. And like other
tools, it is important to understand how to use fungicides
properly so that they can continue to control disease. Other
pests like insects, mites, and to a certain extent weeds, can
often be effectively controlled with pesticides after they are
observed in a field. Effective control of disease on the other
hand, relies on preventing the start of epidemics and with
strawberry this involves the regular application of fungicides.

A monthly newsletter of the University of Florida Institute of
Food and Agricultural Sciences, Gulf Coast Research and
Education Center, and Florida Cooperative Extension Service.
Gulf Coast Research and Education Center
13138 Lewis Gallagher Road, Dover, FL 33527
(813) 744-6630 SC512-1160
Website: http//
Editors: Dan Legard ( & Craig Chandler
(ckc(; Design, Layout & Distribution: Christine Manley
(; Director: Jack Rechciql

However, the regular use of fungicides can lead to
the development of pathogens resistant to fungicides.
Fungicide resistance is the result of a genetic mutation within
the target pathogen that causes it to have a reduced
sensitivity to one or more fungicides. These mutations
typically occur at very low frequencies in the pathogen
population and cause resistance by 1) reducing the uptake of
the fungicide by the fungus, 2) enabling the fungus to
detoxify it or by 3) altering the target site for the fungicide.
When there are resistant individuals in a population, use of a
fungicide will selectively inhibit the sensitive strains
allowing resistant strains to increase and, if not properly
managed, ultimately produce a population of pathogens
resistant to the fungicide
Fungicides can be grouped by mode of action and
by similarities in their chemical structure. I have listed some
fungicides that have been used on strawberry grouped based
on their chemical structure (fungicide class) in Table 1.
Some fungicides are systemic and typically site-specific.
That is they control fungi by disrupting a single metabolic
process or structural function such as cell division, sterol
synthesis, nucleic acid synthesis, etc. Fungicides like
benomyl (Benlate), iprodione (Rovral), and mycobutanil
(Nova) are site specific and a single mutation at the target
site can enable strains of a fungus to become resistant. Other
fungicides like captain and sulfur are mainly protectant (i.e.
they do not get translocated within the plant like systemic
fungicides) and control fungi by interfering with many
metabolic processes. Because they attack multiple target
sites in the fungus it is rare or almost impossible for
resistance to develop.
Intensive or exclusive use of systemic or single-site
fungicides often leads to the development of resistance
problems. Repeated application of a fungicide increases the
selection pressure on the pathogen population. It is also
important to use fungicides at rates that are adequate to
control the disease and reduce the reproduction of the
pathogen. Applying systemic or single target site fungicides
at below labeled rates can also lead to the development of
resistant pathogens.
Production practices that favor increased disease
pressure also increase the likelihood of fungicide resistance.
Using excessive nitrogen and/or tight plant spacing may
result in a dense plant canopy that prevents good coverage
with fungicides or inhibits the ability of harvesters to remove
diseased fruit from the plant, resulting in more disease. The
increase in disease results in the need for additional
applications of fungicide which favors the development of

Berry Times 1


The best way to manage fungicide resistance is
to prevent it from developing. Strategies for managing
fungicide resistance should be based on each fungicide /
pathogen combination and should integrate cultural
practices with proper fungicide use to minimize the
selection for resistant pathogens. Cultural methods such
as using certified disease-free transplants, minimizing
overhead irrigation, using resistant cultivars and proper
row spacing, and avoiding harvest when plants are wet,
reduce disease, reduce the need for fungicides and
ultimately reduce selection of pathogens resistant to
Cautious and judicious use of fungicides is
required on a crop with serious disease control problems
like strawberry. Growers should always consider either
tank mixing or alternating the application of systemic or
single-target site fungicides that have a moderate to high
risk of resistance developing with protectant fungicides
that have multiple target sites. Growers should also
avoid rotating fungicides in the same class (see Table 1)
as they typically have the same target-site and cross
resistance among fungicides can develop.
Basics for resistance management: 1) Use the best
cultural practices available to minimize disease. 2)
Limit the number of applications of high-risk / single-

site fungicides in a single season. Do not apply for a
full season. 3) Alternate or tank-mix high-risk / single
site fungicides with a low-risk / protective fungicide.
4) Apply fungicides at adequate rates (do not apply
below minimum labeled rate). 5) Apply fungicides as a
preventative whenever possible.
Fungicide resistance management for
specific diseases: Anthracnose- Regular applications
of captain throughout the season. Use lower rates early
in the season before disease is seen and the top labeled
rate later in the season when disease pressure is highest.
Add applications of QudiliS at full labeled rate on a
14 day schedule during the late season or when the first
symptoms of disease are observed. Powdery mildew-
Alternate applications of sulfur with Nova on a 7 day
schedule during the early season when conditions
typically favor powdery mildew epidemics. Botrvtis
fruit rot- Apply captain or thiram at lower labeled rates
during the early season when disease pressure is low
and at full labeled rate during mid to late season when
disease pressure is high. During the peak bloom period
that typically begins in mid to late January, alternate
applications of Elevate and Switch every 7 days for 4
to 6 weeks or until the end of February.

Table 1. Characteristics of several fungicides commonly used on strawberry and their relative risk for
developing resistance problems.

Fungicide class Trade name Common name Protectant or systemic Resistance risk

Benzimidizoles Benlate* benomyl systemic high
Topsin M thiophanate-methyl systemic high

Dicarboximides Rovral iprodione systemic moderate

Phenylamides Ridomil Gold mefenoxam systemic high

Sterol inhibitors Nova mycobutanil systemic moderate

Phthalimides Captan, Captec captain protectant low

Strobilurins Quadris azoxystrobin locally systemic moderate high

Hydroxy-anilides Elevate fenhexamid locally systemic moderate (?)

Anilinopyrimidine Switch ** cyprodinil systemic high
+Phenylpyrrole fludioxonil protectant low

Inorganics Kocide copper hydroxide protectant low
Sulfur sulfur protectant low

Other Aliette forsetyl-aluminum systemic low

Benlate is no longer labeled for use on strawberry or other crops.
SSwitch is composed of two active ingredients, a systemic component (cyprodinil) and a protectant (fludioxonil).

Berry Times 2

Strawberries in France -Craig Chandler

In June, I visited some commercial strawberry farms
in the Aquitaine, Limousin, and Loire Valley regions of
France. These areas are as far north as Nova Scotia, but their
winter climate is not as severe because of the influence of the
Gulf Stream, which has a warming effect on all of Western
Europe. Commercial strawberry growers in France use
predominantly an annual protected culture production system
with fruit grown in either greenhouses or large plastic
tunnels. These structures improve early fruit production and
protect the crop from freezes and rain.
'Gariguette' and 'Darselect' are probably the main
short-day cultivars used in central France, although 1.9
million plants of 'Sweet Charlie' were also grown there last
year. Most of the transplants used in France are bare-root
frigo plants. Frigo plants are plants that have been dug from
the nursery after going dormant (between December and
February) and held in cold storage at 28 F (-2 C) until
transplanted. In France, frigo plants are typically set in mid
July. Such plantings may produce some fruit in the fall, but
the main production is from April June the next year). Day
neutral cultivars, such as 'Mara des Bois', 'Seascape', and
'Diamante', are planted in March for late spring, summer,
and fall production.

Raised beds
in large

Traditionally, French growers use raised (ground)
beds covered with black polyethylene mulch. Their tunnels
will cover three or four double-row beds, depending on the
size of the tunnel. Some growers are now using a tabletop
hydroponic system because it eliminates the need for soil
fumigation, and is easier to harvest. According to Frederic
Angier, of Angier International (nursery), a picker can

harvest 88 pounds (40 kg) of fruit per hour from the tabletop
system, while in the raised bed system a picker only averages
55 pounds (25 kg) per hour. Plant densities in the tabletop
system are very high (10 plants pcli in) compared to
plantings in west central Florida (approx. 4 plants per m2).
For an expanded article on strawberries in France, please
visit our web site at

Cover crops in Strawberry John R. Duval

As summer arrives, few crops are grown on farms
that will be planted with strawberries this fall. The use of
cover crops on unused land improves soil quality, suppresses
weed growth, and prevents erosion from heavy summer
rains. The ideal cover crop is one that germinates and grows
quickly to suppress weeds, produces abundant biomass to
improve soil structure, and does not provide a good host for
sting nematode. By quickly forming a dense canopy, cover
crops can shade out weeds and reducing the amount of
herbicide and tillage needed for a field. Green manure crops
will increase soil organic matter which improves the soil's
nutrient and water holding capacity, drainage, and increases
soil aggregation, providing a better substrate for subsequent
crops. Increased soil organic matter promotes the growth of
beneficial bacteria and fungi which help suppress pathogenic
soil microbes. Leguminous cover crops (such as Iron Clay
pea) can provide a source of slow release nitrogen (as the
plant decomposes), and up to 100 lbs. of nitrogen can be
gained per acre. Hairy Indigo (Indigofera hirsuta) can
provide sting and root-knot nematode suppression. Cover
crops should be thoroughly incorporated into the soil several
weeks before the field is to be fumigated and planted to
allow decomposition of large pieces of plant material that
may hamper fumigation and mulch applications. As the
methyl bromide phase out continues, the selection of cover
crops to help minimize soil related problems and improve the
soil environment will become an important BMP (best
management practice) for strawberry field management.

system in
large tunnel

Cover crop planted in strawberry field

Berry Times 3

Center Update Christine Manley

Renovations continue at GCREC with the installation of three new growth chambers and completion of plans for a new
climate controlled greenhouse. Our farm crew finished removing the old irrigation system and we will begin installing a new
computerized irrigation system this summer. We are currently finishing the installation of new air handling systems and other
renovations in the lab and office buildings.

Laser leveling
began June 28

AgriTech 2002 is scheduled for August 28 and 29 at the Arthur Boring Building in Plant City. The program will provide
an opportunity to gain core CEU credits for those who qualify. On Tuesday, August 28 the University of Florida, IFAS will be
presenting several talks from variety updates to nematode management. In addition, GCREC will have educational booths on
entomology, weeds, plant nutrition, and diseases. If you would like more information regarding AgriTech call Florida Strawberry
Growers Association at (813) 752-6822 or visit their website at

The use of trade names in this publication is solely for the purpose ofproviding specific information. It is not a guarantee or warranty of the products named,

and does not signify that they are approved to the exclusion of others of suitable composition. Use pesticides safely. ,, directions on the

manufacturer's label.

The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational

information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin.

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