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Contents...
Quadris Resistance in Gummy Stem
Blight
Curbit Out - Strategy In
Tomato Tolerance to Treflan and Tillam
Sandea for Cucumber & Cantaloupe
Expected
Tifton Plant Disease Clinic _________________________________________________________________________________________
Quadris
Resistance in Gummy Stem Blight Confirmed
David Langston
Extension Vegetable Pathologist - UGA
Gummy stem blight, caused by the fungus Didymella bryoniae
is the most widespread and destructive disease of watermelon in Georgia.
Although watermelon suffers the greatest losses from gummy stem blight,
severe epidemics are observed in cucumber and cantaloupe each year. Management
options for this disease are rotation, deep turning diseased tissue, avoiding
irrigating that prolongs leaf wetness, and preventive fungicide applications.
Of these management options, preventive fungicide applications is the
most effective. Fungicides labeled for control of gummy stem blight are
primarily ethylenebisdithiocarbamates (EBDCs such as Dithane, Maneb, Manzate,
Penncozeb, ect), chlorothalonil (Bravo, Echo, Equus), benomyl (Benlate),
thiophanate methyl (Topsin M), and just recently, azoxystrobin (Quadris),
a fungicide in the new strobilurin class of chemistry. Benomyl or thiophanate
methyl tank-mixed with EBDC=s and alternated with chlorothalonil products
had proven to offer good control of gummy stem blight until resistance
to the benzimidazoles (benomyl and thiophanate methyl) was observed in
the early 1990's. Chlorothalonil products have shown good efficacy on
gummy stem blight but are not used because they have been implicated in
causing phytotoxicity to mature watermelon rinds. Azoxystrobin was shown
to have excellent efficacy on gummy stem blight by several researchers
in the early 1990's and was granted Section 18 emergency exemption status
in Georgia in 1997 and 1998 specifically for gummy stem blight control.
A full Section 3 national label was granted for azoxystrobin use on the
cucurbit crop grouping in March of 1999 which led to the widespread and
routine use of the fungicide to control a broad spectrum of foliar cucurbit
diseases. However, reduced efficacy of azoxystrobin on gummy stem blight
has been noted in several grower fields and in watermelon fungicide trials
conducted in1999 and 2000 in Cordele, GA. In 2001, six applications of
azoxystrobin (Quadris 2.08SC at 12.3 fl/oz) resulted in a 14% increase
in gummy stem blight severity compared to plots not receiving fungicide
treatment. Reduced efficacy may be a result of possible fungicide resistance,
which has been observed in strobilurins in Europe.
Isolates of the pathogen collected in 2000 from watermelon fields in Delaware,
Maryland, and Georgia, where disease control was unsatisfactory, were
recently confirmed by Syngenta to be resistant to azoxystrobin. In 2001,
an extensive survey was conducted to determine the frequency of azoxystrobin-resistant
isolates in commercial watermelon fields in Georgia.
Isolates of the fungus were obtained from samples of infected watermelon
from 25 commercial watermelon fields and research sites in Georgia. Sensitivity
of each isolate to azoxystrobin was determined using a spore germination
assay on water agar (WA) medium amended with azoxystrobin and salicylhydroxamic
acid (SHAM) to inhibit an alternative respiratory pathway in the fungus
that can interfere with the activity of the fungicide. Conidia of each
isolate were transferred to fungicide-amended or unamended agar plates.
After 48 h of incubation at 23-25°C, the percentage of germinated
spores was recorded. Fungicide sensitivity was expressed as the ED50 value
(the fungicide concentration that inhibits spore germination by 50% relative
to the control). As reported by Syngenta, an isolate was considered resistant
to azoxystrobin if the ED50 value was >10 g ml-1.
Preliminary results from 8 of the 25 locations provide evidence of widespread
resistance in the gummy stem blight pathogen to azoxystrobin in Georgia.
Of the 65 isolates tested to date, 54 isolates (83%) were found to be
resistant to azoxystrobin based on the spore germination assay. Isolates
from the remaining 17 locations are currently being tested. Extensive
sampling for the upcoming 2002 season is planned.
Where did this problem originate and what will growers do now? It is very
difficult to determine at this time exactly where the azoxystrobin-resistant
isolates originated. However, overuse of the product both in the greenhouse
and in the field are the primary suspects. When a fungicide of this type
is used repeatedly, without rotating to fungicides with a different mode
of action, the chance of selecting for a fungicide-resistant population
of the target fungus is very great. Rotating to different fungicide chemistries
will hopefully control the resistant populations before they can reproduce
and spread. Growers should now look towards the more traditional fungicides
for protecting their watermelon crops. The mancozeb and chlorothalonil
products both suppress gummy stem blight to some degree. Mancozeb products
alone are usually marginally effective at best and chlorothalonil products
have been implicated with a rind burn when applied within two weeks of
harvest. However, chlorothalonil remains our most effective labeled material
for gummy stem blight suppression.
_________________________________________________________________________________________
Curbit Out - Strategy In
A. Stanley Culpepper
Extension Weed Science - UGA
The registrant of Curbit herbicide has decided to eliminate production
and sale of Curbit for all crops including cucumber, melons, squash, pumpkin,
and watermelon.
Strategy, a new herbicide by UAP, will attempt to replace the loss of
Curbit. Strategy is a mixture of ethalfluralin (active ingredient of Curbit)
and clomazone (active ingredient of Command) and will be labeled for use
in cucumber, melons including cantaloupe, squash, processing pumpkin,
and watermelon for the 2002 season.
Application rates of Strategy will differ from historical use rates of
Curbit. Table 1 allows growers to select the rate of Strategy needed to
provide the rate of Curbit used in past seasons. Strategy rates for most
Georgia soils should be between 2 and 3 pints per broadcast acre.
It is critical that users of Strategy read and follow labeled
directions. Several of the issues addressed on the Strategy label are
noted in Table 2; however, this table does not provide all of the recommendations
and restrictions found on the label.
________________________________________________________________________________________
Tomato Tolerance To Treflan and Tillam
A. Stanley Culpepper
Extension Weed Science - UGA
With the phase out of methyl bromide, many vegetable growers
are forced to search for alternative methods of weed control. In tomato,
several herbicide options including trifluralin (Treflan, others) and
pebulate (Tillam) are available to assist in weed control. Unfortunately,
growers have been concerned with tomato response to Tillam and Treflan
when applied under plastic mulch. Thus, studies were conducted to evaluate
tomato tolerance to these herbicides when applied broadcast and incorporated
with a tilrovator followed immediately by a super-bedder plastic layer.
Treflan stunted tomato 13, 25, and 50% when applied at 0.5, 1.0 and 1.5
pt/A, respectively, at early-season (Table 3). This plant stunting was
still noticeable at late-season when plant measurement heights were taken
from each plot. Treflan at 0.5 pt/A did not affect yield compared to the
non-treated control. However, delayed maturity and reduced tomato fruit
weights were noted with Treflan at 1.0 or 1.5 pt/A. Treflan at 1 and 1.5
pt/A reduced tomato yields by 39 to 53% at the first harvest and 23 to
63% for the total harvest.
Tillam stunted tomato 8, 16, and 30% at 0.75, 2.5, and 5 pt/A, respectively,
at early-season (Table 4). By late-season, only Tillam at 5 pt/A stunted
tomato greater than 10%. Tillam at 0.75 and 2.5 pt/A did not affect yield;
however, 5 pt/A reduced tomato weight yield 20% for the first harvest
and 20% for the total harvest.
When Treflan and Tillam were applied together, herbicide injury was nearly
additive (data not shown). For example, Treflan 1.0 pt/A stunted tomato
growth 25% at early season while Tillam 0.75 pt/A stunted growth 8%, thus
both herbicides applied together stunted tomato growth approximately 33%.
__________________________________________________________________________________________
Sandea
Label For Cucumber and Cantaloupe Expected Anytime
Stanley Culpepper
Extension Weed Science - UGA
A Section 24(c) label request for the use of Sandea herbicide in cucumber
and cantaloupe for Georgia producers has been submitted to the Georgia
Department of Agriculture. Hopefully, with the cooperative efforts of
Gowan Company, the University of Georgia, and the Georgia Department of
Agriculture, Sandea labels for cucumber and cantaloupe will be granted
within the next two weeks. Once I receive these labels, they will be forwarded
by e-mail to Extension Agents with vegetable responsibilities.
For crops labeled, I would strongly encourage the use of Sandea by Gowan
Company and not other products with the same active ingredient. If growers
choose to use other products with the same active ingredient as Sandea,
these labels and all future labels will likely be lost quickly.
At this time, I am unaware of how much a Sandea application
will cost.
__________________________________________________________________________________________
Tifton Plant Disease Clinic
Jason Brock
Plant Disease Diagnostician -UGA
Looking back over the past couple of years, I see that most
diseases diagnosed in Tifton have been soil-borne. When collecting or
examining a sample, be careful to include the roots and stem. Do not pull
a plant out of the ground or container. Instead, dig around the plant
and lift. Wash all soil from the roots and look for necrotic tissue that
might be a symptom of a disease. Please refer to page 12 of the Pest Control
Handbook for information on collecting and shipping samples.
The following is a summary of the commercial vegetable samples diagnosed
during January and February.
| Carrot: |
Environmental
|
| Collard: |
Downy Mildew |
| Cucumber: |
Potyvirus
No Disease
|
| Onion: |
Botrytis Leaf Blight
Stemphylium Leaf Blight
Chemical Phytotoxicity
|
| Watermelon: |
Gummy Stem Blight
Pythium Damping-Off
Chemical Phytotoxicity |
Georgia
Extension Vegetable News
| Volume 1, No.2. |
Jan-Feb 2002 |
| Editor |
David B. Langston, Jr. |
| Production Assistant & Webmaster |
Paul Sumner |
The Georgia Extension Vegetable News is published bimonthly for Jan-Feb,
Sept-Oct, and Nov-Dec and monthly March-Aug. This newsletter is also available
on the World Wide Web at www.cpes.peachnet.edu/veg
or www.ugaveg.org.
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