Downy Mildew in Cucurbits: Occurrence of Qoi Resistance in the 
USA and Impact on Managing Disease
==============================================================

Strains of the cucurbit downy mildew fungus resistant to QoI (also known 
as strobilurin) fungicides were detected in GA and NC in fall 2004 and in 
FL in spring 2005.  The genetic mutation detected is the same as that in 
QoI-resistant cucurbit powdery mildew fungal strains.  These strains 
exhibit qualitative resistance, thus resistant strains are highly 
insensitive to QoIs.  It is not possible to re-gain control by increasing 
the application rate or shortening the interval with this type of 
resistance.

Development of resistance was not surprising.  The cucurbit downy 
mildew pathogen has developed resistance to other fungicides.  Other 
pathogens have developed resistance to QoIs thus demonstrating that 
this fungicide group is at high risk for resistance development.  
Conditions were very favorable for downy mildew in 2004.
Effect of QoI resistance on managing downy mildew in 2005 cannot be 
predicted.  Only 4 samples have been examined, thus the frequency and 
distribution of resistant strains is not known.  In fungicide efficacy 
experiments being conducted in GA and NC where 2 of the tested 
pathogen isolates were collected, QoI fungicides were not as effective 
as expected based on previous results.  However, this could have been 
due to high disease pressure resulting from downy mildew starting to 
develop earlier than usual, and conditions being very favorable for 
disease development. 

Fortunately, there are additional management practices for cucurbit 
downy mildew control.  These include making adjustments to fungicide 
programs (Item 4 below), and attention to other control methods as 
follows:

1. Select cucumber and melons varieties with resistance to downy 
mildew when possible.  
2. Minimize leaf wetness by selecting sites with good air movement 
and without shading.
3. Avoid overhead irrigation during early morning when leaves are wet 
from dew, and during late evening when leaves will not have an 
opportunity to dry before dew forms.  
4.  Non-QoI fungicides labeled for this disease must be combined and 
alternated with QoI fungicides (a) to reduce selection of resistant 
strains and (b) to protect against loss if resistance does occur and 
affect efficacy of the QoI fungicides.  

It is critical to know what fungicides contain an active ingredient in 
the QoI fungicide group, which is Group 11 in the system being used by 
EPA and the Ag Chemical industry.  Some of the products, plus their 
active ingredient(s) and Group number in parentheses, that are 
registered for use on cucurbits include the following:

Amistar (azoxystrobin; Group 11), 
Cabrio (pyraclostrobin; Group 11), 
Flint (trifloxystrobin; Group 11), 
Reason (fenamidone; Group 11), 
Pristine (pyraclostobin; Group 11 + boscalid; Group 7), and 
Tanos (famoxadone; Group 11 + cymoxanil; Group 27).  

Non-QoI fungicides that could be tank-mixed with QoI fungicides (Group 11) 
and applied in alternation with QoI fungicides are:

1.  Translaminar fungicides with some ability to enter and/or move in 
leaves:
Curzate (cymoxanil; Group 27), 
Acrobat (dimethomorph; Group 15), 
Previcur Flex (propamocarb; Group 28), 
Gavel (zoxamide; Group 22, and mancozeb; Group M2), 
phosphorus acid fungicides (Phostrol, ProPhyt, and Fosphite; Group 33), 
Ridomil fungicides (mefenoxam; Group 4)

2.  Protectant fungicides which do not enter leaves:
Bravo (chlorothalonil; Group M4), 
Maneb (maneb; Group M2), 
Dithane (mancozeb; Group M2), and 
copper fungicides (Group M1). 

Curzate reportedly has good curative activity (about 3 day kickback), 
thus it is a good choice for the first application after downy mildew is 
detected. However, it has poor residual activity (only 1-2 days), thus it 
is critical to tank-mix it with a protectant fungicide and to follow-up 
with another systemic fungicide when disease pressure is high. 

Furthermore, although downy mildew can be very destructive, luckily it 
occurs sporadically in the northeastern US, and usually does not affect
 yield because downy mildew develops so late in the season.  
Development of downy mildew in the eastern US plus forecasts for new 
outbreaks that include the northeast region are posted at a North 
Carolina State University web site:
www.ces.ncsu.edu/depts/pp/cucurbit/

See the following web sites for more information on downy mildew and 
its management plus photographs of symptoms: 

vegetablemdonline.ppath.cornell.edu/NewsArticles/Cuc_Downy.htm
vegdis.cas.psu.edu/VegDiseases/Identification_files/cucurbits.html
vegdis.cas.psu.edu/03Diseases/D400_Farm.html

Please Note: The specific directions on fungicide labels must be adhered 
to -- they supersede these recommendations, if there is a conflict.  Any 
reference to commercial products, trade or brand names is for 
information only; no endorsement is intended.

This article was prepared by Meg McGrath, Department of Plant Pathology, 
Cornell University; Tim Elkner and Alan MacNab, Penn State University 
made minor changes for use in PA.

-------------------------------------------------------------
A. A. MacNab,
Plant Pathology Department, The Pennsylvania State University.
June 6, 2005

Where trade names are used no discrimination is intended, and no endorsement by the Cooperative Extension Service is implied.

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