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Guidelines  for  Managing  Cucurbit  Powdery Mildew  in  2006

Margaret Tuttle McGrath

Department of Plant Pathology, Cornell University

Long Island Horticultural Research and Extension Center

3059 Sound Avenue, Riverhead, NY 11901; mtm3@cornell.edu

For current managment recommendations see Powdery Mildew Management Guidelines for 2010

See also:

Successful control of powdery mildew in cucurbit crops challenged by Evolving pathogen (May 2008)

Section 18 specific exemption approved for Quintec to manage cucurbit powdery mildew in NY in 2006

Registration of Procure in NY provides growers with valuable tool to manage cucurbit powdery mildew

Protectant fungicides for managing powdery mildew in cucurbits: how do they stack up?

Guidelines for managing cucurbit powdery mildew with fungicides in 2005

Occurrence of strobilurin resistance and impact on managing powdery mildew of cucurbits

For current managment recommendations see Powdery Mildew Management Guidelines for 2010


For current managment recommendations see Powdery Mildew Management Guidelines for 2010

Guidelines  for  Managing  Cucurbit  Powdery Mildew  in  2006

There are new varieties with resistance to powdery mildew and several fungicides for managing this the most important cucurbit disease occurring every year where these crops are grown.  Fungicides and resistant varieties are the only management tools.  In NY, Procure was recently registered for this use and Quintec was granted a specific exemption under Section 18 of FIFRA. Below are recommendations for 2006 followed by information about these, then separate articles on Procure and Quintec.

  1. Select varieties with resistance when possible.  Lists of resistant varieties are at http://vegetablemdonline.ppath.cornell.edu/Tables/TableList.htm.
  2. Start fungicide applications when symptoms are just beginning to develop (IPM) or when fruit are starting to enlarge (preventive).  The IPM action threshold is 1 of 50 older leaves with symptoms.  Look at the lower as well as upper surface of leaves when scouting. Use a 14-day interval with resistant varieties; 7-day with other varieties.

Preventive (optional). Protectant alone before symptoms develop.

Week 1. Quintec (FRAC Group 13) plus protectant for non-edible peel cucurbits.

Week 2. Procure or Nova (FRAC Group 3) at high label rate plus protectant.

Week 3. Pristine (combination of FRAC Group 7 and 11) plus protectant.

For edible peel cucurbits (summer squash and cucumber), and in states where Quintec is not available, use a two-way alternation without Quintec.

Continue alternating for as long as good powdery mildew control is needed in the crop.

Use protectant alone if powdery mildew is not being controlled well on the lower leaf surface and near the end of the season when level of control needed does not justify the expense of including a mobile fungicide.

The fungicide program recommended for 2006 was selected to manage fungicide resistance as well as ensure effective control especially if resistant strains able to tolerate the fungicide rates used are present.  Managing resistance is a critical aspect of effectively controlling cucurbit powdery mildew because this pathogen has a history of control failure due to developing resistance.  Tactics need to be implemented beginning when a new product is first used because the main goal of resistance management is delaying its development rather than managing resistant strains once present.  General tactics are alternating among products at risk for resistance, which are the mobile fungicides due to their single-site mode of action, and tank-mixing with protectant fungicides which have low resistance risk.  Mobile fungicides are needed to control powdery mildew on the lower leaf surface.

Scout weekly for symptoms by examining both surfaces of at least 50 older leaves beginning at fruit set and continuing until powdery mildew is detected.  Then immediately begin applying fungicides on a 7-day schedule (14-day with resistant varieties).  A critical component of a fungicide resistance management program is not applying at-risk fungicides when disease is established (eradicant or curative application); therefore, where it is not feasible to implement a scouting program, use a preventive application schedule.  Evaluate control during the growing season, especially on lower leaf surfaces.  Report poor control promptly to extension specialists so the possibility of fungicide resistance being the cause can be investigated. Stop using mobile fungicides if powdery mildew is becoming severe; use only protectant fungicides.  Applying fungicides at-risk for resistance when disease is severe creates a lot of pressure for selecting resistant strains.  While spores of the powdery mildew fungus overwintering in soil in the northeast are generally considered an inconsequential source of inoculum for epidemics the following year given the quantity of wind-dispersed spores from southern production areas, just a few spores of a new resistant strain could be very important.

Quintec, Procure and Nova have narrow spectrum activity.  Therefore it is critical to monitor crops for diseases other than powdery mildew.  When other diseases are a concern, use broad-spectrum protectant fungicides like chlorothalonil and copper rather than sulfur or oil.  Pristine contains a QoI (strobilurin) fungicide and thus has broader activity than the other mobile fungicides for powdery mildew.  Quintec is available for use in NY under an emergency exemption (Section 18).  Users must have a copy of this label when they use this product, which can be obtained from local distributors and extension office as well as at http://pmep.cce.cornell.edu/regulation/sec18/2006/index.html.

Approximate cost of the fungicides in this program are $16.89 for Quintec at 4 oz/A, $17.63-$23.50 for Procure at 6-8 oz/A, $21.53 for Nova at 5 oz/A, and $30.73-45.47 for Pristine at 12.5-18.5 oz/A.

Three-way alternation programs with Quintec, Pristine and Nova started after reaching the IPM threshold with a total of 5 weekly applications applied to pumpkin provided excellent control of powdery mildew on upper leaf surfaces (96-97%) and lower (71-74%) when evaluated in 2005 in Riverhead.  The protectant included each week was sulfur (4 lb/A Microthiol Disperss).  The QoI component of Pristine did not appear to be contributing to powdery mildew control because Pristine tested alone was not more effective than Endura, a formulated product not registered for use on cucurbits with the active ingredient boscalid, which is the other active ingredient in Pristine.  For another treatment sulfur was applied twice on a preventive schedule before initiating the fungicide program at the IPM threshold.  These two applications did not delay when threshold was reached which was one objective of this treatment.  Control achieved with this treatment was numerically better (98% and 87% control) but not significantly better than the similar treatment without preventive sprays of sulfur.  Greater benefit may be achieved when an entire planting is treated. 

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.

SEE ALSO: Successful control of powdery mildew in cucurbit crops challenged by Evolving pathogen (May 2008)

For current managment recommendations see Powdery Mildew Management Guidelines for 2010