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Successful control of powdery mildew in cucurbit crops challenged by Evolving pathogen

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

May 2008

Additional evidence was obtained in 2007 of pathogen adaptation to fungicides and also to resistant varieties, the main management tools for cucurbit powdery mildew.  This is arguably the most important disease of cucurbit crops caused by a pathogen with demonstrated ability to overcome these management practices.

Fungicide resistance development has been a major concern with cucurbit powdery mildew for the past few years, and with good reason.  This pathogen has clearly shown ability to develop resistance.  Control failure with Benlate, Bayleton, and QoI fungicides was detected in university fungicide evaluations just 1, 2, and 3 years after these were registered in the US for this use.   Several fungicides are labeled for powdery mildew control, but due to resistance, Procure (FRAC code 3) and Pristine (FRAC codes 11 + 7) were the main fungicides recommended in 2007, with Quintec (FRAC code 13) also recommended for melons, the only labeled crop.

In 2007, there was great concern that the powdery mildew fungus might evolve further and render FRAC code 3 and 7 fungicides ineffective. Procure and Pristine were effective in some university fungicide evaluations in 2007, but, they were not always as effective as Quintec; also control was variable in some other experiments and commercial fields where these fungicides were used.  Fungicide efficacy experiments conducted at LIHREC (Cornell University research facility on Long Island, NY) have included sole use of fungicides at risk for resistance to assess their contribution to fungicide programs (this is not a labeled use pattern).  Control of powdery mildew on lower surfaces of pumpkin leaves with weekly spray intervals was 46%, 88%, and 52-58% with Pristine (14.5-18.5 oz/A) in 2005, 2006, and 2007, respectively. Control was 93%, 34%, and 75-78% with Procure (6-8 fl oz/A) in these years and 88% and 81% with Quintec in 2006 and 2007, respectively.  Fungicide sensitivity monitoring work in 2007 revealed the pathogen population was less sensitive to the active ingredients in Procure and Pristine than in Quintec. Strains were detected with an in-field seedling assay able to tolerate 150 ppm triflumizole (a.i. in Procure), 200 ppm boscalid (Pristine), and 10 ppm quinoxyfen (Quintec).

While attention has been focused on fungicide resistance, powdery mildew may also evolve to overcome host plant resistance, which has not occurred recently.  In 2007, there were indications the pathogen had evolved to overcome host plant resistance in melons and squashes. Cantaloupe varieties with resistance to race 1 and 2 of the pathogen have been providing excellent suppression of powdery mildew, plus with good horticultural characteristics, some are in widespread production.  However, there were reports of powdery mildew becoming severe on these varieties in some states in 2007.  Additionally, squash (Cucurbita pepo) varieties with one gene for resistance were more severely affected by powdery mildew than varieties with two resistance genes, in contrast with previous years. This difference in performance had been previously detected in pumpkin (also C. pepo).  All squash and pumpkin varieties marketed to date have one common main gene for resistance obtained from a wild cucurbit relative, plus there are additional modifying genes.  Hollar Seeds recently announced a new gene.

In 2008, fungicide resistance continues to be a concern. The recommended program for managing powdery mildew and fungicide resistance is:

1. Grow resistant varieties.  Select squash and pumpkin varieties with resistance from both parents when possible.  Cantaloupe varieties should have resistance to races 1 and 2.  See the ‘Resistant Variety’ section at http://vegetablemdonline.ppath.cornell.edu/.

2. Scout regularly and initiate fungicide applications at disease onset or before.  Powdery mildew typically starts to develop early in fruit production, therefore when first fruit appear is a good time to start applications.  The action threshold is 1 leaf with symptoms out of 50 older leaves examined.

3. Alternate among at-risk fungicides in different FRAC Groups. Procure and Pristine are recommended at highest label rates (8 and 18.5 oz/A).  Quintec remains only labeled for use on melons.  Additional crops are anticipated to be labeled in 2009.  Quintec and Procure have narrow spectrum activity.  Therefore it is important to monitor crops for other diseases.

4. Tank mix fungicides at-risk for resistance with protectant (contact) fungicides (e.g sulfur, chlorothalonil, and oils).  Melons are sensitive to sulfur; there are tolerant varieties.

5. Maintain a regular (7-day) application schedule.  When maintaining this schedule through the season for at-risk fungicides is not economical, use protectant fungicides alone late in the season rather than compromising application timing early in the season to save money.  The powdery mildew pathogen does not require leaf wetness for infection as other fungal foliar pathogens do, therefore fungicides are needed under dry conditions.

6. Rate control achieved based on powdery mildew severity on lower surfaces of leaves.  Report poor control despite following these guidelines to a local extension specialist.

Resistance to QoI fungicides (FRAC Group 11) and resistance to MBC fungicides (FRAC Group 1) were common again in 2007, therefore Amistar, Flint, Cabrio, Quadris, Topsin M and other fungicides in these groups are no longer recommended.

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.