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Managing Cucurbit Powdery Mildew

Margaret Tuttle McGrath

Plant Pathology and Plant-Microbe Biology Section, SIPS,

Cornell University, Long Island Horticultural Research and Extension Center

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

January 2019

Up-dated fungicide recommendations for NY and for elsewhere in the Northeast.  

Recommendations with supporting research results  for NY and for elsewhere in the Northeast

Table of fungicides, resistance status, and use guidelines

Up-dated list of biopesticides for organic production

View additional photographs of symptoms at the Long Island Vegetable Pathology website

Vegetable varieties resistant to powdery mildew and other diseases

Additional information about powdery mildew

Effectively managing powdery mildew is essential for producing a high-quality cucurbit crop.  This foliar, fungal disease is common wherever cucurbits are grown, including in the northeastern U.S.  This is because the pathogen produces an abundance of asexual spores (the powdery growth) easily dispersed by wind, thus it can spread widely, and the pathogen can produce a sexual spore in fall that enables it to survive over winter.  Leaves affected by powdery mildew die prematurely which results in fewer fruit and/or fruit of low quality (poor flavor, sunscald, poor storability).

Powdery mildew is managed with resistant varieties and fungicides.  An integrated program with both management tools is the best approach for achieving effective control because the pathogen is adept at evolving new strains resistant to individual tools such as resistant varieties or a specific fungicide.  It is more difficult for new pathogen strains to develop when an integrated program is used, and effective control is more likely.  Powdery mildew management program often needs adjustments as the pathogen and management tools change.

Resistant varieties are now available in most crop groups with new varieties released most years.  Resistance in cucumber is standard in modern varieties and is so strong it is easy to forget this cucurbit type is susceptible until an Heirloom type is grown.  Resistance in other cucurbit crop types is not adequate used alone (without fungicide applications) to prevent impact of powdery mildew on yield.  Melon varieties with resistance to pathogen races 1 and 2 have exhibited very good suppression in experiments conducted at LIHREC until recently.  Squash and pumpkin exhibit a moderate degree of resistance.  Select varieties with resistance from both parents (homozygous resistance) when possible.  This term is used in a few catalogues (for example Outstanding Seeds) whereas others use terms like ‘high resistance’ and ‘intermediate resistance’ or ‘tolerance’ to generally refer to homozygous and heterozygous resistance, respectively.  Degree of disease suppression obtained with a variety also depends on modifying genes present.  Plant breeders are actively searching for new sources of resistance to powdery mildew.

Fungicide program.  The most important component of an effective management program is an effective fungicide program.  And the key to that is using mobile fungicides targeted to powdery mildew.  Mobile fungicides are needed for control on the underside of leaves.  Because these fungicides have targeted activity, additional fungicides must be added to the program when there is a need to manage other diseases such as downy mildew and Phytophthora blight.
Alternate among targeted, mobile fungicides and apply them with a protectant fungicide to manage resistance development and avoid control failure if resistance occurs, and also to comply with label use restrictions (most mobile fungicides are not permitted used exclusively).  The powdery mildew pathogen has a long history of developing resistance to fungicides (it was the first occurrence of resistance in the USA), thus a diversified fungicide program applied to resistant varieties when possible is critical for success.  Always implement a resistance management program; do not wait until there is a problem.  The goal is to delay development of resistance, not manage resistant strains afterwards.

When to apply fungicides. The action threshold for starting applications is one leaf with symptoms out of 50 older leaves examined.  Examine both surfaces of leaves.  Starting treatment after this point will compromise control and promotes resistance development.  Powdery mildew usually begins to develop around the start of fruit production. Protectant fungicides applied before detection will slow initial development.  After detection, continue applying fungicides weekly.  Conditions are favorable for powdery mildew throughout the growing season.

Recommended targeted fungicides. Alternate among targeted, mobile fungicides in the following four chemical groups (principally the first three), and tank mix with a protectant fungicide (see below) to manage resistance development and avoid control failure if resistance occurs, and also to comply with label use restrictions. The first product listed below is the most important one to have in a fungicide program. Note that fungicide recommendations for powdery mildew change frequently as resistance develops to additional chemistry and new fungicides are registered, therefore it is important to obtain current recommendations. See Mobile Fungicides for Mildews and Phytophthora Blight for more information about these and other targeted fungicides. Federal pesticide labels can be viewed and downloaded at: http://www.cdms.net/labelsmsds/lmdefault.aspx. New York state labels are available at: http://www.dec.ny.gov/nyspad/products?0.

Vivando (FRAC Code 50) is the only fungicide with this mode of action. It has exhibited excellent control in fungicide evaluations. Activity is limited to powdery mildew. Vivando re-distributes through volatility. It is recommended used with a silicon adjuvant. Do not mix with horticultural oils. It can be applied three times per year with no more than two consecutive applications. REI is 12 hr. PHI is 0 days. 365 day plant back restriction for non-labeled crops.

DMI fungicides (FRAC Code 3) include Proline, Procure, Luna Experience (not labeled for use on Long Island), Rally, and Inspire Super. Additional products are registered for use outside NY. Resistance is quantitative. Highest label rate is recommended because the pathogen has become less sensitive to this chemistry. Efficacy has varied in fungicide evaluations. Proline is thought to have the greatest inherent activity and Inspire Super the least. Procure applied at its highest label rate provides a higher dose of active ingredient than the other Code 3 fungicides. Five applications can be made at this rate. REI is 12 hr. PHI is 0 days, 7 days for Proline and Inspire Super. Powdery mildew is the only labeled cucurbit disease for Procure and Rally. Proline is also labeled for Fusarium blight and gummy stem blight. Inspire Super, which contains another active ingredient (Code 9), is also labeled for Alternaria blight, anthracnose, gummy stem blight, Plectosporium blight, and Septoria leaf spot. Luna Experience, which contains another active ingredient (Code 7), is also labeled for Alternaria leaf spot, anthracnose, gummy stem blight, and belly rot.

Quintec (FRAC Code 13) has been consistently effective in fungicide evaluations. However, insensitivity to a high concentration of Quintec (similar to the dose when applied in the field) was first detected in several of the pathogen isolates collected from fungicide-treated research and commercial fields at the end of the 2015 growing season on Long Island. Therefore Quintec is now recommended used less than the label permits, which is a crop maximum of four applications. Aerial applications are not permitted and no more than two consecutive applications. Activity is limited to powdery mildew. It is the only mobile fungicide that does not move into leaves: it redistributes to foliage where spray was not directly deposited, including the underside of leaves, through diffusion and a continual process of absorption and desorption in the cuticular waxes of foliage. REI is 12 hr. PHI is 3 days.

Recommended used sparingly if at all:

Carboxamide fungicides (FRAC Code 7) registered in NY are Luna fungicides*, Endura, Pristine and Merivon* (* not labeled for use on Long Island). Last two also contain the same QoI fungicide (Code 11), which is no longer effective for powdery mildew. Resistance to boscalid, the FRAC Code 7 active ingredient in Endura and Pristine has been detected routinely on Long Island since 2009 and likely is the reason their efficacy has varied in fungicide evaluations. Full cross resistance was documented between several carboxamides, including those in Pristine, Merivon and also Fontelis (not registered for use in NY), but not Luna fungicides, through laboratory assays conducted with pathogen isolates resistant and sensitive to boscalid. However, Luna Sensation (FRAC Code 7 and 11) exhibited poor control in a fungicide evaluation conducted in 2017 on Long Island. Luna Experience (FRAC Code 7 and 3) is recommended for powdery mildew in upstate NY primarily when the chemistry is needed for managing other diseases.

Torino (FRAC Code U6) exhibited excellent control in fungicide evaluations until recently. It failed in an experiment in North Carolina in 2016 and on Long Island in 2017. Resistance has not yet been confirmed, but is known to have developed quickly elsewhere (e.g. Japan). Activity is limited to powdery mildew. It can only be applied twice to a field in a 12-mo period. Consecutive applications are not recommended. REI is 4 hr. PHI is 0 days.

Aprovia Top (FRAC Code 3 and 7) is recommended for other labeled diseases, which include anthracnose, Alternaria leaf blight, gummy stem blight, and Plectosporium blight. It is expected to provide some control of powdery mildew. but there are other FRAC 3 fungicides with greater intrinsic activity for powdery mildew that are better choices when this is the only disease developing, This is a new fungicide registered in NY in December 2017.

Fungicide evaluations conducted each year on pumpkin at LIHREC on Long Island include fungicides at risk for resistance tested alone (this is neither a labeled nor recommended commercial use pattern for these fungicides; it is done in efficacy evaluations to determine if resistance affects control). In 2018 Vivando was most effective, in particular for controlling powdery mildew on the lower surface of leaves where the pathogen develops best. This was the only treatment still controlling powdery mildew on lower leaf surfaces at 2 weeks after the last application. Quintec was second most effective: powdery mildew was numerically but not significantly more severe on plants treated with Quintec than Vivando on most assessment dates and based on AUDPC, which is a summation measure of severity for all assessments. Control was achieved with the other treatments tested in 2018, which were Luna Sensation, Gatten (new fungicide), and several fungicide programs including alternation of Vivando, Quintec, and Procure. In 2017 Torino and Pristine were ineffective, Luna Sensation provided limited control, and Vivando was most effective albeit not significantly better than Quintec or Procure. In 2016 Quintec and Procure were as effective as an alternation program while Pristine was substantially less effective. In 2015 Quintec, Pristine, and Vivando were as effective as an alternation program (69-78% control on lower leaf surfaces). Quintec and Vivando were the most effective of the targeted fungicides evaluated in 2014 (96 and 98% control); Pristine was moderately effective (54%); Procure was slightly but not significantly better (70%). In 2013 Quintec, Pristine, and Procure provided excellent control (93-99% control). In 2012 Pristine and Fontelis (FRAC Code 7) were ineffective (albeit treated pumpkins were numerically less severely affected by powdery mildew than the non-treated plots) while Quintec was very effective (96%) and Procure was moderately effective (57%). These experiments have documented year-to-year variation in the pathogen population. See also table of results from these and previous experiments at LIHREC and experiments at other universities.

No longer recommended. Resistant pathogen strains are sufficiently common to render the following fungicides ineffective: Topsin M (FRAC Code 1; MBC fungicide) and QoI fungicides (Code 11), which include Quadris, Cabrio and Flint. Resistant strains continue to be detected very commonly every year on Long Island where monitoring is being conducted. Recommended protectant fungicides. Many fungicides have contact activity for powdery mildew; mancozeb is an exception. They include chlorothalonil, sulfur, copper, mineral oil, and several biopesticides. Many of these products are approved for organic production (see list below). Sulfur is one of the most effective and least expensive products. Its activity is limited to powdery mildew, thus it is especially useful early in disease development when other diseases are not a concern, including as a preventive application. Microencapsulated formulations are recommended. Melons are sensitive to sulfur especially when hot; there are tolerant varieties.

Organic fungicides. Products labeled for cucurbit powdery mildew, in addition to several formulations of copper and sulfur, include:
Actinovate AG. 0.0371% Streptomyces lydicus strain WYEC 108. For best results with applications to foliage, label indicates to use a non-ionic spreader-sticker. OMRI-listed.

BacStop. 2.0% thyme, 2.0% clove & clove oil, 1.5% cinnamon, 1.0% peppermint & peppermint oil, and 1.0% garlic oil. Recommended used with EF400. Exempt from EPA registration.

Companion. 0.03% Bacillus subtilis strain GB03. EPA Reg. No. 71065-3.

Double Nickel 55 LC and WDG. Bacillus amyloliquefacinens strain D747, 98.8% and 25%, respectively. OMRI-listed. EPA Reg No. 70051-107 and 108, respectively.

EF400. 8.2% clove, 8.1% rosemary, and 6.7% peppermint. Exempt from EPA registration. No Ag Label.

JMS Stylet-oil. 97.1% paraffinic oil. OMRI-listed. EPA Reg. No. 65564-1.

Kaligreen . 82% potassium bicarbonate. OMRI-listed. EPA Reg. No. 11581-2.

KeyPlex 350 OR. 0.063% yeast extract hydrolysate from Saccharomyces cerevisiae. Combination of defensive proteins (alpha-keto acids) and secondary and micronutrients.

LifeGard WG. 40% Bacillus mycoides isolate J. Biological Plant Activator. OMRI-listed. v

Mildew Cure (formerly GC-3 Organic fungicide). 30% cottonseed oil, 30% corn oil, 23% garlic extract. OMRI-listed. Exempt from EPA registration.

MilStop. 85% potassium bicarbonate. OMRI-listed. EPA Reg. No. 70870-1-68539.

Organocide. 5% sesame oil. OMRI-listed. Exempt from EPA registration.

OxiDate. 27% hydrogen dioxide. OMRI-listed. EPA Reg. No. 70299-2.

Procidic. 3.5% Citric acid. NOP compliant; registered for use in organic agriculture with Washington State Dept of Ag. Exempt from EPA registration.

Promax. 3.5% Thyme oil. OMRI-listed. Exempt from EPA registration.

Regalia. 5% Extract of Reynoutria sachalinensis. OMRI-listed. EPA Reg. No. 84059-2.

Serenade ASO. 1.34% Bacillus subtilis strain QST 713. OMRI-listed. EPA Reg. No. 264-1152.

Serifel. 9.9% Bacillus amyloliquefacinens strain MBI 600. OMRI-listed. EPA Reg No. 71840-18.

Sil-Matrix. 29% potassium silicate. OMRI-listed. EPA Reg. No. 82100-1.

Sonata. 1.38% Bacillus pumilus strain QST 2808. OMRI-listed. EPA Reg. No. 69592-13.

Sporatec AG. 18% rosemary oil, 10% clove oil, and 10% thyme oil. OMRI-listed. Exempt from EPA registration.

Timorex Gold. 23.8% tea tree oil. OMRI-listed. EPA Reg. No. 70051-2.

Trilogy. 70% clarified hydrophobic extract of neem oil. OMRI-listed. EPA Reg. No. 70051-2.

TriTek. 80% mineral oil. OMRI-listed. EPA Reg. No. 48813-1.

Before purchase for organic production, confirm product is acceptable for agricultural use with your certifier or your NYS DEC regional office.

In summary, to manage powdery mildew effectively in cucurbit crops: 1) select resistant varieties, 2) inspect crops routinely for symptoms beginning at the start of fruit development, and 3) apply targeted fungicides weekly with protectant fungicides and alternate amongst available chemistry based on FRAC Group code, starting at the action threshold of 1 affected leaf out of 50 older leaves. Add new fungicides to the program when they become available; substitute new for older product if they are in the same FRAC group.

Click on images for enlargements

Commerical pumpkin crop with powdery mildew effectively controlled on upper leaf surfaces by the protectant fungicide applied while the disease is not controlled on lower surfaces because of resistance to QoI fungicides.

Please Note: The specific directions on fungicide labels must be adhered to -- they supersede these recommendations, if there is a conflict.  Note that some products mentioned are not yet registered for use on cucurbits in NY.  Check state registration for all products and approval with certifier for organic products.  Check labels for use restrictions. Any reference to commercial products, trade or brand names is for information only; no endorsement is intended.