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Cucurbit Powdery Mildew

Updated: July 2020 Printer-friendly .pdf version of the management information on this page.

See also:

  • LIHREC Cucurbit powdery mildew photo gallery (includes diagnostic images)
  • Research on powdery mildew conducted at LIHREC.
  • Podcast: Avoiding the Powdery Mildew Blues – Meg McGrath, plant pathologist at Cornell’s Long Island Horticultural Research and Extension Center, discusses how with other members of the Great Lakes Vegetable Working Group on 24 June 2020. This and other recordings are in the green-bordered box at the bottom of the page.
  • Listen to Meg McGrath talk about managing powdery mildew in a teleconference hosted by Steve Bogash of Marrone Bio Innovations on 22 July 2020. Dial 515-604-9875. At prompts enter 832191 for access code and 14 for reference number.

Topics on this page: 

Impact and causal fungi

Powdery mildew is a common disease of cucurbits under field and greenhouse conditions in most areas of the world. Although all cucurbits are susceptible, symptoms are less common on cucumber and melon because many commercial cultivars have resistance. This disease can be a major production problem. Quantity of yield is reduced due to a decrease in the size or number of fruit or a decrease in the length of the harvest period. Premature senescence of infected leaves can result in reduced market quality because fruit become sunburnt or ripen prematurely or incompletely.  Such fruit have poor storability (winter squash), low soluble solids with consequent poor flavor (melon), poor rind color (pumpkin), and shriveled, discolored handles (pumpkin). Stress from disease can lead to imperfections on fruit rind such as speckling, raised indentations, and oedema.  In addition, powdery mildew infection predisposes plants to other diseases, in particular, gummy stem blight.

Podosphaera xanthii (previously known as Sphaerotheca fuliginea and S. fusca) and Erysiphe cichoracearum are the two most commonly recorded fungi causing cucurbit powdery mildew. E. cichoracearum was considered to be the primary causal organism throughout most of the world before 1958. Today, P. xanthii is found more commonly worldwide. A shift in the predominance of these two fungi may have occurred or the causal organism may have been misidentified. P. xanthii is a more aggressive pathogen than E. cichoracearum. E. cichoracearum may have a lower temperature optimum since this species is found mainly during cooler spring and early summer periods and P. xanthii appears to progress most rapidly during the warmer months.

The conidia (spores produced asexually) of E. cichoracearum and P. xanthii are difficult to distinguish and cleistothecia, which are sexual fruiting bodies (structures containing spores produced through sexual reproduction), have been observed less commonly. Consequently, these fungi have been confused. The name of the fungus frequently has been reported without valid confirmation. Criteria for differentiating these fungi using the conidial stage were not identified until the 1960s. The main criterion used is presence of fibrosin bodies in conidia of P. xanthii. Based on these criteria, P. xanthii was found to be the predominant fungus, rather than E. cichoracearum as previously claimed, in several countries. During recent surveys E. cichoracearum was found rarely and only at the start of disease development in New York and other eastern states.

Symptoms and signs

White, powdery fungal growth develops on both leaf surfaces, petioles, and stems. This growth is primarily asexual spores called conidia. It usually develops first on crown leaves, on shaded lower leaves and on leaf undersurfaces. Yellow spots may form on upper leaf surfaces opposite powdery mildew colonies. Older plants are affected first. Infected leaves usually wither and die. Plants may senesce prematurely. Fruit infection occurs rarely on watermelon and cucumber. Cleistothecia are dark brown, small (diameter of about 0.003 inches) structures that are barely discernable without a hand lens. They develop late in the growing season. The sexual spores within these structures are protected from adverse conditions.

Disease cycle

Source(s) of initial inoculum for powdery mildew have not been definitively determined. The primary initial inoculum is believed to be airborne conidia dispersed potentially long distances from other affected crops, starting in southern states where cucurbit crops are grown earlier in the year. Conidia remain viable for 7-8 days based on results from laboratory studies. The causal fungi are obligate parasites and therefore cannot survive in the absence of living host plants, except as cleistothecia. Possible local sources of initial inoculum include conidia from greenhouse-grown cucurbits, cleistothecia, and alternate hosts. Cleistothecia have been reported rarely in the United States; however, even when present they can be overlooked. They have been found every year in New York that they were looked for since 1989. Both mating types required for sexual reproduction have been found throughout the United States. Although P. xanthii and E. cichoracearum are described as having broad host ranges, strains of these fungi have been shown to be host-specific. The role of non-cucurbit hosts as sources of inoculum has not been investigated. Verbena, a common ornamental plant, could be an important source of inoculum, especially for cucurbits grown as a crop or transplants in the same greenhouse as verbena.

Powdery mildew develops quickly under favorable conditions because the length of time between infection and symptom appearance is usually only 3 to 7 days and a large number of conidia can be produced in a short time. Favorable conditions include dense plant growth and low light intensity.  High relative humidity is favorable for infection and conidial survival; however, infection can take place as low as 50% RH. Dryness is favorable for colonization, sporulation, and dispersal. Rain and free moisture on the plant surface are unfavorable. However, disease development occurs in the presence or absence of dew. Mean temperature of 68-80°F is favorable; infection can occur at 50-90°F. Powdery mildew development is arrested when daytime temperatures are at least 100°F. Plants in the field often do not become affected until after fruit initiation. Susceptibility of leaves is greatest 16 to 23 days after unfolding.

Pathogenically distinct races of Podosphaera xanthii have been differentiated on muskmelon. Races 1 and 2 have most common in the eastern United States recently.

Managing cucurbit 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.

Cultural and biological controls including resistant varieties

Genetic resistance is used extensively in cucumber and melon, and has been incorporated into most other cucurbit crops. Most resistant squash and pumpkin varieties in the United States contain one or two copies of the same major resistance gene from a wild cucurbit. Genetic of resistance is different in cucumber and melon. Recently a decline in the degree of suppression achievable with resistant varieties has been detected indicating adaptation in Podosphaera xanthii. Successive cucurbit plantings should be physically separated or at least planted up-wind of older plantings because older plants can serve as a source of conidia. Fungicides containing antagonistic fungi for biological control have been developed.

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.

View resistant varieties lists: 

Chemical control – General information

Fungicides should be applied every 7-10 days beginning very early in disease development following detection through an IPM scouting program. Inspect plants weekly beginning at the start of fruit formation (when plants become more susceptible). Examine upper and under surfaces of five older leaves at each of 10 sites or until symptoms are found. Initiate a weekly spray program when symptoms are found. A spring planting of summer squash will become infected first; therefore, when available, it can be used as an indicator of when to begin scouting vine crops and later plantings of summer squash. For a preventive schedule, applications should begin when first female flowers have opened. To obtain adequate control, fungicide is needed on the undersurface of leaves and on leaves low in the plant canopy because the fungus develops best on these surfaces. This can be best accomplished by using mobile materials. Another approach is to improve efficacy of contact materials (i.e. chlorothalonil, sulfur, oil) by maximizing spray coverage on undersurfaces of leaves. Air-assist sprayers are one of the most effective means for increasing coverage and deposits on all leaf surfaces. Coverage produced by traditional hydraulic boom sprayers can be increased by either decreasing nozzle spacing (10 inches is better than 20 inches), increasing volume (75 gpa has worked well), increasing pressure (at least 80 psi), or by changing to smaller nozzle tips that direct sprays at an angle to the canopy. Use water sensitive paper to check spray coverage. Refer to Cornell Integrated Crop and Pest Management Guidelines for Commercial Vegetable Production for an updated list of available fungicides and follow label directions.

Development of fungicide resistance and consequent control failure is always a concern with mobile fungicides due to their single site mode of action. Strains of the powdery mildew fungus resistant (insensitive) to such fungicides have been found throughout the United States, including New York where research on resistance started in 1990. Reduced sensitivity to fungicides from several chemical groups have been detected in other areas of the world as well. Therefore, tactics should always be used to minimize the potential of resistant pathogen strains being selected, starting when a new product is first available: apply mobile fungicides together with a contact fungicide (tank mix), apply them only when needed most to protect yield (which begins at the start of disease development; contact fungicides alone can suffice late in the season), use highest labeled rates, and alternate among as many different mobile fungicides as possible selecting based on resistance occurrence and on modes of action as indicated by their FRAC code (see link to current fungicide information at top of page). In addition, maximize spray coverage and also use nonchemical (cultural) control practices. Frequency of resistant strains can increase rapidly with use during a growing season, especially when products are used exclusively rather than in a resistance management program.

Several biopesticides approved for organic production are registered for this disease in the United States. These products contain natural ingredients such as botanical oils, bicarbonates, hydrogen dioxide, and lipopeptides. Most are contact materials, thus good coverage is critical for effective control. Products evaluated in university trials have exhibited a range in efficacy with some being as effective as conventional contact fungicides. Refer to the link at the top of this page about current fungicide information).

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

Recommended targeted fungicides. Alternate among targeted, mobile fungicides primarily in the following three chemical groups (principally the first two), and apply with protectant fungicide to manage resistance development and avoid control failure if resistance occurs, and also to comply with label use restrictions. All targeted fungicides are at risk of resistance developing; FRAC Code 50 (formerly U8) and U13 are the only chemistries that resistance has not yet been detected to.

View and/or download: Federal pesticide labels | New York State labels (enter product name under ‘Names’ in center of page, then click on Search at bottom of this section).

  • Vivando (FRAC Code 50, formerly U8) has exhibited excellent control in fungicide evaluations. Activity is limited to powdery mildew. 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. Prolivo is a new fungicide with a new active ingredient in this FRAC group registered in NY April 2018.  It was not as effective as Quintec for managing powdery mildew on lower leaf surfaces in a fungicide evaluation conducted at LIHREC in 2016 in which Vivando was not included.
  • DMI fungicides (FRAC Code 3) include Proline*, Procure, Luna Experience*, and Rhyme* (these considered most effective) plus Aprovia Top*, Inspire Super*, Mettle, and Rally.  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. 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 for DMI fungicides. PHI is 0 days for some including Procure; 7 days for others including Proline.  not labeled for use on Long Island.  *Fungicides labeled for additional cucurbit diseases; see section on other diseases.  Inspire Super (FRAC Code 3 and 9) is recommended for other labeled diseases. 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.  TopGuard is labeled but not recommended because it has Code 11 ingredient plus same DMI ingredient in Rhyme.  Additional products are registered for use outside NY.
  • Gatten (Code U13) is the newest fungicide; it was introduced in 2018 and registered in NY in 2020.  REI is 12 hr.  PHI is 0 days.  Activity is limited to powdery mildew.  In NY, Gatten can only be applied twice (five times elsewhere) and it is not allowed used on Long Island.  It was as effective as Vivando for managing powdery mildew on lower leaf surfaces in a fungicide evaluation conducted at LIHREC in 2019 but not in 2018.

Recommended used sparingly:

  • Quintec (FRAC Code 13) was consistently effective in fungicide evaluations conducted on Long Island until 2019 when it was significantly less effective than Vivando for the first time in the fungicide evaluation conducted annually on Long Island. This was not surprising because insensitivity to a high concentration of Quintec (similar to the dose when applied in the field) has been detected in some pathogen isolates collected from commercial fields and/or fungicide-treated research fields at the end of the growing season on Long Island since 2015. Resistant isolates evidently were sufficiently uncommon most of the season in 2015-2018 not to impact Quintec efficacy.  Because resistance has developed, Quintec is now recommended to be 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. Labeled for use on non-edible peel crops: melons, pumpkin, and winter squash.  REI is 12 hr. PHI is 3 days.
  • Carboxamide aka SDHI fungicides (FRAC Code 7) registered in NY are Luna fungicides*, Aprovia Top, Miravis Prime*, Endura, Pristine and Merivon* (* not allowed used 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 has exhibited limited control in fungicide evaluations conducted in 2017-2019 on Long Island. Luna Experience is the best choice because it also contains tebuconazole (Code 3), which needs to be considered when developing an alternation program.  Luna Sensation is not recommended because it also contains trifloxystrobin (Code 11); resistance to this chemistry is very common.  Aprovia Top, Luna Experience, and Miravis Prime are the only Code 7 fungicides recommended.  Limit use.  Aprovia Top and Luna Experience have the advantage that they contain a second active ingredient with activity for powdery mildew (Code 3). All have 12 hr REI. PHI is 0, 7, and 1 day respectively. Maximum number of applications is 2-5, depending on product and rate. Low rate isn’t recommended.

Recommended used sparingly if at all:

  • 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, where resistance to Torino was detected in pathogen isolates. Torino resistance was also detected in 2018.  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.

Fungicides classified for “Restricted Use” in New York: Aprovia Top, Gatten, Miravis Prime, and Procure.

No longer recommended. Resistant pathogen strains are sufficiently common to render the following fungicides ineffective: Topsin M (FRAC Code 1; MBC fungicide), QoI fungicides (Code 11), which include Quadris, Cabrio and Flint, and SDHI fungicides (Code 7) containing boscalid (Endura and Pristine) or an active ingredient that has exhibited full cross resistance in laboratory testing of pathogen isolates (Merivon). 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.

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).  Not every fungicide chemistry prone to resistance is tested every year. In 2019 Vivando was the most effective single product tested providing control statistically similar to the alternation program (Vivando, Quintec, Proline), albeit numerically lower control (75 and 92% control on lower leaf surfaces). Gatten was as effective as Vivando (65%); Luna Sensation and Quintec were less effective (47 and 40%). In 2018 Vivando was most effective albeit not significantly better than Quintec, which was not significantly better than Luna Sensation (54, 42, and 28%)).  In 2017 Torino, Pristine, and Luna Sensation were ineffective, while Vivando was most effective (80%) albeit not significantly better than Quintec (72%) or Procure (54%). In 2016 Quintec and Procure were as effective as an alternation program (98, 91, and 97%) while Pristine was substantially less effective (43%). In 2015 Quintec, Pristine, and Vivando were as effective as an alternation program (69-78%). Quintec and Vivando were the most effective of the targeted fungicides evaluated in 2014 (96 and 98%); Pristine was moderately effective (54%); Procure was slightly but not significantly better (70%). In 2013 Quintec, Pristine, and Procure provided excellent control (93-99%). In 2012 Pristine and Fontelis 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.

Fungicides Labeled for Other Diseases in Addition to Powdery Mildew. 

  • Proline (FRAC 3). Fusarium blight and gummy stem blight.
  • Rhyme (FRAC 3). Gummy stem blight.
  • Luna Experience (FRAC 3 and 7). Alternaria leaf spot, anthracnose, gummy stem blight, and belly rot.
  • Aprovia Top (FRAC 3 and 7). Anthracnose, Alternaria leaf blight, gummy stem blight, and Plectosporium blight.
  • Inspire Super (FRAC 3 and 9). Alternaria leaf blight, anthracnose, gummy stem blight, Plectosporium blight, and Septoria leaf spot
  • Miravis Prime (FRAC 3 and 12). Alternaria leaf blight and spot, gummy stem blight, and scab.

Organic fungicides

Printer-friendly .pdf version of this section on organic fungicides

Following is a list of some biopesticides and other organic fungicides labeled for cucurbit powdery mildew. There are several formulations of copper and sulfur not included.  Each product name is followed by active ingredient. Most products are labeled for managing other diseases which are included. Products listed with OMRI (Organic Materials Review Institute) are NOP compliant. Check state registration: each product may not be registered in all states. Also, always check with your certifier before purchasing any product. ‘No Ag Label’ indicates an agricultural label was not found for the product.

Results of product evaluations conducted at LIHREC and at Cornell AgriTech.

  • AVIV.  0.08% Bacillus subtilis strain IAB/BS03.  Labeled for Alternaria leaf spot, anthracnose, downy mildew, gummy stem blight, powdery mildew, Phytophthora blight and also as soil treatment for root pathogens (Fusarium, Phytophthora, Pythium, and Rhizoctonia) in cucurbits.  Replaces Prevont. OMRI-listed.  EPA Reg. No. 91473-1-86182.  Seipasa S.A.
  • Cease.  1.34% Bacillus subtilis strain QST 713. Broadly labeled for use on greenhouse vegetables. Labeled for angular leaf spot, anthracnose, bacterial fruit blotch, downy mildew, gummy stem blight, and powdery mildew in cucurbits. OMRI-listed.  EPA Reg. No. 69592-19-68539.  BioWorks, Inc.
  • Cinnerate.  60% cinnamon oil.  Labeled for diseases such as powdery mildew and gray mold.  OMRI-listed.  Exempt from EPA registration.  Seipasa S.A.
  • Companion.  0.03% Bacillus subtilis strain GB03.  Broadly labeled for foliar and soil-borne diseases, including bacterial wilt, gummy stem blight, powdery mildew, damping-off, crown and root rot, Phytophthora blight, and Fusarium wilt in cucurbits. EPA Reg. No. 71065-3.  Growth Products, Ltd.
  • Double Nickel 55 LC and WDGBacillus amyloliquefacinens strain D747, 98.8% and 25%, respectively. Broadly labeled for foliar and soil-borne diseases, including downy mildew, gummy stem blight, powdery mildew, damping-off, crown and root rot, Monosporascus vine decline, and charcoal rot in cucurbits. OMRI-listed.  EPA Reg No. 70051-107 and 108, respectively.  Certis USA, LLC.
  • EcoSwing Botanical Fungicide.  82% extract of Swinglea glutinosa. Labeled for powdery mildew. OMRI-listed. EPA Reg. No. 10163-357. Gowan Co.
  • ECOWORKS EC.  70% cold pressed neem oil. Labeled generally for powdery mildew, downy mildew, leaf spot, botrytis, anthracnose, blight, etc. and also several insects and mites. OMRI-listed. EPA Reg. No. 89152-4. ECOSTADT Technologies LLC.
  • GreenFurrow BacStop.  2.0% thyme, 2.0% clove & clove oil, 1.5% cinnamon, 1.0% peppermint & peppermint oil, and 1.0% garlic oil.  Broadly labeled primarily for bacterial diseases including bacterial leaf spot, bacterial wilt, bacterial fruit blotch, downy mildew, and powdery mildew in cucurbits.  Recommended used with EF400 for these and some other diseases.  Exempt from EPA registration.  Mar Vista Resources.
  • GreenFurrow EF400.  8.2% clove, 8.1% rosemary, and 6.7% peppermint.  Broadly labeled including downy mildew and powdery mildew in cucurbits.  Exempt from EPA registration.  Mar Vista Resources.
  • Kaligreen.  82% potassium bicarbonate. Labeled for powdery mildew.  OMRI-listed.  EPA Reg. No. 11581-2. Arysta LifeScience North America LLC.
  • KeyPlex 350 OR. 0.063% yeast extract hydrolysate from Saccharomyces cerevisiae.  Combination of defensive proteins (alpha-keto acids) and secondary and micronutrients.  Elicits systemic acquired resistance in plants against fungal and bacterial pathogens. Labeled for general disease control in vegetables.  EPA Reg. No. 73512-4.  KeyPlex.
  • LifeGard WG.  40% Bacillus mycoides isolate J.  Biological plant activator.  Labeled for anthracnose, downy mildew, gummy stem blight, and powdery mildew. OMRI-listed.  EPA Reg No. 70051-119.  Certis USA, LLC.  Federally registered Jan 2017; not registered in NYS yet.
  • Mildew Cure (also marketed as GC-3 Organic fungicide). 30% cottonseed oil, 30% corn oil, 23% garlic extract. Labeled for powdery mildew. OMRI-listed.  Exempt from EPA registration. JH Biotech, Inc.
  • MilStop.  85% potassium bicarbonate. Broadly labeled including Alternaria leaf spot, anthracnose, downy mildew, powdery mildew, scab, and Septoria leaf spot in cucurbits. OMRI-listed.  EPA Reg. No. 70870-1-68539.  BioWorks, Inc.
  • M-Pede.  49% potassium salts of fatty acids.  Labeled for powdery mildew in greenhouse cucumber plus several insects and mites. OMRI-listed.  EPA Reg. No. 10163-324.  Gowan Co.
  • Organocide. 5% sesame oil. Labeled broadly for several fungal diseases and insects. OMRI-listed.   Exempt from EPA registration.  No Ag Label.  Organic Laboratories, Inc.
  • Organic JMS Stylet-oil.  97.1% paraffinic oil. Labeled for several insect pests, viruses transmitted by aphids, and some fungal diseases in several crops including Alternaria leaf spot, gummy stem blight, and powdery mildew in cucurbits. OMRI-listed. EPA Reg. No. 65564-1. JMS Flower Farms, Inc.
  • OSO.  5% polyoxin D zinc salt.  Alternaria, anthracnose, downy mildew, gray mold, gummy stem blight, powdery mildew, and scab in cucurbits.  OMRI-listed.  EPA Reg No. 68173-4-70051.  Certis USA, LLC.
  • OxiDate 2.  27% hydrogen dioxide and 2% peroxyacetic acid. Labeled for Alternaria, anthracnose, belly rot, downy mildew, Fusarium wilt, gummy stem blight, leaf spot, Phytophthora, powdery mildew, and root rots in cucurbits.  OMRI-listed.  EPA Reg. No. 70299-12. BioSafe Systems, LLC.
  • PerCarb.  85% sodium carbonate peroxyhydrate.  Alternaria leaf spot, anthracnose, downy mildew, gummy stem blight, leaf spot, powdery mildew in cucurbits. OMRI-listed.  EPA Reg. No. 70299-15. BioSafe Systems, LLC.
  • Prestop.  93% Gliocladium catenulatum strain J1446.  Broadly labeled for seed-borne and soil-borne diseases (such as those caused by Phytophthora, Pythium, Rhizoctonia, Sclerotinia, and Verticillium) and foliar diseases (Alternaria, Colletotrichum, Botrytis, Didymella, and Fusarium plus powdery mildew). Can be incorporated in potting mix, applied to soil and to foliage. OMRI-listed. EPA Reg. No. 64137-13.  Lallemand Plant Care.
  • Prevont.  Renamed AVIV.
  • Procidic. 3.5% citric acid.  Labeled for damping-off, foliar diseases caused by fungal and bacterial pathogens, and post-harvest diseases.  Previously marketed as Citrex. Procidic was reviewed and determined to be NOP compliant by Washington State Dept of Ag. Exempt from EPA registration.  Greenspire Global, Inc.
  • Regalia. 5% extract of Reynoutria sachalinensis. Boosts the plant’s natural defense mechanisms against certain fungal and bacterial diseases. Broadly labeled including Alternaria blight, anthracnose, Cercospora leaf spot, downy mildew, gummy stem blight, Phytophthora blight, and powdery mildew in cucurbits. OMRI-listed. EPA Reg. No. 84059-2. Marrone Bio Innovations, Inc.
  • Serenade ASO. 1.34% Bacillus subtilis strain QST 713.  This bacterium colonizes roots when applied to soil and produces compounds that affect pathogens directly and triggers metabolic pathways to activate the plant’s natural defenses and modulate growth.  Labeled for angular leaf spot, anthracnose, downy mildew, gummy stem blight, and powdery mildew and for soil diseases caused by Rhizoctonia, Pythium, Fusarium, Verticillium and Phytophthora.  Serenade ASO has replaced Serenade Soil.  OMRI-listed.  EPA Reg. No. 264-1152.  Bayer CropScience.
  • Serenade Opti. 26.2% Bacillus subtilis strain QST 713. New formulation.  Broadly labeled including downy mildew, gummy stem blight, and powdery mildew in cucurbits. OMRI-listed. EPA Reg. No. 264-1160.  Previous product names and formulations: Serenade, Serenade Max, Serenade Optimum.  Bayer CropScience.
  • Serifel. 9.9% Bacillus amyloliquefacinens strain MBI 600. Broadly labeled for foliar diseases, including anthracnose, Alternaria leaf spot, downy mildew, gummy stem blight, and powdery mildew in cucurbits. OMRI-listed.  EPA Reg No. 71840-18.  BASF Corporation.
  • Serifel NG. 11% Bacillus amyloliquefacinens strain MBI 600. Labeled for use on greenhouse crops applied to soil for root rot pathogens and as foliar spray for powdery mildew. OMRI-listed.  EPA Reg No. 71840-8.  BASF Corporation.
  • Sil-MATRIX. 29% potassium silicate.  Labeled for powdery mildew in cucurbits. OMRI-listed. EPA Reg. No. 82100-1. PQ Corporation.
  • Sonata. 1.38% Bacillus pumilus strain QST 2808. Labeled for downy mildew and powdery mildew. OMRI-listed.  EPA Reg. No. 264-1153. Bayer CropScience (formerly AgraQuest).
  • Sporan EC2. 16% rosemary oil, 10% clove oil, 10% thyme oil, 2% peppermint oil. Labeled for Alternaria leaf spot, downy mildew, and powdery mildew. OMRI-listed.  Exempt from EPA registration.  KeyPlex.
  • Taegro 2. 13% Bacillus subtilis var. amyloliquefaciens strain FZB24. Labeled for suppression of powdery mildew and seedling diseases caused by Fusarium, Phytophthora, Pythium, and Rhizoctonia. OMRI-listed. EPA Reg. No. 70127-12. Novozymes Biologicals, Ind.
  • Thyme Guard.  23% thyme oil extract. Labeled generally for diseases caused by all pathogen groups and also insect pests.  Determined to be NOP compliant by Washington State Dept of Ag. Exempt from EPA registration.  Agro Research International.
  • Timorex Act.  12.5% tea tree oil. Labeled for several diseases, including downy mildew, gummy stem blight, powdery mildew, and soil-borne diseases (damping-off, crown and root rot).  OMRI-listed.  EPA Reg. No. 86182-3-88783.  Summit Agro USA, LLC.
  • Timorex Gold.  23.8% tea tree oil. Labeled generally for several diseases, including anthracnose, bacterial diseases, downy mildew, and powdery mildew in cucurbits.  OMRI-listed.   EPA Reg. No. 70051-2.  Stockton USA, LLC.
  • Trilogy.  70% clarified hydrophobic extract of neem oil. Labeled generally for several insects and diseases, including Alternaria, anthracnose, downy mildew, leaf spot, and powdery mildew in cucurbits. OMRI-listed.   EPA Reg. No. 70051-2.  Certis USA, LLC.
  • TriTek.  80% mineral oil. Labeled for several insect pests and powdery mildew in several crops including cucumber, melon, and squash. OMRI-listed.  EPA Reg. No. 48813-1. Previously named Saf-T-Side.   Brandt Consolidated, Inc.


To manage powdery mildew effectively in cucurbit crops:

  • Select resistant varieties.
  • Inspect crops routinely for symptoms beginning at the start of fruit development.
  • 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.

Please Note: The specific directions on fungicide labels must be adhered to — they supersede these recommendations, if there is a conflict.  If you are farming organically, before purchase make sure product is registered in your state and approved by your certifier. Any reference to commercial products, trade or brand names is for information only; no endorsement is intended.

More information/prepared by:

Margaret Tuttle McGrath
Associate Professor
Long Island Horticultural Research and Extension Center (LIHREC)
Plant Pathology and Plant-Microbe Biology Section
School of Integrative Plant Science
College of Agriculture and Life Sciences
Cornell University

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