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Managing Downy Mildew in Cucurbits

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

Up-dated fungicide and variety recommendations for New York

Up-dated fungicide and variety recommendations for outside New York

Mobile fungicides for managing downy mildew and other major cucurbit diseases

List of biopesticides for organic production

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

See also ‘Assorted Foliar Diseases of Cucurbits. Fact Sheet Page: 732.90 Date: 7-1992
and ‘Identifying initial downy mildew symptoms in cucurbits is critical for successful management

Downy mildew is a potentially devastating disease that can begin to develop at any time during cucurbit crop development in the northeastern US. Fortunately it has occurred sporadically in this region, usually appearing late enough in the growing season that yield is not impacted. However, in 2004, this disease appeared here much earlier than usual causing extensive defoliation. By late June, growers in NC were ‘going through the worst epidemic of downy mildew on cucumber that anybody can remember seeing’ and the disease had already appeared in NJ. Downy mildew was first noticed on Long Island about 4 days after a rainstorm on 31 July that was the remnant of the first hurricane of the season. Some pumpkin fields were severely affected, especially where fungicides had not been applied beforehand. Subsequently downy mildew developed throughout upstate NY, becoming more widespread than in the previous 25 years. Downy mildew was also a major problem in 2005. With reports of the disease widespread in NC by 7 July 2006 and already occurring in MI, OH and Ontario, downy mildew will be a concern this year in the Northeast.

Only leaves are affected (figs 1-10). Spots are angular being delineated by leaf veins (fig 6). Often several spots occur together in a coalesced group. Initially spots are pale green, then yellow (fig 1) before the tissue dies (fig 3). Affected tissue in pumpkin can be more orange than yellow. On the leaf underside spots typically appear water-soaked at first (fig 2). Extensive defoliation can occur when conditions are favorable. Leaf petioles often remain green and upright after the leaf blade has died and drooped (fig 5). In contrast with powdery mildew, spores of the downy mildew fungus are darker (purplish gray) and develop only on the underside of leaves (fig 3). Spores are not always present (fig 4) and symptoms can vary greatly, thus diagnosis can be challenging.

Fig 1. Yellow spots on the upper surface of these pumpkin leaves are early symptoms of downy mildew. These are not diagnostic as similar spots can occur with other diseases, notably powdery mildew. Click on photos for enlargement
Downy mildew on pumpkin Downy mildew on pumpkin Downy mildew on pumpkin
Photo Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Center, Cornell University Photo Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Center, Cornell University Photo courtesy of T.A. Zitter. Cornell University
Downy mildew on pumpkin Downy mildew on pumpkin  
Photo Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Center, Cornell University Photo Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Center, Cornell University  
Fig 2. Early lesions of downy mildew in melon usually appear water-soaked on the underside of leaves.Click on photo for enlargement
downy mildew on melon
Photo courtesy of J. G. Kantzes, Professor Emeritus, Department of Botany, University of Maryland

Fig 3. Dark purplish gray spores of the downy mildew fungus only develop on lower surfaces of leaves and are easily distinguished from white spores of the powdery mildew fungus. These spores are diagnostic but unfortunately are not always present (see Fig 4). Note that downy mildew lesions usually do not enlarge beyond major veins, giving spots an angular appearance.Click on photo for enlargement
downy mildew on pumpkin downy mildew on pumpkin
Photos Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Center, Cornell University

The downy mildew fungus exists as pathotypes varying in ability to infect the various cucurbit types. Some can infect all types while others are able to infect cucumber and cantaloupe but not watermelon, squash or pumpkin. Races and strains have been described within pathotypes based on variation in virulence and fungicide sensitivity. These develop in response to selection pressure from management practices. Major change evidently occurred recently in the downy mildew fungal population in the US based on detection in 2004 of resistance to the QoI fungicides and observations of downy mildew developing on resistant cucumber varieties.

Manage downy mildew by planting resistant varieties, monitoring disease occurrence and weather forecasts, inspecting crops for symptoms weekly, and applying broad-spectrum protective fungicides before detection and systemic narrow-spectrum fungicides when downy mildew is present.

Many cucumber varieties, a few melons, and one butternut squash have resistance to downy mildew. See variety tables. Most cucumber varieties described in catalogues have resistance developed to strains of the pathogen present before 2004.  Against current pathogen strains these varieties exhibit limited resistance contrasting greatly with the very high level of resistance that they exhibited to previous strains.  While these old resistant varieties are still a useful component of a downy mildew management program, fortunately, new source of resistance has been found and cucumber varieties with this are starting to become available.  Reports of resistant cucumber variety evaluations conducted in NY are available below the variety tables.  One slicer type that exhibited good resistance in NY and also MA is DMR 401. It was developed by Cornell plant breeders and is available at http://commonwealthseeds.com/varieties-offered/ along with a downy mildew resistant butternut squash.  Bristol also performed well in these evaluations.  Cucumber variety evaluations conducted each year in NC include downy mildew ratings.  Trial summaries are posted near the bottom of http://cucurbitbreeding.com/cucumber-breeding/.

Cultural practices can be a useful component of a downy mildew management program.   Choose planting sites with good air movement and without shading. Avoid overhead irrigation in early morning when leaves are wet from dew or late in the day when leaves will not have an opportunity to dry before dew forms. Maintain ample but not excessive nitrogen fertility.

This fungal pathogen does not survive winter in the Northeast, thus it only occurs in the region when conditions favor spore production, release, and movement by wind from where the disease is occurring, plus cloud cover during spore movement to provide protection from uv radiation and favorable conditions for disease development where the spores land. These factors have been used to forecast where downy mildew will occur in the eastern USA. Forecasts are posted three times a week at a North Carolina State University web site (http://cdm.ipmpipe.org) when the program is funded. Even when the forecasting program is not supported, as occurred in 2004, a map of the current active sources and a table describing each is maintained at this web site.  Forecasts enable timely fungicide applications.  On the left side of the home page are links to many valuable pages including Current Forecasts and Archived Forecasts, thus past occurrences and forecasts can be examined.  Considering the potential for spores to be dispersed to the northeastern US at any time during the cucurbit growing season, the destructive potential of this disease, and the fact these crops are susceptible from the cotyledon stage, regularly checking the forecast is a critical component of downy mildew management.  Growers can subscribe to receive customizable alerts by e-mail or text message.  Information is also maintained at the forecast web site of cucurbit crop types being affected by downy mildew, which is needed for determining risk of downy mildew for specific crop types other than cucumber.  When risk of downy mildew is forecast for a location, to determine if the risk includes cucurbit crops other than cucumber, look at the origin of the trajectory on the map, then look at the Epidemic History page to see what cucurbit crops downy mildew has been reported on in the area covered by the trajectory.

Fig 4. Lower surface of pumpkin leaf affected by downy mildew lacking the pathogen’s characteristic purplish gray sporulation. Diagnostic spores may form when an affected leaf is placed with damp paper towel in a closed plastic bag for 12-24 hours.Click on photos for enlargement
downy mildew downy mildew
Photos Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Center, Cornell University
Fig 5. Leaf tissue affected by downy mildew can change quickly from yellow to brown as it is killed. Click on photos for enlargement
downy mildew downy mildew downy mildew
Photo Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Center, Cornell University Photo Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Center, Cornell University Photo Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Center, Cornell University
downy mildew
Photo courtesy of J.P. Gibbons, Cornell Cooperative Extension

downy mildew
Photo courtesy of J.P. Gibbons, Cornell Cooperative Extension

 
Fig 6. Angular, necrotic, downy mildew spots on cucumber leaf.Click on photo for enlargement
downy mildew

Photo Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Center, Cornell University

Fig 7. Necrotic spots due to downy mildew on zucchini leaf.Click on photo for enlargement
downy mildew

Photo courtesy of T.A. Zitter. Cornell University

Fig 8. Chlorotic and necrotic spots due to downy mildew on cantaloupe leaves.Click on photo for enlargement
downy mildew

Photo courtesy of J. G. Kantzes, Professor Emeritus, Department of Botany, University of Maryland

Recommended targeted fungicides.  Use in alternation and tank mixed with a protectant fungicide.  Label directions for some state to begin use before infection or disease development.  The forecasting program helps ensure this is accomplished.  There is a table of fungicides for this and other key diseases of cucurbit crops at
http://vegetablemdonline.ppath.cornell.edu/NewsArticles/Cucurbit%20Fungicide%20List%202018-NY.pdf

Orondis (FRAC Code 49, previously U15).  The novel active ingredient, oxathiapiprolin, has exhibited excellent activity in fungicide evaluations.  It is formulated with mandipropamid (FRAC 40) as Orondis Ultra (REI is 4 hr) and with chlorothalonil (M5) as Orondis Opti (REI is 12 hr).  PHI is 0 day.  Make no more than 2 consecutive applications of either before rotating to a different fungicide.  When at least 3 applications for downy mildew will be made, Orondis fungicides can be no more than 33% of the applications, or a maximum of 4 applications per planting, whichever is fewer.  Orondis Opti is labeled for several other diseases because it contains chlorothalonil.  It is only recommended used for these diseases when downy mildew is also present.  Orondis Ultra is also labeled for Phytophthora blight.  Another fungicide, Orondis Gold 200, is only labeled for application to soil for Phytophthora blight.  Its use in a crop prohibits foliar application of Orondis fungicides for downy mildew.

Ranman (21). Use organosilicone surfactant when water volumes are less than 60 gallons per acre.  REI is 12 hr. PHI is 0 day.  Apply no more than 6 times in a season with no more than 3 consecutive applications. 

Omega (29).  REI is 12 hr. PHI is 7 days for squash/cucumber subgroup, which includes pumpkin, and 30 days for melons.  Apply no more than 7.5 pts/A to a crop or 4 applications applied at highest label rate of 1.5 pts/A.

Curzate or Tanos (27).  These have some curative activity (up to 2 days under cool temperatures) but limited residual activity (about 3-5 days).  They can be a good choice when it was not possible to apply fungicide at the start of a high risk period when temperature is below 80 F.  Apply another targeted fungicide 3-5 days later.  Both must be tank-mixed with a protectant.  REI is 12 hr. PHI is 3 days.  Apply no more than 4 times in a season (6-9 for Curzate depending on rate); no consecutive applications of Tanos are permitted.  Tanos also has a FRAC Code 11 ingredient.  It is recommended used only when this ingredient is needed for other diseases that are also occurring, such as Plectosporium blight. 

Zing! or Gavel (22).  These are the only products that consist of a targeted fungicide and a protectant fungicide (chlorothalonil or mancozeb). REI is 12 hr and PHI is 0 days for Zing!.  REI is 48 hr and PHI is 5 days for Gavel.  Apply no more than 8 times in a season.  Some cantaloupe varieties are sensitive to Gavel (see label).  Workers must be notified that a dermal sensitizer is applied both orally and by posting at entrance to treated area 24 hours before the scheduled application and for 4 days afterwards.  The amount of chlorothalonil in Zing! is an intermediate rate (1.18 lb/A chlorothalonil) of the labeled rate range for downy mildew in products with just chlorothalonil (1.125-1.5 lb/A).  Chlorothalonil is labeled for use at a higher rate (1.5-2.25) to manage several other diseases including powdery mildew.  Growers trying to manage these diseases as well as downy mildew should apply additional Bravo to bring the amount of chlorothalonil up to the higher rate.  To obtain an application rate of 1.5-2.25 lb/A chlorothalonil,  tank mix Bravo WeatherStik at 0.43-1.43 pt/A with Zing!.

Zampro (40 + 45) or Revus (40).  Zampro is the best choice, but it is not labeled for use on Long Island due to groundwater contamination concern.  Apply Zampro no more than 3 times in a season with no more than 2 consecutive applications before switching to a fungicide with different FRAC code.  Revus can be applied 4 times; there is no label restriction on number of consecutive applications, but more than 2 is discouraged for resistance management.  REI is 12 hr and PHI is 0 day for both products.  There is a different FRAC code 40 fungicide ingredient in Zampro and Revus which may have slightly different mode of action, thus there may be benefit to using both in a fungicide program. Revus must be applied with a spreading/penetrating type adjuvant.  Revus has exhibited differential activity, being effective for downy mildew in pumpkin but not in cucumber.  Therefore it is not recommended for use in cucumber.  Forum (40) is no longer recommended because it was ineffective in recent fungicide evaluations (see table).

Targeted fungicides no longer recommended.   Resistance has been documented in the USA in the cucurbit downy mildew pathogen to the following fungicides.  They have provided limited to no control of downy mildew when tested alone in recent university fungicide evaluations, in contrast with excellent control provided in the past (see table).  Poor control has also been reported in commercial crops.

Additionally, fungicides with mefenoxam and metalaxyl (FRAC 4), e.g. Ridomil, or a strobilurin active ingredient (FRAC 11), e.g. Cabrio, have not been recommended since 2004 as they have been ineffective due to resistance.

Recommended protectant fungicides. Chlorothalonil and mancozeb are the main protectant fungicides for downy mildew.  Copper is not as effective. 

Generally, although symptoms were severe in early August when downy mildew was first observed on Long Island in 2004, growers were able to avoid excessive loss of leaves by applying these fungicides that have systemic activity for this pathogen. This was clearly demonstrated in a field where there were edge areas that the spray boom did not reach. Powdery mildew was more severe than downy mildew in early September (fig 10). Although the canopy looked adequate in September, apparently in some fields there had been enough loss of foliage to downy mildew that the older portions of vines had died resulting in rotten handles on fruit (fig 11).

Fig 9. Downy mildew can be a very destructive foliar disease, resulting in leaf death before fruit mature, as shown here on pumpkin.Click on photo for enlargement
downy mildew
Photo Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Center, Cornell University

Fig 10. Despite symptoms of downy mildew being severe in early August before systemic fungicides were applied on Long Island in 2004, this disease was under better control than powdery mildew on 3 September.Click on photos for enlargement
downy mildew downy mildew
Photos Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Center, Cornell University

Fig 11. Fruit with rotting handles on 16 September in a field where downy mildew was severe in early August before fungicides were applied.Click on photos for enlargement
downy mildew downy mildew
Photo Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Center, Cornell University

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. Original page prepared July 2006, updated subsequently. Download the ‘up-dated recommendation’ file for the most current information about fungicides and varieties (link is near top of page).

See Also Identifying initial downy mildew symptoms in cucurbits is critical for successful management

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