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Update on Managing Downy Mildew in Cucurbits

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

July 2006

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 occurs early in crop production.

Most cucumber varieties and a few melons have resistance to downy mildew. See variety tables. Resistant cantaloupes include Allstar, an Eastern type that is also resistant to Fusarium wilt and powdery mildew. Although with the new race cucumber varieties do not exhibit the very high level of resistance that they did to previous races, resistant varieties are still a valuable component of downy mildew management. Cucumber variety evaluations each year in NC now include downy mildew ratings. Best pickle type varieties based on horticultural type as well as downy mildew resistance in 2005 were LB 1 (Baker Seeds), Pershing (Nunhems), and Bejo-2759.

See Table 21 at http://cuke.hort.ncsu.edu/cucurbit/cuke/cktrials/ckrpt05pk4.html for ratings of all entries. Best Slicer types were BCS-003 (Harris Moran), Dasher II, and NUN-2002 (Nunhems). See Table 36 at http://cuke.hort.ncsu.edu/cucurbit/cuke/cktrials/ckrpt05sl4.html.

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 there when conditions favor spore production, release, and movement by wind from where the disease is occurring plus 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 at a North Carolina State University web site (http://www.ces.ncsu.edu/depts/pp/cucurbit/) 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. Click on ‘Current Forecasts’ on the left side of the home page. The calendar on the right side can be used to look at last year’s forecasts. 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. This web site also has fungicide evaluation results and photographs of symptoms. The North Carolina State University web site can also be accessed through the NEWA site (http://newa.nysaes.cornell.edu). It is under cucurbits in the list on the left.

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

Broad-spectrum contact protectant fungicides (Bravo, Maneb, Dithane, copper) provide some downy mildew control. Researchers in NC regularly conducting fungicide efficacy trials for downy mildew rated chlorothalonil (4 rating) better than mancozeb and maneb (3) and also copper (1).

Mobile (systemic, translaminar) fungicides with an active ingredient that specifically targets oomycete fungi are recommended beginning when downy mildew is forecast to occur in the area or symptoms have just started to develop. Apply every 5-7 days depending on disease severity. Fungicide resistance is a concern with this pathogen and with these fungicides due to their specific mode of action; therefore, alternate among systemic fungicides in different chemical classes and tank-mix with protectant fungicides when the systemic is not formulated with a protectant. Fortunately several systemic fungicides are now available. Their efficacy was similar when compared in recent fungicide efficacy experiments

Curzate (cymoxanil, FRAC Group 27 fungicide) is labeled for use at 3.2 oz/A on a 10-14 day schedule for a maximum of 9 applications. It has a 12 hr REI and 3 day PHI. Since this product reportedly has good curative activity (about 3 day kickback), it is a good product to use first after downy mildew is detected. However, it has poor residual activity (only 1-2 days), thus it is critical to tank-mix it with a protectant fungicide and to follow-up with another systemic fungicide when disease pressure is high. Curzate should be used as soon as possible after rain if not applied before (2 hr rainfastness). Cost of product per application is about $8.50/A. Tanos is another fungicide with cymoxanil.

Forum (dimethomorph, Group 15), a new formulation replacing Acrobat, is labeled for use at 6 oz/A tank-mixed with protectant fungicide on a 5-10 day schedule for a maximum of 5 times with no more than 2 sequential applications. PHI is 0 days. REI is 12 hours. Tank-mix with protectant fungicide. Cost of product per application is about $8.91/A.

Gavel (mancozeb and zoxamide, Group 22) can be used on cucumber, melon, summer squash, and watermelon but not currently on pumpkin and winter squash because it contains mancozeb. Gavel is labeled for use at 1.5–2.0 lb/A, which will cost about $7.73-10.30/A, every 7 to 10 days or when conditions are favorable for disease for a maximum of 8 applications.

New phosphorus acid fungicides (Phostrol, ProPhyt, and Fosphite)(cyazofamid; Group 33) are more effective than Aliette. They have a 12 hr REI and can be applied to all cucurbits at 2.5-5 pt/A, which will cost about $12.50-25.00/A, on a 7-14 day interval up to 6-7 times/crop. Phosphite ion, the active ingredient for these fungicides, effects fungal pathogens directly and promotes the plant’s defense system.

Previcur Flex (propamocarb, Group 28) is labeled for use at 1.2 pts/A on a 7-14 day schedule for a maximum of 6 pts. PHI is 2 days. REI is 12 hours. Recommended tank-mixed with protectant fungicide. Cost of product per application is about $11.10/A.

Ranman (cyazofamid; Group 21) is labeled for use at 2.1-2.75 fl oz/A on a 7-10 day schedule for a maximum of 6 applications (16.5 fl oz) with no more than 3 consecutive applications followed by at least 3 applications of fungicide in another FRAC group. It has a 12 hr REI and 0 day PHI. Recommended tank-mixed with protectant fungicide. Cost of product per application is about $16.76/A for the highest label rate which is the label rate for Phytophthora blight.

Tanos (cymoxanil, Group 27, plus famoxadone, Group 11) is labeled for use at 8 oz/A, tank-mixed with protectant fungicide, on a 5-7 day schedule for a maximum of 4 applications of Group 11 fungicides including Tanos. It has a 12 hr REI and 3 day PHI. Cost of product per application is about $10.31/A. Curzate is another fungicide with cymoxanil.

Forum, Ranman, Gavel, Tanos and phosphorus acid fungicides are also labeled for Phytophthora blight, which is caused by a pathogen related to the downy mildew fungus.

Fungicides with mefenoxam (Ridomil Gold Bravo, Ridomil Gold Copper, Group 4) are highly effective but more at-risk for resistance than fungicides mentioned above. Ridomil Gold Bravo applied at 2 lb/A will cost $34/A. PHI is 5 days. REI is 48 hours.

QoI (aka strobilurin) fungicides (Group 11) are no longer recommended for downy mildew because resistant strains of the pathogen have been detected in the US. Although impact of resistance on efficacy is not known, there are several other effective, mobile fungicides. Tanos is one Group 11 fungicide that is still recommended because it contains an additional active ingredient, cymoxanil. Other Group 11 fungicides include Amistar, Cabrio, Flint, and Pristine. When compared for managing pathogen strains without resistance to this group, Cabrio has been more effective than Amistar. Reason (fenamidone) has a federal label and could soon be registered in NY.

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.

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

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