Identifying Initial Downy Mildew Symptoms in Cucurbits Is Critical for Successful Management

Margaret Tuttle McGrath
Department of Plant Pathology, Cornell University
Long Island Horticultural Research and Extension Centre
3059 Sound Avenue, Riverhead, NY 11901

Knowing how to identify the first symptoms of downy mildew in cucurbit crops is a critical component of an effective management program. Level of control achievable with fungicides can decline greatly when applications are delayed after downy mildew onset. Downy mildew can begin to develop at any time during crop development, including the cotyledon stage. In contrast, powdery mildew typically begins to develop in field-grown cucurbit crops over a narrow period in crop development around the time of initial fruit formation; earlier in severely stressed plants and later in early-spring summer squash crops. Applying fungicides preventively throughout crop development for a disease that could begin to develop at any time, including late in the season when yield will not be impacted, is undesirable and not economical. Fungicides for downy mildew that are mobile in the plant are most effective but unfortunately they have narrow-spectrum activity, in particular lacking efficacy for the most common cucurbit foliar disease, powdery mildew, thus these fungicides are only recommended when downy mildew is present.

Initial symptoms of downy mildew observed include small yellow spots (figs 1 and 3) and small irregular black spots with yellow-green border (fig 4 and 5). First symptoms also can be a water-soaked spot (fig 2). Infection does not expand beyond veins thus spots develop an angular appearance as the spots expand (figs 3 and 7). Sometimes several spots occur together forming a yellow patch that can have an orange tint, especially in pumpkin (fig 6).

With a good hand lens it may be possible to observe pathogen spores on the underside of leaves opposite these initial symptoms of downy mildew, thereby confirming the diagnosis, but spores are not always present in addition to being very small and thus hard to see. Spore production can be encouraged by putting leaves on damp paper towelling in a plastic bag for about 24 hours. Spores are lemon-shaped, dark grey with a purplish tint, and form on short specialized hyphae (conidiophores). As spots develop, further spore production may occur resulting in the underside of spots becoming visibly fuzzy due to the dark spores without the need for magnification (fig 7); however, spores do not always form, even when leaves are incubated in a moist environment.

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.

Photo Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Centre , Cornell University	Photo Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Centre , Cornell University	Photo courtesy of T.A. Zitter. Cornell University

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

Fig 2. Early lesions of downy mildew in melon usually appear water-soaked on the underside of leaves.

early symptoms of downy mildew

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

Fig 3. Yellow spots on cucumber leaves due to downy mildew.

Photos Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Centre, Cornell University

Fig 4. These small irregular black spots with yellow borders are early symptoms of downy mildew in pumpkin.

Photos Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Centre, Cornell University

Fig 5. Early symptoms of downy mildew on the upper and lower surface of a winter squash leaf.

Photos Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Centre, Cornell University

Fig 6. Patches of yellow tissue on winter squash and pumpkin leaves due to downy mildew.

Photos Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Centre, Cornell University

Fig 7. Purplish dark grey 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. Note that downy mildew spots often have an angular appearance because they do not enlarge beyond major veins. This is most evident on the lower surface. Yellow spots are on the upper surface of this leaf opposite where spores have developed on the lower surface. One section of this leaf has died because of downy mildew.

Photos Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Centre, Cornell University

Managing Downy Mildew in Cucurbits

Margaret Tuttle McGrath
Department of Plant Pathology, Cornell University
Long Island Horticultural Research and Extension Centre
3059 Sound Avenue , Riverhead , NY 11901 ;

Downy mildew is a potentially devastating disease that can begin to develop at any time during cucurbit crop development. 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 favourable. 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 grey) 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.

Photo Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Centre , Cornell University	Photo Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Centre , Cornell University	Photo courtesy of T.A. Zitter. Cornell University

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

Fig 2. Early lesions of downy mildew in melon usually appear water-soaked on the underside of leaves.

downy mildew on melon

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

Fig 3. Dark purplish grey 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.

Photos Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Centre , Cornell

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 Qol 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. 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.

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.

Fig 4. Lower surface of pumpkin leaf affected by downy mildew lacking the pathogen�s characteristic purplish grey sporulation. Diagnostic spores may form when an affected leaf is placed with damp paper towel in a closed plastic bag for 12-24 hours.

Photos Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Centre, Cornell University

Fig 5. Leaf tissue affected by downy mildew can change quickly from yellow to brown as it is killed.

downy mildew

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

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

Fig 6. Angular, necrotic, downy mildew spots on cucumber leaf.

downy mildew

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

Fig 7. Necrotic spots due to downy mildew on zucchini leaf.

downy mildew

Photo courtesy of T.A. Zitter. Cornell University

Fig 8. Chlorotic and necrotic spots due to downy mildew on cantaloupe leaves.

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 favourable 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.

Qol (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.

Generally, when downy mildew is first observed, growers are 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 March (fig 10). Although the canopy looked adequate in March, 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.

downy mildew

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

Fig 10. Despite symptoms of downy mildew being severe in early February before systemic fungicides were applied, this disease was under better control than powdery mildew in March.

Photos Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Centre, Cornell University

Fig 11. Fruit with rotting handles in a field March, where downy mildew was severe in early February before fungicides were applied.

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

Please note: The specific directions on fungicide labels must be adhered to . Any reference to commercial products, trade or brand names is for information only; no endorsement is intended.

QoI

QoI, or Quinone outside inhibitors are a group of fungicides used in agriculture. They represent the most important development made in fungicides by the chemicals industry. QoI are chemical compounds which act at the Quinol outer binding site of the cytochrome bc1 complex.

QoI's are the resulting fusion of three fungicides families, the well-known family of strobilurins and two new families, represented by fenamidone and famoxadone. Some strobilurins are azoxystrobin, kresoxim-methyl, picoxystrobin, pyraclostrobin and trifloxystrobin.

These fungicides are used on a wide range of crops, such as cereals, vines, pome fruits, cucurbits, tomatoes and potatoes.

For example, they are used as fungicides for cereals, against Erysiphe graminis f.sp tritici responsible for the powdery mildew in wheat or against Septoria tritici, responsible for septoria leaf spot in wheat.

They are also commonly used for vine culture, against Plasmopara viticola, responsible for downy mildew or in o�dium treatment.

All these fungicides are in the same cross-resistance group (same mode of action) and must be managed carefully to avoid the appearance of fungicide resistance. Some fungicide resistance has been observed in most crops (such as in the case of wheat powdery mildew), so the application of QoI products should respect effective rates and intervals to provides time and space when the pathogen population is not influenced by the product selection pressure.