Cucurbit Downy Mildew – get out and scout!

By Katie Goldenhar, Horticulture Crop Pathologist, OMAFRA

Cucurbit downy mildew has not been identified in Ontario or the Great Lakes region, but sporangia have been detected in Michigan so growers should be on high alert and scouting often for this disease. Cucumber and muskmelon crops are the most susceptible, especially if there is heavy overhead irrigation or significant rainfall.

If you suspect downy mildew in your field, send for confirmation immediately. Contact the OMAFRA specialists listed below if you have any questions about detection or management.

What to look for Downy mildew symptoms first appear as water‑soaked lesions on the topside of leaves. The lesions can start anywhere in the canopy. These initial lesions are best seen during a heavy dew. The centre of the lesion then turns yellow (chlorotic) and eventually tan or brown (necrotic) (Figure 1). In cantaloupe, lesions appear irregularly shaped (Figure 2). As the disease progresses, lesions expand and multiply, causing foliage to become necrotic, and leading to plant death.

Under humid conditions, a downy growth that resembles “dirt” often develops on the underside of the initial water-soaked lesions. This growth on the underside of the leaf can also be seen before any symptoms on the upper leaf (Figure 2). This downy growth is particularly noticeable in the morning after a period of wet weather or when conditions favour dew formation.

Spore trapping

Michigan State University annually conducts spore trapping and reports their findings here. New this year, Dr. Hausbeck’s lab is reporting the clade associated with each spore detection. Clade 1 infects pumpkins, squash and watermelons and is not a concern most years in Ontario. Clade 2 infects cucumbers and cantaloupe and is seen annually in the region. Clade 2 spores have been identified in Michigan on May 17, 2021.  

In Ontario, passive spore traps have been placed across the province to detect for cucurbit downy mildew spores. Reports will be posted on the onvegetables.com blog.

Management

For optimum control, use a preventative downy mildew management strategy. The broad-spectrum fungicides listed in Table 1, Downy mildew multi-site, broad spectrum fungicides, provide protection against downy mildew infections under low disease pressure conditions. Apply the first application no later than the vine development stage. Consider an earlier application under high risk downy mildew conditions.

Under higher risk conditions, refer to Table 2. For late crop cucumbers, be prepared to begin the preventative fungicide program soon after crop emergence. Banded applications on small plants greatly reduces the cost of the fungicide program. Trials in Michigan and Ontario have shown the three most consistently effective downy mildew fungicides are Orondis Ultra, Torrent and Zampro. Where possible, tank mix each application with chlorothalonil (Echo/Bravo) or mancozeb (Dithane, Penncozeb, Manzate). Never make back-to-back applications of the same product or products from the same chemical family. Follow a 5 to 7-day application interval, and rotation of the three products can be repeated as necessary.

See crop labels for listed cucurbit crops registered on the products below. For more information, refer to publication 838.

Table 1. Downy mildew multi-site, broad spectrum fungicides

Common name (FRAC group)	Trade Names	Rate per hectare (Rate per Acre)	PHI (days)	Re-entry interval	Max applications
mancozeb (M03)	Dithane Rainsheild, Penncozeb 75DF Raincoat Manzate Pro-Stick	1.1-3.25 kg (0.4-1.3 kg)	14	12 hours	–
chlorothalonil (M05)	Bravo ZN Echo	4.8 L (1.9 L)	2	12 hours	2

High risk downy mildew conditions include downy mildew has been identified in the Great Lakes Region, prolonged periods of cool, wet weather, cooler night-time temperatures and heavy dew fall followed by warm, windy days and extended periods of leaf wetness due to dew, rain or overhead irrigation. Apply a preventative fungicide before a rainfall event or prior to overhead irrigation.

Table 2. Downy mildew specific fungicides

Common name (FRAC group)	Trade Name	Rate per hectare (Rate per Acre)	PHI (days)	Re-entry interval	Max applications
cyazofamid (21)	Torrent 400SC	150-200 ml (61-81 ml) + NIS or organosilicone surfactant	1	12 hours	6
amectotradin (45) + dimethomorph (40)*	Zampro	0.8-1 L (0.3-0.4 L)	1	1 day for hand harvesting, pruning or thinning, 12 hours for other activities	3
oxathiapiprolin (49) + mandipropamid (40)*	Orondis Ultra	0.4-0.6 L (162-243 ml)	0	12 hours	4

*resistance is known to the group 40 fungicides, mandipropamid and dimethomorph. Only the premix partners oxathiapiprolin and amectortradin in Orondis Ultra and Zampro, respectively, are effective on downy mildew.

For more information, check out the new cucurbit downy mildew factsheet.

Contacts

Elaine Roddy, Vegetable Crop Specialist, elaine.roddy@ontario.ca, 519-401-5890

Katie Goldenhar, Pathologist—Horticulture, katie.goldenhar@ontario.ca, 519-835-5792