Effectively Managing Cucurbit Downy Mildew in the Mid-Atlantic and Northeast Regions in 2013

June 14, 2013 in Vegetable Crops

Meg McGrath (Cornell), Beth Gugino (Penn State), Kate Everts (Univ. Maryland), Steve Rideout (Virginia Tech), Nathan Kleczewski (Univ. Delaware), and Andy Wyenandt (Rutgers)

Producing a high-quality cucurbit crop necessitates effectively managing downy mildew. This foliar disease is common in the mid-Atlantic and northeast because the pathogen produces a large quantity of asexual spores that are easily dispersed long distances by wind, which enables it to spread widely. Although the pathogen cannot survive between growing seasons where winter temperatures kill cucurbit crops, it moves throughout the eastern USA each year via its asexual spores. The pathogen does not affect fruit directly; however, affected leaves die prematurely which results in fewer fruit and/or fruit of low quality (poor flavor, sunscald, poor storability).

The most important component of an effective management program for downy mildew is an effective, properly-timed fungicide program. The key is to apply mobile fungicides targeted to the pathogen starting when there is a risk of the pathogen being present in your area. Locally systemic (or translaminar) fungicides are needed to manage the disease on the underside of leaves. Because these fungicides have targeted activity towards oomycete pathogens like downy mildew and Phytophthora blight, different fungicides must be used to manage other diseases, such as powdery mildew.

Resistant varieties are another tool for managing downy mildew. Resistance was the main tool for cucumbers until a new strain of the pathogen developed. Since 2004, varieties with this resistance, which include most hybrids, have provided some suppression of the new strains, but substantially less than the excellent suppression that was achieved against strains present before 2004. However, these resistant varieties are still considered a worthwhile component of an integrated program. Fortunately, a new source of resistance has been found and cucumber varieties with these new genes for resistance are being developed. Seminis Vegetable Seeds is one company developing new varieties. The first ones released in 2012 did not have the resistance successfully incorporated and did not exhibit greater suppression of downy mildew than previously developed resistant varieties.

Fungicide Program
Alternate among targeted, mobile fungicides from different FRAC groups and apply with a protectant fungicide to manage resistance development and to help avoid control failure if resistance occurs. Remember to comply with label use restrictions. The pathogen has demonstrated the ability to develop resistance to fungicides, thus a diversified fungicide program applied to resistant varieties when possible is critical for success.

When to Apply Fungicides
An important resource for determining when fungicide applications are warranted is the NCSU Cucurbit Downy Mildew Forecasting (CDM ipmPIPE) website at http://cdm.ipmpipe.org. The forecasting program monitors where downy mildew is currently active and predicts where the pathogen likely will be successfully spread and cause disease based on the current and forecasted weather conditions. The risk of downy mildew occurring throughout the eastern USA is forecast and posted three times a week (Monday, Wednesday, and Friday). Forecasts enable timely fungicide applications based on the risk of disease development. Growers can subscribe to receive customizable alerts by e-mail or text message. Information on the cucurbit hosts affected is also available. This is important because the pathogen exists as pathotypes that differ in their ability to infect different cucurbits. All pathotypes can infect cucumber; while only some can also infect melons and squashes. Success of the forecast system depends on knowledge of where downy mildew is occurring; therefore prompt reporting of outbreaks by growers is critical.

Recommended Downy Mildew Specific Fungicides
Use in alternation and tank mixed with a protectant fungicide. Label directions for some products state to begin use before infection or disease development. The forecasting program helps ensure this is accomplished and lets you know when your crops are at risk. The updated FRAC Guidelines and Efficacy Table for cucurbits is now available.

Ranman (FRAC code 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.

Previcur Flex (28). This fungicide is more systemic than others and has good activity for downy mildew, but it is not effective for Phytophthora blight, which usually is also a concern in cucurbit crops. REI is 12 hr. PHI is 2 days. Apply no more than 5 times in a season.

Zampro (40 + 45) and Revus (40). While in the same fungicide chemical group, these products may have a slightly different mode of action, thus there may be benefit to using one early in a season-long fungicide program and then switching to the other product later in the program. REI is 12 hr. PHI is 0 day. Apply no more than 3 times (4 for Revus) in a season with no more than 2 consecutive applications (none with Revus). Revus must be applied with a spreading/penetrating type adjuvant. Revus has exhibited differential activity among cucurbit types. It is very effective for downy mildew in pumpkin but not in cucumber and therefore it is not recommended for use in cucumber.

Curzate (27) or Tanos (11 + 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 a fungicide application is not possible 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. Curzate is not labeled for Phytophthora blight.

Gavel (22). This is the only product that consists of a targeted fungicide (zoxamide) and a protectant fungicide (mancozeb). REI is 48 hr. PHI is 5 days. Apply no more than 8 times in a season. Some cantaloupe varieties are sensitive to Gavel. Workers must be notified that a dermal sensitizer was applied both orally and by posting at entrance to treated area for 4 days.

Presidio (43). Until recently, Presidio had been the most effective fungicide in several university fungicide evaluations*. Control was moderate to poor in several fungicide efficacy trials conducted in the eastern USA (FL to NJ) in 2011 and especially in 2012 suggesting that resistance likely has developed. In sharp contrast, Presidio was highly effective in trials conducted in OH and MI in 2012, providing 91-100% control versus 12-43% control in trials in the eastern USA. This finding indicates the pathogen population in the mid-west differs from that in the east. Until resistance develops in the mid-west, in production areas where the pathogen could come from the south or the mid-west, such as western to central NY and PA, growers will want to use the CDM ipmPIPE forecast website to determine where the pathogen is originating as this will dictate the utility of including Presidio in the fungicide program.

*Presidio and other fungicides were tested alone in these experiments, which is neither a labeled nor recommended commercial use pattern for these fungicides; it is done in efficacy evaluations to determine if resistance affects control.

It is prudent where Presidio is included in the fungicide program to use it judiciously with limited applications in a good rotation program. Presidio has a long rotational interval of 18 months for non-labeled crops, which can be a constraint on production. The label has been expanded and now includes all cucurbits, fruiting vegetables, leafy vegetables, brassica (head and stem), bulb vegetables, sweet potatoes and root vegetables (except carrot, sugar beet, potato). REI is 12 hr. PHI is 2 days. Apply no more than 4 times in a season with no more than 2 consecutive applications. Presidio must be applied with another fungicide.

Recommended Protectant Fungicides
Chlorothalonil and mancozeb are the main protectant fungicides for downy mildew. Copper is not as effective. Dithane has a supplemental label that includes pumpkin, winter squash and gourd.

No Longer Recommended
Resistance to mefenoxam and metalaxyl and to strobilurins is sufficiently common that fungicides with these active ingredients (e.g. Ridomil and Cabrio), which used to be highly effective, are now ineffective and should not be applied for managing downy mildew.

In summary, to manage downy mildew effectively in cucurbit crops: 1) select resistant cucumber varieties, 2) sign-up to receive alerts about downy mildew occurrence and routinely check the forecast web site to know where the disease is occurring and what crops are affected, 3) inspect crops routinely for symptoms beginning at the start of crop development, and 4) apply targeted fungicides tank-mixed with protectant fungicides weekly and alternate among available chemistries based on FRAC code, starting when there is a risk of downy mildew for the specific crop based on the CDM ipmPIPE forecast. 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. Note that some products mentioned are not yet registered for use on cucurbits. Check labels for use restrictions. Any reference to commercial products, trade or brand names is for information only; no endorsement is intended.

click image below for pdf of theĀ 2013 Fungicide Resistance Management Guidelines for Cucurbit Downy Mildew and Powdery Mildew Control in the Mid-Atlantic & Northeast regions of the United States

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