Vegetable Notes 2024 Vol. 36:8

Managing Cucurbit Downy and Powdery Mildew

--written by Meg McGrath.and updated for 2024 by UMass Extension Vegetable Program.

For many years, we’ve published the results and recommendations from Meg McGrath’s annual bioassays and fungicide trials for resistance development in the cucurbit downy and powdery mildew pathogens. Meg retired last year from her position as plant pathologist at the Cornell Extension Long Island Horticultural Research and Extension Center (LIHREC). She and her work will be missed by all her colleagues and the many growers she’s helped over the years. Her fact sheets and other resources on downy and powdery mildews are still available on the Cornell Extension website, and you can find the full collection of resources from the LIHREC here. Below are Meg’s most recent downy and powdery recommendations, from 2022, with updates for 2024 by Sue Scheufele.

Cucurbit powdery mildew (PM) is a fungal disease of cucurbit crops that begins developing in New England in mid-summer every year. The fungus produces round patches of powdery white sporulation on the tops and bottoms of cucurbit leaves and severe infections will lead to extensive defoliation, reducing yield. The similarly named cucurbit downy mildew (DM) is a different pathogen; powdery mildew is a true fungus, whereas downy mildew is a fungal-like organism called an oomycete. Downy mildew spores land on host leaves, infect and grow within the leaf, and produce fuzzy gray sporulation only on the undersides of leaves. Lesions are angular because the pathogen cannot grow across the leaf veins. Both pathogens can be transmitted over long distances by wind. Neither downy nor powdery mildew infects cucurbit fruit directly; however, leaves infected by either pathogen will die prematurely, resulting in significant yield losses and decreased fruit quality. While several of the management recommendations are the same for preventing disease spread by these organisms, the distinction is important because not all the fungicides that control powdery mildew will control downy mildew, and vice versa.

Also, downy mildew is caused by an obligate parasite, meaning that it must have a living host to survive. For that reason, it does not overwinter in the Northeast, where winter temperatures kill cucurbit crops. Instead, it travels north from warmer regions on storms as the season progresses. The powdery mildew pathogen, on the other hand, can produce a sexual spore in the fall that enables it to survive over winter in New England.

Powdery & Downy Mildew Management

The most important components of an effective management program for powdery and downy mildews are resistant varieties and properly timed fungicides. Both diseases develop best on the undersides of leaves, so mobile (or translaminar) fungicides are needed to achieve successful control. Resistance to certain fungicides is widespread for both pathogens; fungicide recommendations change as new resistance develops or as new products are released. Always implement a resistance management program; do not wait until there is a problem. The goal is to delay development of resistance, not manage resistant strains afterwards. Again, because downy mildew is an oomycete and not a true fungus, targeted fungicides that control powdery mildew will not control downy mildew, and vice versa. Phytophthora blight, also caused by an oomycete, will usually also be controlled by fungicides that are effective for downy mildew.

• Select resistant varieties. There are many PM-resistant varieties of many types of cucurbits available, and DM-resistant cucumber varieties available.
• Inspect crops routinely for symptoms of both powdery and downy mildew, beginning at the start of crop development. Scouting routinely for early symptoms is important to ensure targeted fungicides are applied starting at the onset of disease development. Click here to view images of symptoms of downy and powdery mildews.
• Monitoring disease outbreaks is important for determining when fungicide applications are warranted. Cucurbit plants are susceptible to downy mildew from emergence; however, this disease usually does not start to develop in the Northeast until later in crop development when the pathogen is dispersed by wind into the region. The pathogen is thought to only be able to survive over winter in southern Florida, and from there spreads northward. There has been no evidence that the pathogen is surviving between growing seasons where winter temperatures kill cucurbit crops (outdoors above the 30th latitude). Tracking occurrences and paying attention to forecasted storms can alert you to when protectant and/or targeted sprays are warranted. Sign up to receive alerts about downy mildew occurrence and routinely check the Cucurbit Downy Mildew website to know where the disease is occurring and what crops are affected or read our weekly Pest Alerts for a summary update of powdery and downy mildew outbreak status.
• Make preventive and targeted pesticide applications based on forecast and reported risk. For both powdery and downy mildews, apply protectant fungicides weekly before symptoms develop in your crop. For powdery mildew, begin these preventive sprays when crops start producing fruit or when powdery mildew is reported in the area. For downy mildew, begin when the disease has been reported in the area and weather forecasts indicate that storms may be moving in your direction. When you first detect PM in your crop by scouting, add a PM-targeted material. When the DM risk level increases in your area, add a DM-targeted material. Targeted materials will be different for PM (a true fungus) and DM (an oomycete). Rotate between FRAC groups for the targeted materials.
• Add new fungicides to the program when they become available; substitute new for older products if they are in the same FRAC group, unless efficacy data indicate they are not as effective.

Protectant materials include:

• Sulfur: very effective, inexpensive product for PM. Has no efficacy for DM or other diseases.

• Oils: Effective for PM but not DM. Several botanical and mineral oils are available (search Table 23 in the New England Vegetable Management Guide for “oil”).

• Chlorothalonil and copper: Effective against both PM and DM. Copper is less effective against DM than chlorothalonil or mancozeb but is effective against bacterial diseases and OMRI-approved formulations are available.
• Mancozeb: Recommended when only DM is occurring.

Powdery mildew fungicides recommendations:

When powdery mildew is present, apply targeted fungicides weekly with contact fungicides (sulfur, chlorothalonil, and oil are more effective than copper) and alternate amongst available chemistry based on FRAC Group code. It is prudent to decide now what products to use each week and have a plan in place ready to implement. Use the highest label rates to control moderately resistant isolates if present.

• Vivando (FRAC 50) can be applied up to 3 times with no more than 2 sequential applications. It is a good choice for the last application considering the long residual activity observed in the 2018 fungicide evaluation; it is also recommended to be used at other times during the season.
• DMI fungicides (FRAC 3) are also a good choice in the program, particularly for the first application. Proline is the most effective. It is also labeled for Fusarium. Crop limit is 2 applications. Procure is also very effective. It can be applied 3 times, or 4 times to direct-seeded crops when the intermediate rate is used. Trionic is a newer product, which may be more effective, but efficacy data is lacking.
• Gatten (FRAC U13) was introduced in 2018, and so is a relatively new chemistry. REI is 12 hr. PHI is 0 days. It can be applied 5 times. Activity is limited to powdery mildew. It was as effective as Vivando for managing powdery mildew on lower leaf surfaces in a fungicide evaluation conducted at LIHREC in 2019 but not in 2018.
• Luna fungicides* contain fluopyram (FRAC 7) pre-mixed with other active ingredients to yield a somewhat confusing proliferation of magical sounding products each labeled for a different set of diseases and crops – Luna Sensation (+ FRAC 3), Luna Experience (+ FRAC 11), Luna Flex (+ FRAC 3). Luna Flex has the widest range of cucurbit crops and includes a DMI (recommended) pre-mix as opposed to a QOI (not recommended due to resistance) and is therefore the recommended choice for cucurbit PM (Luna Sensation is only labeled for watermelon). Luna Flex has a 12-hour REI and a 0-day PHI and can be used twice per crop per season. Velum Prime is another fluopyram product that can be used through drip chemigation.
• Miravis Prime* (FRAC 7+12) has a 12-hour REI and a 0-day PHI and can be used twice per crop per season.
• Orondis Opti* (FRAC 49 + M05) is a pre-mix of a new chemical, oxathiapiprolin, with the protectant fungicide chlorothalonil. Use Orondis Opti (or any other FRAC 49-containing product) in no more than 33% of the applications, or a maximum of 4 applications per planting, whichever is fewer. Orondis Opti has a 12-hour REI and a 0-day PHI.

Quintec, Torino, Endura, and QoI fungicides (FRAC 3) are not recommended due to resistance.

*Newer products that are less likely to have resistance since they have not been recommended for as long

Example recommended alternations of targeted fungicides:

• 6 applications: Orondis Opti, Vivando, Proline, Vivando, Luna Flex, Vivando
• 6 applications: Proline, Vivando, Luna Flex, Vivando, Procure, Vivando
• 8 applications: Orondis Opti, Proline, Vivando, Luna Flex, Proline, Orondis Opti, Vivando, Luna Flex

Assess control achieved on the underside of leaves toward the end of the season when there is still a good canopy.

Downy mildew fungicide recommendations:

There is more information available about each material, including maximum number of sprays and application recommendations, available at Dr. McGrath’s Cucurbit Downy Mildew Management website.

Targeted fungicides below are currently recommended:

• Elumin*, Zing! or Gavel (FRAC 22): 12-hour REI and 2-day PHI
• Orondis Opti (FRAC 49 + M05): 12-hour REI and 0-day PHI
• Zampro (FRAC 40 + 45): 12-hour REI and 0-day PHI
• Omega (FRAC 29): 12-hour REI and 30-day PHI for melons and 7-day REI for cucumbers and squash.
• Previcur Flex (FRAC 28): 12-hour REI and 2-day PHI. Can be applied foliar or via drip chemigation.
• Curzate, or Tanos (FRAC 27): 12-hour REI and 3-day PHI
• Ranman (FRAC 21): 12-hour REI and 0-day PHI in the field and 1-day PHI in the greenhouse.

*Newer products that are less likely to have resistance since they have not been recommended for as long

Bold indicates cucurbit DM “A-team” products per Mary Hausbeck, pathologist at Michigan State University

Targeted fungicides below are not recommended:

• Presidio (43)
• Revus and Forum (40)

Striped Cucumber Beetle: Focus On Early Control

As we head towards the end of May, striped cucumber beetles (SCB) are active and feeding on cucurbit foliage in parts of New England and have been reported in high tunnels and greenhouses in Massachusetts. Adult beetles overwinter in plant debris at field edges and move rapidly into cucurbit crops with the onset of warm weather. High tunnel and greenhouse cucumbers draw beetles first, followed by early field crops. Densities can be very high, especially in non-rotated fields or fields close to last year’s cucurbit crops. Adult beetles feed on cotyledons and young leaves, which can kill young plants causing stand reduction, delay plant growth, and reduce yield. They lay eggs in the soil near plant stems, and larvae feed on plant roots. This hidden damage reduces plant vigor and yield. Striped cucumber beetles also vector the bacterium Erwinia tracheiphila, the causal agent of bacterial wilt – this disease can be more damaging than direct feeding injury. It is important to focus on early, effective control to avoid yield impacts and protect pollinators.

Cultural controls

Crop rotation can reduce the impact of cucumber beetles, as can using transplants and protecting young cucurbits with row covers. In addition to insect protection, row covers also provide extra early-season heat, though it is important to remove them during times of excessive heat and at flowering to allow for pollination. Insect netting prevents beetle damage and does not trap in heat.

Scouting and thresholds: Cucurbit plants at the cotyledon and 1-to 2-leaf stages are more susceptible to infection with bacterial wilt than older plants. Thus, it is especially important to keep beetle numbers low before the 5-leaf stage. Scout frequently (at least twice per week up to SCB emergence, and for two weeks after) and treat after beetles colonize the field. Scout at least 25 plants to monitor the number of beetles and damage. Use this UMass Cucurbit Scouting Form to help keep track of what you find. The economic threshold depends on the crop. To prevent bacterial wilt in highly susceptible crops such as cucumber, muskmelons, summer squash, and zucchini, we recommend that beetles should not be allowed to exceed 1 beetle for every 2 plants. Less wilt-susceptible crops (butternut, watermelon, most pumpkins) will tolerate 1 or 2 beetles per plant without yield losses. Spray within 24 hours after the threshold is reached. Proper timing is key.

Perimeter trap cropping is another cultural control that has been shown to reduce or eliminate the need for  insecticide sprays while providing effective beetle control. Trap cropping exploits the fact that SCB are more attracted to Cucurbita maxima crops (e.g. buttercup and hubbard squashes and giant pumpkins) than C. pepo or C. moschata crops (e.g. pumpkins, summer squash, butternut squash, other winter squash). Note that some specialty pumpkin varieties are C. maxima types and very attractive to beetles. Plant 1 or 2 rows of a C. maxima variety in an unbroken perimeter around the field, using 2 rows near woods or last year’s fields, and space plants no wider than the between-row spacing that is used in the main crop. Trap crops planted in rows within the main field also work as trap crops that draw beetles as they move within the field. Do not use a crop that is highly susceptible to bacterial wilt (see scouting and thresholds) in the border. Beetles must be killed in the border, either by applying foliar insecticide when beetles first arrive or using a systemic insecticide at planting. Regularly scout both borders and the main crop to assess beetle numbers. Repeat perimeter-sprays if needed to prevent influx into the main crop, and spray the main field if thresholds are exceeded.

Insect netting in high tunnels: SCB can be especially damaging early in the season in high tunnels. An increasing number of growers are experimenting with covering high tunnel sides and doorways with insect netting to exclude SCB. We’ve heard some success stories with this tactic, and some frustrations about ripping expensive netting, reduced airflow in netted tunnels, and incomplete exclusion that leads to resident high tunnel SCB populations. This tactic is best combined with crop rotation out of cucurbits in a netted tunnel so that the beetles that do make it into your tunnel one year will be unable to find food the following year. Consider using shade cloth to reduce heat in the tunnel when netting is used.

Beneficial nematodes: Some growers have expressed interest in applying entomopathogenic nematodes to the soil to control SCB. These nematodes would target the soil-dwelling larval and pupal stages of SCB. Significant research has not been done on this topic, but theoretically it would be difficult to entirely control SCB using nematodes because of the high populations of SCB in the environments surrounding a field—if the SCB larvae and pupae in the soil in a field were successfully killed, more adults will continue coming in from field edges throughout the season.

Chemical Control

Foliar insecticides: There are a number of broad-spectrum conventional insecticides which can be used for foliar control of adult beetles, including carbamates (e.g. Lannate), pyrethroids (e.g. Warrior II, neonicotinoids, and diamides (e.g. Harvanta). All are highly toxic to bees and should only be used before bloom. Avoid using foliar neonicotinoid sprays if the same class was used as a seed or soil treatment (see below). See the cucurbit insect management section of the New England Vegetable Management Guide for more details.

Systemic insecticides including neonicitinoids and diamides can be applied to seed (e.g. thiamethoxam or ‘Farmore’ treated seed) or as an in-furrow, banded, drench, or drip irrigation application to the seed/seedling root zone during or after planting/transplanting operations e.g. Verimark (IRAC group 28). These offer about 21 days of protection right at the critical establishment period and before flowering. Note specific application methods and rates on labels.

Organic insecticides: Kaolin clay (Surround WP), pyrethrin (Pyganic Crop Spray 5.0 EC), and Azera (mixture of pyrethrin and azadiractin) are labeled for SCB. Surround does not kill the beetles but instead acts as a physical deterrent. With direct-seeded crops, apply Surround as soon as seedlings emerge if beetles are active. Transplants can be sprayed or dunked before setting out in the field—consult with your organic certifier about dunking beforehand to ensure they will allow this use pattern. As with other non-systemic insecticides, Surround must be re-applied after heavy rain and on new growth. Pyganic is a contact insecticide that provides a short-term knock-down of adult beetles with no residual effect. Spinosad (Entrust) is not labeled for and is not effective against SCB.

Reducing risk to pollinators: The New England Vegetable Management Guide describes many steps that growers can take to protect honey bees and native pollinators when using insecticides. The issue of neonicotinoids in particular has received a great deal of attention in recent years. This is a group of insecticides that have a chemical structure very similar to nicotine. They have been widely used in agriculture because they are effective against a wide range of insects, have lower mammalian toxicity compared to older classes of insecticides, and because they can be absorbed by roots and moved through the entire plant, reducing the need for foliar sprays. This trait allows for applications to be made to soil or on seeds, with less exposure to humans and to natural enemies of insect pests. Neonicotinoids are highly toxic to bees, and label requirements prohibit use on blooming crops or where there are blooming weeds or borders. Additional concern about impact on bees arises because research has shown that detectable, low concentrations of neonicotinoids can move into pollen or nectar. These are present at sublethal concentrations but may affect the foraging behavior of bees or suppress their immune system. The long-term or colony effects of sublethal concentrations of neonicotinoids are difficult to assess in the field because bees from each colony travel long distances and forage in many different habitats and types of plants. In cucurbits, both native bees (e.g., squash bees and bumble bees) and honey bees visit flowers to gather both pollen and nectar and are essential to crop pollination. Research in cucurbits has shown that higher levels of neonicotinoids were found after foliar treatments and chemigated insecticides were applied during flowering. Lower levels were detected in treatment regimens that involved a single application at planting via seed treatment, a drench application to transplant trays, or transplant water treatment. Thus, growers should avoid high rates and multiple applications, especially as the crop approaches flowering.

--UMass Extension Vegetable Program

2023 Cucurbit Downy Mildew Cucumber Variety Trial

Every year, the UMass Extension Vegetable Program evaluates cucumber varieties for resistance to cucurbit downy mildew (CDM). Cucurbit downy mildew (CDM) is a devastating disease of cucurbit crops that develops annually in the Northeast. There are several strains of CDM that affect different cucurbit crops; cucumber is susceptible to all strains, so we see CDM in cucumber without fail every year. CDM is caused by Peronospora cubensis, an oomycete pathogen that overwinters in the southern US (and potentially in greenhouse cucumbers in southern Canada) and is blown northward on storms every year. It usually arrives in the Northeast around mid-August. The pathogen causes interveinal chlorosis (leaf yellowing that is trapped between the leaf veins) that is visible from the top side of the leaf, and fuzzy gray sporulation on the undersides of leaves. As the disease progresses, leaf spots coalesce and leaves eventually die completely and severe defoliation can occur.

Prior to 2004, cucumbers were bred to have resistance to CDM and farmers could easily grow cucumbers through October. However, in 2004, the pathogen overcame that resistance and breeders have been working hard to develop new resistant varieties ever since. UMass trials have helped to identify CDM-resistant varieties appropriate for New England markets, including Bristol, DMR401, NYS264, and Citadel.

Cucumber seeds were sown in the greenhouse for the 2023 trial on June 15 in 50-cell flats and moved outside to harden off on July 5. Soil was amended with 130, 106, and 80 lbs/A of N, P, and K, respectively, in the form of Kreher’s pelleted chicken manure (5-4-3) prior to bed formation. Raised beds were formed and covered with 1-mil white-on-black plastic mulch with one drip irrigation line per bed. Seedlings were planted by hand into the beds on July 7, with plots consisting of 10 ft of bed with 6 plants in a single row at 18-in. spacing and 10-ft unplanted buffers between plots within a bed. Plots were arranged in a randomized complete block design, with beds on 15-ft centers, to keep varieties separate even after vine run. We measured downy mildew severity in every plot weekly, and total and marketable yield twice a week. Fruit was considered unmarketable if it was misshapen or had excessive bird or insect damage.

CDM arrived in the trial relatively late in the season but developed quickly once it arrived. The disease was first observed on August 8 on Stronghold, Gatehouse, and EWS-CUC-060, followed by the remaining varieties on August 15. Plant pathologists commonly compare disease incidence over time using a single value, called the Area Under the Disease Progress Curve, or AUDPC. A higher AUDPC value means more disease over time, and a lower AUDPC means less disease over time.

• Chaperon, SVCS0029, and EWS-CUC-059 had the lowest AUDPC values, and Chaperon and EWS-CUC-059 also had the highest marketable yields, making them good options for late summer cucumber production when DM pressure is high.
• EWS-CUC-060 had the highest AUDPC values.
• TSX-CU248AS, TSX-CU247AS, and TXS-CU249 had the lowest marketable yields, in part due to striped cucumber beetle feeding damage, which was severe on these varieties.

Click on images below to enlarge.

--G. Higgins, UMass Extension Vegetable Program ​