Cucurbit Powdery Mildew: A Little Late this Year but Start Scouting Now

We are just now finding powdery mildew on squash and pumpkins, several weeks later in the season than we have seen it during the past few years. So far disease incidence and severity have been relatively low in commercial fields, as well as in OSU research plots.  The fungus that causes cucurbit powdery mildew does not overwinter in Ohio, so the disease does not appear until spores arrive on wind currents from warmer growing areas.  This fungus is an unusual plant pathogen in that it is inhibited by free water – so the frequent rains we have been experiencing may have kept this disease at bay for the time being. However, it is here now and will undoubtedly flare up in susceptible cucurbits unless they are treated with fungicides. Signs of infection are small circular powdery growths (mycelium and spores of the pathogen) on either side of the leaf. These spots enlarge and can eventually cover most of the leaf surface and kill the leaves.  Stems and leaf petioles are also susceptible, but the disease is not observed on fruit.  In pumpkins, powdery mildew may also attack the “handles”, which can be further damaged by secondary pathogens.

Click on table to enlarge.

Powdery mildew is managed using powdery mildew-resistant varieties and fungicides.  Development of insensitivity to overused fungicides is common in populations of the fungus that causes this disease, so it is important that a fungicide resistance management program is followed. Remember to alternate fungicides in different FRAC (Fungicide Resistance Action Committee) groups, indicating different modes of action against the fungus. It is important to apply fungicides when the disease first appears and incidence is low. Fungicides that are effective against cucurbit powdery mildew can be found in the Midwest Vegetable Production Guide for Commercial Growers; product ratings are on page 117.  Our evaluations of efficacy of powdery mildew fungicides at three locations (Wooster, Columbus, South Charleston) in Ohio in 2016 indicated that five products consistently provided very good control (> 93%) of powdery mildew  on pumpkins in all three locations (see table).  Three products were very good in Wooster and Columbus but fair in South Charleston; control by Fontelis was 73% in South Charleston, while the others provided less than 55% control at that location.  Both Bravo and Pristine performed poorly in all three locations.

Cucumber Downy Mildew Confirmed in Southeastern Ohio – Belmont County

We confirmed downy mildew in cucumbers collected yesterday from a small commercial field in Belmont County, near Barnesville.  Disease incidence and severity were high in this field, which had not been treated with any fungicides. The outbreak followed several days of intense rainfall in the previous week.

Map of cucurbit downy mildew outbreaks – CDM ipmPIPE, July 26, 2017.

This is the first report of cucumber downy mildew in central or southern Ohio, and was found during a field walk sponsored by OSU Extension and the Captina Produce Auction.  Ohio growers should assume that cucumber/melon downy mildew is more widespread than we have been able to report, and should protect these crops with appropriate fungicides as listed in my post on July 28.  We have not seen downy mildew on squash, pumpkins or watermelon in Ohio to date, and the downy mildew pathogen population currently affecting cucumbers and melons in Ohio is not likely attack pumpkins or squash.  A pumpkin field near the cucumber planting that was highly diseased was not affected, and we have not found downy mildew in our sentinel plots, despite the presence of infected cucumbers for several weeks.  However, downy mildew populations that can damage pumpkins and squash are likely to move in from the southeastern U.S. later in the season.

We depend on county educators, growers and consultants to let us know when cucurbit downy mildew is suspected, particularly in counties where it has not been reported.  Our lab will diagnose samples at no cost to Ohio growers, so we appreciate receiving samples that, if downy mildew is confirmed, will enable us to alert the cucurbit growing community.




Downy Mildew Confirmed on Melons in Wayne County and Cucumbers in Henry County, OH

Downy mildew on cantaloupe.

Downy mildew continues to spread on cucumbers in Ohio, with a confirmed report in Henry County this week.  The disease is likely to be widespread on cucumbers in northern Ohio, particularly after last week’s rainy, humid weather.  As usually happens within a few weeks of cucumber downy mildew outbreaks, we are now finding downy mildew on cantaloupe.  Chris Smedley and the Wayne County IPM Scouting team found widespread and fairly severe downy mildew in commercial melons in the northwestern part of Wayne County, where we first reported downy mildew on cucumbers on June 28.  As noted in previous posts, it is imperative that growers protect melons as well as cucumbers with downy mildew-effective fungicides such as Orondis Opti and Ranman if they are in an area where downy mildew risk is high, such as northern Ohio.  See my June 28 post for a list of recommended fungicides. Remember to follow label instructions and alternate products with different modes of action.

More Cucumber Downy Mildew – Wood County, OH

We have just reported to the Cucurbit Downy Mildew ipmPIPE the first case of downy mildew on cucumbers in Wood County. While the sample was just received today, the grower estimated that symptoms were first present July 7, about a week after the first report of downy mildew on cucumbers upwind in southeastern Michigan. The multiple rainstorms last week likely delivered the downy mildew pathogen from long distances and also moved it about locally, while the overcast, humid conditions favored pathogen survival and infection. Although predicted weather patterns for this week in northern Ohio are not as favorable for downy mildew as last week, growers should assume the pathogen is present and take  or continue measures to protect cucumbers and melons from downy mildew.  We have not yet found or had reports of downy mildew in central or southern Ohio on any crops, and no reports of the disease on squash, pumpkins, cantaloupe or watermelon in northern Ohio. That being said, cantaloupe is second to cucumber in susceptibility to downy mildew, and should be protected by fungicides in this area.  See my posts on June 28 and July 8 for more information on cucurbit downy mildew management.

Single lesion of downy mildew on cucumber leaf from Wooster sentinel plot.

We also found downy mildew at a very early stage of development in our sentinel plot in Wooster today (see photo).  There were only a few lesions present and it took a keen eye to spot them.  When scouting for downy mildew in cucumbers, look for this type of early lesion.  If the diseases progresses significantly it will be difficult to avoid yield losses despite fungicide applications.

If you suspect downy mildew in any cucurbit or basil, please send us a sample for confirmation.  It is best if the sample is wrapped lightly in a damp paper towel and shipped in a box by overnight mail or courier to: Sally Miller, OSU-OARDC, Dept. Plant Pathology, 1680 Madison Ave, Wooster, OH 44691; 330-466-5249.


Preventing Anthracnose in Peppers and Tomatoes

Hot rainy weather and fields with a history of anthracnose mean high risk for for this disease in peppers and tomatoes.  Anthracnose, caused by species of the plant pathogenic fungus Colletotrichum,  causes no obvious leaf lesions on tomato foliage and only occasionally on pepper foliage under high disease pressure.  However, pepper and tomato fruits are very susceptible to the disease.  The fungus can be introduced on seeds, and survives over the winter in temperate climates associated with crop debris.  Fruits are infected when green; pepper fruits develop large lesions with salmon-colored spores when green or ripe, but tomato fruits do not develop the typical sunken lesions until they begin to ripen.  Spores of the fungus are moved about by splashing rain, so rainstorms can promote disease spread throughout a field.  Mechanically harvested processing tomatoes are particularly prone to anthracnose problems since fruits ripen at different rates but are harvested all at once.  Management practices include thorough scouting, sanitation/removal of diseased fruits, and fungicide applications.  There are some differences in susceptibility of pepper and tomato varieties to anthracnose, but none are highly resistant.

It is time now to start protecting plants from anthracnose – fungicides must be applied as soon as fruits begin to set, and continued on a weekly schedule as fruits develop.  Fungicides  labeled for use against anthracnose in fruiting vegetables (eggplant, pepper, tomato) are listed in the Midwest Vegetable Production Guide for Commercial Growers.  Several studies have shown the best results with Aprovia Top, Quadris, Quadris Top, Cabrio or Priaxor alternated with chlorothalanil or mancozeb.  Some labels may recommend a spreader-sticker; be sure to read and follow label instructions.

Phytophthora Blight Already a Problem in Cucurbits

Cucumber seedling with Phytophthora blight lesions on the cotyledons; pictured on the left is a positive immunostrip test for Phytophthora.

Ohio cucurbit growers have been battling Phytophthora blight in squash for at least the past 2 weeks, and yesterday we diagnosed an unusual outbreak in cucumber seedlings. Phytophthora can attack any part of cucurbit plants, but we don’t usually see seedling disease in cucumbers in Ohio.  This may be due in part to an “escape” scenario, since many cucumber crops are planted early before Phytophthora blight pressure builds. In addition, cucumber plants usually hold up well to Phytophthora blight, although fruits are highly susceptible. It looks like this may be another season of high Phytophthora blight pressure.

Management of Phytophthora blight in cucumbers and other cucurbits is complicated by the fact that downy mildew may also be a problem.  Currently we are only seeing downy mildew on cucumbers in northern Ohio, although we expect to find it in melons soon – see my posts on June 28 and July 8. Phytophthora blight is different from downy mildew in a number of ways, and similar in others. One big difference is that Phytophthora movement through the air is limited, unlike downy mildew. Another is that the pathogen survives over the winter in Ohio. Third, Phytophthora has to be introduced into a field, usually by contaminated water or movement of soil. Phytophthora tends to thrive in hot weather and downy mildew in cooler weather, but there is considerable overlap. Phytophthora and the downy mildew pathogen are related; some of the fungicides that are effective against downy mildew are effective to a degree against Phytophthora.  All cucurbits are susceptible to Phytophthora blight, as are peppers and some other vegetable crops. No varieties of cucurbits are resistant to Phytophthora blight, so cultural practices (raised beds, good drainage, clean irrigation water; no cull piles) and fungicides are needed to manage this disease. The pathogen is a “water mold” and moves around readily in wet, and especially flooded fields. The sporangia that contain the zoospores that swim to and infect plants can also be splashed onto stems, leaves and fruit.

Keeping ahead of both diseases is important, but any efforts to manage them can be undone by long periods of wet weather, when it is particularly important to keep a tight schedule of fungicide applications. The Midwest Vegetable Production Guide for Commercial Growers lists the products labeled for these diseases on cucurbits – see page 117 in the 2017 Guide for a chart on relative efficacy of fungicides against different diseases of cucurbits. Dr Mary Hausbeck, Michigan State University, recommends applying fungicides when pickles are 1”, 3” and 5” long to suppress Phytophthora blight. Fungicides with 0-days pre-harvest interval (PHI) – Orondis Ultra, Ranman, Zampro and Zing! – will be useful when fruits are close to harvest. Make sure to note re-entry intervals. While copper in itself is not very effective against Phytophthora blight, tank mixing with a copper fungicide (e.g. Kocide 3000 – also 0 days PHI) improves activity against Phytophthora. Dr. Mohammad Babadoost’s research at the University of Illinois indicates that Revus tank mixed with a copper fungicide and alternated with Ranman + copper, Tanos + copper, or Zampro + copper (7-day intervals) is effective in suppressing Phytophthora blight.

Be sure to read and follow label directions for application of fungicides and other pesticides.  Alternating fungicides with different modes of action is critical to reduce the risk of resistance development in pathogen populations.

Click on table to enlarge.

Cucumber Downy Mildew Spreading in Northern Ohio

Downy mildew pathogen sporulation on underside of a cucumber leaf.

On July 5, downy mildew was found on cucumbers in Huron County, one week after the first 2017 Ohio report (Wayne County) of the disease. In this case, disease symptoms had just become visible in a low spot in the field, but the sporulation was very strong, indicating a good potential for the pathogen to spread. The disease was also confirmed in Ontario on June 27 and in Michigan on June 29. Given the favorable weather conditions of the last week, with high humidity, overcast skies and rainstorms moving through the state, growers in the northern third of Ohio should assume that the risk of downy mildew in cucumbers and melons is very high. See the Cucurbit Downy Mildew ipmPIPE website for a detailed forecast; you can also find the map of outbreaks on this site. As indicated in my post on June 28, growers need to scout fields carefully, especially in low or shady spots that may hold moisture longer than other parts of the field. The only way to manage this disease at this point is through application of effective fungicides – and recommendations are provided in the June 28 post.

There were reports of downy mildew on cucumbers in Erie County in western NY and Columbia County in central PA, but so far there have been no reports of downy mildew on pumpkins or squash in the US Midwest or Northeast.

Reminder: I report outbreaks of cucurbit downy mildew and other veggie diseases as soon as we know about/confirm them on Twitter: @OhioVeggieDoc.