CDM.ipmpipe map of pumpkin downy mildew reports, August 10, 2022

We have had a confirmed report today from Bill Holdsworth of Rupp Seeds of downy mildew on pumpkins in research plots near Wauseon, Ohio, in Fulton County. This is the first report of downy mildew on pumpkins, not only in Ohio but in the Great Lakes region and in fact a big swath of the Midwest and Northeast

CDM.ipmpipe map of cucumber downy mildew reports, August 10, 2022

(map on left). Compare this to the cucumber downy mildew map on the right, with widespread distribution. As a reminder, these maps are constructed from voluntary reports of downy mildew. The disease is likely more widespread than the maps show. Bill has also seen downy mildew on gourds in the area.

The downy mildew pathogen, Pseudoperonospora cubensis, exists as two clades. Clade 2 is quite specific to cucumbers and melons and tends to circulate early in the Great Lakes region, having possibly overwintered on cucumbers in greenhouses. It also makes its way up the Eastern US from southeastern states. Clade 1 has a broader host range, attacking all cucurbits, and also originates in the Southeast, moving up the through the eastern states. Due to typical wind currents from west to east, we don’t usually see clade 1 outbreaks in the Midwest until mid-August or September, when spores can be carried in storms that are remnants of hurricanes originating in the Southeast. We don’t know with certainty yet if the Fulton County outbreak was caused by clade 1 strains, but this is likely. We hope to conduct molecular tests to confirm the clade in the near future.

Downy mildew symptoms on a pumpkin leaf (top)

Downy mildew symptoms on the underside of a pumpkin leaf.

Downy mildew can be a bit harder to diagnose in pumpkins, squash and other Cucurbita species since the lesions may be smaller and not always crisply angular as in cucumbers. Downy mildew can also cause a lot of damage on pumpkins, squash, gourds and other Cucurbita spp.

Growers should be scouting all cucurbits for symptoms of downy mildew. The best time to do this is in the morning before 9 or 10 am before the lesions dry out and the sporangia disperse. If you find suspicious symptoms you may text me photos of the underside and top of symptomatic leaves (330-466-5249) and/or send us samples for confirmation.

Growers of any cucurbits throughout Ohio should apply appropriate protectant fungicides such as chlorothalanil (e.g. Bravo). Growers in Fulton County should begin a fungicide program with downy mildew-effective fungicides now. See my July 11 post for a list of recommended fungicides.

Downy mildew on basil

Downy mildew was spotted in a garden in Wooster, Wayne County, Ohio and confirmed by Dr. Francesca Rotondo, interim director of the OSU Plant and Pest Diagnostic Clinic. This is the first report in Ohio this summer but it has likely been here for a few weeks. The pathogen, Peronospora belbahrii, is related to but different from the cucurbit downy mildew (CDM) pathogen, Pseudoperonospora cubensis. These pathogens don’t cross-infect hosts: P. belbahrii does not infect cucurbits and P. cubensis does not infect basil. However, their biology is similar; they are both obligate parasites that require living plant tissues to survive. They disappear from the outdoor environment in northern areas during the winter and are introduced the following spring or summer from infected plant material or via spores carried on wind currents and rain. Cucurbits or basil grown over the winter in greenhouses can be a source of inoculum. Last year we found basil plants with severe downy mildew symptoms in a big box store in Wooster. We tend to begin

Light microscope image of sporangia of the downy mildew pathogen Peronospora belbahrii. Image by Francesca Rotondo.

seeing basil downy mildew in Ohio in July or August. Rainy, cloudy weather favors spore (sporangia) transport and infection; the sporangia are sensitive to UV light and tend to be killed by sunshine.

Management of basil downy mildew, like CDM, is entirely preventative. There are resistant varieties and a number of fungicides and biologicals are available for conventional and organic production systems.  Fungicides and biologicals are only effective if applied before infection. Home gardeners should keep an eye on their basil plants and harvest non-diseased leaves as soon as downy mildew is observed. More specifics can be found in last year’s July 17 post.

Cucumber downy mildew in a sentinel plot at OSU NCARS, Sandusky County. Photo by our intern Raven Schaffter.

Northern Ohio counties are falling like dominoes to cucumber downy mildew. The disease was detected today in our sentinel plot on the OSU North Central Agricultural Experiment Station in Fremont, Sandusky County. This is the sixth county in northern Ohio for which we have confirmed reports of cucumber downy mildew. Management information can be found in yesterday’s post and previous posts on this site.

Confirmed reports of cucurbit downy mildew. https://cdm.ipmpipe.org

Downy mildew developed in a commercial fresh market cucumber field in Huron County this past weekend. That makes five counties in northern Ohio with confirmed reports of downy mildew, all on cucumbers. It is likely that downy mildew is widely distributed in cucumbers in northern Ohio, but we are not always alerted to its appearance. Since our first report of downy mildew on cucumbers in Wayne County on July 13, it has been reported in several other fields, including one of our research plots in Wooster. So far there are no confirmed reports of downy mildew on melons, pumpkins, squash or other cucurbits in Ohio.

Downy mildew must be managed preventatively with resistant varieties (there are a few, see chart below) and more commonly with fungicides. In our experience, fungicides applied after infection are significantly less effective than the same ones applied before infection. Cucumber and melon growers in northern Ohio should be applying effective fungicides  on a 5-7 day schedule depending on the label requirements. Growers elsewhere in the state should be applying protectants such as chlorothalanil, e.g. Bravo, to all cucurbits. This will also help prevent other diseases such as Alternaria leaf spot and gummy stem blight. More details and a fungicide efficacy table can be found here.

We are seeing many reports of yellowing and/or wilting squash, pumpkin and other cucurbits in commercial fields and gardens this month. Chances are that the cucurbits have been affected by cucurbit yellow vine decline (CYVD) or bacterial wilt. Both of these diseases are caused by bacteria transmitted to plants during the feeding of their insect vectors. Once infected, the plants cannot recover; these diseases must be managed preventatively by controlling the insect vectors, ideally early in the season. At this point, insecticides may be applied to prevent the diseases from spreading to healthy plants. It is also useful to remove and destroy symptomatic plants that serve as sources of bacterial inoculum.

Sticky bacterial ooze from a cucumber vine with bacterial wilt

Bacterial wilt of melon

Bacterial wilt affects many cucurbits including cucumber, melon, pumpkin and squash. The causal agent, Erwinia tracheiphilia, overwinters in the digestive system of spotted and striped cucumber beetles.  When these beetles emerge in the spring, the pathogen is spread from beetle feces to healthy cucurbits mainly via wounds caused by insect feeding.  Bacterial wilt occurs almost every summer in Ohio, but is less severe after very cold winters that reduce overwintering beetle populations. Symptoms begin as discoloration and wilting of individual leaves.  As the disease progresses, the entire plant begins to wilt and collapse as the bacteria clog the xylem vessels.  When the stem is cut along the base, clear to white elastic strands comprised of the bacteria and “gum” are visible when cut ends of stems are slowly pulled apart. More details can be found here. Insecticides labeled for cucurbits and effective against the beetles can be found in the Midwest Vegetable Production Guide for Commercial Growers. Covering plants with netting or floating row covers until flowering can also protect plants from early infection.

Cucurbit yellow vine decline in summer squash

Cucurbit yellow vine decline in pumpkins.

Cucurbit yellow vine decline (CYVD) is caused by the bacterial pathogen Serratia marcescens, transmitted by squash bugs. It is uncommon in some years but in others can do a lot of damage. Bright yellowing of leaves, followed by by wilting and death of plants is indicative of possible CYVD. If squash bug adults, nymphs or eggs are found on the underside of leaves, this is a good clue that the symptoms are caused by CYVD. A cross-section of the vine may show a light tan discoloration of the vascular tissues. CYVD is managed by applying insecticides (see Guide link above). For both CYVD and bacterial wilt, fields and gardens should be scouted regularly for the insect vectors beginning soon after transplanting or seedling emergence.

The OSU Plant Disease Diagnostic Clinic in Wooster can provide a definitive diagnosis of CYVD using a PCR assay.  Testing for this and other diseases and pests is free of charge for Ohio commercial vegetable growers thanks to financial support from the Ohio Produce Growers and Marketers Association.

Eggs of the squash bug

Cross-section of squash vine with light browning of the vascular system caused by the bacteria

Reports of cucurbit downy mildew as of July 21, 2022. cdm.ipmpipe.org

A new report came in this week of cucumber downy mildew in Fulton County, as well as another report from Medina County, this time from a different farm in the Homerville area.  Downy mildew is favored by cooler conditions so despite many hot days, nights were cool enough for the pathogen to get a foothold in northern Ohio. In addition, we have seen a number of storms across the area; spores of the pathogen travel by air and clouds protect them from UV light, while rain drops them to earth.

Cucumber and melon growers in northern Ohio should scout fields for downy mildew and apply appropriate fungicides on a 7-10 day schedule, depending on the fungicide. See this post for fungicide recommendations.

Many thanks to Bill Holdsworth and Frank Becker for reporting these outbreaks.

Cucurbit downy mildew incidence report, July 20, 2022. https:/cdm.ipmpipe.org

Downy mildew was reported on cucumbers in NE Ohio (Medina and Wayne counties) and southern Ontario last week, suggesting widespread occurrence possible in northern Ohio. This morning Marty Hofbauer, CCA and Agronomist at Luckey Farmers, Inc., discovered downy mildew in fresh market cucumbers near Tiffin, Ohio in Seneca County. Marty sent me excellent photos of leaf lesions taken with his smartphone that allowed me to confirm downy mildew without the need to send in a sample. By taking the pictures early in the morning, the sporulation on the undersides of leaves was clearly visible in young lesions. Sometime about mid-late morning the sporangia will be discharged. Older lesions tend to dry out and are not necessarily diagnostic for downy mildew. Several of his pictures are included here as examples of what to look for.  Angular leaf spot, caused by a bacterium, also makes angular lesions, but the grey fuzzy growth and black dots (sporangia) on the lower side of the lesion is diagnostic for downy mildew.

Cucumber downy mildew – young lesions. Photo by Marty Hofbauer.

Underside of cucumber downy mildew lesion showing sporulation of the pathogen. Photo by Marty Bauer.

I continue to be amazed at the increasingly high quality and resolution of smartphone images. For a disease like downy mildew, where quick diagnosis and treatment is crucial, taking high high quality pictures that can be sent to us for confirmation saves both time and money. Ohio cucurbit growers, consultants and others may send me pictures such as these by text (330-466-5249) or email (miller.769@osu.edu) for diagnosis. Physical samples may be sent to Dr. Francesca Rotondo in the OSU Vegetable Pathology Lab – see submission instructions here. Vegetable and fruit diagnostics are free to Ohio growers due to financial support from the Ohio Produce Growers and Marketers Association’s Ohio Vegetable and Small Fruit Research and Development Program.

Cucumber downy mildew July 13, 2022, Wayne County, OH. Photo by OSU diagnostics intern Vansh Khatri.

Quick on the heels of our report of cucumber downy mildew on Monday in Medina County, we have diagnosed the disease on cucumbers in the Fredericksburg area of Wayne County. With thunderstorms rolling across northern Ohio this afternoon, we expect the disease to spread to cucumbers and melons throughout this part of the state. See my post on July 11 for fungicide recommendations.

Reported cucurbit downy mildew, July 13, 2022. http://cdm.ipmpipe.org

Cucurbit downy mildew map, July 11, 2022. cdm.ipmpipe.org

Today OSU plant diagnostician Francesca Rotondo diagnosed the first cucumber sample of the 2022 growing season with downy mildew. Downy mildew has been a bit slow to appear, likely due to the high temperatures and often sunny and dry conditions. Last year our first report of cucumber downy mildew in Ohio was on July 12. I expect that conditions last week – rain and overcast skies in northern Ohio -promoted spore transport, deposition and infection. The sample came from an organic farm in the Homerville area, in Medina County – it was just getting started in the field and disease incidence and severity were low. Cucumber and melon growers in northern Ohio should ramp up their spray programs to include highly effective fungicides against downy mildew such as Orondis Opti, Ranman, Omega, Previcur Flex, and Elumin (see efficacy table below). Curzate was effective in our 2021 tests but has been variable in efficacy over the years. It is recommended to tank mix these products with chlorothalanil. Check the labels carefully for use instructions and restrictions. Remember to alternate products in different FRAC groups. Fungicides must  be applied preventatively – they  are far less effective if applied after  infection.

Growers in central Ohio should intensify scouting of cucumbers and melons and apply a protectant fungicide. Look for yellow or tan angular lesions delimited by veins on the top surface of leaves, and fuzzy grey/brown growth on the undersides of the lesions. With a good hand lens or a smartphone camera with high magnification you may be able to see small dark brown/purple spots within the fuzzy growth. These are the spores of the downy mildew pathogen.

Smartphone image of a downy mildew lesion with the pathogen sporulating on the underside of a cucumber leaf.

If you suspect downy mildew in cucumber or melon please text or email pictures to Sally Miller (330-466-5249; miller.769@osu.edu) of both sides of lesions, with the underside in the highest possible magnification. I can often confirm downy mildew from photos, but if not will ask you to send a sample to the OSU Vegetable Disease Diagnostic Lab for confirmation. Instructions for sample submission are here. Thanks to financial support from the Ohio Produce Growers and Marketers Association’s Ohio Vegetable and Small Fruit Research and Development Program, there is no fee for this service for Ohio vegetable growers.

An old post updated:

Septoria leaf spot has been found in Ohio recently, not surprisingly after heavy rains in many areas. This excellent photo taken by Dr. Francesca Rotondo is diagnostic for Septoria leaf spot: round tan to brown spots on the leaves and leaf yellowing. In the more mature spots, margins are dark brown and small round black dots can be seen in them through a hand lens or the lens of your smartphone camera. The tiny black dots are called pycnidia, the fruiting bodies of this fungus. Pycnidia are flask-shaped structures with a small hole at the top, partially submerged in leaf tissue. Pycnidia contain large numbers of spores held in a gelatinous matrix; when humidity is high or free water is on the leaf surface, the spores ooze out of the pycnidia like toothpaste being squeezed out of a tube. The spores are dispersed by rain or irrigation water to other leaves on the same plant and to other nearby tomato plants.

Septoria lycopersici is seedborne and also survives at least 1-2 years in soil. Septoria leaf spot is favored by moderate temperatures, high humidity and rain or overhead irrigation. While Septoria does not cause spots on tomato fruit, it can rapidly defoliate the plant. If this happens early the plant is likely to die. Later on, defoliation leads to small fruit, poor ripening and problems with sunscald. Even large, previously healthy, vigorous plants can be completely defoliated.

Commercial growers can manage Septoria leaf spot by including a strobilurin fungicide such as Quadris or Cabrio in a fungicide program that also includes a protectant such as chlorothalanil or mancozeb. A list of labeled fungicides to manage Septoria leaf spot can be found in the Midwest Vegetable Production Guide for Commercial Growers. Cultural practices to minimize Septoria leaf spot should be undertaken, including crop rotation of 3 years or more, planting tomatoes on raised beds, using adequate spacing, using drip irrigation, pruning foliage to allow good air movement through the canopy, and removing diseased plants from the field. Septoria leaf spot is rarely a problem in high tunnels, greenhouses and other structures that protect plants from rain.

Organic growers need to follow the cultural practices described above and may consider growing tomatoes in protected culture. Copper-based fungicides formulated for organic production can suppress disease development if applied soon after initial symptoms appear.

Home gardeners should adopt the cultural practices described above and should also remove and destroy leaves with symptoms. This is really only effective when symptoms first appear. Always avoid the foliage when watering plants. Fungicides containing chlorothalanil or copper can be applied to slow disease spread.