Downy mildew of cucumber.

Downy mildew was confirmed today in Saginaw County, MI. Dr. Mary Hausbeck’s team at Michigan State University also detected spores of the downy mildew pathogen, Pseudoperonospora cubensis, in air samples in several MI counties.  Although we haven’t seen or heard of downy mildew on cucumbers yet this year in Ohio, it is likely already here or soon will be. Cucumber and melon growers in northern Ohio should start adding fungicides very effective against downy mildew to their spray programs now. These are Orondis Opti, Ranman, Omega, Previcur Flex, and Elumin. More information on these fungicides can be found here.

Cucurbit downy mildew map – click on map to enlarge it. https://cdm.ipmpipe.org/

Downy mildew was found on cucumbers in Haldimand-Norfolk, Ontario on July 1 and Chatham-Kent, Ontario on July 4. Even though it has been hot recently, cooler nighttime temperatures and cloudy skies can allow the spores of the downy mildew pathogen to survive during transport from source plants in Ontario on air currents, and to infect cucumbers and melons. The spores are usually deposited on plants during rainfall events. In our experience, we can find downy mildew in northern Ohio 1-3 weeks after outbreaks in these counties in Ontario, depending on the weather. Normally we only see Clade 2 isolates in northern Ohio during this time. Clade 2 isolates attack only cucumbers and melons e.g. cantaloupe, and circulate primarily in the Great Lakes region. Clade 1 isolates attack watermelon, pumpkins, squash and other cucurbits and work their way up the east coast from Florida and into the eastern parts of the Midwest. We see them mainly in southern and central Ohio in August or later in most years. The clades of the downy mildew pathogen differ somewhat in sensitivity to fungicides, so Ohio growers should use recommendations for fungicides from labs in this region.

Before symptoms appear in northern Ohio, cucumbers and melons should be protected with a chlorothalanil or mancozeb product. Fields should be scouted often; once downy mildew is reported in the northern Ohio counties, growers should add an effective downy mildew fungicide to their protectant (chlorothalanil or mancozeb) spray program. Good options for Ohio are Orondis Opti (FRAC 49+M05; contains chlorothalanil so don’t tank mix with additional chlorothalanil), Previcur Flex (FRAC 28), Elumin (FRAC 22), Ranman 4SC (FRAC 21), Omega (FRAC 29), and Zampro (FRAC 45+40). For more information on these fungicides, see here (OSU data until 2021) and here (Michigan State University 2022).

See my June 24, 2023 post for pictures of symptoms and instructions for submitting live or digital samples to OSU for diagnosis. Diagnosis is free for Ohio growers thanks to a grant from the Ohio Produce Growers and Marketers Association’s Ohio Vegetable and Small Fruit Research and Development Program.

Early symptoms of cucumber downy mildew. Photo by Raven Schaffter.

With the recent mild, cloudy weather, we could see an early arrival of downy mildew in northern Ohio.  Downy mildew is favored by moderate temperatures, overcast skies and rain. The pathogen, Pseudoperonospora cubensis, does not overwinter in northern regions and is introduced via sporangia (a type of spore) on wind currents from other areas. In Ohio and other Great Lakes states and Canada, the first “wave” of downy mildew likely originates in greenhouse production systems in the region. This wave rarely extends to central or southern Ohio. Over the years the first report of downy mildew on cucumbers in northern Ohio has been about 7-10 days before or after July 4. Melons are also susceptible to this pathogen “type” that occurs in the first wave. Pumpkin, squash, watermelon and other cucurbits are usually not affected. The second wave of this pathogen occurs later in the summer, with a variant of the pathogen arriving on wind currents from the south, affecting all cucurbits. This variant affects mainly southern and central Ohio, although it may spread north.

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

Cucumber and melon growers in northern Ohio should intensify scouting these crops and apply a protectant fungicide such as chlorothalanil. 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.  Once downy mildew is reported in the area, growers should ramp up their spray programs to include fungicides highly effective against downy mildew such as Orondis Opti, Ranman, Omega, Previcur Flex, and Elumin. More information on these fungicides can be found here.

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 C. Wayne Ellett Plant and Pest Diagnostic Clinic (CWEPPDC), now in Wooster. Instructions for sample submission are here. Digital images may also be sent to the CWEPPDÇ.

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.

Verticillium wilt symptoms in high tunnel tomatoes. Photo by Anna Testen.

Tomatoes are prone to damage by many foliar, fruit and root diseases. While producing tomatoes in high tunnels protects them from diseases spread by rainsplash such as bacterial spot and Septoria leaf spot, diseases favored by high humidity, including  Passalora leaf mold and Botrytis grey mold, can be exacerbated. Producing tomatoes year after year in high tunnels often results in a slow decline over the years in crop productivity due to a buildup of soilborne tomato pathogens. In Ohio, corky root rot, black dot root rot, Fusarium wilt, Verticillium wilt and root-knot nematode are quite common. Our fact sheet describing these diseases can be found here. Our research has shown high levels of these diseases in Ohio high tunnel tomatoes.

Corky root rot symptoms in high tunnel tomatoes. Photo by Anna Testen.

If tomatoes have been produced year after year in the same place, even if there are no obvious aboveground symptoms, a few plants in the high tunnel should be dug up as the season comes to a close and their roots washed off and inspected. Healthy roots are abundant and white with an intact taproot and many smaller feeder roots. Plants with corky root rot, for example, may have rotted tap roots and banded dark or “corky” lesions.

What to do if tomato roots appear diseased?

First, find out which diseases/pathogens are present. The Ohio State University Vegetable Pathology Lab and the USDA-ARS Application Technology Research Unit in Wooster are offering free soil testing this Fall for soilborne tomato pathogens.  See the flyer here: Soil_testing_flier_1Sep2022

Secondly, consider management tactics for next season. Anaerobic soil disinfestation (ASD) is a non-chemical soil treatment that reduces the populations of soilborne plant pathogens. It is very suitable for conventional and organic high tunnels but should be done while the soil is fairly warm; in Ohio this means ASD treatment should begin by late September. More information on ASD can be found here (factsheet) and here (step by step video).

Once the pathogens have been identified, growers can also choose disease-resistant rootstocks and produce or purchase grafted seedlings. Information on rootstocks and propagators can be found here (see Resources).

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.