Late Appearance of Phytophthora Blight in Peppers and Cucurbits Wreaking Havoc

Bell pepper fruits with Phytophthora blight, received in late August 2023 by the OSU Plant and Pest Diagnostic Clinic. Photo by Francesca Rotondo.

Phytophthora blight on pumpkins received in October 2021 by the OSU Plant Disease Diagnostic Lab in Wooster. Photo by Francesca Rotondo.

After a hot dry early summer in many parts of Ohio this year, growers may have thought that they escaped the scourge of Phytophthora blight, and for a while this appeared to be true. However, several intensive rainfall events during the past several weeks, some bringing as much as 10 inches of rain over a weekend, resulted in flooded soil conditions perfect for spread of Phytophthora blight. Unfortunately, this late in the season, growers have invested a lot of money in these crops and may see significant reductions in yield.

The cause of Phytophthora blight is Phytophthora capsici,  a soilborne oomycete pathogen that thrives in rainy weather. It produces sporangia that release motile spores (zoospores) that are attracted to plants, then form a structure that allows them to infect, and aggressively attack any type of plant tissue. Sporangia and zoospores can be splashed onto leaves, stems and fruits during rain events and overhead irrigation. Phytophthora blight must be  managed preventatively.  This includes the use of resistant varieties (partially resistant varieties are available for pepper but not for cucurbits), cultural practices and fungicides.

We have received several reports of fruit infections of peppers and pumpkins this week. Once fruits have become infected with Phytophthora, nothing can be done to rescue them. Additionally, some lesions on pepper fruits may not be obvious initially but develop during shipping, putting healthy fruits at risk of infection. For pumpkins, if Phytophthora blight was detected in a field at any time during the season, growers are advised to harvest mature, uninfected fruits as early as possible.  These fruits need to be laid out individually (not touching, so bins are not acceptable) in a shaded area with good ventilation so that they can cure.  A barn floor would be an ideal location since they would not get rained on, but outside under a tree (to prevent sunscald) would be better than nothing.  If putting them outside, do NOT put them on a tarp or plastic that would tend to hold rainwater and spread the disease to the other fruits.  If any of these fruits start to show signs of infection (discolored areas or white, cottony growth – see photo) remove them from the area immediately and discard them in an area away from the fields or curing location.

Dr. Meg McGrath of Cornell University found that hosing pumpkins off first to remove soil (using a garden hose with a trigger spray nozzle) was the second most important step in reducing disease incidence (getting them out of the field being the most important).  Washed fruits need to be dried as quickly as possible. Dipping fruit in 10% Clorox, GreenShield or Kocide was no better than just hosing them off, and these products are not labeled for this use.

Pepper and cucurbit fruits with symptoms in the field should be removed and destroyed away from the field and surface water sources. Leaving them in the field will contribute to inoculum buildup; if Phytophthora blight was present in a field, practice rotation of at least four years away from susceptible crops including all cucurbits, peppers, tomatoes, and beans.

[Updated from previous posts. Information on fungicides to manage Phytophthora blight in peppers and pumpkins is available in previous posts.]

We thank the Ohio Produce Growers and Marketers Association’s Ohio Vegetable and Small Fruit Research and Development Program for financial support of the OSU PPDC.

More First Reports – Downy Mildews of Basil and Cucumber in Ohio

Basil plants with severe downy mildew symptoms in a big box store garden center in Wooster, OH, 2021. Photo by F. Rotondo.

We received a report on August 8 from an Ohio Master Gardener of sweet basil downy mildew in a garden near the  OSU main campus in Columbus, Franklin County. We often see this disease in Ohio for the first time in late July or early August, although our reporting, aside from sentinel plots in northwest and central Ohio, depends on the growing and gardening communities giving us a heads up when observed. So, downy mildew may have been present in Ohio earlier than this first report. The pathogen, Peronospora belbahrii, does not overwinter in Ohio and arrives most years on air currents from the south. The pathogen is also seedborne and has been introduced earlier on basil seedlings and transplants in nurseries and retail stores as we observed in 2017 and 2021. For future plantings, there are sweet basil varieties now available with good resistance to downy mildew. These include Prospera Compact DMR, Prospera DMR, Prospera Red DMR, Rutgers Devotion DMR, Rutgers Obsession DMR, Rutgers Passion DMR, and Rutgers Thunderstruck DMR, available from a number of seed companies as organic or non-organic seeds. Resistance in the varieties may break down under severe disease pressure from favorable weather conditions (cool, overcast, high humidity, rainy) and high inoculum levels, so crop protectants may also need to be applied. A detailed listing of fungicides and biologicals registered for downy mildew management was published recently by Dr. Andy Wyenandt, Rutgers University.

Downy mildew was also reported on cucumbers in Huron County, Ohio this week.

Downy Mildew Reported on Pumpkins in Fulton County, OH

Partial map of 2023 cucurbit downy mildew reports as of August 7, 2023. Counties in red indicate new reports (<7 days), while counties in green have older reports. cdm.ipmpipe.org.

Bill Holdsworth of Rupp Seeds confirmed a severe outbreak of downy mildew on pumpkins in his field trials in Fulton County, Ohio today. Downy mildew was extensive in this field, indicating that it had begun some time ago, perhaps a week or two. Last year, Bill reported pumpkin downy mildew in the same county on August 10. It was likely caused by Clade 1 of the downy mildew pathogen Pseudoperonospora cubensis, which has a broad host range among cucurbits, preferring pumpkins, squash and watermelons, but also infecting cucumbers and melons. Clade 2 isolates infect and cause damage to cucumbers and melons, and are seen in northern Ohio first. As in 2022, the Fulton County outbreak on pumpkins was unusual for the Great Lakes region. There are no reports of downy mildew on cucurbits other than cucumber in the Midwest, Northeast or Canada at this time – although not being reported doesn’t mean it is not there. We have not seen downy mildew on pumpkins, squash, melons or watermelons in sentinel plots in Sandusky and Franklin Counties, and Jim Jasinski, OSU Extension, has not found downy mildew in scouted pumpkin fields on OSU’s Western Agricultural Research Station in South Charleston.

Downy mildew can kill cucurbit foliage, including that of pumpkins, which will stop the fruit from maturing unless it is controlled preventatively. Pumpkin, squash, and watermelon growers should be applying at least a protectant fungicide such as chlorothalanil (Bravo, Eqqus, etc.) and amp up scouting efforts. Growers in Fulton and surrounding counties should apply downy mildew-targeted fungicides (Orondis Opti, Ranman, Omega, Previcur Flex, and Elumin) posted here and here in a spray program alternating fungicides with different modes of action/FRAC group.  Follow all label instructions.

Bacterial spot on pumpkin – can be mistaken for downy mildew.

Downy mildew symptoms on a pumpkin leaf (top)

If you suspect cucurbit downy mildew you can 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. It is harder to confirm downy mildew in pumpkins and squash from photos because lesions are less distinctive and sporulation is less than in cucumbers.  So you may need to send a sample to the OSU C. Wayne Ellett Plant and Pest Diagnostic Clinic (CWEPPDC) in Wooster for confirmation. 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.

Powdery Mildew Management in Cucurbits

Powdery mildew colonies on the underside of a pumpkin leaf. Fungicide applications should start when these colonies are first observed during scouting. It is important to check both surfaces of the leaves. Photo by Josh Amrhein.

[Updated from 2022 post] Powdery mildew usually appears on pumpkins and other cucurbits in Ohio beginning in early July, but this year it appears to have come in a little later. The pathogen, Psudoperonospora cubensis, does not overwinter in Ohio; infections result from spores blown into the area on the wind.  Powdery mildew is favored by moderate to high temperatures and high humidity. However, unlike most other fungal plant pathogens, it is inhibited by free moisture on the leaf surface. Scouts observed a small number of powdery mildew colonies about 2 weeks ago on squash in our downy mildew sentinel plot at OSU’s North Central Agricultural Research Station in Fremont.

Signs of infection are small circular powdery growths 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.

Powdery mildew is managed using disease-resistant varieties and fungicides. Pumpkin and squash varieties vary in resistance to powdery mildew; in general, the more susceptible the variety, the more fungicide needed. The choice of fungicide is important because insensitivity to overused fungicides is common. It is critical that a fungicide resistance management program is followed. Alternate fungicides in different FRAC (Fungicide Resistance Action Committee) groups, indicating different modes of action against the fungus. Fungicide applications should begin when the disease first appears and incidence is low (rule of thumb: at least one leaf of 50 scouted). Fungicides that are labeled for use against cucurbit powdery mildew can be found in the searchable Midwest Vegetable Production Guide for Commercial Growers.

OSU evaluations of efficacy of powdery mildew fungicides in Ohio in 2021 indicated that Aprovia Top, Luna Experience, Inspire Super, Rally, Miravis Prime, Luna Sensation, Microthiol Disperss, Vivando and Procure provided very good control of powdery mildew on pumpkins (see table in color below).  Velum Prime, Cevya, Prolivo and Gatten provided good control of powdery mildew on upper leaf surfaces but poor control on the lower surfaces.

Quintec provided good control in 2021 but in other years and other states has failed due to resistance. Fontelis, Bravo Weather Stik, Merivon Xemium, Pristine, and Torino have been shown to provide poor or variable control in Ohio or other states and are not recommended.

Jim Jasinski, OSU Extension, has been running field trials in central Ohio for many years to assess fungicide efficacy against powdery mildew on pumpkins. Some effective fungicide combinations, based on 2021 and 2022 data, are shown in the second table. Some of these fungicides were not effective in our bioassays (table in color), but when paired a broad spectrum protectant like Manzate may prove more effective than when applied alone. Most experts suggest adding a broad spectrum protectant fungicide like Manzate to more powdery mildew-targeted fungicides to reduce the risk of fungicide development, boost fungicide efficacy and protect the crop from other diseases.

Former Buckeye Dr. Andy Wyenandt (Rutgers Univ.) has suggested that spray programs for cucurbit powdery mildew should contain multiple fungicides with different modes of action/FRAC numbers. He suggest a schedule of A-B-C-D-E-A-B-C-D-E where A, B, C, D, and E represent fungicides from different FRAC groups applied at weekly intervals. A protectant fungicide like Manzate should be tank mixed with the powdery mildew-targeted fungicide. This program is primarily to reduce the risk of resistance to any single fungicide, but it will also provide some cover if the pathogen has come in already resistant to one of the fungicides in the program.

Always check the label for full list of allowed crops and use recommendations and restrictions.

Effective Fungicide Treatments to Control PM in Pumpkin in Ohio in 2021 and 2022.

Treatment Crop
Cevya alt. Quintec; all with Manzate Pro-Stick (FRAC 3,13) Pumpkin
Gatten alt. Quintec; all with Manzate Pro-Stick (FRAC U13, 13) Pumpkin
Procure + Vacciplant alt. Vivando; all with Manzate Pro-Stick (FRAC 3, 50) Pumpkin
Cevya alt. Merivon; all with Manzate Pro-Stick (FRAC 3, 11,7) Pumpkin
Gatten alt. Merivon; all with Manzate Pro-Stick (FRAC U13, 11,7) Pumpkin
Procure alt. Quintec; all with Manzate Pro-Stick (FRAC 3,13) Pumpkin
Procure alt. Vivando; all with Manzate Pro-Stick (FRAC 3, 50) Pumpkin
Inspire Super* (FRAC 3,9) Pumpkin
Aprovia Top* (FRAC 7,3) Pumpkin

*sequential applications for research only, must be rotated per label for grower use.

 

Cucumber Downy Mildew Spreading

Map of 2023 cucurbit downy mildew reports. Red = reported <1 week ago; Green = reported > 1 week ago. cdm.ipmpipe.org

Downy mildew is now confirmed in cucumbers in five Ohio counties: Sandusky, Fulton, Medina, Wayne and Knox. The latest report, from Sandusky County, was from our sentinel plot on OSU’s North Central Agricultural Research Station (NCARS) in Fremont. Our interns Raven and Audrey scouted the week before and saw no symptoms or signs of downy mildew. One week later, downy mildew symptoms and signs were present on leaves of every cucumber plant in the sentinel plot (see photos).  This illustrates the explosive potential of this disease on a highly susceptible host like cucumbers and the necessity

Downy mildew on cucumbers in the OSU Sentinel plots at NCARS, Fremont, OH, July 24, 2023. Photo by R. Schaffter.

Underside of cucumber leaf showing downy mildew lesions and signs of the pathogen, Pseudoperonospora cubensis. NCARS, July 24, 2023. Photo by Raven Schafter.

of managing it preventatively.  This is mainly done through applications of appropriate bound spectrum fungicides such as chlorothalanil and mancozeb plus specific fungicides (e.g. Orondis Opti, Elumin, Ranman) targeted against the downy mildew pathogen and other oomycete pathogens. See previous recent posts on management tactics for cucurbit downy mildew.

Cucumber Downy Mildew Confirmed in Three Ohio Counties

Severe cucumber downy mildew in Medina County, OH, July 24, 2023. Photo by Frank Becker.

Map of cucurbit downy mildew outbreaks. First reports for this year in Ohio: Medina, Wayne and Knox counties. cdm.ipmpipe.org.

Cucumber downy mildew was confirmed yesterday in three Ohio counties (Wayne, Medina and Knox), following a report last week from Michigan and several weeks ago from Ontario. While these reports are later in July than average for Ohio, it is likely that infections occurred at least a week earlier. Thanks to OSU’s Plant Pest and Disease Clinic Director Dr. Francesca Rotondo and Wayne County Extension Educator Frank Becker for these finds. Knox County is in central Ohio, so I’ll amend my July 19 post to include cucumber and melon growers statewide, who should add fungicides very effective against downy mildew to their spray program now if they have not already done so. Waiting until symptoms appear may be too late to avoid yield losses; effective fungicides should be applied preventatively. The best ones, according to research in Ohio, Michigan  and other Great Lakes states and provinces are Orondis Opti (FRAC 49+M05), Ranman (FRAC 21), Omega (FRAC 29), Previcur Flex (FRAC 28), and Elumin (FRAC 22). These should be tank mixed with chlorothalanil (Bravo, Equus, etc.) or mancozeb (Dithane, Manzate, etc.). Orondis Opti is a premix already containing chlorothalanil, but at a reduced rate.  Fungicides have restrictions on how much product can be applied and how often, so follow the label. The more effective fungicides should be rotated to avoid resistance development in the pathogen.  More information can be found here and here.

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 vegetable growers thanks to a grant from the Ohio Produce Growers and Marketers Association’s Ohio Vegetable and Small Fruit Research and Development Program.

Cucumber Downy Mildew Confirmed in Michigan

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.

Cucumber Downy Mildew Reported in Ontario, Canada

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.

Start Scouting Cucumbers in Northern Ohio for Downy Mildew

 

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.

High Tunnel Tomato Root and Soil Health Checkup

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).