Downy Mildew Reported on Cantaloupe and Acorn Squash in Ohio

As of Monday, most reports of downy mildew in our neighboring states – MI, KY, PA – were in cucumbers, as were our previous reports. However, we found downy mildew on acorn squash in our sentinel plot on the OSU North Central Agricultural Experiment Station in Sandusky County and on cantaloupe in our sentinel plot on the OSU Muck Crops Experiment Station in Huron County. While we have not had reports from most Ohio counties, it is likely that downy mildew is widespread in Ohio. Now we have evidence that the strain that infects most cucurbits, including squash and pumpkins, is present here. The cucurbit growing season is winding down, but if cucurbit crops are expected to be in the field in the next few weeks they should be protected with fungicides.  See my blog post on August 22 below for fungicide recommendations.

The downy mildew pathogen does not infect cucurbit fruits but if plants are defoliated the fruits are at risk of sunburn. Sunburn may not be obvious immediately in the field but may develop in storage. In fields with significant defoliation, pumpkins and winter squash should be removed from the field and into shade for curing.

Finally, the downy mildew pathogen does not survive over the winter in the absence of living cucurbit plants. However once harvesting is completed, plants should be disked as soon as possible to kill remaining green tissues that may otherwise be infected and serve as sources of inoculum. This will reduce downy mildew inoculum that can spread to cucurbit crops near and far.

Good and Bad Birds on the Farm

Do you have a bird problem on your farm?  Do you want to encourage beneficial birds as an IPM tool? If so, consider attending this free webinar.  Registration information below:

Webinar: Supporting Beneficial Birds and Managing Pest Birds

Supporting Beneficial Birds and Managing Pest Birds

Webinar: Tuesday, October 1 at 11AM Pacific, 12PM Mountain, 1PM Central, 2PM Eastern Time

Register in advance at https://oregonstate.zoom.us/webinar/register/WN_Yey2HdAZQ8S3CSKUuR8FIg

Join eOrganic for the first webinar of our fall season! The topic is Supporting Beneficial Birds and Managing Pest Birds, by Jo Ann Baumgartner of the Wild Farm Alliance, Sara Kross of Columbia University, and Sacha Heath of the Living Earth Collaborative.

Beneficial birds can help farmers keep pest insects, rodents, and pest birds at bay. They act the same way that beneficial insects do in helping with pest control. The overwhelming majority of songbirds are beneficial during nesting season because they feed pest insects to their voracious nestlings. Farmers may be able to reduce their pest-control costs by providing habitat for these beneficial birds and by only targeting detrimental birds at the right time and place. Wild Farm Alliance and two avian ecologists will present on: a) How birds’ diets, foraging strategies, and nesting periods affect the farm, b) How best to manage and co-exist with pest birds, c) Why on-farm habitat and the surrounding landscape influences pest control, and, d) What farmers can do to make farms more bird-friendly and resilient. With this webinar and the associated Supporting Beneficial Birds and Managing Pest Birds booklet, we aim to help all farmers and farm consultants make the most of birds on farms.

Cucumber Downy Mildew Confirmed in Northeast Ohio

cdmipmpipe.org, August 22, 2019

Following reports this week of downy mildew on cucurbits in Michigan, central Pennsylvania, Wisconsin and Indiana, we have our first confirmed report of downy mildew on cucumbers in northern Wayne County. This is very late for this area – we usually see downy mildew on cucumbers in early July in northern OH. Many growers have been spraying preventatively due to the seriousness of downy mildew on cucumbers and other cucurbits. All of the reports this week from MI, WI and PA were from cucumber, although the report from southwestern Indiana was from watermelon.

Cucurbit growers who have not transitioned from applying only protectant fungicides such as chlorothalanil or mancozeb to downy mildew fungicides should now do so.  The environmental conditions – cooler temperatures, high humidity, overcast skies and rain showers- expected in much of Ohio during this part of the season are conducive to downy mildew. Effective fungicides against downy mildew are Ranman, Elumin, Orondis Opti and Zampro.  Tank mix these with chlorothalanil or mancozeb, with the exception of Orondis Opti, which includes chlorothalanil inn the pre-mix.

Alternate products on a 7-10 day schedule.  Follow the label regarding limitations on number and timing of applications.  If you have already applied Orondis Ultra or Orondis Gold for Phytophthora blight management you may have reached the limit on Orondis applications.  Cucurbit crops must be protected from downy mildew in advance – applying fungicides after the disease is well-established is not effective and yield losses are likely.

July 29, 2019 cdm.ipmpipe.org

Downy mildew was confirmed today in cucumbers in an unsprayed MSU research plot in Benton Harbor, MI. In our experience, when downy mildew is identified in Michigan, it is probably already or soon to be seen in Ohio, and vice versa.  Cucurbit growers should have been applying protectant fungicides such as chlorothalanil or mancozeb , but should now consider transitioning to downy mildew fungicides.  Moderate temperatures, humidity, overcast skies and rain showers expected in much of Ohio in the next few days are conducive to downy mildew spore movement, deposition and infection. MSU’s recommendations for effective fungicides against downy mildew are shown here:

  • Ranman + chlorothalonil or mancozeb
  • Orondis Opti (chlorothalonil is part of the premix)
  • Elumin + chlorothalonil or mancozeb
  • Zampro + chlorothalonil or mancozeb

Alternate products on a 7-10 day schedule.  Follow the label regarding limitations on number and timing of applications.  If you have already applied Orondis Ultra or Orondis Gold for Phytophthora blight management you may have reached the limit on Orondis applications.  Cucurbit crops must be protected from downy mildew in advance – applying fungicides after the disease is well-established is not effective.

Look-alikes Spotted but No Cucurbit Downy Mildew in Ohio Yet

Cucurbit downy mildew has been moving up the east coast, with some westward movement in the South, but there have been no reports of downy mildew on cucurbits in Ohio, its surrounding states, or Ontario. Downy mildew pathogen (Pseudoperonospora cubensis) spore trap counts from the Hausbeck Lab at MSU have been very low – and only a few (8) P. cubensis spores were counted from one location on June 28, and none before or since.

We have seen many, many examples of bacterial diseases in vegetable crops this season, a consequence of excessive rainfall this spring and summer. Some bacterial diseases cause leaf spots that can be mistaken for those caused by fungi or oomycete pathogens. Angular leaf spot of cucumber is a prime example: the angular lesions are very similar to those of downy mildew. In downy mildew, the spores of the pathogen may be observed on the lower side of leaves with a good hand lens, but sometimes they are difficult to find. If downy mildew is suspected but can’t be confirmed with a hand lens, it should be confirmed with a lab microscope. Downy mildew spores can be visualized easily with a microscope. If on the other hand the symptoms are caused by bacteria, bacterial streaming from the lesions can be seen using a microscope. This process can be completed in minutes in an experienced lab. Ohio vegetable growers can send or drop off samples for diagnosis to the OSU Vegetable Pathology Lab in Wooster; a fillable PDF sample submission form can be found here.  Samples can also be dropped off at OSU-OARDC Experiment Stations in Celeryville or Fremont, or on the OSU main campus in Columbus (Kottman Hall).

Downy mildew of cucumber

Angular leaf spot of cucumber – a bacterial disease

Applying fungicides is an important tactic in downy mildew management, but spraying them when the disease is caused by a bacterial pathogen is a waste of time and money. Regular scouting, confirmation of symptoms and being alert to reports of outbreaks of downy mildew in the vicinity should be the focus now.

Cucurbit Powdery Mildew – Start Scouting Now

Powdery mildew arrived this week on squash in Wayne County, Ohio. It is early – usually we see it first in mid-July. 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. 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.

Powdery mildew is managed using disease-resistant varieties and fungicides. Organic production systems need to rely heavily on resistant varieties but there are OMRI-approved fungicides and biologicals that can reduce disease severity.  These options were summarized in this blog in 2018.  In conventional systems, 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 and FRAC codes are on page 128. Our evaluations of efficacy of powdery mildew fungicides in Ohio in 2018 indicated that Inspire Super, Procure, Rally, Aprovia Top and Quintec provided very good control of powdery mildew on pumpkins in all three locations (click on graph to enlarge).  In this test, a bioassay, Bravo Weather Stik and Fontelis provided moderate control and Pristine provided poor control. Merivon Xemium and Torino did not perform well in this test (data not included); however efficacy ratings in the Midwest and Southeast vegetable production guides are “good”. Growers should take this into account when choosing products for powdery mildew management.

Downy Mildew Confirmed in Pumpkins in Clark County, OH

A severe outbreak of downy mildew was confirmed on pumpkins from a field trial at OSU-OARDC Western Agricultural Research Station. This is the first confirmed outbreak of downy mildew on pumpkins in Ohio, although it is likely elsewhere in central Ohio, if not even more widespread.  Symptoms on pumpkins are somewhat different than on cucumber – the lesions on pumpkins are smaller than on cucumber, although both are angular, look watersoaked on the underside of leaves (upper right photo) and yellow on the upperside  (upper left photo) initially. On pumpkins the older lesions appear bronze-brown in color (lower right photo). Pumpkin leaves can be completely destroyed if not treated with effective fungicides.

With cooler temperatures expected for the rest of this week, as well as rain showers and storms, downy mildew risk is high for most of Ohio and all cucurbits should be protected with fungicides that are effective against downy mildew. Although the season is winding down, if pumpkins still need some time to reach maturity, the foliage should be protected. Information on fungicides can be found in the Midwest Vegetable Production Guide for Commercial Growers 2018; in addition, fungicide efficacy rankings from our 2017 bioassays can be found here. Control of downy mildew requires preventative fungicide application – inadequate control is often observed when fungicides are applied after infection, even if symptoms have not started to appear.

Cucumber Downy Mildew in Medina County, OH

Tape mount from the underside of a cucumber leaf with downy mildew. Characteristic branched sporangiophores (center) and oval, brown sporangia. Micrograph by Francesca Rotondo.

Downy mildew was confirmed today on cucumbers in Medina County – the field is in the Homerville area and symptoms were just beginning to show.  The pathogen that causes cucurbit downy mildew, Pseudoperonospora cubensis, was sporulating well on the underside of leaves (see photo).

This is the second confirmation of downy mildew on cucumbers in Ohio this year – the first was on August 11 in Huron County.  Please see my August 11 post in this blog for management recommendations.

We still have not confirmed downy mildew on squash or pumpkins, but we have received quite a few lookalikes, most of which were bacterial spot or angular leaf spot (also a bacterial disease). Bacterial diseases will not be controlled using any of the fungicides recommended for downy mildew, with the exception of copper-based products. However, these are only partially effective against downy mildew and bacterial diseases. If you are not sure about your diagnosis, send samples to the OSU Vegetable Pathology Lab – diagnoses are free for OH growers.

Organic Options for Cucurbit Powdery Mildew Management

Powdery mildew is a scourge of summer for squash, pumpkins, and other cucurbits.  Organic growers should always start with varieties with some degree of resistance to powdery mildew – seed catalogues often call partial resistance “tolerance”.  Although resistance will generally not be complete, efforts to manage powdery mildew with organic-acceptable products will be more productive if growers start with a variety that can put up a fight on its own than one that is highly susceptible.

Dr. Meg McGrath, Cornell University, has summarized field research results throughout the US for organic-approved products tested against various diseases of vegetables and herbs.  Her summary for zucchini powdery mildew research in NY includes the following:

Best results are obtained when these products are used preventatively or at the very first signs of powdery mildew, usually in mid-July in Ohio.  If you wait until powdery mildew has progressed to the stage you see in the photo above, it will probably be too late to get it under control.

Cucumber Downy Mildew – First Ohio Report for 2018

Cucumber downy mildew – early 

This has been a very unusual year for cucurbit downy mildew. The disease usually appears on cucumbers like clockwork on or around July 4 in one of the northern Ohio counties, but this year we found it for the first time only yesterday, August 10, in Huron County – with just two mature lesions in one of our cucumber research plots on the OSU OARDC Muck Crops Experiment Station in Celeryville. We have been expecting it due to reports in MI, IN, PA, and Kentucky during the last few weeks.

Bacterial spot of pumpkin

We have been receiving many samples of cucurbits suspected of downy mildew during the past month, including cucumbers, squash and pumpkins, but nearly all of these had bacterial spot or angular leaf spot.  While we expect that these bacterial diseases will continue to be a problem, growers and scouts should be on the lookout for downy mildew in all cucurbit types.  Symptoms caused by bacterial diseases, Alternaria and sometimes anthracnose can look like downy mildew.  If you are unsure, send a sample to the OSU Vegetable Pathology Lab in Wooster for confirmation. There is no fee for Ohio residents. You can also text or email photos – please be sure the images are sharp, as close up as possible, and include both the upper an lower side of the leaf – to me at 330-466-5249 or miller.769@osu.edu.  We can’t always diagnose from photos but they can be a good place to start.

Most growers have been protecting cucurbits for the last few weeks with a protectant fungicide such as chlorothalanil (Bravo, Echo, Equus, Initiate versions), which will also help manage anthracnose and Alternaria leaf spot.  At this late date and with confirmed cases in Ohio and our surrounding states, growers should consider including additional fungicides in their spray programs.  The chart below shows our 2017 bioassay results for fungicide efficacy against downy mildew. Always rotate fungicides with different modes of action and follow label instructions. Remember that Orondis Opti applications are restricted to 1/3 of the total fungicide applications. Under highly conducive environmental conditions, apply fungicides on a 5-7 day schedule.  When the risk is lower due to hot, dry, sunny weather, or downy mildew has not been reported in the area, the schedule may be stretched to 7-10 days.

Information on fungicides for vegetables, including Fungicide Resistance Action Committee (FRAC) code and greenhouse use can be found in a table beginning on page 79 of the 2018 Midwest Vegetable Production Guide for Commercial Growers. Pre-harvest intervals are shown for each crop/fungicide combination throughout the guide.

Options for organic cucurbit production are limited.  Many organic-approved products include cucurbit downy mildew on their labels, but most are not very effective or ineffective.  A copper-based fungicide such as Champ usually is the most effective in research trials, but generally control is not complete. These products must be applied preventatively, before the downy mildew pathogen infects the plants. Cultivars with some resistance to downy mildew should be used. Dr. Meg McGrath of Cornell University has summarized recent research throughout the US on organic-approved products for control of downy mildew and other diseases of vegetable and herb crops.