Downy Mildew Confirmed in SW Michigan

cdm.ipmpipe.org July 29, 2019.

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 Cucumber 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 – 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.

First 2018 Report of Cucurbit Downy Mildew in Ohio

Cucumber downy mildew

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 only yesterday, August 10, for the first time 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.

 

Managing Cucurbit Powdery Mildew

Powdery mildew arrived this week on squash, pumpkins and other cucurbits throughout Ohio. It is a little late – we often see it by early- to 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.  This fungus is an unusual plant pathogen in that it is inhibited by free water – so frequent rains may delay powdery mildew’s appearance, at least to a notable level.  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 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 and FRAC codes are on page 125.  Our evaluations of efficacy of powdery mildew fungicides at three locations (Wooster, Columbus, South Charleston) in Ohio in 2017 indicated that Procure, Quintec, and Rally consistently provided very good control of powdery mildew on pumpkins in all three locations (see table).  Approvia Top and Inspire Super were very good in two locations but fair in a third; and Merivon Xemium, Fontelis and Torino were very good in one location and fair in two. Both Bravo and Pristine performed poorly in all three locations.

No Cucurbit Downy Mildew Reported Yet in Ohio, but be Prepared

Cucurbit downy mildew reports as of July 12, 2018. cdmipmpipe.org

Cucurbit downy mildew is marching up the U.S. eastern seaboard from Florida to central Pennsylvania, but has not been reported in Ohio, Michigan or Ontario as of July 12.  This is later than normal for Ohio – last year our first cucumber downy mildew report was on June 28. Excessive heat followed by sunny days have likely contributed to the delay.  However, cucurbit growers should be vigilant and scout their fields regularly for downy mildew.  These crops should be protected now with an effective protectant fungicide such as chlorothalanil (Bravo, Echo, Equus, Initiate versions), which will also help manage anthracnose and Alternaria leaf spot – we have seen an unusually large amount of Alternaria leaf spots in various crops this summer. When we start experiencing cooler, rainier weather with high humidity and overcast skies, the downy mildew risk will increase.  We evaluated numerous fungicides for efficacy against cucumber downy mildew in 2017 (see chart below).  Ranman 400SC, Orondis Opti, Omega 500F and Gavel 75 DF performed best in these tests.  Bravo Weather Stik 6F, Zampro 52SSC, Tanos 50DF, Presidio 4SC and Curzate 60DF were intermediate in efficacy and can be used as rotational partners in a fungicide program, particularly under low to moderate disease pressure.  The poorly-performing fungicides are not recommended for downy mildew management.  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. Cucumber and cantaloupe downy mildew risk is much higher in northern than in central and southern Ohio at this time.

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.

 

 

 

Pepper Phytophthora Blight – First 2018 Report in Ohio

Phytophthora blight was diagnosed last week in pepper plants from Huron County, Ohio.  This is several weeks earlier than we normally see Phytophthora blight in northern Ohio, but heavy rains and periods of high temperatures likely contributed to an early appearance of the disease.  Growers should scout both peppers and cucurbits for typical symptoms of Phytophthora blight  Phytophthora is a water mold that thrives under conditions of high moisture and high temperature. It produces 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. Zoospores can be splashed onto leaves, stems and fruits during rain events and overhead irrigation. Phytophthora blight is often seen first in low spots or other poorly drained areas of production fields, but the disease also occurs on well-drained, even sandy soils if the environmental conditions are right.  An integrated, preventative program to manage Phytophthora blight is more effective than in-season rescue treatments with fungicides.  During the growing season, fungicide application is the main option for management of Phytophthora blight (see below). In small plantings prompt removal of diseased plants is also recommended.

Effective management of Phytophthora blight in peppers requires an integrated approach:

Crop rotation.  Phytophthora produces structures called oospores that can survive for a number of years in the soil.  Plan to rotate out of peppers, cucurbits or green beans for 4-5 years if Phytophthora blight has been a problem.

Resistant varieties.  A few pepper varieties are resistant to the root rot phase of the disease.  In general, these varieties are susceptible to the crown rot phase, which affects foliage and fruits. Varieties with moderate to good resistance to Phytophthora blight are: Paladin, Aristotle, Declaration, Intruder, Vanguard (bell); Hechicero (jalapeño); and Sequioa (ancho).

Well-drained soil. Avoiding standing water is critical to limiting the movement of Phytophthora from plant to plant.

Avoid surface water for irrigation. We have found Phytophthora in irrigation ditches and ponds as early as late June in vegetable production-intensive areas in Ohio.  Using surface water for irrigation is risky, especially if Phytophthora is present in fields near surface water sources.

Plant on raised beds. Prepared properly, raised beds will help prevent standing water near pepper plants.  If possible beds should be domed, and there should be no depressions in the soil surrounding the plants.

Sanitation.  Phytophthora can be moved from an infested field to a clean one on soil clinging to boots, equipment, etc.  Power washing to remove soil is a good first step, followed by rinsing with a sanitizer.  Do not build cull piles containing discarded peppers or cucurbits – plant material needs to be disposed of, preferably by burying, far from fields and surface waters.

Fungicides.  There are a number of fungicides labeled for use on peppers to manage Phytophthora blight  (see table below).  The newest product, Orondis, has very good efficacy against this disease. It is available as a pre-mix with either Revus (Orondis Ultra), Ridomil (Orondis Gold) or Bravo (Orondis Opti). There are many restrictions on the use of Orondis – including the number of applications (no more than 1/3 of total applications for Phytophthora blight) and when it can be applied (to the soil or to the foliage but not both).  Orondis Ultra and Orondis Gold can be applied in transplant water or through the drip, although Orondis does not move much in soil and emitters need to be right next to the plant.  If the pepper variety is susceptible to Phytophthora blight, it may be a good idea to apply Orondis Gold or Orondis Ultra at planting, and follow up later with a program containing at least two of the fungicides with activity against Phytophthora (see table). If the pepper variety is resistant to Phytophthora, any of the three Orondis products can be used in a foliar fungicide program that includes other effective fungicides. The Bravo component of Orondis Opti will not help with Phytophthora blight, but will control anthracnose.  Orondis Gold is considerably more expensive than Orondis Ultra and Orondis Opti, and resistance in Phytophthora to the Ridomil component of Orondis Gold has been found in numerous locations.

For in-season control where an at-plant application of one of the Orondis products has not been made, foliar applications can be very effective if undertaken preventatively. Results of our research in 2016/2017 on squash indicated that foliar applications of Orondis Ultra could be alternated with Presidio, Ranman or Tanos + Kocide with equivalent results (see chart Squash Phytoph Orondis foliar 1 slide-2eauh2g).