2019 has come and almost gone, and the results of our vegetable disease management field trials can be downloaded here: OSU Vegetable Disease Management Research Reports 2019. It was a tough year for research, with flooding in some research plots and a surprising lack of disease development in some others. I will summarize some of the more interesting trials in future posts. These are the report titles:
Evaluation of biological and chemical products for the control of bacterial spot of processing tomatoes, 2019
Evaluation of biorational products for the control of bacterial spot of processing tomatoes, 2019
Evaluation of resistance of pepper cultivars to bacterial canker, 2019
Evaluation of resistance of pepper cultivars to Pseudomonas leaf spot, 2019
Integrated management of Phytophthora blight using grafted peppers and fungicides, 2019
Evaluation of resistance of cucumber varieties to downy mildew and Phytophthora fruit rot, 2019
Bioassay for sensitivity of Pseudoperonospora cubensis to fungicides in OH, 2019
Management of Plectosporium blight of pumpkins, 2019
Evaluation of Regalia and Stargus for the control of black rot of cabbage, 2019
Evaluation of biorational products for the control of bacterial leaf spot of cilantro, 2019
Vegetable Disease Diagnostics, 2019
Northern root-knot nematode galls on tomato
As the vegetable growing season winds down, now is a good time to dig up plants in high tunnels and open fields and determine the health of the roots. Root diseases that may not kill plants outright can nonetheless stunt plant growth and reduce yields. Look for plants that appear less vigorous than others and remove them with a shovel, taking care to maintain root integrity. Shake off the soil and rinse the roots gently with water. Then examine roots for symptoms.
While some soilborne diseases such as Phytophthora blight can “explode” in the field from relatively low initial inoculum levels, many others build up slowly from season to season. Root-knot, caused by the the plant pathogenic nematode Meloidogyne spp., is increasing in prevalence in Ohio in high tunnel tomatoes and other crops in open fields. Root-knot is fairly easy to identify – galls are clearly visible on roots. The Northern root-knot nematode, which predominates in northern Ohio, causes small galls, while the southern species, which occurs in southern Ohio, causes large galls. Since root-knot nematodes have a very broad host range, crop rotation may not be helpful, although certain cover crops such as sudangrass are toxic to nematodes, and wheat and corn are not hosts of northern root-knot nematodes. Anaerobic soil disinfestation (ASD) is highly effective against root-knot.
Corky root rot of tomatoes
End-of-season root health checks are especially important for high tunnel tomatoes, which are often produced in the same soil year after year. Root-knot, Verticillium wilt, corky root rot, black dot root rot, and Pythium root rot are among the root diseases that can predominate in long-term non-rotated tomatoes. The roots should be evaluated every year to assess disease development and the need for control measures such as grafting on disease-resistant rootstocks and ASD. A fact sheet describing tomato soil borne diseases and their management can be found here.
Downy mildew was found this week in organic pumpkins in Harrison County. Options for downy mildew control are limited in organic cucurbits, and at this point in the growing season the damage may not significantly affect yield. Downy mildew affects pumpkin leaves but not vines or fruit. The main danger, once fruits have matured, is defoliation and subsequent sunburn of the fruit. If plants are defoliated, and sunny weather is expected, fruits should be removed from the field and stored in the shade.
Previously this summer we found downy mildew on cucumbers and cantaloupes in our sentinel plots and on commercial farms. The disease appeared on cucumbers/melons much later in the growing season than expected, first detected on August 22. The strain of the downy mildew pathogen that appears early, usually in early July, is thought to originate in greenhouses in the Great Lakes region. This strain only affects cucumbers and melons, and we don’t know why the early introductions did not happen this year. The strain that infects all cucurbits originates in the South, always arrives in August or later, and is likely the main culprit this year. We are currently testing isolates to identify the strain type from plants we’ve collected 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.
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.
cdmipmpipe.org Thursday August 22, 2019
Following recent 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.
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.
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.
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.
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.