Cucurbit Downy Mildew & Gypsy Moth

In this episode, we are joined by Katie Goldenhar, Pathologist for horticulture crops to talk about the early activity of cucurbit downy mildew (CDM). For more information on CDM, visit Cucurbit Downy Mildew Forecast on ipmPIPE and the Michigan State University Hausbeck Plant Pathology Research Lab. Looking to report CDM in Ontario? Contact Andew Wylie (andrew.c.wylie@ontario.ca) or Katie Goldenhar (katie.goldenhar@ontario.ca). 

As well, Jennifer Llewellyn, Nursery & Landscape Specialist sits in the 'Horticulture Hotseat' to shine some light on why we're seeing such high populations of gypsy moth in Ontario this summer. 

Plus, Ontario crop updates for July 17th, 2020. If suspect allium leaf miner damage in garlic, contact Travis Cranmer (travis.cranmer@ontario.ca).

For more updates on Ontario horticulture visit:

• www.onvegtables.com
• www.onfruit.ca
• https://onspecialtycrops.wordpress.com/

Have a question or a topic you'd like us to cover? Email us at ONhortcrops@gmail.com 

Music: Aspire by Scott Holmes

Transcript

0.00
Kristy: This is What’s Growing ON? A show where we dig up the latest dirt on Ontario horticulture production, helping producers navigate best management practices and taste the sweet success of a quality crop. My name is Kristy Grigg-McGuffin.

Cassie: And I’m Cassie Russell. Join us as we talk to specialists in the field of fruit, vegetables and specialty crops to find out what’s really growing on.

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0.42
 Kristy: Welcome to episode 6 of What’s growing ON. We have a shorter episode for you today, following our extended irrigation episode last week. Cassie, do you want to give us a sneak peak maybe of who we are going to hear from today? 

Cassie: Well, I got to do a quick interview with Katie Goldenhar, the pathologist for horticulture, on downy mildew since that has now arrived in Ontario. So, although a bit more of a vegetable focus with it being so damaging to cucumbers, it still might be relevant to melon or squash growers in the province to get some more information about this disease. 

Kristy:  Yeah for sure, it’s a really important topic right now. After that, we are going to hear quickly about a pest many have noticed this year called the Gypsy Moth, and to talk to us about that will be Nursery and Landscape Specialist, Jennifer Llewellyn. But lets just get right to it and get thing started with our weekly crop updates as of Friday, July 17h, 2020.

From the fruit side, I actually don’t have a lot new to update from last week. While strawberry harvest is winding down, raspberry and blueberry harvest is ramping up. Tart cherry harvest has also started with plum and apricot not far behind. And by the looks of it, we may even see the start of peach harvest soon and the first of the early apple varieties in a few weeks.

Those areas that got the much needed rain last week saw a little relief. Though irrigation is probably picking back up again shortly. Unfortunately, not all regions got much rain at all and remain very dry. Despite that, fruit still seems to be sizing well. 

We continue to see pest pressure building for those that thrive in hot, dry conditions such as mites, potato leafhopper and Japanese beetle. It’s been difficult for some to find windows of time long enough with temperatures below 25C to spray. 

Efficacy of spray and phytotoxicity issues can really become a concern in this extreme weather. Plants under water stress can respond different to sprays and some classes of pesticides may have a reduced residual in high temperatures. If you’re concerned about the performance of a pest control product, speak to your local retailer or sales rep. Also keep in mind, hot and dry conditions increase drift because droplets rapidly evaporate and become fine droplets, vapour or particles of concentrated pesticide. 

Handing it over to you, Cassie for a vegetable update.

Cassie:  For beans and peas, high humidity can be conducive to Sclerotinia white mould. Make sure you’re checking the inside the canopy for the development of white, cottony mycelial growth. Potato Leafhopper has been reported in several areas: scout for nymphs and adults and watch for hopper burn which can resemble drought stress.

In, brassica crops make sure you’re scouting for Alternaria, aphids, and thrips. Lepidopteran pests are present in many fields, mainly imported cabbage worm and diamondback moth. Refer to the newsletter on onvegetables.com from June 18 for information on specific lepidopteran pest thresholds. Incorporate all crop residue into the soil from a block once it is harvested.

Celery – Black heart, caused by a calcium deficiency has been seen in some fields as well as some carrot weevil damage. Leaf tip dieback has been observed from the heat last week and these wounds may be entry points for bacteria to enter the plant over the upcoming weeks. Also Be on the lookout for celery leaf blights and celery leaf curl and a really good thing to keep in mind is to avoid scouting the field when the leaves are wet and that’s to lower the chance of pathogen transmission throughout the field.

Cucurbits – Downy Mildew has been found in many areas of Ontario: Outbreaks have been confirmed in Kent County (July 6th), Elgin County (July 7th), Norfolk County (July 7th), and most recently Lambton County (July 14th). Unconfirmed cases are being investigated in Haldimand and Niagara regions. There are also confirmed cases in Michigan, Ohio, and New York so pressure is high. More information on CDB can be found at the onvegetable.com blog and we will also be chatting with pathologist Katie Goldenhar as mentioned, a little later in our episode about this topic. If you do suspect that you have downy mildew symptoms in any cucurbit crops including melons, pumpkin, and squash, please contact Andrew Wylie or Katie Goldenhar –as they are tracking cases I through the province. And their email addresses will be provided in the show notes. 

Cucumber harvest is well underway in all cucumber growing regions, and larger cucurbits are growing well. Cucumber beetle flushes have been continuous this summer, and the percentage of cucumber beetles harbouring the causative agent of bacterial wilt tends to increase over the season.

Garlic harvest is underway across the province. While harvesting, be on the lookout for any insect damage to the leaves, stem or bulb. Leek moth causes wounds to the scape and leaves and larvae can be found in the bulb. Allium leaf miner can cause similar damage in garlic, however, brownish-red pupae can also be found in the stem when the layers are peeled apart (below). If you suspect you have Allium leaf miner damage please contact Travis Cranmer, once again email will be provided in the show notes.  After harvest, avoid storing harvested bulbs in direct sun as this has shown to reduce storage life. Storage pathogens and storage insects, such as bulb mites, find it easier to survive when the crop is dried out over a prolonged period of time. To reduce the survival of these pests, speed up the curing process. A reduction in humidity is more important than heat when it comes to curing garlic. Often heat is used to increase the water-holding capacity of air, however, garlic can be cured properly at a lower temperature so as long as the humidity is low. Take the time to cull/remove bulbs with rots or defects before they go into storage. The higher the quality of crop that goes into storage, the longer it will last.

In Onions – Weather has been conducive for Stemphylium development and Purple blotch and pink root has been observed in some fields. Onion and seedcorn maggot are active in many regions across the province and will be laying eggs. With all the hot weather, the population of thrips has potential to greatly increase over the next week.  Apply no more than two consecutive insecticides from the same IRAC crop as thrips have a relatively short life cycle with multiple generations through the summer months and are at a high risk of developing insecticide resistance. In most onion growing regions, conditions have not been favourable for downy mildew development.

Peppers – The second generation flight of European corn borer (ECB) is expected shortly in Essex and Kent regions, and any preventative chemical controls should be initiated soon. Continue to monitor for adults and scout for larval entry holes in fruits. ECB pheromone traps are a great tool to help time product applications. If you have a trap set up, consider adding it to the Great Lakes and Maritimes Pest Monitoring Network 2020. As of Thursday, July 16th 2020, no pepper weevil have been caught on any outdoor traps in SW Ontario.

Sweet corn – Lepidopteran and other pest populations continue to build in sweet corn: for detailed information refer to the Great Lakes and Maritime Pest Monitoring Network for updates. Look for European Corn Borer, Corn Earworm, Western Bean Cutworm, and Corn Leaf Aphids. Common Stalk Borer has also been found so check areas adjacent to grasses. 

 And lastly for tomatoes, growers should be on a good foliar disease program by now as rains and humidity are expected over the next week and will be favourable for disease development. Check back to the updated Fungicide Efficacy Tables by Cheryl Trueman for information to help manage diseases in field tomatoes. High nighttime temperatures have caused some flowers to drop, but overall fruit set has been good especially with recent rains.

Kristy: For more detailed information on these and other fruit and vegetable crops, check out our weekly crop updates at onvegetables.com and onfruit.ca.

9.43
SEGMENT 1: Cucurbit Downy Mildew 

Cassie: Cassie: Very excited to welcome back a familiar guest to our podcast, Katie Goldenhar, the Pathologist for Horticulture with OMAFRA. Thanks for joining us again, Katie. 

 Katie: Hi Cassie, nice to be back! 

 Cassie: So I’ know on our first episode you gave us rundown on fire blight in apples and pears, but today were switching gears and chat about disease in vegetables. Specifically, cucurbits because we’ve been hearing a lot about cucurbit downy mildew over the past couple weeks so, yeah why don’t you start us off by telling us why now is the time to be chatting about this pathogen. 

 Katie: Lots going on with CDM as we are recording this on July 10th, 2020. CDM was identified in the great lakes region on June 22nd in Michigan and then again on the east side of the state, closer to Ontario, on June 30. CDM has been confirmed in Kent county on July 6th, and then Elgin and Norfolk the following day. Now we have CDM confirmed in the major cucurbit growing regions in the province. This is the earliest it has been in the field since 2017 and is present in the major cucurbit growing regions. In 2018 and 2019, the disease was only seen at the end of August and into September, when the crops were almost finished. With it showing up earlier this year, growers are given an added challenge to their production.

 Cassie: So what is cucurbit downy mildew and specifically why is it so important to track its arrival into the province? 

 Katie: It’s a really interesting disease for a pathologist but not so exciting to a grower. It is caused the oomycete Pseudoperonospora cubensis which is an obligate parasite. Meaning it needs to survive on living host tissue so in this case on cucurbits. Since we have winters, it cannot survive outdoors in Ontario and is confined to frost free regions in the southern US and Mexico during the winter months. It can also survive if there are cucumbers planted throughout the winter months in greenhouses. 

 It’s an oomycete which is different than fungi although oomycetes are still often referred to as fungi which drives pathologists crazy. They’re actually more closely related to algae but that’s a topic for another day. They are commonly called water mould because they love water, meaning they need it for their reproduction. P. cubensis needs leaf wetness in order for the sporangia, which are the spore like structures that spread in the wind, to germinate and release zoospores. These zoospores need to swim to reach a stomate where they can infect the leaf. They feed for the nutrients in the leaf than then there are structures known as sporangiophores that come on the underside of the leaf, these contain the sporangia, which are released and go infect other leaves. 

 It has occurred annually since 2005. Each year it shows up in the Great Lakes Region at different times. It isn’t uncommon for it to show up late June to early July, but growers have to be aware right away so they can use preventative measures to control the disease before it gets out of hand. In uncontrolled infections, it can lead to foliar death within 2 weeks. 

 Cassie: Wow, just two weeks – that is completely devastating. Sounds a bit similar in nature to the late blight we have to deal with in tomatoes and potatoes. 

 Katie: Yeah, for sure it is similar, it can be equally as devastating. And that’s another oomycete!

Cassie: Now, you mentioned that it’s been around each year since 2005. Was it not detected prior to that or just not of concern each year? 

 Katie: Prior to 2005, it was a sporadic issue on cucurbits other than cucumbers, which are not as susceptible to the disease as cucumbers. Breeding efforts in the 50s introduced a resistance gene to cucumbers called the dm-1, which conferred complete resistance to CDM. In 2004 in the southern US, there was an outbreak of DM, devastating growers crops. In 2005, the disease occurred for the first time in the northeastern US and Ontario where it devastated our crops. Something about the pathogen changed or shifted to allow for the disease to occur in plants that had this dm-1 gene. Since then there has been no complete host resistance, however there are some breeding initiatives to introduce tolerant varieties of pickling cucumbers that’ve been recently commercialized and could be a fit if the agronomics are acceptable to the processors. 

 Growers, especially field cucumber growers, have had to invest significantly in managing this disease. They had to buy new spraying equipment to mange with fungicides not to mention the dollars spent on targeted fungicides. 

 Cassie:  And, just going back, you mentioned that this pathogen has other cucurbit hosts, is it not of as big of an issue in those crops in Ontario and just really of economic concern in cucumbers?  

 Katie:  CDMs known hosts are cucumber, muskmelon, watermelon, squash, pumpkins, zucchini and gourds as well as a few wild cucurbit hosts. However, the majority of outbreaks are in cucumber fields. Recent research has shown that the pathogen seems to separate into two groups known as clades. These groups appear to be host specific and clade 2 appears to be more aggressive and infect cucumbers and muskmelons whereas clade 1 infects the other major cucurbit crops including pumpkin, zucchini, squash, watermelon. While CDM may occur in crops other than cucumbers and muskmelons, the outbreaks seem to be less severe.  When we need more information on P cubensis biology, it makes is an extremely challenging pest to study, it can’t just be grown in a plant in a lab. More research is needed to understand the population structure, but it takes time.

 Cassie: Sounds like a big job. Now are there options other than fungicides for growers to manage this disease? 

 Katie: There are some, but there are more efforts into managing this disease with less reliance on fungicides. But they need to be implemented at planting rather than when the disease shows up. Tolerant cultivars are a really good way to limit the disease severity in pickling cucumbers however the attributes have to be favourable for the processor and growers in order for them to be used. Cucumber crops are the most susceptible so once the pathogen is in the region, they rely on fungicides to keep the crop alive till and through harvest. In other cucurbits that are less susceptible such as watermelons, squash and pumpkins may benefit from cultural management including using drip irrigation instead of overhead irrigation, ensuring airflow is achieved through the canopy – all things to keep the crop foliage as dry as possible. 

 Cassie: OK so for growers experiencing a downy mildew outbreak or when the disease is in the area, do growers have many options when it comes to fungicides? 

 Katie: I think we should start with more about the pathogen biology as it sets the stage for the fungicide issues. The pathogen P cubensis has a very high reproduction frequency, needing moisture to infect but can infect and release spores in a wide range of temperatures, up to 30C. It can complete it’s lifecycle in one week, releasing hundreds more sporangia from very few. It has a high risk of developing fungicide resistance.  Now that this pathogen is here to stay, growers should be following a weekly fungicide application. In Ontario, there are three targeted fungicides that are registered and effective, Orondis Ultra, Torrent and Zampro according to efficacy studies in Michigan and Ontario. P cubensis has developed resistance to multiple fungicides since 2005 when the disease re-emerged. The high reproduction frequency of P. cubensis means there is an inherent risk when using fungicides to manage this disease. During reproduction, genetic mutations naturally occur at very low rates. When a pathogen like P. cubensis produces so many spores in a short period of time, there is a higher probability that a spore will receive a mutation that confers resistance to a specific fungicide. If that fungicide is continually sprayed, that resistance mutation will be selected for and can then make up the predominate population that causes downy mildew. So, it is important to follow resistance management strategies when using fungicides for downy mildew to keep these products effective for seasons to come. 

 The main tactics growers can use to reduce resistance development once the disease is established in the region, is to rotate effective targeted products and tank mix these with multisite fungicides that are active on downy mildew. The effective multisite fungicides are mancozeb and chlorothalonil. This year, cucurbit growers are in a pickle when it comes to tank mixing with these multisite fungicides. The recent re-evaluation of chlorothalonil means that as of May 10, 2020 cucurbit growers can only use 2 applications of chlorothalonil per year. Mancozeb is another option to tank mix, but it has a 14-day pre-harvest interval, making it very challenging to be used when fruit are continuously hand-harvested. Growers should tank mix with mancozeb until they are within the 14-day pre-harvest interval and then use chlorothalonil to tank mix for two applications. Ontario growers still have a significant season ahead of them and cucurbit downy mildew threatens their yields. 

 Cassie: How can growers know that the population hasn’t developed resistance to the product they’re using? 

 Katie: Right, it is very challenging to tell if a product is still bringing efficacy to the table when used in rotation and tank mixed. This is why we need local efficacy research studies. These researchers look at each product individually in order to inform local recommendations on which products to use in rotation for downy mildew control. This is how we knew that Presidio was no longer working or Revus or Ridomil. These trials are important to track the changes in this high-risk pathogen. It is also important to note that there are more options for growers in the US not based on efficacy but based on fungicide registrations. Canadian growers are dealing with the same aggressive disease but with fewer fungicide options. 

 Cassie: when it comes to the temperatures, we’ve certainly seen extended weeks of daytime highs over 30C, is the pathogen still able to infect at this temperature? 

 Katie: With the extended hot, dry weather through July, the mindset that “it is too hot for disease” has allowed us all to be caught off guard when downy mildew arrived. Studies have shown that P. cubensis germinates, infects, and produces sporangia the quickest at 15 to 20°C, which have been the average overnight temperatures. Additionally, with extended leaf wetness from heavy dew, high humidity, overhead irrigation (and rain for the lucky few), the pathogen can infect at temperatures up to 30°C. At higher temperatures, all the sporangia are not killed, they are often hiding in the cooler canopy of the cucurbit crop, waiting until nighttime where temperatures often reach around 20°C.  

 Cassie: That’s a great point. I know a lot of us have been getting out into the field early to avoid the insane heat and you see first hand, you still have quite a bit of leaf wetness from humidity. So basically, growers should just always be on the lookout for the disease once the spores are present.  

 Katie: absolutely, not only do some plants only need those heavy dew periods to infect, the plants are more stressed from the lack of moisture which gives some of the more abundant pathogens the ability to infect the weakened plants.  They’re sneaky like that. 

 Cassie: And what are some of the ways growers can stay updated on this disease in Ontario? 

 Katie: There are quite a few resources. To track the movement of confirmed diseases including which crops they were found, use cdm.ipmpipe.org this is good to get a general idea of how the disease is progressing up from the southern US. Dr. Mary Hausbeck at Michigan State University leads a spore trapping network for the state and posts the results at veggies.msu.edu, where there is also the confirmed cases listed. It’s a good resource to keep track of the disease in our neighbouring state. They grow the largest acreage of pickling cucumbers which as mentioned before are highly susceptible. 

When it comes to local recommendations, for registered products in Canada growers can check out OMAFRA publication 838, guide to crop protection in vegetable crops. And we also monitor for this pathogen in Ontario, so for up to date information follow onvegetables.com. I’m also active on twitter and often tweet about findings as well as other cool pathology related information, and the occasional dog photo. My handle is @kegoldenhar

 Cassie: That great, lots of helpful resources to help our cucurbit growers overcome downy mildew challenges and I’ll be sure to include those websites as links in the show notes. One of the things I always like to say is that pests don’t observe boarders. And cucubit downy mildew is definitely another example of that. So it’s great that we have these networks and are able to work with researchers and extension specialists in neighbouring states such as Michigan.  

Katie: It’s unfortunate it isn’t stopped for non-essential travel. 

 Cassie: Well thanks again Katie, this was great and it was great to have you on the podcast again!   

 Katie: Thanks, Cassie!

Cassie: I was just speaking with Katie Goldenhar, pathologist for horticulture with the Ontario Ministry of Agriculture, Food and Rural Affairs. 

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27.20
SEGMENT 2: Gypsy Moth (The Horticulture Hotseat) 

Kristy: If you’ve noticed this year those small brown caterpillars in early spring that became those large brown caterpillars defoliating trees and now those brown moths frantically hitting your windows, flying into your face and maybe even filling your monitoring traps – I’m not holding a grudge, I swear – you are not alone. Jennifer Llewellyn, OMAFRA’s Nursery and Landscape Specialist gives us a quick lesson on gypsy moth and why we are seeing such large numbers this year across the province in a fun segment we’re calling the Horticulture Hotseat. 

Jennifer: So, basically gypsy moth is a moth, larva or caterpillar. In the immature larval or caterpillar stage, it's a very significant defoliator of over 500 species of plants. It's in the Erebidae family and it's in there with some other hairy moths as well. Tiger moth, you might know, or the woolly bear moth which are those orange and brown, fuzzy fat looking larvae that you see crawling around in the fall looking for an overwintering site. They're actually in the same family. But gypsy moth, they have very long hairs associated with the larval stage. And actually, some people can be a little bit allergic to the hair. Sometimes they'll get a skin rash reaction to that. So very hairy caterpillars that can be anywhere from one centimeter to five centimeters long, depending on what developmental stage or instar, they're in. On their backs, they'll have paired spots that are blue - about five of those paired blue spots - and then behind that, six pairs of red spots that will appear on their backs. That makes them kind of unique and how they look in terms of identification of the caterpillar stage. 

Mostly we're going to find them feeding on deciduous trees in Ontario – so, oak trees, maple, birch, linden are probably some of their most favorites. However, when we have really high populations like we do in 2020, they will feed on just about anything because they just become a lot less picky when they're literally starving. They're looking for another food source. And a lot of people don't realize they can be a pest on Colorado spruce. Of all the evergreens Colorado spruce is one that we do see them on. Sometimes you can lift up a branch and you'll see the larval stage hanging out during the day. 

And their behavior as a caterpillar is really interesting. Quite often they'll feed during the night time or dark hours or cooler hours and during the day - so in the morning - they'll actually travel down the trunk and they'll hide vertically in bark crevices and underneath large branches. They'll kind of just hang out there during the day and then in the later part of the day, they'll move back into the canopy. Not all the caterpillars do that, but a lot of them do. So that makes an opportunity for collection culturally. You can knock some of those caterpillars during the day into a bucket of soapy water and this soapy water will smother them so that they'll die. You can also just manually squish them and kill them that way also.

And then in terms of the stage that we're in right now, most of our gypsy moths have emerged and we've got these brown moths flying in a zigzag pattern relatively close to the ground just like crazy all over the place. And those are actually the male adult gypsy moths. The females are white. They're quite striking to see. But they don't fly; they're actually flightless. So, they would emerge from their pupal case and then potentially crawl up the tree, hoping to be intercepted by a male. They'll throw off some pheromones and attract the males. This makes it a really good idea for people to put sticky bands around their trees at this time, because as the female crawls, she'll get stuck on a sticky band. And hopefully, she'll lay her eggs on that sticky band and attract males that will get stuck on that sticky band. So it's a really nice cultural method. 

In terms of management though, we'll also do some spraying for the early instar larvae. So for the young larvae - so any larvae that are, let's say, below an inch in length or below two and a half centimeters in length, Bt.k, the biological insecticide, Bacillus thuringiensis kurstaki, is very, very effective on gypsy moth larvae. You just need really, really good coverage. And we find that it works very well. And once they get to be over an inch long, it usually takes some more traditional synthetic insecticide, such as something that contains permethrin to be very effective. 

So, we've actually found quite significant populations this year in 2020. We knew they were coming. We saw really heavy egg mass counts in the fall of last year. The gypsy moth larvae will actually start to hatch around the time that the trilliums are blooming in the spring, and they'll feed up until the sambucus, or the elderberry are just sort of at the latter stages. And this is a peak year and this happens in a lot of organisms. So just like anything else, there's great food source, great growing conditions, and not a lot of parasites, parasitoids or pathogens are really caught up with them. And so they've been able to really expand their population. And then we'll find a population crash when there's a lot of predation of them or conditions aren't as good for them. And there's a couple of bacteria, virus and actually fungal pathogens that normally attack the larval stage of gypsy moth. But it was so dry this spring and early summer, that it was really the worst conditions for those natural pathogens of the larvae. So there was virtually no larval mortality from natural biological control. And so the larval populations were all very successful at pupating into adults, unfortunately. 

What's really going to be important for 2020 in terms of managing trees that have sustained significant amounts of defoliation is going to be irrigation. Irrigation is going to be very, very important this year. So please do give a good inch, or two and a half centimeters of water to your trees in the landscape, especially if they're not getting regular rainfall events. But we aim for about an inch of water about once a week, and that's really going to help the trees recover and put out another set of foliage. And don't forget to irrigate them. You know, in August and September, we sometimes forget but even early fall is a really important period for soil moisture and root growth. If it is a dry fall, continue to irrigate your trees with that one inch of water per week or so into, I would say, early October would be great.

Kristy: That was Jennifer Llewellyn, Nursery and Landscape Specialist with Ontario Ministry of Agriculture, Food and Rural Affairs.

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34.06
Cassie: Thanks for tuning into our episode today.  This has been Cassie Russell -

Kristy: - and Kristy Grigg-McGuffin, for the What’s Growing ON podcast. For more information on horticulture grown in Ontario, check out the links to our fruit, vegetable and specialty crop blogs in the show notes.

Cassie: Thanks goes out to Katie Goldenhar and Jennifer Llewellyn for joining us on our episode today. And a big shout to Michael Pupulin for the editing of our episodes. Music from this episode is the track Aspire from Scott Holmes.

Kristy:  We will be back soon with an all new episode of What’s growing ON. In the meantime, if you have questions, comments or suggestions for a topic you would like us to cover, Please send us an email us at ONhortcrops@gmail.com. We would love to hear from you.