Storms: Planning for more than just your chips

 

It may seem early to start thinking about downy mildew or some of those other nasty fungal pathogens that are sure to be coming our way soon. However, as storm systems make their way up the Eastern Seaboard they usually bring spores with them from diseased fields all along the coast. In addition to transporting spores to our province they usually provide ideal conditions for fungi to colonize plants as these systems often bring with them sustained wet, cool, and humid conditions.

Downy mildew was positively identified in New Jersey last week and while it doesn’t look like storm Bret and the newly developing storm Cindy will be tracking over Nova Scotia anymore, it’s important to monitor systems coming up the Eastern Seaboard to correctly time preventative fungicide sprays and other IPM techniques such as trapping and scouting for new pests and ensuring physical barriers such as insect netting are in place as needed.

Over the last 2 years downy mildew has become a serious problem for cucurbit growers here as it is devastating when it gets a foothold in your field. Over the last 2 years both of the first confirmed occurrences in Nova Scotia happened in August, however, we need to be prepared for earlier occurrences as it is only one storm system away once it is detected along the eastern U.S coast. Along with fungal spores, these systems can bring with them novel insect pests to be mindful of as well.

Please reach out to your commodity support specialists if you have any disease or pest concerns!

Click here for excellent information on effective disease management, fundamentals of fungicides, herbicides and using weather stations for decision making on your farm

here for more information on downy mildew in Nova Scotia

Tim

Cucurbit Downy Mildew was detected in New Jersey on June 13th, 2023

The first incidence of Cucurbit Downy Mildew was detected in New Jersey, United States on June 13th, 2023. Previously (in 2021 and 2022) Clade II has been detected in Nova Scotia, in the Annapolis Valley region, which predominantly infects cucumbers and cantaloupes. Clade I, which affects watermelon, pumpkin, squash, and zucchini has not yet been detected, but monitoring should still be undertaken in these crops on the chance that it does make an appearance.

It is important that growers remain vigilant and scout their fields regularly for this disease, as spores may be blown into the province by storm systems in the United States. We are not recommending protectant fungicides at this point in time, but strongly recommend weekly scouting for disease.  Downy mildew may be identified initially as water-soaked lesions that appear on the top side of the leaf, which may first appear on any region of the canopy. The most ideal time to identify these lesions are during or right after a heavy dew. The centre of the lesion initially appears chlorotic or yellow before eventually the tissue dies, becoming brown or necrotic (Fig 1 and 2). This disease is very fast spreading, the lesions will continue to spread across the canopy, and quickly kill the entire plant if left unchecked.  When conditions are humid, a “downy growth” may be observed on the underside of the initial water-soaked lesions. These symptoms may first appear before lesions on the upper leaf. This growth is particularly apparent in the morning, after a period of wet weather or dew formation.

Growers must stay vigilant, scout often, and report any positive findings they may see.

 

Figure 1: Chlorotic lesions associated with downy mildew infection on cucurbits.

Figure 2: Necrotic lesions associated with downy mildew infection on cucurbits.

If you suspect downy mildew in your field, please contact Dustin MacLean, the Field Plant Pathologist at dustinmaclean@perennia.ca or 902 324-9623.

 

 

Posted by Dustin Maclean, Horticulturalist and Field Plant Pathologist with Perennia

First Occurrence of Downy Mildew in Nova Scotia in 2022

 

Downy mildew (DM) was diagnosed on cucumber Monday, August 29^th, 2022, in Annapolis County, Nova Scotia by both visual inspection and spore identification using microscopy. The strain of DM that affects cucumber also affects cantaloupe. At this time, cucurbit crops other than cucumber and cantaloupe are not at risk.

Figure 1: Downy mildew infected cucumbers

Growers should now add DM-targeted fungicides to cucumber and cantaloupe spray programs. Rotate between FRAC groups of targeted materials and combine targeted materials with broad-spectrum, preventative materials to prevent resistance development.

Where possible, tank mix Zampro (maximum 3 applications per year, 1 day PHI) or Orondis Ultra (maximum 4 applications per year, 0 day PHI) with chlorothalonil (maximum 2 applications per year, 2 day PHI) or mancozeb (maximum 3 applications per year, 14 day PHI). Never make back-to-back applications of the same product or products from the same chemical family. Follow a 5 to 7-day application interval, and rotation of the three products can be repeated as necessary.

Downy mildew can often be confused with angular leaf spot, bacterial leaf spot and Alternaria leaf spot. Downy mildew can be identified by dark sporangia growing on the underside of the leaf.

If you would like confirmation that downy mildew is present in your crop you can contact Dustin MacLean, Perennia Field Plant Pathologist at dustinmaclean@perennia.ca or call 1 902 324-9623

Figure 2: Sporangia developing on underside of cucumber leaf.

Table 1. Downy mildew multi-site, broad spectrum fungicides

Common name (FRAC group)	Trade Names	Rate per hectare (Rate per Acre)	PHI (days)	Re-entry interval	Max applications
mancozeb (M03)	Dithane Rainsheild, Penncozeb 75DF Raincoat Manzate Pro-Stick	3.25 kg (1.3 kg)	14	12 hours	3*
chlorothalonil (M05)	Bravo ZN Echo	4.8 L (1.9 L)	2	12 hours	2

 Table 2. Downy mildew specific fungicides

Common name (FRAC group)	Trade Name	Rate per hectare (Rate per Acre)	PHI (days)	Re-entry interval	Max applications
amectotradin (45) + dimethomorph (40)*	Zampro	0.8-1 L	1	1 day for hand harvesting, pruning or thinning , 12 hours for other activities	3
oxathiapiprolin (49) + mandipropamid (40)*	Orondis Ultra	0.4-0.6 L	0	12 hours	4

 

 

Be on the Lookout for Cucurbit Downy Mildew in Nova Scotia

Cucurbit downy mildew was detected in Nova Scotia for the first time in 2021 in Annapolis, Kings, and Hants Counties. It has not yet been found in Nova Scotia in 2022, but has been reported in New Jersey, USA on June 11^th, 2022. Currently, only Clade II has been discovered in Nova Scotia, which predominantly infects cucumbers and cantaloupes. Clade I, which affects watermelon, pumpkin, squash, and zucchini has not yet been detected, but monitoring should still be undertaken in these crops on the chance that it does make an appearance.

It is important that growers remain vigilant and scout their fields regularly for this disease, as spores may be blown into the province by storm systems in the United States. We are not recommending protectant fungicides at this point in time, but strongly recommend weekly scouting for disease.  Downy mildew may be identified initially as water-soaked lesions that appear on the top side of the leaf, which may first appear on any region of the canopy. The most ideal time to identify these lesions are during or right after a heavy dew. The centre of the lesion initially appears chlorotic or yellow before eventually the tissue dies, becoming brown or necrotic (Fig 1 and 2). This disease is very fast spreading, the lesions will continue to spread across the canopy, and quickly kill the entire plant if left unchecked.  When conditions are humid, a “downy growth” may be observed on the underside of the initial water-soaked lesions. These symptoms may first appear before lesions on the upper leaf. This growth is particularly apparent in the morning, after a period of wet weather or dew formation.

Growers must stay vigilant, scout often, and report any positive findings they may see.

 

Figure 1: Chlorotic lesions associated with downy mildew infection on cucurbits.

Figure 2: Necrotic lesions associated with downy mildew infection on cucurbits.

If you suspect downy mildew in your field, please contact Dustin MacLean, the Field Plant Pathologist at dustinmaclean@perennia.ca or 902 324-9623.

 

 

Posted by Dustin Maclean, Horticulturalist and Field Plant Pathologist with Perennia

Status report on Cucurbit Downy Mildew

As of Wednesday, September 1st, cucurbit downy mildew has been found in Annapolis, Kings, and Hants Counties.  While all Cucurbits are susceptible to downy mildew, there are different Clades which affect different subsets of the Cucurbit Family.  Clade I predominantly infects watermelon, pumpkin, squash, and zucchini.  Clade II predominantly infects cucumbers and cantaloupes.  So far, we have only found downy mildew in cucumber fields, suggesting that we have Clade II. 

There is a great Cucurbit Downy Mildew Diagnostic Key, which might help you determine if you have downy in your field. 

Downy mildew spores blow in on storm systems from the United States, so it is possible that Thursday's storm will bring in Clade I, so growers should be aware.  Downy mildew cannot overwinter in Nova Scotia unless the infection spreads to greenhouses.  Please reach out to Rosalie Gillis-Madden, Perennia’s Vegetable Specialist by email or cell (902-670-9655) if you want support around managing this disease. This late in the season, growers of squash and pumpkins might choose not to put on a protectant spray.  

• Growers who do not yet have downy in their fields and are hoping to continue picking their cucumbers and cantaloupes are advised to put on a protectant spray (i.e. Zampro or Torrent for conventional growers, Serenade Opti or copper for organic growers).   
• If you have downy mildew in your cucumber or melon field, and you wish to keep picking, an application of Orondis Ultra is advised (copper for organic growers).  Conventional growers should rotate to a different FRAC group and spray again 7 days after the first spray as advised by the pesticide label (suggested Torrent as a second application for conventional growers, and another copper spray for organic growers.)  Please also call Rosalie so we can document the spread of this disease. 
• If you are hoping to keep picking zucchini and watermelons for a few more weeks, a protectant spray is advised as downy mildew spores from Clade I might blow in on Thursday’s storm (i.e. Zampro or Torrent for conventional growers, Serenade Opti or copper for organic growers).  You should rotate to a different FRAC group and spray again 7 days after the first spray as advised by the pesticide label.
• If you find downy in your squash, pumpkins, watermelons, or zucchini, please contact Rosalie right away.   

Please note that powdery mildew sprays ARE NOT effective against downy mildew.  Downy mildew can very quickly breed fungicide resistance so rotating FRAC groups and following the label closely is key to management.  

If you are currently picking crop, please pay close attention to the PHI listed on the fungicide label.  

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Updates on Cucurbit downy mildew

Monday, August 30th, 2021 downy mildew was found in King's County.  Please read the original blog post here.  Downy mildew that infects Cucurbits is specific to Cucurbits - it won't infect onions, grapes, hops, lettuce, spinach, basil, etc.

There is a great Cucurbit Downy Mildew Diagnostic Key, which might help you determine if you have downy in your field.  Please do not hesitate to reach out if you need support in identification and you are a registered farm in Nova Scotia.

So far, downy mildew has only been confirmed here on cucumbers.  The Nova Scotia Vegetable Blog will be updated if that changes.  There are two different host-specific clades of downy mildew.  Clade I mostly infects watermelon, pumpkin, squash, and zucchini.  Clade II mostly infects cucumbers and cantaloupe.  It is possible that we currently only have Clade I in Nova Scotia.  However, there is a storm system blowing in on Thursday/Friday that will certainly bring downy conducive conditions (i.e. prolonged leaf wetness), and perhaps more infections spores of either Clade I or Clade II.  

Cucumber and melon growers should apply protective fungicides as outlined in yesterday's blog post.  Pumpkin, squash, zucchini, and watermelon growers can take a gamble - either apply a protectant as previously mentioned; do nothing and hope there isn't downy that will infect the pumpkin/squash subset; or harvest early.  For growers that do not have established drive rows in their winter squash and pumpkins - especially if they are close to harvest - it may be worth the risk to wait see and thereby avoiding driving over crop to make a fungicide application.  Harvesting early is a good option if the crop is mature and you have curing facilities, especially in light of the forecasted precipitation coming our way on Thursday/Friday.  Downy mildew does not affect fruit, however in the case of cantaloupes, the fruit will be less sweet.  

Cucurbit downy mildew!!!

Fig 1. Heavily infected cucumbers.

ALERT!  Downy mildew has been found in Cucurbit fields throughout Kings County!  

We are lucky here in Nova Scotia, we do not usually get downy mildew in our Cucurbit fields.  This disease is catastrophic - it can take out a field in a matter of days (Fig. 1).  It affects all Cucurbits: melons, watermelons, cucumbers, pumpkins, and squash.  It can very quickly breed fungicide resistance so rotating FRAC groups and following the label closely is key to management.  

Fig. 2. Downy mildew
sporulating on the
underside of a cucumber
leaf.  Sporangia can vary in
colour from light grey to
deep purple with a downy
appearance.

Monday's cool, drizzly weather has been perfect downy breeding conditions.  Abundant sporangia are produced if humidity is at 100% for 6 hours with temperature between 15-20˚C (Fig. 2). These sporangia are wind blown or splashed onto susceptible host tissue and germinate if leaves are wet for at least 2 hours at 20-25˚C.  Spores can also be carried on your tools, clothes and hands so DO NOT move from infected fields to healthy fields. Spores do not last indefinitely on non-host surfaces, for example exposing spores to sunlight for 4.5 hours kills 63% of the spores.  Spores can survive where there is moisture, for example on wet clothes or farm equipment, but needs living plant tissue to survive for any length of time.  It does not overwinter in our region.  This infection likely blew in from the Southern US.  

Fig. 3a. Early downy infection on
cucumber, leaf surface.

Pale green or yellow spots can be seen on the upper leaf surface as an early sign of infection.  These spots later turn brown and spread.  The spots are typically bound by leaf veins giving it a bit of an angular appearance, especially in cucumbers.  As the lesions spread, the whole leaf will turn brown and look like it's been zapped by frost.  

If downy mildew develops in fields nearby, but not in yours, use weekly rotations of Manzate + Zampro (FRAC group 40, 45) or Manzate +Torrent 400 SC (FRAC Group 21).  If downy mildew is present in your field, apply Phostrol (FRAC Group 33) or Orondis Ultra (FRAC Group 49, 40).  

For organic growers, if you have downy, use copper.  If you don’t have it, Serenade Opti (44), Serifel (BM02– cucumber only) with a liberal dose of prayer.

Fig 3b.  Underside of cucumber leaf.

For all pesticides, be sure to read the label carefully, and follow exactly.  If the label differs from what you read here, always follow the label.  Up-to-date labels can be found here.  

Do you know about Perennia's Pest Management Guides?  Every year we update pest management options for the major crops in Nova Scotia.  A complete listing can be found here (click on Vegetable Crops for the drop down menu).

Fig. 4a. Older lesions of downy
mildew on cucumber leaf surface

Fig 4b. Advanced downy mildew
infection, underside of cucumber leaf.

Fig 5. For contrast, this is powdery
mildew on squash. Powdery mildew
sprays are NOT effective against
downy mildew.