Dirty rotten scoundrels – harvest rots in vineyards, 2026

Managing Botrytis bunch rot

Berries become susceptible to direct infection by Botrytis once fruit reaches veraison.  Under conditions of high humidity, a single berry infection can quickly spread to involve the entire cluster. Frequently bunch rot infections start at the centre of the cluster and, by the time you see it, the entire cluster is compromised. This is why applying the berry touch spray before cluster close is so important. Every berry should be covered in a protective fungicide coat. Applying after the cluster has closed up means only the berries on the outside of the cluster will be protected. With the challenging conditions for spraying this year, this window could easily have been missed, leaving internal berries vulnerable to infection.

Botrytis also has the ability to move from cluster to cluster if they are in close proximity. This is especially an issue in crops with very heavy fruit set and overlapping clusters. Not only can Botrytis spread, these clumped up clusters further impede complete coverage of berries. Thin clusters so that they are not in close contact to reduce cluster-to-cluster spread.

To reduce the risk of fungicide resistance, if sporulating infections are present, first dry up the infections first with a fruiting zone spray of MilStop (potassium bicarbonate) followed by a spray with one of the products with Botrytis activity (see table below).  MilStop will increase the pH, which may reduce the effectiveness of a Botrytis fungicide so check with the company to confirm whether a tank mix is recommended.  MilStop alone will not provide protection – its suppression rating is due only to the desiccating action it has on infected berries. Tank mixing MilStop with other fungicides is not recommended due to the high pH that is created. 

I’ve heard reports that Folpan or other WP fungicides may also dry up Botrytis infections and these can be tank mixed with a Botrytis product unless specified otherwise on the label.  Be aware of preharvest intervals when considering a desiccating tank mix partner.

Make sure that you consider fungicide group rotations and avoid using products from the same group in back-to-back sprays.

Fungicide	Group	Botrytis	Maximum Applications	REI	PHI
Kenja	7	4	3	12 hr	14 days
Inspire Super	3+7	4	2 (3 L/ha)	7 days	14 days
Luna Tranquility	7+9	4	2	24 hr	45 days
Scala	9	4	3	24 hr	7 days
Miravis Prime	7+12	4	2	12 hr	14 days
Switch 62.5 WG/Impala	12	4	3	48 hr	7 days
Intuity	11	4	3-4 (2.631 L/ha)	12 hr	10 days
Elevate 50 WDG	17	4	3	5 days (leaf pulling)	7 days
Diplomat 5 C	19	2	3 L/ha	When dry	0 days/12 hrs1
Allegro 500 F	29	4	2	9 days (leaf pulling)	21 days
ProBlad Biofungicide	BM1	3	5	when dry	0 days
Serenade OPTI	BM2	3	not specified	when dry	o days
Serifel	BM2	3	not specified	4 hrs	0 days
Stargus	BM2	1	not specified	4 hrs	0 days
Botector	NC	2	4	4 hrs	0 days/4 hrs1
Double Nickel	NC	2	not specified	when dry	0 days
Oxidate 2.0	NC	1	8	4 hrs	0 days
Timorex Gold	NC	3	not specified	when dry	when dry
MilStop	NC	2	10	4 hrs	0 days/4 hrs1
Regalia Maxx	P5	1	not specified	when dry	0 days
1 Machine/hand harvest

Waxy grape berries are difficult to wet. Some research suggests that the use of wetters (a type of adjuvant) may improve late-season control of botrytis. Read the product label or ask the chemical supplier as most product formulations already contain adjuvants and too much wetter added to the tank mix can result in less chemical being deposited on bunches.

Managing Sour Rot

Sour rot is caused by acetic acid bacteria and yeasts that reside on the outside of the fruit.  Infection can occur anywhere there is an opening in the skin – from a fully split berry to the slightest pull of a berry off the stem. It frequently starts in the centre of the cluster where berries are squeezed as they swell.  Drosophila spp (“fruit flies”) are excellent vectors of these organisms and can rapidly spread the disease throughout a vineyard.

While some fungicides may list “sour rot” control on the label, in all except Oxidate, this refers to sour rots caused by fungi that are not the causal organisms in Ontario. Research in Ontario and New York has shown that applications of antimicrobials such as potassium metabisulphate (KMS at 5 kg/1,000 L) or peroxide (Oxidate) in combination with insecticides were the most effective way to manage the disease.  These products sterilize the surface of berries but provide no protection from yeasts and bacteria that can be easily reintroduced by Drosophila (fruit flies) and other insects. There haven’t been any reports of issues with tank mixing KMS or Oxidate with Botrytis fungicide, although ProBLAD might be an issue because pH of 5.5 must be avoided.

Get the bugs out

It is critical to manage not just the yeasts and acetic acid bacteria that cause sour rot, but also the Drosophila flies (spotted wing Drosophila as well as the common “fruit fly) that can carry and spread them.

Wasps or yellow jackets are attracted to the juice that leaks from damaged berries. They also have mouthparts that are capable of puncturing berry skins to get to the sweet insides.

Earwigs may be a problem in table grapes, not because they cause damage, but because of consumer acceptance (or lack thereof).  If you have table grapes, choose products that are effective against both Drosophila and Earwigs.

The most effective labeled treatments for SWD (and common fruit flies) are Delegate and Success and they both have activity vs wasps and earwigs, but not MALB.

Insecticides	SWD	Wasps	MALB	Earwigs	Applications	REI	PHI
Malathion 85 E	1	1	4*	3	1	4 days	3-4 days2
UP-Cyde 2.5 EC, Ship 250 EC	3	4*	4*	3	2/31	12 hr	2-7 days2
Delegate	4*	3	0	3	3	12 hr	7 days
Entrust	3*	3	0	3	3	7 days	7 days
Success	4*	3	0	3	3	7 days	7 days
Harvanta	4*	0	0	0	3 (4.8 L/ha)	12 hr	7 days
* Labeled for the pest on grapes. 1 Maximum applications for hand/mechanical harvest. 2 PHI for machine vs hand harvest

Coverage is critical

Since Botrytis and sour rot often start in the middle of clusters, it is very important to make sure that spray penetrates to the rachis. This will require high volumes of water in a fruiting zone application. In research trials, I used a rate equal to 600 L/ha to try to achieve this.

Dropping crop

If you are dropping crop to reduce the amount of volatile acidity (VA) in the crop, do so as close as possible to when the harvester will go through. Otherwise, infected fruit lying on the ground in the vineyard can still act as a source of inoculum and will also attract Drosophila. If possible, bury the dropped fruit or remove it from the vineyard. Probably not practical but the best approach if it can be managed, possibly in premium blocks.