Bitter Rot Management: Results of 2024-2025 Field Trials

Introduction

Bitter rot is an important summer disease that can significantly affect apple production in Ontario. Studies from the University of Guelph have confirmed that three Colletotrichum species are associated with this disease in the province: C. fioriniae, C. nymphaeae, and C. godetiae. Similar to reports from other regions of the Northeast, C. fioriniae appears to be the dominant bitter rot species in Ontario, despite the presence of other Colletotrichum species. Comprehensive province-wide incidence data are still pending and will be reported in a future article.

These pathogens can infect fruit early in the season without producing visible symptoms. As fruit approach maturity or during storage, infections can develop rapidly, appearing as characteristic sunken lesions that can result in fruit loss (Figure 1 & 2). Recent research indicates that apples are susceptible to Colletotrichum spp. infection from fruit set through harvest, highlighting the importance of season-long disease management. To learn more on bitter rot timing, read Timing of Infection and Management of Bitter Rot in Ontario.

To support improved bitter rot management for Ontario growers, a fungicide efficacy trial was conducted during the 2024 and2025 growing seasons at the University of Guelph’s Ontario Crop Research Centre – Simcoe. Several fungicide treatments were evaluated to determine their effectiveness in reducing bitter rot under Ontario growing conditions.

Experimental Set-Up

The trial was conducted using a randomized complete block design with two cultivars, Empire and Ambrosia on M.9 rootstock. Five trees were used per replicate and four replicates per treatment. Treatments (Table 1) were selected in collaboration with project partners.

Table 1. Treatments for 2024-2025 bitter rot fungicide efficacy trials, Ontario Crop Research Centre – Simcoe

Treatment	Trade Name	Active Ingredient(s)	FRAC Group	Rate Applied
1	Untreated Check (UTC)	water	—	—
2	Folpan 80 WDG	folpet (80%)	M4	3.75kg/ha
3	Aprovia	benzovindiflupyr (100 g/L)	7	500 mL/ha
4	Cyclone Plus* + Agral 90	lactic acid (1.4%) + citric acid (2.54%)	P7	1.4% v/v + 0.1% v/v
5	Switch 62.5 WG*	cyprodinil (37.5%) + fludioxonil (25%)	9+12	975 g/ha
6	Commercial Standard (CS)
	Pristine WG	boscalid (12.8%) + pyraclostrobin (25.2%)	7+11	1.2 kg/ha
	Allegro 500 F	fluazinam (40%)	29	1 L/ha
	Supra Captan 80 WSP	captan (80%)	M4	3 kg/ha

The first fungicide application was made 10 days after petal fall.

For Empire, applications began on:

• May 23, 2024 – when fruit size was approx.. 11 mm
• May 26, 2025 – when fruit size was approx.. 7 mm

For Ambrosia, applications began on:

• May 30, 2024 – when fruit size was approx.. 14 mm
• June 3, 2025 – when fruit size was approx.. 10 mm

Subsequent fungicide applications were applied approximately after 9-17 days or after 50 mm or more rainfall). Spray schedule and rainfall summary for 2024 and 2025 are provided in Tables 2 and 3, respectively.

Applications were made using a CO₂-powered backpack sprayer equipped with a TeeJet XR11002-VS nozzle, delivering spray volumes of 500–800 L/ha. Weather data was monitored using an on-site Weather INnovations System (Tables 2 & 3).

To ensure disease pressure, trees were inoculated in 2024 on May 24 and May 31 for Empire and Ambrosia, respectively, and in 2025 on May 27 and June 4 for Empire and Ambrosia, respectively. Mesh bags containing Empire fruit inoculated with C. fioriniae spores were hung from the top trellis wires to act as a source of inoculum (Figure 3).

A total of nine spray applications were made in 2024 and eight in 2025 for each cultivar (Tables 2 & 3). Empire fruit were harvested on September 9, 2024, and September 11, 2025, while Ambrosia fruit were harvested on September 26 in both years. In-field bitter rot incidence was assessed by recording the number of infected fruit and total fruit per tree in between each spray application and at harvest. In 2024, all fruit were harvested, whereas in 2025, a subsample of 100 fruit per plot was collected.

Table 2. Application dates for 2024 bitter rot fungicide efficacy trial, Ontario Crop Research Centre – Simcoe

Application	Date	Cultivar(s)	Interval (days)	Rain (mm)	Commercial Standard Rotation
1	23-May	Empire	–	–	Supra Captan 80 WSP
2	30-May	Ambrosia, Empire	7	59.8	Allegro 500 F
3	11-Jun	Ambrosia, Empire	12	38	Pristine WG
4	24-Jun	Ambrosia, Empire	13	9.6	Supra Captan 80 WSP
5	3-Jul	Ambrosia, Empire	9	66.2	Supra Captan 80 WSP
6	18-Jul	Ambrosia, Empire	15	91.2	Pristine WG
7	31-Jul	Ambrosia, Empire	13	15.4	Allegro 500 F
8	14-Aug	Ambrosia, Empire	14	28	Supra Captan 80 WSP
9	27-Aug	Ambrosia, Empire	13	35.6	Pristine WG
10	10-Sep	Ambrosia	13	35.8	Supra Captan 80 WSP

Table 3. Application dates for 2025 bitter rot fungicide efficacy trial, Ontario Crop Research Centre – Simcoe

Application	Date – Empire	Interval (days) – Empire	Rain (mm) – Empire	Commercial Standard Rotation – Empire	Date – Ambrosia	Interval (days) – Ambrosia	Rain (mm) – Ambrosia	Commercial Standard Rotation – Ambrosia
1	26-May	–	–	Pristine WG	3-Jun	–	–	Pristine WG
2	12-Jun	17	11	Allegro 500 F	17-Jun	14	7.6	Supra Captan 80 WSP
3	25-Jun	13	32.4	Supra Captan 80 WSP	4-Jul	17	88	Pristine WG
4	4-Jul	9	56	Pristine WG	15-Jul	11	42.29	Allegro 500 F
5	15-Jul	11	42.49	Allegro 500 F	1-Aug	17	80.4	Supra Captan 80 WSP
6	1-Aug	17	80.4	Supra Captan 80 WSP	15-Aug	14	6.8	Pristine WG
7	15-Aug	14	6.8	Pristine WG	29-Aug	14	42.8	Allegro 500 F
8	29-Aug	14	42.8	Allegro 500 F	12-Sep	14	–	Pristine WG

Following harvest, fruit were stored in cold storage for four months and subsequently held at room temperature for 7–14 days to allow symptom development. Fruit were then evaluated for symptoms of bitter rot. Mean bitter rot incidence (% fruit with at least one lesion) was used for treatment comparisons using SAS statistical software.

2024 Results

In-Field Assessments

Bitter rot incidence remained low early in the season on both cultivars but increased steadily towards late August and September (Figure 4). Across assessment dates, fungicide treated plots had lower bitter rot incidence compared with the untreated check (UTC).

Based on pre-harvest rating (9-September) in Empire, FOLPAN and APROVIA provided the highest control (>95%), followed by Commercial Standard (CS) (~90%). SWITCH (67%) and CYCLONE PLUS (74%) provided moderate suppression.

Based on the pre-harvest rating (26-Septmeber) in Ambrosia, FOLPAN and CS again provided the highest control (>95%), followed by APROVIA (~93%). SWITCH provided 78% control of bitter rot incidence, and CYCLONE PLUS showed the lowest control (~62%) under higher disease pressure.

Post-Storage Assessments

In post-storage ratings (Figure 5), UTC showed extremely high disease incidence, exceeding 90% on Empire and approaching 100% on Ambrosia, confirming severe disease development in the absence of fungicide protection during the growing season.

On Empire, FOLPAN, APROVIA, and CS provided 80% or higher control, while SWITCH provided intermediate control. CYCLONE PLUS did not provide effective control and was similar to the UTC.

In Ambrosia, FOLPAN, SWITCH, APROVIA and CS provided 76% and above control. CYCLONE PLUS did not show effective control with values statistically similar to UTC.

2025 Results

In-Field Assessments

Bitter rot incidence remained low on Empire; no treatment differences were observed (Figure 6). Based on pre-harvest rating (11 September) in Empire, FOLPAN and CS provided near complete control (~100%).  APROVIA and SWITCH provided strong control (~88–92%), and CYCLONE PLUS showed lower suppression (~42%).

On Ambrosia, disease pressure was moderate in August and September; all fungicide treatments significantly reduced bitter rot incidence compared with UTC (Figure 6). Based on the pre-harvest rating (26 September), CS, APROVIA, and FOLPAN provided excellent control (>95%). SWITCH provided good control (~84%) and CYCLONE PLUS provided moderate control (~75%).

Post-Storage Assessments

After storage, bitter rot incidence was high, even with low disease pressure seen during the field season (Figure 7).

In Empire, treatments of FOLPAN, SWITCH, APROVIA, and CS provided good control, while CYCLONE PLUS showed little control (2.9%) and was statistically similar to UTC.

For Ambrosia, disease incidence was around 90% in UTC. Treatments of FOLPAN, SWITCH, APROVIA, and CS again provided high control, whereas CYCLONE PLUS provided poor control of 14% compared with the UTC.

Conclusions

FOLPAN, APROVIA, and CS (a rotation of PRISTINE, ALLEGRO, and CAPTAN) consistently provided the most effective bitter rot control in both 2024-2025.

SWITCH provided effective control in 2025 when the disease pressure was low and moderate suppression when the disease pressure was high in 2024.

CYCLONE PLUS offered moderate in-season suppression in both years but no effective control in post-storage assessments.

Fungicide label expansions are underway to include control of bitter rot on the Aprovia and Folpan 80 WDG labels.

Management Considerations for 2026

This research has demonstrated that apples can be infected at any stage of development, even if symptoms are not seen until after storage. Spores are active as early as May and fungicides need to be applied preventatively – fungicides cannot treat an existing infection.

Bitter rot targeted fungicides should start at petal fall and continue on a 14-21 day interval to keep fruit protected. If favourable weather persists (frequent rains with warm conditions), shorten the application interval. If possible, time an effective fungicide application prior to a rain to protect healthy fruit from rain-splashed spores.

The following products are currently registered for bitter rot:

• Allegro / Downforce (FRAC 29, PHI 28 days)
• Pristine (FRAC 11 & 7, PHI 5 days)
• Merivon (FRAC 11 & 7, PHI 0 days)
• Maestro/Supra Captan (FRAC M4, PHI 15 or 19 days depending on orchard density)
• Regalia Maxx (FRAC P5, suppression only, PHI 0 days)

As indicated from this research, products like Folpan/Follow (FRAC M4, PHI 0 days) or Aprovia (FRAC 7, PHI 30 days) may provide some efficacy on bitter rot when the product is applied at the registered rate for diseases listed on the product label. Always rotate fungicide FRAC groups to reduce the potential for resistance development.

Orchard sanitation to reduce inoculum is important for bitter rot management. Mulch or remove fruit on the orchard floor following hand thinning and harvest to reduce inoculum and the potential of spreading the disease for the following year. Removal of dead wood, cankers produced by other diseases such as fire blight, and fruit mummies (where possible) may also reduce the disease.

Acknowledgements

University of Guelph Pathology Team (Simcoe): Asifa Munawar, Lisa Weber, John Watson, Katerina Jordan

Project Collaborators: Kristy Grigg McGuffin (Ontario Ministry of Agriculture, Food & Agribusiness – OMAFA), Katie Goldenhar (OMAFA), Kelly Ciceran (Ontario Apple Growers), Ali Shawkat (Agriculture & Agri-Food Canada, Nova Scotia), Mary Ruth McDonald (University of Guelph).

The author is grateful to our team members and collaborators for their efforts in making this project successful. The author also thanks Ontario Apple Growers and Ontario Agri-Food and Innovation Alliance for funding this proposal.