Managing Mycosphaerella (Ascochyta) Blight
The main disease to be controlled by foliar fungicide in peas is mycosphaerella (ascochyta) blight (Figure 1). Fungicides also offer control or suppression of white mould, powdery mildew and rust, depending on the product. However, these diseases are less frequent and less severe in Manitoba-grown peas. Scout for mycosphaerella symptoms from V10 (10th true node) to R2 (beginning bloom), during mid-June to late July.
Foliar fungicide application for control of mycosphaerella blight should be made at early flower (R2), when one flower is open on most plants across the field. The goal is to get good fungicide coverage on leaves in the lower canopy. This means spraying just before the canopy begins to close. However, in drier years, it may be beneficial to delay application. There is a fungicide decision worksheet available to determine if a fungicide application is likely to be beneficial and timely.
If a score is below 65, or above 65 but disease is not yet present, revisit the field in a few days and reassess. If the field is scoring above 65 but disease is not present, conditions are suitable for disease to develop and the field should be revisited in a few days to catch any developing symptoms early.
Continue to monitor for Mycosphaerella and its progression up the pea canopy from early bloom (R2) to full pod (R4) to determine if a second application may be warranted. If symptoms spread into the mid to upper canopy and warm, humid weather persists, consider a second fungicide application 10-14 days later using a different mode-of-action.
To aid the decision of whether or not to apply fungicide, follow the worksheet below to determine your risk level of mycosphaerella blight development:
Symptoms of Mycosphaerella Blight vs. Bacterial Blight
Mycosphaerella blight and bacterial blight symtpoms are commonly confused. Bacterial blight will not be managed by a fungicide so accurate identification is key.
Mycosphaerella blight:
- Early lesions appear as small purple-brown freckles or as large circular, ringed lesions.
- Initial infection typically begins in the lower canopy on the bottom-most leaves and move up the plant as it progresses.
Bacterial blight:
- Lesions are broken by leaf veins, resulting in an angular appearance.
- Initially, lesions have water-soaked edges.
- Most often, occurs in the mid or upper canopy that was exposed during storms. Wounds from high winds, storms or mechanical damage are sites where bacterial blight colonizes.
On-Farm Fungicide Research Results
Since 2017, 27 randomized and replicated, field-scale trials have been conducted across Manitoba to evaluate foliar fungicide in peas. Of the 27 trials, 17 have compared single vs. no fungicide, two have compared double vs. single vs. none, seven have compared double vs. single and one has compared double vs none (Table 6). Mycosphaerella blight and white mould are the targets of foliar fungicide in peas. However, Mycosphaerella is the main concern in most years.
These on-farm trials have shown us that field peas are more responsive to foliar fungicide than soybeans or dry beans. According to the aggregated results, yield increases occurred 33% of the time, but profit increases only occurred 18% of the time (Figure 2). Overall, the economic responsiveness of peas has been relatively low in these last few drier years.
When we examine the different trial types in Table 6, there have been more frequent, larger and more reliable economic responses from double fungicide application. However, this does not mean that double application is always the best practice. It simply means that double application may be necessary at times to control substantial disease pressure.
Pea yield response to fungicide varies among farms and will only be economical if the disease pressure is there, or if conditions are conducive to its development. Use the new Fungicide Decision Worksheet for Managing Mycosphaerella Blight in Field Peas to help you decide if fungicide will be beneficial in your field.
Small Plot Fungicide Research Results
A small-plot study testing various pea inputs was conducted at Minto and Hamiota in 2015 and 2016. These trials tested no fungicide vs. one application (Headline EC at 10% flower) vs. two applications (Headline EC at 10% flower + Priaxor 12-13 days later).
In 2015, there was a significant effect of fungicide application compared to no fungicide. However, there was no difference between one application and two applications of fungicide (Table 2). At Hamiota (2016), there was a significant yield response to two applications of fungicide and no response to one application of fungicide. At Minto (2016), there was a significant yield response to both one and two applications of fungicide. Two applications of fungicide resulted in the highest yield; however, the overall yield potential was low at this site.
Table 1. Field pea yield response in small-plot trials across Manitoba from 2015-2016.
Foliar Disease Survey Results
In 2020, 14 pea fields were surveyed for Mycosphaerella blight, bacterial blight, downy mildew, white mould, powdery mildew, anthracnose, rust and Septoria leaf blotch. The number of fields was lower compared to past years due to COVID-19 constraints. Surveying took place during mid- to late-July at the R3 to R4 (flat to full pod) stages.
Find a detailed summary of 2020 results in Table 4 and a summary of Mycosphaerella blight, bacterial blight and white mould results from 2015 to 2020 in Table 5. Mycosphaerella blight has been found in every surveyed pea field since 2015 (Table 5). Over the past five years, the greatest severity of Mycosphaerella blight occurred in 2016 due to wet conditions and a high frequency of summer storms. A score of 6.0 indicates that, on average, plants had symptoms on < 20% of the upper canopy, 21–50% of the mid canopy and on 51–100% of the lower canopy. Not reported in Table 5 are powdery mildew, anthracnose and Septoria leaf blotch, which have not been detected in Manitoba in the past five years.
In 2020, bacterial blight was easily identified in 71% of surveyed fields. High infection levels were due to early-season storms and strong winds that wounded plants, creating openings for bacterial blight to infect peas. Thankfully, only 1% of leaf tissue was infected, on average.
Additional Resources
Managing Ascochyta Blight in Field Peas – Pulse Beat Issue 87, The Pea Report by Serena Klippenstein (MPSG)
The Effects of Seeding Rate and Fungicide Application on Field Peas in Manitoba – Manitoba Agronomists Conference poster, Greg Bartley and Laryssa Stevenson (MPSG)
Fungicide Decision Support Checklist for Ascochyta & Mycosphaerella Blight in Pea – Saskatchewan Pulse Growers fact sheet by Sherrilyn Phelps (SPG), Sabine Banniza (University of Saskatchewan) and Faye Dokken-Bouchard (Saskatchewan Ministry of Agriculture)
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