Pulse Beat Individual Articles

Foliar Diseases in Soybeans, Peas and Dry Beans

Results from the 2019 late-season disease survey and On-Farm Network foliar fungicide trials

Laura Schmidt, Production Specialist – West and Megan Bourns, Agronomist – On-Farm Network, MPSG

Each year a representative sample of soybean, pea and dry bean fields are surveyed for root, foliar and stem diseases throughout Manitoba. These surveys are a collaborative effort between Agriculture and Agri-Food Canada, Manitoba Agriculture and Resource Development and Manitoba Pulse & Soybean Growers. Root rot results from the 2019 survey can be found in the spring 2020 issue of Pulse Beat.

Soybean Foliar Diseases

Sixty-eight soybean fields were surveyed in 2019 at R3 (beginning pod) and R6 (full seed). Soybeans were visually assessed for infection by common diseases like bacterial blight, septoria brown spot and downy mildew, as well as less common diseases like white mould, pod/stem blight, anthracnose and frogeye leaf spot.

Similar to previous years, bacterial blight and septoria brown spot were found in the majority of fields surveyed. Average severity levels of these two diseases were low in 2019, scoring 1.4 and 1.5 respectively, on a scale of 0 to 5 (Table 1). At these levels, bacterial blight and septoria brown spot are not expected to cause yield loss. While bacterial blight and septoria brown spot are common in Manitoba, they are rarely present at levels which cause economic concern. However, some fields will experience greater severity and incidence of these diseases. In these cases, it’s important to verify which diseases are present and causing symptoms. Fungicides will only be effective against fungal diseases like septoria brown spot, but management practices like crop rotation and tillage could be used to manage both fungal and bacterial diseases.

Downy mildew was present in roughly a third of fields surveyed, also at low severity levels (Table 1). Pod/stem blight, anthracnose and frogeye leaf spot were present but less common. In 2019, white mould was not detected in any of the soybean fields surveyed.

On-Farm Evaluation of Fungicide in Soybeans

MPSG’s On-Farm Network (OFN) has conducted 59 foliar fungicide response trials in soybeans since 2014. These randomized and replicated strip trials compared soybeans with and without a single foliar fungicide application, where fungicide was intended to control fungal disease including septoria brown spot, white mould and frogeye leaf spot. Applications were made at recommended label rates and timings of either R1 (beginning bloom) or R2 (full flower).

Among the 59 trials, there were nine statistically significant yield responses, eight of which were positive (14%). Significant yield responses occurred within three growing seasons — 2015, 2017 and 2018. At most responsive sites within this study, septoria brown spot and white mould disease pressure were reduced by fungicide application.

This type of variability in soybean yield response to fungicide application is to be expected, as the extent and severity of disease pressure is inconsistent across years and dependent on growing season conditions. Fungicide application is also expected to protect yield only when fungal disease pressure is severe enough to be limiting.

In Manitoba, soybean yield is not often limited by septoria brown spot and white mould. This is one reason for even greater variability in the yield response to fungicide application. It also calls into question the economic benefit of regular fungicide applications in soybeans if yield benefits are small or insignificant.

The OFN will continue to conduct soybean fungicide trials in 2020. For more information on each of the OFN soybean fungicide trial results, visit manitobapulse.ca/on-farm-network.

 

Field Pea Foliar Diseases

Forty-four pea fields were surveyed in 2019 during mid- to late- July when peas were at the podding stages of R3 to R4 (Table 2). Foliar diseases were identified based on their symptoms. The severity of mycosphaerella blight, white mould and anthracnose were estimated using a scale of 0 (no disease) to 9 (whole plant severely diseased). Powdery mildew, downy mildew, rust and bacterial blight were rated as the percentage of foliar area infected.

Similar to previous years, mycosphaerella (ascochyta) blight was the most prevalent foliar disease and was present in all pea fields surveyed. Disease severity ranged from 2.3 to 6.3, averaging 3.8. The impact of disease severity on yield will depend on how early the disease sets in and how quickly it develops. For scouting and management information on mycosphaerella blight, read The Pea Report — Managing Ascochyta (Mycosphaerella) Blight in Field Peas in the summer 2019 issue of Pulse Beat available here.

Downy mildew was detected in 64% of fields (29/44) and the percentage of leaf area infected averaged 0.4%. Bacterial blight was observed in 39% of the crops surveyed (17/44) with the percentage of foliar area infected averaging less than 0.1%. White mould, powdery mildew, rust and anthracnose symptoms were not observed in any of the fields surveyed in 2019.

On-Farm Evaluation of Fungicide in Field Peas

MPSG’s OFN has conducted 19 trials evaluating the effectiveness of foliar fungicide application in peas since 2017 (Figure 2). Typically, foliar fungicide application in peas targets control of mycosphaerella blight. However, fungicide application can also offer control or suppression of other diseases including white mould, powdery mildew and rust, depending on the product.

This pea fungicide study includes 11 trials of single vs. no application, where foliar fungicide was applied at R2 (beginning bloom). There have been six single vs. double application trials, where the single application is applied at R2 and a second application is applied one to two weeks later. This study has also included two trials comparing all three treatments — single vs. double vs. no fungicide application.

To date, four out of 11 trials have resulted in significant yield increases from single application of foliar fungicide compared to peas with no application. Two out of six trials have had a signifi­cant yield increase from a double application of fungicide compared to a single application. Of the two trials that compared all three treatments, one had significant yield differences between treatments. At this trial, both the single and double fungicide applications significantly increased pea yield compared to untreated peas. However, double and single fungicide treatments yielded similarly to one another.

Dry Bean Foliar Diseases

Forty dry bean fields were surveyed in 2019 (Table 3). Most fields surveyed were located in southern Manitoba, with 10% of fields located outside of the traditional bean growing regions. Foliar and stem diseases were assessed when dry beans started to mature during mid-August, except for halo blight, which was assessed during flowering in mid- to late July.

Foliar diseases were identified visually by their symptoms. Common bacterial blight (CBB) severity was assessed on a scale of 0 (no disease) to 5 (50 to 100% of leaf area covered in lesions), while anthracnose, rust, white mould and halo blight severity were assessed as percentages of infected plant tissue.

CBB was the most widespread foliar disease throughout the province. CBB symptoms were observed in every field surveyed. The incidence of CBB leaf infection, or the average percentage of plants showing infection within infected fields, ranged from 5 to 37% with an average of 16%. Severity ranged from 0.3 to 3.3, with an average of 1.8.

Halo blight was observed in five of the 40 dry bean fields surveyed (13%) with an average of 5% leaf area infected. White mould symptoms were detected in only one field in 2019, with less than 1% of tissue infected. Rust and anthracnose were not detected in any of the fields surveyed.

Bacterial blights (CBB and halo blight) first appear as small water-soaked spots on the underside of leaves. Lesions eventually coalesce and become necrotic, surrounded by a yellow halo. Infection occurs through leaf damage caused by hail, high winds or mechanical cultivation through the field. These diseases can spread rapidly from plant to plant favouring humid conditions. Halo blight is favoured by cool temperatures and CBB by warm temperatures.

The best way to manage CBB and halo blight is to plant disease-free seed, avoid working in fields when foliage is wet, keep equipment clean and incorporate residue. As these diseases are bacterial, foliar fungicide will not be effective for control.

On-Farm Evaluation of Fungicide in Dry Beans

Since 2016, 15 trials have been conducted by MPSG’s OFN investigating foliar fungicide application in navy and pinto beans to control white mould. A single application of foliar fungicide was applied at R2 (early pin bean) and compared to an untreated control.

To date, there have not been any significant dry bean yield responses to a single foliar fungicide application. The impact of fungicide on yield depends on the extent of fungal disease pressure in the field. Across all 15 trials, disease pressure was generally low. Under different environmental conditions that facilitate higher disease pressure, fungicide could have a significant impact on yield. The OFN will continue these trials to measure the effects of fungicide in dry beans across different environmental conditions and levels of disease pressure.

For more information on each of the OFN dry bean fungicide trial results, visit manitobapulse.ca/on-farm-network.