Taking the complexity out of the Ascochyta/Mycosphaerella complex in peas

Laura Schmidt, MSc, PAg, Production Specialist – West, MPSG – Summer (June) 2021 Pulse Beat

THE COMPLEX

The Ascochyta/Mycosphaerella disease complex in peas consists of Ascochyta pisi causing leaf and pod spot, Phoma pinodella causing foot rot and Mycosphaerella pinodes (Peyronellaea pinodes or Ascochyta pinodes) causing the majority of symptoms and yield loss in Manitoba field peas. It is a complex, but research has shown that M. pinodes is responsible for most of the crop damage, accounting for roughly 95% of infections. M. pinodes is the most aggressive species, followed by P. pinodella and then by A. pisi.

Historically, A. pisi was a bigger problem. In the 50s, it accounted for roughly 85% of infections. Ten years later, that number was down to about 5% thanks to the advent of a resistant variety, Century, in 1961. Unfortunately, that was also about the time when M. pinodes took off. Over the past ten years, A. pisi has become more prominent again, but it has not been a major contributor to yield loss in field peas.

Mycosphaerella pinodes overwinters on crop residues and in soil. It is the only species of the Ascochyta complex to form a sexual spore stage (called ascospores), allowing disease transmission over long distances by wind dispersal. In early summer, ascospores are produced and spread by wind. Symptoms develop within two to eight days after infection. Secondary infection from asexual spores within the plant canopy amplifies the disease. Most of the infections in western Canada are due to initial infection by wind-blown spores, with secondary infection occurring later in the season after canopy closure.

This sexual stage of the life cycle also imparts more genetic diversity. This means that any resistance mechanisms that crop breeders have been able to identify in the pea germplasm are quickly overcome in the field. Breeding was an effective tool to manage A. pisi, but the hunt for resistance to M. pinodes continues. Most varieties grown nowadays have moderate resistance to Mycosphaerella. Lodging resistance and Mycosphaerella resistance have been positively correlated, so selecting varieties with improved lodging resistance may help as well.

PRIMARY SPORE SOURCES

Primary inoculum produce the ascospores that are responsible for the initial infection of Mycosphaerella blight in pea plants. There are four sources of primary inoculum — infected pea residue/stubble, soil, nearby alternate hosts or volunteers and seed. Wind-borne spores released from infected pea residues are the main source of inoculum, followed by soil-borne inoculum.

Infected residues are specifically the residue or stubble left above ground and unburied following pea harvest. Inoculum from these residues is initially high but drops to very low levels after one year. When planning to grow peas, consider the field history of surrounding fields. If the neighbouring field was peas last year and had high disease pressure, peas might not be the best option there since the crop residue will actively be releasing spores. One option to manage this source of inoculum is by burying residue two inches deep following pea harvest, reducing spore production and disease development in subsequent nearby pea crops.

On the other hand, soil-borne inoculum persists for a long time. Research from Australia found that soil-borne inoculum decreased by 15% per year, suggesting that a break period of six years is necessary to reduce the inoculum borne in the soil. M. pinodes and P. pinodella can survive and overwinter in the soil, while it is rare for A. pisi to be soil-borne.

In areas where peas are common and thus where there are more infected pea residues producing spores, inoculum from alternate hosts has been deemed a minor source of infection. Recent research infected 20 legume species with M. pinodes and found that Mycosphaerella caused visible symptoms on 19 of the species evaluated — the exception being dry beans. Peas were the most susceptible, followed by lentils, lupins, medics, clovers, fenugreek and vetches. Ascochyta species of other pulses like lentils and chickpeas are more specific and only infect their lentil and chickpea hosts, respectively. The Ascochyta species infecting faba beans are intermediately specific — infecting mainly fabas, but also slightly infecting soybeans, dry beans, clovers and common vetch.

The fourth source of inoculum is seed-borne. Seed infection with Ascochyta/Mycosphaerella does not contribute substantially to above-ground disease symptoms. However, very high levels of seed infection do reduce seedling emergence. In western Canadian growing conditions, infected seeds are not regarded as a source of inoculum.

DISEASE AMPLIFICATION

Ascospores (sexual spores) and pycnidiospores (asexual spores) are released throughout the season by infected plant tissues within the crop. Spores are transmitted by rain splash onto neighbouring plant tissues and disease progresses up the plant canopy, becoming more severe. Only a small portion of these secondary spores escape the plant canopy.

FUNGICIDES

Fungicides are the main line of defence against Mycosphaerella blight. The optimal timing for application is at R2 (beginning bloom) stages, but can be delayed if conditions are not conducive to disease development. Use the Field Pea Fungicide Decision Worksheet to determine if an application is recommended. Once peas reach R4 (full pod), fungicide application is no longer recommended since disease is no longer expected to influence yield and peas are within the preharvest interval of several products.

Infection timing directly relates to yield loss. Infections that initiate at mid-flowering and at the 8–10 node stages result in greater yield loss than those that start at pod fill stages. If symptoms do not progress beyond the lower third of the plant canopy by the flowering stage, large yield losses are not expected.

A single application is often adequate for disease control if conditions are drier. Two applications are more common in wet years and have been shown to be beneficial if wet conditions persist and disease is progressing up the plant canopy. Find the results of On-Farm Network testing of fungicides in peas on page 35.

INSENSITIVITY TO STROBILURIN FUNGICIDES

Resistance to the strobilurin fungicides (FRAC group 11) has been confirmed in Mycosphaerella, with resistance reported in North Dakota, Saskatchewan and Alberta. Though it has been reported and confirmed in those regions, it is currently unknown how widespread this resistance is in Manitoba.

Since all fungicides in FRAC group 11 use the same targeted mode of action, resistance to one likely means resistance to all of the fungicides in that group. There are two types of fungicide resistance that develop — quantitative, where the pathogen is less sensitive to the fungicide and higher rates or additional applications still work, and qualitative, where the pathogen is completely insensitive to the active ingredient. Strobilurin resistance is typically qualitative, meaning these products will not provide control of the pathogen (Table 3).

In 2016, Robyne Bowness confirmed the first report of M. pinodes insensitivity to pyraclostrobin (Headline) at low to moderate levels of resistance in SK and AB. This was followed up by research by Dr. Bruce Gossen, where surveys found 72% of Mycosphaerella isolates from SK were insensitive to strobilurins, indicating they were likely no longer effective in the field. On-going research in ND is finding widespread insensitivity as well. In chickpeas, resistance developed throughout the Ascochyta rabiei pathogen population completely in two to three years, and it seems that resistance is developing quickly in M. pinodes too.

Since we don’t know what the spread of resistance is in the Manitoba pathogen population, I would caution that if you’re applying a group 11 fungicide on peas, monitor fields for management of the disease. Wind-blown ascospores are a main vector of disease, so it is likely that these resistant populations are here. Select fungicides with multiple modes of action and rotate to fungicides of a different group for sequential applications.

Most fungicide options for field peas are a combination of a group 3 or 7 with an 11 (Table 3). Products with a group 3 or 7 alone have a medium risk of resistance development down the road and contact products like chlorothalonil and copper octanate have a low risk. In previous ND studies (2011-2012), products containing prothioconazole, fluxapyroxad+pyraclostrobin and azoxystrobin have provided excellent control of Mycosphaerella, while boscalid and penthiopyrad were less effective. There is a need in Manitoba to test fungicide efficacy, especially with newer products on the market and the likelihood that a fungicide-resistant pathogen population has developed since these products were last tested.

SEEDING RATES

Reducing seeding rate does not appear to be an effective tool to manage Mycosphaerella blight severity in field peas. In trials at Minto, Hamiota and Morden, lowering seeding rates only marginally reduced disease severity and often only did so at very low plant populations (< 50 live plants/m2). The influence of pea seeding rates on blight severity will be further investigated in the On-Farm Network this year.

INTERCROPPING

Intercropping may have the potential to reduce Mycosphaerella blight disease levels. In Spain, growing peas with faba beans, barley, oats, triticale and wheat reduced disease — both the amount of diseased tissue per plant and the progression up the canopy.

Intercropping peas with faba beans or triticale provided the greatest suppression, reducing disease by 60%. Oats, wheat and barley, had low to moderate suppressive effects. Intercrops were credited with less disease presence due to the combination of less pea biomass to infect, an altered crop microclimate and physical barriers to spore dispersal.

Local research from WADO at Melita has indicated that oats are a promising companion crop with peas in Manitoba conditions. Preliminary results of intercropping peas with barley resulted in lower leaf disease and earlier maturity in barley, ease of harvest and increased barley and oat protein. Intercropping may also provide further benefits through reduced blight severity.