Pea diseases

Main causal agents of pea diseases

Causal agents of pea diseases:

• Aphanomyces euteiches;
• Colletotrichum pisi;
• Ascochyta pisi;
• Didymella pinodella;
• Didymella pinodes;
• Pseudomonas syringae pv. pisi;
• Thielaviopsis basicola;
• Botrytis cinerea;
• Peronospora pisi, Peronospora viciae (downy mildew);
• Fusarium oxysporum f. pisi, other races of Fusarium oxysporum;
• Fusarium solani f. sp. pisi;
• Phoma medicaginis var. pinodella;
• Erysiphe polygoni, other species of Erysiphe spp. (powdery mildew);
• Pythium ultimum, other species of Pythium spp.;
• Uromyces pisi, Uromyces fabae, Uromyces trifolii;
• Sclerotinia sclerotiorum;
• Septoria pisi.

Viruses:

• Bean leaf roll virus bean (Bean Leaf Roll Virus);
• Bean yellow mosaic virus (BYMV), pea strain (@ln-english[Bean Yellow Mosaic Virus, BYMV</span>);
• Pea enation mosaic virus (Pea Enation Mosaic Virus);
• Pea streak virus (Pea Streak Virus);
• Pea stunt virus (Pea Stunt Virus);
• Pea seed-borne virus (Pea Seed Borne Virus);
• Pea early browning virus (Pea Early Browning Virus);
• Pea top yellow virus (Pea Top Yellow Virus);
• Common pea mosaic virus (Common Pea Mosaic Virus);
• Cucumber mosaic virus (@ln-english[Cucumber Mosaic Virus, CMV</span>).

Root rots

Root rots (English: Foot rot diseases) — the most damaging complex of soilborne root infection pathogens includes several fungi that can cause severe seedling or root rot of peas in virtually all traditional pea-growing regions of the world.

The effect on the crop has been described as pea root rot (Biddle, 1983). Fungi accumulate in the soil as a result of frequent cropping, and because resting spores break down more slowly than the return interval of peas, the soil population increases to such an extent that the crop becomes heavily infected.

In the UK, three main organisms are involved in the root rot complex, each of which can infect alone or together. Several Fusarium species may be present, the main one being Fusarium solani f. sp. pisi. The pathogen itself penetrates the roots early in the growing season; during its development the roots are destroyed and the vascular system is blocked by toxins produced by the fungus. This causes premature senescence and subsequent death.

The second pathogen is Didymella pinodella (syn. Phoma medicaginis var. pinodella) (Chen and Cai, 2015), an ascomycete fungus that can be transmitted through soil by producing thick-walled chlamydospores, or through seed when infection develops on leaves and pods. Initially, Didymella pinodella infection causes girdling of the stem base, which blackens and eventually disintegrates. After the initial infection, leaf and pod spotting may appear on neighbouring plants, and pods and seeds become infected during periods of wet and rainy weather.

The third pathogen is Aphanomyces euteiches, which causes root rot. It is a phycomycete fungus that produces zoospores that are motile in moist soil. After infection, the root cortex is destroyed and sloughs off from the vascular cylinder. Oospores formed in the cortex can remain in the soil for many years. Moist soil conditions favour it, and it is the main pathogen on lighter, finer-textured soils in the Paris Basin in France, where frequent pea cultivation on farms often caused major pea losses from root rot.

There are no universal control measures against these soilborne fungi, and the only effective control is to avoid growing peas more often than once every 5 years (Biddle, 1983). In the UK, where serious problems arose in the 1970s, the interval between pea crops was increased to 6 or even 7 years to prevent the development of damaging fungal populations.

In the UK, a test was developed to predict soilborne diseases, which is used as a guide to identify fields with a high pathogen population (Biddle et al., 1988). In France and Scandinavia, a similar method is used to select fields with high levels of Aphanomyces euteiches. Much work is underway to breed varieties with resistance to these pathogens, but because of the complexity of the problem, no variety with combined resistance to all of them has been bred (Ali et al., 1994), although work is currently underway in Europe together with a research cooperative to identify resistance to Fusarium and drought stress.

Fusarium root rots

Causal agents are imperfect fungi of the genus Fusarium spp.

The forms of disease manifestation are varied: plant damage from the seedling phase to seed formation. On seedlings, Fusarium root rots manifest as browning of the hypocotyl and root tip of the seedling. Seedlings turn yellow, stem constriction appears, root rot develops — and they die. Later, the roots and basal part of the stem are affected, and reddening of the main and lateral roots is detected. On the underground part of the stem, longitudinal cracks appear, as a result of which the roots die.

In Fusarium wilt, the fungi release toxins and clog the vessels of the conductive system (tracheomycotic type of damage). Through the vascular-conductive system, the fungus sometimes reaches the upper parts of the plant, resulting in yellowing, wilting, drying and death. On a cross-section of affected stems and roots, browned vessels are visible. Under moist conditions, a coating of conidial sporulation of the fungus — macro- and microconidia — appears on the affected areas. The disease is harmful under conditions of a wet spring, as well as a dry hot summer with insufficient soil moisture. But the disease manifests especially strongly during flowering — the beginning of fruit formation. Increased soil acidity also contributes to the progression of Fusarium. Sources of infection are affected plant residues in the soil and seeds. In the case of infection of pods and seeds, the infection can persist in the seed material.

Ascochyta blight

Leaf and pod spot (English: Leaf and pod spot), or ascochyta blight (Ascochyta), is a widespread disease affecting leguminous vegetable crops.

On peas there are three closely related fungi that make up the ascochyta leaf spot disease complex: Ascochyta pisi, Mycosphaerella pinodes and Didymella pinodella (Phoma medicaginis var. pinodella), and all of them can cause leaf and pod spotting of peas. In addition to these pathogens, two types of ascochyta blight can occur on peas: light spot (causal agent — Ascochyta pisi (Lib.)) and dark spot (causal agent — Didymella pinodes). All are seed-borne and can survive on plant residues; Mycosphaerella pinodes and Didymella pinodella are also soilborne. The symptoms of each are different, but Ascochyta pisi causes a more characteristic type of leaf and pod spotting and is more common worldwide.

Seedlings obtained from seeds infected with Ascochyta pisi show symptoms soon after emergence, especially if the weather has been wet. Brown or grey sunken lesions develop on the stem, leaves or petioles. As the plant develops, pycnidia form in the centre of the lesions, and ascospores are released under the influence of rain or water splashes. The spores then infect surrounding plants, eventually forming lesions on pods, and deep-seated infection continues to infect developing seeds.

Light spot ascochyta on leaves, stems and pods appears as large, rounded, yellowish spots surrounded by a brown border with a lighter centre and small pycnidia on them. Affected leaves dry out prematurely, and stems break. In affected seeds, the surface is wrinkled, with light yellow spots. Such seeds do not germinate or produce diseased seedlings. On cotyledons and stems, ascochyta appears as yellow-brown spots, with some seedlings dying.

In dark spot ascochyta, dark spots form on stems and pods, almost black in the centre, sunken, irregular in shape. In the centre of the spot, brown, few pycnidia are observed — the fruiting bodies of the pathogen. Affected stems break, and when pods are infected, the seeds in them are shrivelled or do not form at all.

The spread of Mycosphaerella pinodes occurs from infected seeds, but infection can also occur from soilborne plant residues or from chlamydospores that infect the base of the stem, leading to girdling of the stem and subsequent leaf and pod spotting with smaller, more purple lesions that are not as deep as those of Ascochyta pisi. Didymella pinodella most often infects from the soil, where thick-walled chlamydospores can remain viable for several years. The disease primarily causes foot rot, but leaf and pod spotting may occur later in the season if the soil is moist, with frequent rains and warm temperatures.

The main source of primary infection is seeds, and the use of healthy seeds is the only way to avoid infection with Ascochyta pisi. Since both Mycosphaerella pinodes and Didymella pinodella are soilborne, a long crop rotation in which peas are grown not too often will delay the accumulation of soilborne populations. Seed crops grown in arid areas are less prone to infection, but in other areas with a temperate climate the risk of disease is higher, where summer rainfall is more erratic. Fungicidal seed treatment is effective for controlling a large proportion of ascochyta-infected seeds, but the use of heavily infected seeds, even with treatment, does not always produce fully healthy seedlings.

Resistance to Ascochyta pisi is currently being studied, but because of the presence of a whole range of pathotypes, it is difficult to breed varieties with complete resistance (Tivoli et al., 2006; Muehlbauer and Chen, 2007).

Powdery mildew

Powdery mildew (English: Powdery mildew), or true powdery mildew, or erysiphosis. The causal agent is the fungus Erysiphe pisi (syn. Erysiphe communis Grev. pisi (Dietrich.)) — a very widespread pathogen in all areas where peas are grown. Severe infection reduces pod weight and yield, and seeds cannot fully realize their potential in infected pods.

The disease is favoured by warm daytime temperatures and cool moist nights. Strong development of powdery mildew is observed in a dry summer. Late-sown peas are more susceptible to infection, and it is on these crops that it manifests most strongly. The incubation period under favourable conditions is 4–5 days. Fungal spores mature and germinate better in dry air and bright light, and the plant is less resistant to disease.

The main type of damage is the formation on leaves and stems of an uneven white powdery coating consisting of mycelium and conidial sporulation. Disease development begins from the lower leaves and the base of the stem, but then quickly spreads to all parts of the plant and pods. By the end of the growing season, fruiting bodies — cleistothecia — form on the affected tissues, and these persist on plant residues. In spring, the cleistothecia open, and ascospores are ejected from the asci, infecting young plants. The mycelium, penetrating inside the leaf, causes tissue death.

Ripening of peas grown for dry harvest is delayed, and peas grown for the fresh market are spoiled and unsuitable for sale.

The fungus overwinters on crop residues and alternative host plants such as vetches and other wild legumes.

Varietal resistance to powdery mildew is mainly transmitted by three recessive genes, er-1, er-2 and er-3, with er-1 most often used by breeders (Fondevilla and Rubiales, 2012). Several commercially available varieties are fully resistant to this disease in the field, and breeders continue work to expand their number.

Very few fungicides are available for controlling powdery mildew, but regular spraying with elemental sulphur can provide some protection.

Downy mildew

Downy mildew (English: Pea downy mildew) is widespread in temperate countries where peas are grown, especially in Northern Europe, Scandinavia, New Zealand, and the northern and midwestern USA, although in drier states this disease is rare.

Causal agent: fungus Peronospora viciae.

The disease causes significant plant losses early in the season, when weather conditions are more favourable for infection, since the fungus prefers moist, cool conditions. Later, infection weakens the plant and reduces the number of pods and seeds. Pods are damaged and become unsuitable for sale on the fresh market, and peas for processing are often damaged and inferior. Oospores form in infected tissue and, after returning to the soil, remain viable for many years. Dense pea crops are the main cause of severe infection.

Taylor et al. (1989) reported that in the UK there are 11 races or pathotypes of Peronospora viciae, and most fields with a history of the disease contain mixtures of them. Therefore, breeding for complete resistance has not been achieved, but several varieties show different levels of field resistance to infection, and on this basis varieties in the UK are classified by their relative resistance. As for peas, very few varieties demonstrate good resistance, and all of them are regularly treated with fungicidal seed treatments. Before the introduction of acylalanine fungicides, which are active against Peronospora spp., many early pea varieties sown for early maturity were severely affected by the disease, and yield losses were significantly reduced, making the cultivation of early varieties practically unviable. In contrast, most combining pea varieties grown in the UK and Europe have good field resistance and are grown without seed treatment.

White mould

White mould (Sclerotinia sclerotiorum).

White mould (English: White mould). The causal agent Sclerotinia sclerotiorum has a very wide host range and can affect most broadleaf (dicotyledonous) crops, including vegetables, potato, flax (Linum spp.) and oilseed rapeseed in most regions of the world. Infection is common in some areas where susceptible crops are present in the rotation, and occurs in many regions of the world, except in the warm humid tropics.

The disease spreads rapidly in warm humid weather and causes rot of stems, leaves, pods and peas.

Infection is characterized by the appearance of wet rot, which is quickly followed by the formation of dense white cottony mycelium. Infected stems look discoloured, and sclerotia develop inside the stems and pods.

After infection is established, the sclerotia return to the soil during harvest, where they can remain viable for 5 years or more. Under suitable conditions, the sclerotia produce apothecia on the soil surface, which release ascospores that germinate on fresh green leaves, stems or wilted flower petals; the pathogen then becomes virulent and infects the rest of the plant.

Control of the disease is based on crop rotation and avoiding growing peas in the immediate vicinity of, or in close rotation with, previously infected plants. Most pea varieties are susceptible, although early sowings may escape infection, since it usually occurs late in the season when the temperature rises. Modelling of infection outbreaks based on oilseed rape petal sampling and local meteorological data has been carried out in the UK and other countries for oilseed rape, and similar modelling has been carried out for lettuce (Clarkson et al., 2014). For peas, the period of susceptibility to infection is very short, and models generally indicate a wide window of opportunity for treatment. The use of a predictive model as a decision support system for peas has been studied in the UK, but peas are susceptible to infection only at the flowering stage, which is very short (HDC, 2013).

Pea rust

Rust affects many leguminous plants.

Causal agent: fungus Uromyces pisi (Schrot.), also affects grass pea — an obligate parasite characterized by a local type of disease manifestation. The intermediate host is all species of spurge, on which the spring (aecial) stage of the pathogen develops in the form of cup-shaped receptacles on the underside of the leaf. Aeciospores are carried by wind to peas and other leguminous crops.

Rust usually appears in mid-summer. On leaves, stems and pods, orange-brown convex uredinia form, often powdery, thus infecting plants during the growing season. By the end of summer, the pustules become dark, almost black, in which unicellular thick-walled winter spores (teliospores) form, ellipsoid or obovoid. Affected leaves turn yellow and dry out.

The causal agent persists as teliospores on plant residues. The mycelium of heteroecious fungi overwinters on the rhizomes of spurge. In spring, the teliospores germinate and form basidia with basidiospores, which infect broad beans and beans. In heteroecious fungi, the basidiospores infect spurge, on which the aecial stage develops, and then the aeciospores infect the primary host.

Bacterial blight

Bacterial blight (English: Pea bacterial blight) can be important for autumn-sown crops or crops grown under irrigation. In Europe and New Zealand, where autumn sowing is common in some areas, the disease can be very severe, and yield losses are not rare. Under dry conditions, the disease is not serious enough to cause losses; in spring-sown crops, significant damage is rare unless there has been frost or hail damage.

Causal agent: bacterium Pseudomonas syringae pv. pisi (erroneously “fungus Pseudomonas pisi (Bergey et al.)”; Autko).

The causal agent is seed-borne, and symptoms can appear at any time during the growing season, especially after physical or mechanical damage to pea leaves. The bacteria, penetrating into plant tissues through stomata or mechanical wounds, spread through intercellular spaces, destroying cells. Infection entering the vessels causes wilting of leaves and of the whole plant. On leaves and pods, small dark green, water-soaked spots first appear, which then increase in size. As the affected tissues dry out, the spots become reddish-brown. The bacteria can penetrate into the pedicels and cause death of the flowers. Young pods affected by bacterial blight become wrinkled and dry out. On severely affected pods, a slimy mass appears or ulcers with “fatty” spots form. Seed infection can occur during their ripening.

The main source of bacterial blight is infected seeds and post-harvest plant residues.

There are seven races of bacterial pea blight. In the UK, race 2 is most common on spring varieties, and races 4 and 6 occur on winter peas. In the USA, race 4 appears to be most common on spring peas.

There are no chemical control measures, and the use of healthy seeds is the most effective means of preventing infection. The bacterium does not survive on crop residues for more than 1 year. There are no varieties resistant to all seven races, but several varieties are resistant to races 2 and 4.

Fusarium vascular wilt

Fusarium vascular wilt (English: Fusarium vascular wilts).

Several races of Fusarium oxysporum f. sp. pisi occur in many countries where peas are grown. The disease caused by Fusarium oxysporum often leads to vascular wilt over large areas during flowering and seed set.

The first race of Fusarium wilt is characterized by stunted plant growth, as well as a change in leaf colour, which first acquire a greyish tint, then wrinkle and lead to plant death. Race 2 is known as near wilt, and plants are affected either individually or in scattered patches across the field. Roots often die, leading to wilting and premature ageing of the plant. Race 6 has been recorded in the USA and Scandinavia and probably sometimes occurs in Europe and Australasia. The symptoms are very similar to those caused by the soilborne root fungus. Other races, including race 5, have been described in the USA (Kraft, 1994), but the symptoms are very similar to those of other races, with race 5 symptoms similar to those of race 1.

Pea wilt became a serious problem in the USA in the early 20th century, and in most years in Wisconsin much of the yield was lost. The disease was first described by Linford (1928), but was later identified as race 1 of Fusarium oxysporum f. sp. pisi in 1935. Only when breeders discovered resistance to wilt (Wade, 1929) did the disease become manageable, and many varieties with resistance to race 1 have now been bred, some of which are also resistant to other races.

In the UK, all newly registered varieties are tested for resistance to race 1, and this information can be obtained from breeders or from the recommended list of pea varieties (PGRO, 2015). Crop rotation is of little effectiveness when the disease is present in the soil, but a rotation including peas no more than once every 5 years will help prevent the development of the disease.

Viruses

Of the important pea viruses in Europe, the USA and Australasia, four aphid-borne viruses can cause the most serious damage, loss of yield or loss of product quality. The pea aphid (Acyrthosiphon pisum) is a vector of a number of viruses, including pea enation mosaic virus (PEMV), pea top yellow virus (PTYV), alfalfa mosaic virus (alfalfa (pea streak)) and pea seed-borne mosaic virus (PSbMV), which is also seed-borne.

The most significant damage from PEMV and PTYV arises as a result of early invasion by the pea aphid, which brings these diseases to the crop from overwintering hosts. PEMV has a range of overwintering hosts, which includes wild and cultivated legumes, such as broad beans (Vicia faba), chickpea (Cicer arietinum), sweet pea (Lathyrus oderatus), lentil (Lens culinaris), Medicago arabica and common vetch (Vicia sativa).

PTYV is also known as bean leaf roll virus and has wild hosts such as clover and bindweed, as well as alfalfa (Medicago sp.) and winter vetch (Vicia faba). PSbMV is transmitted mainly by seed and is transmitted between neighbouring peas only during the movement of aphids into the crop. In some cases, it is assumed that the cereal aphid can also transmit PSbMV from an infected seedling to the rest of the surrounding crop.

Several varieties are resistant to one or both of PEMV and PTYV, and a small number of varieties have also been bred for resistance to PSbMV. A possible source of resistance is the difference in the ability of the virus to penetrate the developing seed (Roberts et al., 2003). In general, effective aphid control is necessary to avoid infection, but in the case of PSbMV only the use of healthy seeds guarantees the absence of this disease. The main symptom of PSbMV is stunting of plant growth, and on seed plants spots with a white border often appear, which can be compared to the markings on a tennis ball. Affected peas can cause crop rejection by frozen pea processors, and in the UK in the mid-1980s seed stocks of the petits pois variety, which was widely used in the UK and Europe, were severely infected. In 1987, a research programme was started to detect the presence of the virus in seeds, which led to the widespread use of a testing programme using an ELISA-based seed test to identify seed stocks carrying infection. This programme proved effective and made it possible to produce healthy seeds of this variety, which remains one of the most important petits pois varieties grown in Europe and the USA.

In Europe, in some seasons, a nematode-transmitted virus occurs — pea early browning virus (PEBV). Free-living nematodes, including the vector Trichodorus sp., are often found in freely draining sandy soil and graze on pea roots during periods of high soil moisture in spring. The virus develops in peas, causing characteristic leaf mosaic, purple stems, stunted growth and necrosis of the main shoot.

The disease can also be transmitted through seeds. Seed health procedures can be effective in reducing the risk of introducing the virus into other areas where trichodorid nematodes are present. A detailed review of the virus was written by Boulton (1996).

Sources

1. Autko A.A. et al. (2012). Modern technologies in vegetable growing. Edited by Dr. Agric. Sci. A. A. Autko. National Academy of Sciences of Belarus, Institute of Vegetable Growing. Minsk: Belaruskaya Navuka.
2. Biddle, A. J. (2017). Peas and beans. Crop production science in horticulture. UK.