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Diseases of peas

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Main pathogens of pea diseases

The causative agents of pea diseases:

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

Viruses:

  • Bean Leaf Roll Virus;
  • Bean Yellow Mosaic Virus (BYMV);
  • Pea Enation Mosaic Virus;
  • Pea Streak Virus;
  • Pea Stunt Virus;
  • Pea Seed Borne Virus;
  • Pea Early Browning Virus;
  • Pea Top Yellow Virus;
  • Common Pea Mosaic Virus;
  • Cucumber Mosaic Virus (CMV).

Root rot

Root rot, Foot rot diseases, is the most damaging complex of soil pathogens of root infection and includes several fungi that can cause severe sprouting or root rot of peas in almost all traditional pea growing areas of the world.

The impact on the crop has been described as pea root rot (Biddle, 1983). The fungus accumulates in the soil as a result of frequent sowing, and because the rate of decay of dormant spores is slower than the return time of peas, the soil population increases to the point where the crop becomes heavily infested.

In the United Kingdom, three major organisms are involved in the root rot complex, each of which can infect individually 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, producing thick-walled chlamydospores, or through seeds when infection develops on leaves and pods. Initially, D. pinodella infection causes girdling at the base of the stem, which turns black and eventually collapses. After initial infection, leaf and pod spotting may occur on neighboring 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 conditions. Once infected, the root bark is destroyed and peels away from the vascular sheath. The oospores produced in the bark can remain in the soil for many years. Moist soil conditions are favorable for it, and it is a major pathogen in light soils with finer texture in the Paris Basin in France, where large losses of peas from root rot have often occurred with frequent farming.

There are no universal control measures for these soil fungi, and the only effective means of controlling them is to avoid cultivating peas more frequently than once every 5 years (Biddle, 1983). In the United Kingdom, where serious problems arose in the 1970s, the interval between pea crops was extended to 6 or even 7 years to prevent the development of damaging fungal populations.

In Great Britain, a test for predicting soil-borne diseases was developed and used as a guide to identify fields with high pathogen populations (Biddle et al., 1988). In France and Scandinavia, a similar method is used to select fields with high levels of A. euteiches. Much work is being done to develop varieties with resistance to these pathogens, but no varieties with combined resistance to all have been developed because of the complexity of the problem (Ali et al., 1994), although work is in progress in Europe with a research cooperative to identify resistance to Fusarium and drought stresses.

Fusarium root rot

The causative agents are imperfect fungi of the genus Fusarium spp.

The disease manifestation is different: plants are affected from the phase of seedlings to seed formation. On seedlings, Fusarium root rot appears as browning of the cotyledon and root tip of seedlings. The seedlings turn yellow, overstemming and root rot appear, and they die. Later, the roots and root part of the stem are affected, reddening of the main and lateral roots is revealed. Longitudinal cracks appear on the underground part of the stem, as a result of which the roots die off.

In fusarium wilt, fungi release toxins and clog blood vessels of the conductive system (tracheomycosis type of lesion). The fungus sometimes reaches the upper parts of the plant via the vascular conductive system, resulting in yellowing, wilting, desiccation, and death. Transverse section of diseased stems and roots shows fulvous vessels. In humid conditions, single plaque of conidial sporonose fungus, macro- and microconidia, appears on the affected areas. The disease is harmful under conditions of wet spring and dry hot summer with insufficient moisture in the soil. But the disease is particularly strong during flowering and the beginning of fruit formation. Increased soil acidity also contributes to the progression of fusariosis. Sources of infection are infected plant residues in the soil and seeds. If beans and seeds are infected, the infection can persist in the seed.

 

Leaf and pod spot (Ascochyta)

Leaf and pod spot, or Ascochyta, is a widespread disease that affects leguminous vegetable crops.

There are three closely related fungi that make up the Ascochyta pisi, Mycosphaerella pinodes and Didymella pinodella (Phoma medicaginis var. pinodella) disease complex on peas, all of which can cause leaf and pod spot in peas. In addition to these pathogens, two types of ascochytosis can appear on peas: light spotted (Ascochyta pisi (Lib.)) and dark spotted (Ascochyta pinocles (Veg. Et Bl.) Jones). All are seed-borne and can survive on plant debris; M. pinodes and D. pinodella are also soil-borne. The symptoms of each are different, but A. pisi causes a more characteristic type of leaf and pod spotting and is more common worldwide.

Seedlings derived from seeds infected with A. pisi show symptoms soon after emergence, especially if the weather has been wet. Brown or gray depressed lesions develop on the stem, leaves, or stems. As the plant develops, pycnidia form in the center of the lesions, and the release of ascospores occurs when exposed to rain or splashes of water droplets. The spores then infect surrounding plants, eventually forming lesions on the pods, and the deep-seated infection continues to infect developing seeds.

Light-spotted ascochytosis on leaves, stems, and beans appears as large, rounded, yellowish spots surrounded by a brown border with a lighter middle and small pycnidia on them. Affected leaves wither prematurely and stems break. Affected seeds have wrinkled surface with light yellow spots. Such seeds do not sprout, or they produce sick sprouts. Ascochytosis appears as yellow-brown spots on cotyledons and stalks, and some shoots die.

Dark spots, almost black in the center, depressed, irregularly shaped, appear on stalks and beans. Few brown pycnidia, fruiting bodies of the pathogen, are observed in the spot center. Affected stems become crushed, and when beans are infected, their seeds become stubby or are not formed at all.

M. pinodes spreads from infected seeds, but can also be infected from soil plant residues or from chlamydospores that infect the stem base, resulting in stem girdling and subsequent leaf and pod spotting with smaller, more purplish lesions that are not as deep as those of A. pisi. D. pinodella is most commonly infected from soil, where thick-walled chlamydospores can remain viable for several years. The disease primarily causes foot rot, but leaf and pod spotting can occur later in the season if the soil is moist, with frequent rainfall and warm temperatures.

Seeds are the primary source of primary infection, and using healthy seeds is the only way to avoid A. pisi infection. Since both M. pinodes and D. pinodella are soil-borne, a long rotation in which peas are not grown too often will delay the accumulation of soil-borne populations. Seed crops grown in arid areas are less prone to infestation, but in other temperate areas the risk of disease is higher where summer rainfall is more erratic. Seed fungicide treatments are effective in controlling most seed-borne Ascochyta, but the use of heavily infested seeds, even with seed treatments, does not always result in complete healthy seedlings.

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

 

Powdery mildew (Erysiphe pisi)

Powdery mildew, or true powdery mildew, or Erysiphos. The causative agent is the fungus Erysiphe pisi (syn. Erysiphe communis Grev. pisi (Dietrich.)) – A very common pathogen in all areas where peas are grown. Severe infection reduces pod weight and yield, and seeds cannot reach their full potential in infected pods.

The disease is favored by warm daytime temperatures and cool, wet nights. Strong powdery mildew development is seen in dry summers. Late-seeded peas are more susceptible to infection, and it is on these crops that it appears strongest. The incubation period under favorable conditions is 4-5 days. The fungus spores in dry air and in bright light better mature and germinate, and the plant is less resistant to disease.

The main type of infestation is the formation of an irregular white powdery plaque on leaves and stems, consisting of conidial sporulation fungus. Disease development begins on lower leaves and stem base, but then spreads rapidly to all parts of the plant and beans. By the end of vegetation, fruiting bodies, cleistothecia, are formed on the affected tissues, which remain on plant debris. In spring, the cleistothecia open, and the sacs expel sacospores that infect young plants. The fungus, penetrating inside the leaf, causes tissue death.

Ripening of peas grown for dry picking is delayed, and peas grown for the fresh market turn out to be spoiled and unfit for sale.

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

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

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

Pea downy mildew (Peronospora viciae)

Pea downy mildew is common in temperate pea-growing countries, especially in Northern Europe, Scandinavia, New Zealand, and the northern and midwestern United States, although the disease is rare in the drier states.

The causative agent is the fungus Peronospora viciae.

The disease causes significant plant losses early in the season when weather conditions are more favorable for infection because the fungus prefers moist, cool conditions. Later in the season, the infection weakens the plant and reduces the number of pods and seeds. The pods become damaged and unsuitable for sale in the fresh market, and peas for processing are often damaged and incomplete. Oospores form in infested tissue and remain viable for many years after returning to the soil. Dense crops of peas are a major cause of severe infection.

Taylor et al. (1989) reported that there are 11 races or pathotypes of P. viciae in the United Kingdom, and most fields with a history of the disease contain mixtures of them. Therefore, selection for complete resistance has not been achieved, but several varieties show different levels of resistance in the field to infection, and on this basis, varieties in the UK are classified according to their relative resistance. As for peas, very few varieties show good resistance, and all are regularly treated with seed fungicide preparations. Before the introduction of acyl-aniline fungicides, which are active against Peronospora spp., many early-ripening pea varieties were severely affected by the disease and yield losses were greatly reduced, making growing early varieties virtually unviable. In contrast, most combination pea varieties grown in the United Kingdom and Europe have good field resistance and are grown without seed treatment.

White mold (Sclerotinia sclerotiorum)

Sclerotinia sclerotiorum has a very wide host range and can infect most broadleaf (dicotyledonous) crops, including vegetables, potatoes, flax (Linum spp.) and oilseed rape in most regions of the world. The infection is common in some areas where susceptible crops are present in the crop rotation and is found in many regions of the world except the warm, humid tropics.

The disease spreads rapidly in warm, humid weather and causes stem, leaf, pod, and pea rot.

The infection is characterized by the appearance of wet rot followed quickly by the formation of dense white cotton-like mycelium. Infected stems appear discolored, and sclerotia develop inside the stems and pods.

Once infection is established, the sclerotia return to the soil at harvest time, where they may 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 affects the rest of the plant.

Control of the disease is based on crop rotation and refusal to grow peas in close proximity or in close rotation with previously infected plants. Most pea varieties are susceptible, although early crops can avoid infection because it usually occurs late in the season when temperatures rise. Modeling of outbreaks based on oilseed rape petal sampling and local meteorological data has been done in the UK and elsewhere for oilseed rape, and similar modeling has been done for lettuce (Clarkson et al., 2014). For peas, the susceptibility time to infection is very short, and models tend to indicate a wide window of opportunity for treatment. Using a predictive model as a decision support system for peas has been studied in the UK, but peas are only susceptible to infection during the flowering stage, which is very short (HDC, 2013).

White mold (Sclerotinia sclerotiorum)
White mold (Sclerotinia sclerotiorum)
Source: commons.wikimedia.org
©Rasbak (CC BY-SA 3.0)

Pea rust (Uromyces pisi)

Pea rust affects many legumes.

Pathogen: fungus Uromices pisi (Schrot.), which also affects chinna, is an obligate parasite characterized by local type of disease manifestation. As an intermediate host, the spring (ecidial) stage of the pathogen develops in the form of cup-shaped receptacles on the underside of leaves. The ecidiospores are wind-borne on peas and other legumes.

Rust usually appears in mid-summer. Orange-brownish, convex, uredopustules form on leaves, stems, and beans, often powdering, thus infecting plants during the growing season. By the end of summer, pustules become dark, almost black, in which ellipsoidal or obovoid winter spores (teliospores) with thick shell are formed. Affected leaves turn yellow and dry out.

The pathogen persists as teliospores on plant debris. Mycelium of fungi overwinters on rhizomes of thrush. In spring, the teliospores germinate, forming basidia with basidiospores that infect beans and beans. In mixed-farm fungi, the basidiospores infect the thrush, on which the eccidial stage develops, and then the eccidiospores infect the main host.

Bacteriosis (Pseudomonas syringae)

Bacteriosis, Pea bacterial blight, can be important in crops sown in the fall or crops grown under irrigation. In Europe and New Zealand, where fall sowing is common in some areas, the disease can be very severe and yield losses are not uncommon. In dry conditions, the disease is not severe enough to cause losses; on crops sown in the spring, significant damage is rare unless there has been damage from frost or hail.

Pathogen: bacterium Pseudomonas syringae pv. pisi (erroneously “Pseudomonas pisi fungus (Bergey ct all)”; Autko).

The pathogen is seed-borne, and symptoms can occur at any time during the growing season, especially after physical or mechanical damage to pea leaves. Bacteria enter plant tissues through stomata or mechanical damage, spreading through the intercellular spaces, destroying cells. Infection, entering the vessels, causes wilting of leaves and the entire plant. Small, dark green, watery spots initially appear on leaves and beans, which then increase in size. As the affected tissues dry out, the spots turn reddish brown. The bacteria can penetrate the pedicels and cause flower death. Young beans affected by bacteriosis shrivel and dry out. Severely infested beans exhibit a slimy mass or sores with “fatty” spots. Infestation of seeds can occur during their ripening.

The main source of bacteriosis is infected seeds and postharvest crop residues.

There are seven races of bacterial infestation in peas. In the United Kingdom, race 2 is most common on spring varieties, and races 4 and 6 occur on winter peas. In the U.S., race 4 appears to be most common on spring peas.

No chemical means of control exist, and use of healthy seeds is the most effective means of preventing infestation. 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 wilts (Fusarium oxysporum)

Several races of Fusarium oxysporum f. sp. pisi are found in many countries where peas are grown. The disease caused by F. oxysporum, often results in 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 discoloration of the leaves, which first take on a grayish hue, then shrivel up and cause the plant to die. Race 2 is known as close wilt, and plants are affected either individually or in scattered patches throughout the field. Roots often die off, causing the plant to wilt and age prematurely. Race 6 has been reported in the United States and Scandinavia and probably occurs occasionally in Europe and Australasia. Symptoms are very similar to those caused by the soil root fungus. Other races, including race 5, have been described in the United States (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 U.S. in the early 20th century, and in most years many crops were lost in Wisconsin. The disease was first described by Linford (1928), but was later isolated as race 1 F. 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, and some are also resistant to other races.

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

Viruses

Of the important pea viruses in Europe, the United States and Australasia, four aphid-borne viruses can cause the most serious damage or loss of crop or product quality. The pea aphid (Acyrthosiphon pisum) is a vector of several viruses, including pea enation mosaic virus (PEMV), pea apical yellowing virus (PTYV), alfalfa mosaic virus (pea stripe) and pea seed-borne mosaic virus (PSbMV), which is also transmitted by seeds.

The most significant damage from PEMV and PTYV results from early invasions of pea aphids, which bring these diseases into the crop from overwintering hosts. PEMV has a range of overwintering hosts that includes wild and cultivated legumes such as forage beans (Vicia faba), chickpeas (Cicer arietinum), sweet (chickpea) (Lathyrus oderatus), lentils (Lens culinaris), Medicago arabica and seed vetch (Vicia sativa).

PTYV is also known as bean leafroll virus and has wild hosts such as clover and creeper, and on alfalfa (alfalfa) and winter V. faba. PSbMV is mainly seed-borne and is only transmitted between neighboring peas during aphid movement into the crop. In some cases, it has been suggested that forage cereal aphids may also transmit PSbMV from an infected seedling to the rest of the surrounding crop.

Several cultivars are resistant to one or both PEMV and PTYV, and a small number of cultivars have also been bred for resistance to PSbMV. A possible source of resistance is a 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 ensures that the disease is not present. The main symptom of PSbMV is stunting of the plants, and the seedpods often have spots with a white border that can be compared to the marks on a tennis ball. Affected peas can cause crop rejection by processors of frozen peas, and in the mid-1980s, seed stocks of the variety petits pois, which was widely used in Great Britain and Europe, were severely infected. In 1987, a research program to detect the presence of the virus in seed was initiated, which led to a widespread testing program using an ELISA-based seed test to identify seed stocks carrying the infection. This program has been effective and has produced healthy seeds of this variety, which remains one of the most important petits pois cultivars grown in Europe and the United States.

In Europe, a nematode-transmitted virus, Pea Early Brown Spot Virus (PEBV), occurs in some seasons. Free-living nematodes, including the vector Trichodorus sp., are often found in free-draining sandy soil and graze on pea roots during periods of high soil moisture in spring. The virus develops in peas, leading to characteristic leaf mosaic, purple stems, stunting and necrosis of the main shoot.

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

 

Sources

Modern technologies in vegetable production / Dr. A.A. Autko [etc.]; edited by A.A. Autko. – National Academy of Sciences of Belarus, Institute of Vegetable Growing. – Minsk : Belarus. nauvuka, 2012. – 490 p., [16] l. ill.

Peas and beans. Crop production science in horticulture / Antony J. Biddle. 2017. UK.