Field Pea – Foliar Diseases

Ascochyta/Mycosphaerella Blight Ascochyta on Field Pea Watch the Video: Ascochyta in Peas
CAUSED BY	This disease complex is caused by three fungi: Mycosphaerella pinodes (Berk & Blox.) (the perfect stage of Ascochyta pinodes), which causes blight; Ascochyta pisi Lib., which causes leaf and pod spot; and Phoma medicaginis var. pinodella (Jones), which causes root rot. The three species often occur together and can be difficult to tell apart. These pathogens can be seed, stubble or soil-borne. Seed-borne infection by M. pinodes is considered to be the primary source of infection in virgin pea fields. P. medicaginis var. pinodella and M. pinodes are both quite persistent, surviving on pea straw fragments and in the soil. When moisture and temperature conditions are favourable, residual pycnidia from M. pinodes mature, new pycnidia develop and pycnidiospores are released. These can infect healthy plants by rain splash (ascospores are also produced and can be carried by wind for a kilometre or more). A. pisi, on the other hand, competes very poorly with other microflora in the soil and overwinters very poorly – the main source of infection is from seed-borne spores.
Symptoms	A. pisi lesions are partially sunken, tan coloured and surrounded by a well-defined dark brown margin: circular lesions are found on pods and leaves while lesions on stems are more elongated; numerous pycnidia are usually found in the lesions; and lesions of A. pisi are rarely found on any plant parts below the soil line. If lesions on leaves, stems and pods are widespread and severe, M. pinodes is the cause. If lesions are severe on roots, P. medicaginis var. pinodella is the cause. M. pinodes produces small, brown to purplish, irregular flecks without definite margins, initially appearing on pods, leaves, stems and the cotyledonary area – these lesions enlarge if weather conditions are favourable (15˚ to 18˚C and high humidity). Dark brown to black specks (pycnidia) are eventually produced and form a distinct concentric tan and brown ring pattern as the lesions enlarge – this is often more pronounced on leaves and pods than on other plant parts. Lesions developing on stems tend to form long, wide purple to bluish-black streaks that eventually coalesce and may completely girdle the stems, pod attachments or tendrils – these streaks are more common near the nodes and on the lower portion of the stem. Early pod infection can lead to seed infection, which may show no visible symptoms if infection is light, to varying degrees of shrinkage and discolouration if severe. Under drier conditions, the concentric ring pattern of the symptoms is less pronounced and may show up only as a uniform yellowing of lower leaves – if the blossom becomes infected, girdling of the sepal often occurs, leading to pod drop or distortion.
PREVENTION AND CONTROL	Have a four to five-year rotation between pea crops. Have pea seed examined for Ascochyta presence by an accredited seed lab. Use only pathogen-free seed – if pathogen-free seed cannot be found, use seed with as low a level of Ascochyta as possible, and have the seed treated. Avoid seeding next to any previous year’s pea fields since spores can spread by wind. Bury all crop residue, to prevent the fungus being spread by wind and rain. For further advice in managing Ascochyta, please refer to resource below.
IMPACT	M. pinodes is the most common on field pea in Western Canada – average yield losses run to 10%, but losses of up to 80% have been reported. For most pea varieties, expect a 5% to 6% yield reduction for every 10% of the stem area affected.
RESOURCES	Managing Ascochyta (Mycosphaerella) Blight in Field Pea.

Sclerotinia Stem Rot
CAUSED BY	Caused by Sclerotinia sclerotiorum (Lib.) de Bary (often called white mould or pod rot). Prevalent when pea and canola are grown in the same rotation. Dense canopies and high humidity favour sclerotinia outbreaks. Ascospores cannot infect pea plants directly because to germinate, they need a source of dying tissue (such as stamens or flower petals) – these are then colonized by threadlike running hyphae, especially under moist conditions. Ascospores can be released over long distances (up to several kilometres) and long periods of time. The critical infection period is during the flowering stage. When pea seed is harvested, sclerotia are either harvested with the seed or fall to the ground with the vines. It overwinters as sclerotia, small black resting bodies, in the soil, apart from the host. Only sclerotia in the top 6 cm of soil can produce apothecia. Sclerotia buried deeper remain dormant and can survive in the soil for five to seven years, until conditions for apothecia formation are favourable.
SYMPTOMS	First sign is the appearance of a light brown discolouration on the stem, leaves or pods. With Pod Rot, this occurs at the base of the style with a small water-soaked lesion appearing on green tissue at the end of the pod. With Stem Rot, the symptoms are more common in the node area. Whitish, threadlike mats develop over the affected areas and over time, cause the tissue underneath to turn soft and decay – black, hard bodies (sclerotia) form in the mat and within the stem and pods. Affected plants often appear wilted and ripen prematurely due to rotting stems – lodging is common in affected areas. When humidity is high and moisture is abundant (such as within a thick canopy), tan, fleshy, golf-tee shaped apothecia emerge on the sclerotia – numerous ascospores develop within the apothecia and are released into the air to infect nearby healthy plants. Lesions appear four to five days after infection, and within eight days, dark to greyish-brown lesions ranging from 1 to 5 mm can be seen on the infected tissue.
PREVENTION AND CONTROL	Four to five-year rotation out of susceptible crops (canola, mustard, sunflower, dry bean). Bury sclerotia bodies deep. Planting semi-leafless pea varieties to allow greater air flow through the canopy. There are fungicides registered to protect the pea plants from Sclerotinia infection. Once symptoms are noted in a pea crop, it is too late to apply a fungicide. Fungicide application has to be based on a forecast of risk.
IMPACT	If infection occurs late in the growing season, there may be little effect on yield, however, the buildup of sclerotia in a field may have a negative impact on subsequent broadleaf crops.

Downy Mildew
CAUSED BY	Caused by Peronospora viciae, this disease likes cool, wet growing conditions and usually occurs on early-seeded crops. Pathogen can be seed or soil-borne and can survive in the soil for 10 – 15 years. Seed-borne infection often causes failed germination.
SYMPTOMS	Can be expressed systemically, locally (on leaves, tendrils and flowers) or as pod infections. Systemic infections: produce the most severe effects, causing stunting, distortion and the proliferation of white, cottony growth on the plant; profuse sporulation of the fungus occurs on the surface of the plant, and plants often die before flowering; if pods are produced, they are flattened, yellow, distorted and rarely set seed. Local infections: appear as fluffy, white to bluish, cottony patches on the undersides of leaflets, while the upper sides turn yellow and die. Pod infection: first appears as blotches that become brown with green areas inside them, which cause small, brown, sunken spots on the seeds and if severe enough, can lead to seed abortion – a cottony growth can occur inside the pod and may contain numerous oospores. Infections on older plants may only appear as spotting on the leaves.
PREVENTION AND CONTROL	All varieties registered in Canada must be resistant to this disease. Use four- to five-year crop rotations to reduce the soil-borne inoculum, but this will not completely eliminate it as spores can be long lived in the soil. Practice deep burial of crop residue. Use of disease-free seed.
IMPACT	This disease should no longer be an issue as all varieties registered in Canada must be resistant to this disease. Downy Mildew Fungal does not usually cause serious losses unless cool, wet, and humid conditions exist. Pathogen can survive anywhere from 10 to 15 years in the soil.

Fusarium Wilt Fusarium Wilts, blights, rots, and damping-off in field pea, Photo Credit: Mary Burrows, Montana State University, Bugwood.org Fusarium Wilt on Field Pea
CAUSED BY	Caused by Fusarium oxysporum Schl. f. sp. pisi (Van Hall) (Snyd & Hans). Of the disease’s many races, races 5 and 6 are most important in Western Canada, they can thrive under soil temperatures of 20˚ to 21˚C.
SYMPTOMS	Race 5 and 6 symptoms are most often found in small, circular patches in fields. In roots cut longitudinally, a yellowish, orange colour is seen in the vascular tissue, which can extend into the basal area of the stem. Downward curling of the leaves and stipules. Thickening of the basal internode and brittle leaves and stem. Yellowing of the leaves progressing from the base of the plant to the top of the foliage (plants often die due to loss of the root system).
PREVENTION AND CONTROL	Grow pea varieties with resistance to Fusarium wilt. Plant as early as possible using only disease-free seed. Early planting allows the crop to develop when soil temperatures are below the optimum for development of the fusarium wilt disease. Use a five-year rotation or longer, using crops other than pea, plus preventing the movement of all pea vine debris to other gardens, will help to prevent buildup of wilt fungi in the soil. Once wilt fungi has been established in a pea, it can be controlled only by growing wilt resistant varieties.
IMPACT	Plants become stunted and may die. Pathogen can be seed or soil-borne and can infect soils for 10 or more years.

Powdery Mildew Powdery Mildew
CAUSED BY	Caused by Blumeria pisi f.sp. pisi. Borne on seed or stubble. Seldom a problem until late in the season when environmental conditions are favourable. Summer rains damage the fruiting bodies, causing them to burst instead of germinate – in late-summer or early-fall, when dews occur without rain, the disease can spread very quickly. Severe infections reduce plant weight, seed weight, number of seeds per pod, pods per plant and plant height.
SYMPTOMS	First appear as off-coloured spots on the upper surface of the lowest and oldest leaves. Spreads rapidly to cover entire surface of leaves, stems and pods with a fine, powdery, bluish-white mildewy growth – underneath the mildew, the tissue often turns purplish. As plants age, tiny pinhead size fruiting bodies (cleistothecia) develop – these are brownish at first, eventually turning black at maturity. Infection can also wither foliage – if infection occurs on the pods, penetration of the pod may occur and cause the seed to turn greyish-brown (severe infections can cause hollow seeds).
PREVENTION AND CONTROL	Seed early and use earlier maturing varieties; use crop rotation practices; and bury the crop residue. All varieties registered in Canada must be resistant to this disease.
IMPACT	This disease should no longer be an issue due to resistant varieties. Without this built-in resistance, yield losses can be high (in excess of 60%).

Other foliar diseases that are less common include Alternaria Blight, Bacterial blight, Black Leaf, and Rust.

Alternaria Blight
CAUSED BY	Alternaria alternata (Fr.) Keissl. is the causal organism of this disease. Favoured by prolonged high humidity (more than three days) and warm temperatures (16˚ to 24˚C).
SYMPTOMS	Foliar lesions are oval with, a concentric ring pattern inside – they are tannish-brown in the centre and fade out towards the margins. Up to half or more of the leaves and stipules can be covered in lesions when an outbreak occurs. Pod lesions are smaller, raised and brownish in appearance – some slight stunting of pea plants can also occur.
PREVENTION AND CONTROL	Crop rotation with non-host crops is the best means of prevention.

Black Leaf
CAUSED BY	Caused by Fusicladium pisicola.
SYMPTOMS	Small, white spots on the underside of leaves are the first symptoms, and these spots penetrate the leaf as they enlarge, forming a concentric ring pattern progressing from tan in the centre to grey to black at the edges – spots eventually darken, as specks (conidia) form on the underside of the leaf. Black, elongated lesions then streak the leaves – this starts with bottom leaves and progresses upwards (blackening is due to closely-packed conidia on the leaf surface). Bottom leaves eventually turn completely black, dry out and become shredded in appearance.
PREVENTION AND CONTROL	Use resistant varieties.

Botrytis Grey Mould
CAUSED BY	Caused by Botrytis cinerea Pers.exFr., grey mould can occur as a rot on pods, stems and leaves on pea plants or on fresh seed and pods in transit or storage. Infection occurs quickly under conditions of 16˚ to 21˚C and 100% humidity.
SYMPTOMS	Fuzzy, grey lesions are the first symptoms to appear on lower areas of the stem and spread until the entire lower foliage turns a fuzzy, grey colour – this appearance is caused by the abundance of conidia produced on leaves and stems. As the disease progresses, leaves become shriveled and dry, and defoliation of bottom leaves occurs. Pod infection causes the most damage, and clinging blossoms provide a humid environment from which the tip of the young pod can be infected – small, oval, water-soaked lesions develop and spread up the pod (these lesions are tan at first but turn greyish with age and often develop sclerotia in the form of small black specks).
PREVENTION AND CONTROL	Potassium fertilizer in potassium deficient soils reduces the severity of grey mould.

Rust
CAUSED BY	Rust is caused by Uromyces appendiculatus. Rust is less common in Alberta, and tends to infest late-maturing crops. Rust fungus can survive on infected pea crop residue and is also spread by wind-borne spores (rust fungus is not transmitted as a seed-borne disease).
SYMPTOMS	Rust first appears as small, white spots on the lower surface of the leaves – these spots break open within a few days to expose rust-coloured fungus spores on both upper and lower leaf surfaces Severely infected leaves turn yellow, then brown and soon die (pods and stems may also be attacked).
PREVENTION AND CONTROL	Follow a crop rotation that allows a minimum of three years between pea crops. After harvest, turn under all pea residue as completely as possible.