Disease can severely impact lentil yield and quality if inoculum is present and environmental conditions are conducive to disease development.
Root rot, seed rot, seedling blight, damping-off, and wire stem are soil-borne fungal diseases that can infect lentil seedlings and can be caused by species of Rhizoctonia, Pythium, Fusarium, and/or Botrytis. These pathogens are present in all Saskatchewan agricultural soils, and can infect and kill individual seedlings from germination to the early flowering stage. Lesions may develop on the base of the stem, causing discolouration and constriction of the stem. Diseased plants turn yellow and die. Usually only scattered plants are infected, so these diseases rarely cause economic loss. Seedling stress or damage due to environmental or herbicide injury can lead to an increase in the incidence of seedling blight, especially wire stem.
Root rots can also be caused by Aphanomyces species and Aphanomyces euteiches has been identified across most of Saskatchewan. There are no seed treatments that control A. euteiches, and it can survive in the soil for many years. It is the most difficult to control among the root rot pathogens, can infect the plant at any stage, and is most aggressive under wet conditions. For more information on root rots see Root Rot in Peas and Lentils in Western Canada.
Ascochyta blight: is a serious foliar disease of lentils in Western Canada. It can be seed-borne or residue-borne, resulting in infection of leaves, stems, pods, and seed. Lesions appear as tan or grey spots with dark margins, and have tiny black fruiting bodies (pycnidia) in the centre. Cool, rainy weather is conducive to infection and spread of the disease. It is most damaging to maturing pods and seeds if prolonged wet weather occurs during July and August. Severely infected seed lots may not be marketable or will be downgraded due to discoloration. Most lentil varieties now have some level of resistance to ascochyta blight.
Ascochyta blight inoculum overwinters on lentil residue, so producers should not plant lentil on lentil stubble.
Anthracnose is a foliar and stem disease found in most lentil producing areas in Western Canada caused by Colletotrichum truncatum. Research has identified two races of anthracnose. A few lentil varieties have been introduced with resistance to Race 1 (Ct1), but no varieties are resistant to Race 2 (Ct0). Infection results in sunken grey to cream coloured lesions on leaves and stems. Lower leaflets turn yellow and brown, and drop to the soil surface. The lower stems become cankered by the disease and the plants die prematurely.
Lesions and dead plant tissue may contain tiny black resting bodies (microsclerotia) similar in appearance to ascochyta blight pycnidia, but smaller, more numerous, and irregular in shape. Diseased patches in the crop can expand rapidly and appear as yellowed or grey patches within an otherwise green field. The disease is favored by warm, moist weather and commonly kills the infected lentil plant before seed is produced.
Anthracnose can be spread on wind-borne residue and dust during harvest, and can be residue-borne in fields for a number of years. There is no research showing that the disease is frequently transferred from the seed to the lentil seedling. However, producers should attempt to use seed with low infection levels, as there are no seed treatment fungicides effective at controlling seed-borne anthracnose. Extend crop rotations to avoid planting lentils in the same field for at least four years.
There are a few foliar fungicides registered for ascochyta blight and anthracnose in lentils. Generally, a fungicide should be applied before the plant canopy closes completely, which usually coincides with the timing of first flower. However, follow label directions for correct time of application. The goal is to protect healthy plant material if disease inoculum is present in the field and weather conditions favour the disease. More than one application may be required if conditions favouring the disease persist.
A disease decision support checklist for ascochyta and anthracnose has been developed by Agriculture and Agri-Food Canada as a working tool for producers to help in establishing thresholds for fungicide application on lentil to control anthracnose and ascochyta in lentil.
Botrytis grey mould causes stem and pod rot during the flowering and seed filling stages and can cause economic losses. Soil-borne inoculum is present in all fields, but this disease is typically only a problem in heavy vegetative stands that have lodged due to wet, cool weather. Leaves wilt and drop off, pods fail to fill, and infected areas turn grey to brown. Clouds of grey spores are dispersed into the air as infected areas are harvested. There are foliar fungicides registered for the control of the foliar blight caused by botrytis grey mould.
Sclerotinia white mould may occur in maturing lentil crops under high moisture conditions that promote vegetative growth and lodging, and can cause economic losses. Lentil crops are at increased risk to sclerotinia infection if grown in rotation with other susceptible crops, such as canola, pea, or sunflower. There are foliar fungicides registered for the control of sclerotinia stem rot on lentils. As with botrytis, achieving the necessary coverage of infected stems often makes treatment difficult.
Research completed at the University of Saskatchewan revealed that all lentil tissues, (i.e. leaves, stems, pods, and flowers), can be infected by spores of sclerotinia. Testing of plants at various ages that were infected with sclerotinia showed that plants older than six weeks were significantly more susceptible. This decrease in resistance, combined with wet weather late in the growing season and a heavy plant canopy, may explain why sclerotinia is more of a problem in maturing lentil crops.
Stemphylium blight has been identified in a number of lentil fields in Saskatchewan. The foliar disease has similar leaflet drop symptoms as anthracnose, and lesions on leaves similar to ascochyta blight. It has not yet been confirmed as causing significant yield losses because the disease tends to show up later in the growing season. The fungus thrives under warm (25°C to 30°C), wet conditions, but spores can germinate as low as 5°C, which may indicate infection can occur under cool, wet weather as well. There have been differences noted between lentil varieties regarding their susceptibility to stemphylium blight. Infection can cause seed staining, reduced seed size, and decreased germination rates.
Management of lentil diseases begins with crop rotation. Some pathogens can survive in the absence of a host crop (botrytis and stemphylium) or will affect other host crops in the rotation (sclerotinia). Varieties producing excessive vegetative growth usually have more frequent problems with late season botrytis and sclerotinia in years or locations with higher moisture.
Viral diseases do not typically case economic impact in Saskatchewan lentil crops. Lentil seed-borne mosaic virus is a potential threat to lentils, as it may be introduced with infected lentil seed and spread by aphids.
Heat canker occurs when young lentil seedlings are exposed to hot soil surface temperatures. With heat canker, the seedlings wilt very quickly on extremely hot spring days. The pinched stem usually remains white, and often new shoots are started from the scale nodes.
Managing Fungicide Resistance
As with herbicides and weed resistance, managing fungicide use to prevent or slow the development of fungicide resistance is important. Use a foliar fungicide only when disease risk and potential loss are considered to be economically damaging (i.e. greater than the cost of control).
Development of resistance of several fungal pathogens to the strobilurin group (Group 11) of fungicides in other crops has been reported in Europe and in Saskatchewan, and is of great concern. No more than two applications per year of any strobilurin fungicide should be made to the same field. The continuous use of strobilurin fungicides without fungicide rotation greatly increases the threat of disease resistance.
Any fungal pathogen population may contain some strains naturally insensitive to various fungicides. A gradual or total loss of disease control may occur over time if these fungicides are used repeatedly in the same fields.
The following strategy should be considered and implemented to delay fungicide resistance/insensitivity:
- Rotate the use of fungicides from different groups that control the same pathogens.
- Tank mix fungicides that have a high risk of developing insensitivity with other fungicides from a different group where both are effective on the target pathogen.
- Do not apply more than the maximum number of applications listed on the label. Avoid consecutive sprays of the same fungicide, or other fungicides in the same group, in a season. Fungicides belonging to the strobilurin group should not be applied more than twice a season in the same field.
- Fungicide application should be based on an integrated pest management (IPM) program that includes scouting and accurate record keeping.
- Monitor treated fungal populations for signs of fungicide insensitivity. If disease continues to progress after treatment with a product, do not increase the use rate. Discontinue use of the product and switch to another fungicide with a different target site of action.