Disease Management

Disease can severely impact lentil yield and quality if inoculum is present and environmental conditions are conducive to disease development. 

Seed rot, seedling blight, damping-off, wire stem, and root rot: these 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 them 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 rot can also be caused by Aphanomyces species and Aphanomyces euteiches is the one that 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 pathogen among the root rot pathogens and is most aggressive under wet conditions. For more information on root rots see the Root Rot in Pea and Lentil in Western Canada.

Heat canker: this 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.

Ascochyta blight: is a serious foliar disease of lentil in Western Canada. It can be seed-borne or residue-borne, resulting in infection of leaves, stems, pods and seed. Lesions appear as tanor grey spots with dark margins, and often have tiny black fruiting bodies (pycnidia) in the centre. Cool, rainy weather is conducive for 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 severely 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 plant 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 lentil in the same field for at least four years.

There are a few foliar fungicides registered for ascochyta bight and anthracnose in lentil. Generally, a fungicide should be applied before the plant canopy closes completely. 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. Up to three applications 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 (Table2).

Table 2. Disease Decision Support Checklist for Ascochyta and Anthracnose

  Inspect at least 10 locations in the lentil crop at 10 per cent flowering.    

1.    Plant Stand 

a. Thin (high weed pressure, low yield expectations)

b. Moderate (some weeds, possibly low yield)

c. Normal (about 12 lentil plants/ft2 or 136/M2)

d. Dense (more plants than normal, lush growth)

Risk Factor

0

5

10

15

2. Number of days with rain in the last 14 days

a. 0 days

b. 1-2 days

c. 5-6 days

d. 7 or more days

Risk Factor

0

5

10

15

3. The five-day weather forecast

a. Dry

b. Unpredictable

c. Light showers

d. Rain

Risk Factor

0

5

10

15

4. Symptoms of anthracnose and ascochyta blight on lentil plants

a. No visible symptoms

b. Few lesions on the lower half of the foliage (up to 10 per cent infected)

c. Lesions on lower half of the foliage (up to 25 per cent infected)

d. Lesions on lower (up to 25 per cent) as well as upper foliage (up to 10 per cent)

d. Lesions on lower foliage and premature leaf drop characteristics of anthracnose

d. Flowers and/or peduncles infected, characteristics of ascochyta blight

Risk Factor

0

5

15

25

25

25

Choose the risk values that most closely describe the plant stand, number of days with rain in the past 14 days, and the five-day weather forecast. Average the disease symptoms over at least 10 sites in the field. A risk value is then calculated as A+B+C+D.

  •  If the risk value is less than 50, a fungicide application is not recommended, but a new assessment should be made at three- to five-day intervals until the crop is no longer flowering.
  •  When the risk value is 50 or above, a fungicide application is recommended.
  • The optimal time for control of anthracnose is the 10 to 12 node stage, or early flowering when premature leaf drop first occurs. An application may be warranted at low levels of stem infection to protect those plants that are still healthy.

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 in wet, cool summer 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. Producers are cautioned to use dust masks to prevent breathing difficulties. There are foliar fungicides registered for the control of the foliar blight caused by botrytis grey mould.

Sclerotinia white mould may occasionally 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 lentil. All lentil tissues, including leaves, stems, pods, and flowers, can be infected by spores of sclerotinia.

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. Although infection may occur later, it can still cause economic loss

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 summer. 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, smaller seed size, and reduced germination rates.

Management of botrytis, sclerotinia, and stemphylium, as with other lentil diseases, starts with crop rotation. However, it should be noted that these particular 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 in years or locations with higher moisture.

A number of foliar fungicides are registered for the control of botrytis grey mould and sclerotinia white mould. These diseases may appear late in the season with minimal impact to yield, which should be considered when making an economical decision on whether to spray or not. It is too late to apply fungicide to control sclerotinia white mould once symptoms are observed, so forecasting to determine risk is necessary.

Viral diseases are not severe in lentil in Saskatchewan. Lentil seed-borne mosaic virus is a potential threat to lentil, as it may be introduced with infected lentil seed and spread by aphids.

Managing Fungicide Resistance

As with herbicides and weed resistance, managing fungicide use to prevent or slow the development of fungicide resistance is in the long-term interests of all lentil growers. Use 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 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, as disease resistance could develop. 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 with others 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.
  • 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 recording related to pesticide use and crop rotation.
  • 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.