Ascochyta blight and anthracnose are the two most important diseases followed by botrytis grey mould, sclerotinia white mould, and stemphylium blight. These diseases become most problematic from flowering to maturity. Moulds such as Botrytis and Sclerotinia are widespread but not economically important, except where lodging due to wet conditions occur.
Lentil varieties classified as "resistant" are not completely immune to the diseases. These varieties resist infection and damage to a reasonable extent and under certain conditions the crop may show few symptoms and suffer little damage. However, if conditions are favourable for disease development and spread, the crop may succumb to the pressure and suffer significant damage. Hence, fungicide application may be needed to assist the plant and control the disease.
Ascochyta Blight – Appears as grey to tan spots or lesions on leaflets, stems, flowers and pods, with dark margins and often with tiny black fruiting bodies (pycnidia) in the centres (Fig. 1). Lesions first appear on lower leaflets close to the soil surface and spread up the plant canopy. Lesions on stems can girdle the plant resulting in wilting. Leaves may turn brown and die off.
Anthracnose – Appears as white to grey or cream-coloured spots on leaflets and stems (Fig. 2). First appear on lower leaves and stem and move up the canopy. Leaves and entire plants may die back and stems of mature and dead plants often blacken. Leaflets litter the soil surface. Leaf symptoms appear between eight and 12 node stages - approximately one week before flowering. Lesions on stems can girdle the plant resulting in wilting. Yellow patches appear in fields and enlarge as the disease spreads and plants die.
Botrytis Grey Mould – Infected seeds produce infected seedlings which die soon after emergence, resulting in reduced plant density. For the rest of the crop, symptoms appear later in the season and include wilting, premature ripening, failure of pods to fill, and dead, infected crop areas. Grey, mouldy growth on stems and pods is visible throughout the canopy (Fig. 3). These are the sources of the clouds of spores seen dispersed during harvesting.
Sclerotinia White Mould – Root system and base of plant become brown and rotten and may have white mould growing on them (Fig. 3). Plants first turn yellow, later, infected areas become bleached and shredded.
Stemphylium Blight – Symptoms appear initially as small, light-beige lesions on leaves/leaflets. Small lesions coalesce to produce large, irregularly shaped lesions that kill entire branches (Fig. 4).
Early identification of diseases is critical, as it allows for timely decisions to commence fungicide application and minimize crop damage. Begin scouting at the eight to 10 node stage (prior to flowering) and scout every five to seven days.
When scouting for ascochyta and anthracnose, start with high risk-areas and fields. Select and flag five to 10 random locations (in an "M" pattern) in the field. Early symptoms are similar for both diseases. At each point, look for small (pinhead-size) brown to dark-brown spots at the lower parts of the plants. Use a magnifying glass and field guides with photos. Under humid conditions, these spots expand quickly into lesions described under symptoms above. Note that anthracnose lesions are more commonly observed on stems.
It is advised that you use field guides with photos or seek the assistance of a qualified agronomist. For conclusive identification, send a sample to the Provincial Crop Protection Lab (306-787-8130).
A number of foliar fungicides are registered for the control of both ascochyta and anthracnose. See the Guide to Crop Protection for details on foliar fungicides.
Note: in lentils, fungicide applications are generally ineffective in controlling sclerotinia white mould and botrytis grey mould because these diseases do not develop until the crop canopy is too dense to allow penetration of the fungicide.
Table 1. Fungicide Decision Support System for Lentil
|Plant stand||Days of rain in past 14 days||5-day weather forecast||Anthracnose and ascochyta severity|
|0 = thin||0 = 0 days||0 = dry||0 = no disease symptoms
|5 = moderate||5 = 1-2 days||10 = unpredictable||5 = Lesions on 1-5% of lower leaflets|
|10 = Normal||10 = 3-4 days||15 = light showers||15 = Lesions on 6-10% of lower leaflets|
|15 = dense||15 = 5-6 days||20 = rain||25 = Lesions on > 10% of both lower and upper leaflets|
|20 => 7 days||25 = Premature leaf drop and few small stem lesions|
|Risk value = A + B + C + D||25 = Flowers and peduncles infected|
The optimal time for control of ascochyta and anthracnose is at the 10 to 12 node stage or early flowering. It is too late to control anthracnose when severe lesions can be found at the stem base and when the crop is no longer flowering. The threshold for fungicide application is set at a risk value of 50 (Table 1). If the risk value is less than 50, a fungicide application is not warranted, but a new risk assessment should be made at three-to-five day intervals until the crop is no longer flowering. If well timed, a single fungicide application is usually sufficient in controlling lentil diseases.
If ascochyta spreads to the top of the canopy and wet conditions prevail, infection may lead to flower and pod abortion, resulting in significant yield losses - as much as 50 per cent. More economic losses are incurred due to reduction in grain quality. Anthracnose is capable of causing yield losses in excess of 50 per cent and is much more destructive than ascochyta.
Crop rotation is key to preventing these diseases, as it allows time for the lentil residue on which the fungi survive to decompose. Allow four years between lentil crops. The highest levels of anthracnose are usually observed on fields with a history of lentils every second year. Avoid planting lentil adjacent to the previous year's lentil fields to reduce residue and windborne spread of the pathogens. Plant seed that has been tested at an accredited lab and known to have zero or acceptable levels of seed-borne diseases and high germination. Use seed treatments to protect the seed and seedlings from seed-borne diseases. Choose varieties classified as disease resistant when available.