The fungal pathogen Ascochyta lentis causes ascochyta blight of lentils and is an important disease in most lentil producing countries. The disease can reduce crop yields under severe conditions, but mainly affects seed quality and hence marketability. Seed discoloured as a result of ascochyta infection is often heavily discounted in price and may be rejected by some buyers. Ascochyta is favoured by cool, wet conditions and is likely to be a problem in years with wet winter conditions and in high rainfall areas.
All above ground parts of lentil plants can be affected by ascochyta blight. Symptoms may appear on plants from the seedling to mature stages. The disease appears as spot-like lesions which are initially light grey but become tan with a dark brown margin, (Figure 8.1).
The centres of lesions become speckled with pycnidia (tiny, dark fruiting bodies), (Figure 8.1). The presence of pycnidia is the best way of identifying ascochyta blight lesions from those caused by other diseases such as botrytis grey mould or stemphylium blight. Heavy infections of ascochyta blight will cause premature leaflet drop and stem dieback at the growing tips giving plants a blighted appearance.
Pod infection can lead to seed infection and discolouration of the grain. Infected seed generally has brown patches on the seed surface, but may show no symptoms at all. Compared to healthy seed, heavily infected seed is purplish-brown, shriveled and reduced in size, (Figure 8.2).
Figure 8.1 Pod lesion caused by ascochyta infection. Note dark brown margin and fruiting bodies present in the centre of the lesions
Figure 8.2 Lentil seed infected with ascochyta blight (left), healthy seed (right)
Figures 8.3 and 8.4 Characteristic leaf lesions caused by ascochyta blight on lentils
The fungus may either be seed borne or survive on infected stubble from previous diseased crops, (Figure 8.5). While the disease can become established as a result of sowing infected seed, it may also infect new crops when spores of the fungus from infected stubble are carried by the wind from old lentil paddocks. Infection can occur at any stage of plant growth, however, moisture is essential. Cool wet weather promotes sporulation, spore dispersal and infection. Secondary spread within crops occurs when spores produced on diseased plants are carried by wind and rainsplash onto neighbouring plants. Heavy rainfall late in the season provides ideal conditions for pod infection and subsequent seed discolouration.
Figure 8.5 Disease cycle of ascochyta blight of lentil. (Illustration by Kylie Fowler)
Using old or damaged seed can reduce seedling vigour and increase susceptibility to infection. Only sow seed with less than 5 percent ascochyta infection and preferably use seed with nil infection.
Use varieties with the greatest resistance. There are a range of varieties available that are MR or R to ascochyta infection with attributes to suit most lentil growing areas in Victoria (Table 8.1).
Infected crop residues can harbour Ascochyta lentis. Therefore, avoid planting this season’s crop near old lentil stubble. A program of stubble reduction may also be undertaken by grazing or burying to reduce the carry-over of infected stubble into the following season. Allow a break of at least 3 years between lentil crops.
Use a registered seed treatment for the control of seed-borne diseases in lentil. Seed treatments can have a deleterious effect on rhizobia. Therefore, seed should be treated with fungicide and then inoculated with rhizobia in two separate operations. Rhizobia should be applied to seed immediately before sowing, especially on acid soils.
Early sowing encourages early infection and increased levels of the disease. Follow the recommended sowing dates for your district.
In areas of high risk, it may be necessary to apply foliar fungicides to protect crops especially if a susceptible variety is grown. Use a registered product; remember that most fungicides are protectants and are most effective if applied before disease development.
Fungicides should be applied according to label directions for use, ensuring the key points of spray timing and frequency are observed as well as grazing and harvest withholding periods.
DISEASES IN LEGUMES: LENTILS
BOTRYTIS GREY MOULD OF LENTIL