Ascochyta Blight of Lentil

Ascochyta blight of lentil and is an important disease in most lentil producing countries and is caused by the pathogen Ascochyta lentis. The disease can reduce crop yields and affect seed quality and hence marketability. Seed with discolouration caused by Ascochyta infection is often heavily discounted in price and may be rejected by some buyers. Ascochyta blight develops in cool, wet conditions and is likely to be a problem in years with a wet winter or high rainfall areas. Heavy rainfall during podding can result in pod infection and is likely to develop when the disease is already present in the crop.

What to Look For

All above ground parts of lentil plants can be affected by Ascochyta blight. Symptoms can appear on plants at any growth stage, with infection looking more severe when plants are smaller. The disease appears as spot-like lesions which are initially light grey and then develop a tan to dark brown margin. The centre of lesions become speckled with pycnidia (tiny, dark fruiting bodies). 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.

Crops which have leaf or stem lesions are at greater risk of developing pod infection. 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, shrivelled and reduced in size.

 

Lentil seed infected with ascochyta blight. Note dark brown margin and fruiting bodies present in (left), healthy seed (right) the centre of the lesions. (Photo: Unknown)

Disease Cycle

The causal of Ascochyta blight, Ascochyta lentis can carry over in infected seed or survive on infected stubble from previous diseased crops. It is important to keep in mind even with sowing clean seed and a good paddock rotation, A. lentis can infect new crops when spores 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 a carried by wind and rain-splash onto neighbouring plants. Heavy rainfall late in the season provides ideal conditions for pod infection and subsequent seed discolouration.

Management

Seed Selection

Using old or damaged seed can reduce seedling vigour and increase susceptibility to infection. Only sow seed with less than 5% Ascochyta infection and preferably use seed with no infection.

Variety Selection

Sow 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. It is important to check the latest resistance ratings as they may change as resistance breaks down. Refer to the latest Victorian Pulse Disease Guide or the NVT Website for up to date ratings.

Paddock Selection

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.

Seed Treatment

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.

Sowing

Early sowing encourages early infection and increased levels of the disease. Follow the recommended sowing dates for your district.

Foliar Fungicides

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 and remember many 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.

 

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