Ascochyta Disease in Lentils
Ascochyta blight, caused by the fungus Ascochyta lentis, is a serious disease of lentil causing yield losses and severe seed discolouration in epidemic years. Yield losses may be as high as 50% in susceptible cultivars. Additional economic losses occur due to loss of grade. The Ascochyta fungus has become much more aggressive in recent years.
Appearance: symptoms appear on leaves, stems, and pods as light gray or tan coloured spots, sometimes with a darker margin. The centres of the spots are often speckled with tiny, black fruiting bodies. When disease becomes severe, premature leaflet drop may occur and the crop takes on a distinctly blighted appearance. Seed discolouration develops later in the season after pods are infected. Seeds become brownish purple in colour and may be shriveled. In moist weather conditions, discolouration can continue to develop even after the crop is swathed.
Disease Cycle: The fungus over-winters in seed and infected lentil crop residue. Infected seed is often discoloured but sometime-healthy looking seed may also carry the fungus.
When infected seed is planted, small proportions of the seedlings emerge diseased. The fungus then spreads from plant to plant by rain-splashed spores. The disease is favored by moist conditions. Frequent showers can result in an epidemic.
a) Use disease-free seed – Many epidemics originate from infected seed. There is no safe level for seed infection. Under favorable environmental conditions, even seed with as low as 1-2% Ascochyta infection can result in an epidemic. An accredited seed lab using an agar plate test can determine the level of Ascochyta in the seed.
b) Crop rotation – Allow three years between lentil crops on the same field and avoid planting next to the previous year’s lentil fields.
c) Chemical control. Seed treatments and foliar sprays exist for the control of ascochyta blight and anthracnose on lentil. More than one application of the foliar may be necessary, starting at early bloom.
d) Resistance – There are Lentil cultivars that have some resistance and suffer less yield loss than susceptible varieties.
Ascochyta Disease in Field Peas
Ascochyta blight of field pea is a disease complex caused by three closely related fungi; Ascochyta (Mycosphaerella) pinodes, A. pinodella and A. pisi. The disease complex is usually most severe in growing seasons with high rainfall. Yield losses in field peas of up to 80% have been reported due to A. pinodes. Ascochyta infected seeds usually have low germination and poor emergence.
Appearance: Seedlings form infected seeds may show a blackening of the stem tissue from the soil line upward for 5 to 15 cm. Seedlings usually survive but in a weakened condition. Symptoms on leaves, stems, flowers and pods vary, depending on the fungus involved. On leaves and stipules early infections of A. pinodes are seen as purplish flecks with irregular margins. On older leaves or under favorable weather conditions the spots may enlarge and cause the leaf to dry up. On stems, A. pinodes produces purplish black lesions which eventually may girdle the stem causing extensive blighting. Petal infection causes the blossoms to fall off. Infected pods usually show a purplish-brown discoloration. Seeds within diseased pods may exhibit discoloration or shrinkage.
A. pinodella lesions are similar in appearance to those of A. pinodes but develop near or just above the soil line, causing so-called foot-rot.
Spots caused by A. pisi on leaves, stems and pods are tan or brown in the centre with a darkened margin. Large stem and pod lesions tend to be sunken. The centres of lesions may be speckled with black fruiting bodies.
Disease Cycle: All three fungi can be seed-borne. Infected seed is the most important means of transmission for A. pisi, which is a weak saprophyte and does not survive long in soil. A. pinodes and A. pinodella are vigorous saprophytes colonizing pea residues both on and below the soil surface. Spores of A. pinodes and A. pinodella are spread by wind and rain splash. Spread of A. pisi is almost entirely by rain splash.
a) Use disease-free or lightly infected seed. Seed infected by Ascochyta species usually has lower germination and poorer plant emergence than healthy seed. This is especially true under environmental conditions such as low temperature and high moisture content. If heavily infected seed must be planted, a chemical seed treatment may be beneficial.
b) Crop rotation – Grow peas no more than once in four years in a given field and avoid planting next to the previous year’s pea fields.
c) Resistance – Some field pea cultivars are more tolerant to the disease complex than others. More research is required in this area.
d) There are foliar spray fungicides registered. More than one application may be necessary, beginning at early bloom.
In a normalized crop year the following information for Ascocyta percentage infection of field pea seed should be used as a guide for your seeding considerations:
0-7% – the seed may not need to be treated with a registered seed treatment.
8-14% – the seed should be treated with a registered seed treatment.
> 14% – the seed should not be planted.