Ascochyta blight is a widespread fungal disease that impacts various plants, particularly pulse crops. It is caused by several species within the Ascochyta genus and related fungi, including those affecting chickpeas, faba beans, and field peas. Each specific strain of the fungus affects only its particular host crop. The disease can lead to significant yield losses and reduced crop quality in susceptible plants.
Identifying Ascochyta Blight
Identifying Ascochyta blight involves recognizing specific symptoms that appear on different parts of the plant. On leaves, initial symptoms appear as small, pale green-yellow patches or water-soaked spots. These spots expand into circular or irregular lesions, with a dark brown margin and a pale grey or tan sunken center. A distinguishing feature is the presence of small, black, pinhead-sized fruiting bodies called pycnidia, which form in concentric rings within these lesions.
The disease also affects stems, where lesions appear as oval-shaped or elongated brown to black cankers. These stem lesions can weaken the plant, potentially causing girdling or breakage of the stem. On pods, the spots are similar to those found on leaves, as round, sunken areas with pale centers and dark margins. Pod infection can lead to poor seed set, as well as discoloration and shrinkage of the seeds themselves.
Common Host Plants
Ascochyta blight is a concern for several pulse crops. Chickpeas are among the most frequently affected hosts, with Ascochyta rabiei being the primary pathogen for this crop. The disease also affects lentils, field peas, and faba beans.
While pulse crops are the main focus, the disease can also occur in other plants. For instance, some Ascochyta species affect turfgrass and various other grass species. Ascochyta rabiei has also shown pathogenicity on cowpea and common bean.
The Disease Cycle and Spread
The fungus responsible for Ascochyta blight persists between growing seasons primarily in infected crop debris left in the field, on infected seeds, or in volunteer plants. The mycelium and pycnidia can survive in host residue for several years. When infected seeds are sown, they can lead to early disease establishment in emerging seedlings.
Primary infection occurs when spores are released from these overwintering pycnidia. These spores are dispersed by rain splash, which can carry them from infected debris or lower leaves to new, healthy plant tissue. Wind can also spread airborne spores over longer distances. Cool, wet, and humid conditions, especially with prolonged leaf wetness, are favorable for disease development and spore production.
Management and Prevention Strategies
Managing Ascochyta blight involves a combination of cultural practices and chemical controls to reduce disease pressure. Implementing a crop rotation of at least three years with non-host crops, such as cereals, helps break the disease cycle and reduces fungal inoculum in the soil and stubble. Removing or burying infected crop debris after harvest can also minimize the carryover of the fungus into the next season. Proper plant spacing in fields promotes better air circulation, which helps to reduce humidity within the plant canopy and can slow disease spread. Avoiding overhead irrigation can also limit leaf wetness, further discouraging fungal development.
Using certified disease-free seed is a primary prevention strategy, as infected seeds are a significant source of initial inoculum. Planting resistant or more tolerant varieties, when available, can minimize disease severity and maximize yields. Seed treatments with fungicides are also recommended to protect seedlings from seed-borne infection and soil organisms.
Fungicides are a common tool in commercial agriculture for managing Ascochyta blight, often applied preventatively before rainfall events or after the first signs of disease. Many foliar fungicides are effective. It is important to rotate fungicides with different modes of action to prevent the development of fungicide resistance in the pathogen population.