Ascochyta Blight

 - Written by Robert Harveson, Extension Plant Pathologist

Pathogen

Fungal structures: pycnidia, conidia, ascospores, and pseudothecia. Ascochyta blight is caused by the fungus Ascochyta rabiei. Ascochyta blight is considered to be the most important disease of chickpeas worldwide. The pathogen attacks all aerial portions of the plant, including leaves, stems, and pods. It is specific to chickpeas and will not carry over to other crops used in rotation. The pathogen survives in infected residue for at least 2 years under dry conditions. Viability of the pathogen is lost rapidly under conditions of high humidity (65-100% RH). The fungus is seed borne, which serves as the most important source of inoculum for initiating new infections, and is the primary mechanism for its spread throughout the world. Between 50 and 80% of seeds from infected pods can be infected by the pathogen. Superficial and deep-seated, internal infections are possible; both are equally capable of transmitting A. rabiei to developing plants. The pathogen remains viable in seed for at least 5 years. The pathogen has both sexual (ascospores formed in pseudothecia) and asexual (conidia formed in pycnidia) life cycles but the presence and disease role of the sexual cycle in Nebraska is unknown.

Seed Borne

Disease Symptoms

Symptoms consist of circular lesions on leaves and pods and elongate lesions on stems. As disease progresses, pycnidia are formed and appear embedded in dark brown lesions arranged in concentric circles like a target board. Early in the epidemic, the disease can be recognized by individual infected plants that turn yellow and die. These diseased plants are a result of planting infected seeds. They serve as foci from which the pathogen spreads to other plants in the field if not halted by either environmental conditions or disease management techniques. When comparing infected vs. uninfected plants, it has been shown that yield reductions up to 50% are common. The reductions result from lower numbers of pods produced per plant and therefore, lower total seed yields. Even if seeds produced in infected pods are not infected, they are still greatly reduced in size compared to seeds from uninfected pods. This affects profits negatively because growers are paid premiums for large seed sizes.

Circular Lesions	Circular Lesions on Pods
Elongate Lesions	Concentric circles
Individual Infected Plants	Reduced Size

Favorable Environmental Conditions

Disease development is most rapid at temperatures between 68°-70° F, and high relative humidity. No blight develops at temperatures outside the range of 40° and 90° F, or leaf wetness periods of less than 6 hours. Spores (conidia) are released from pycnidia during humid weather and are spread locally throughout fields by rain splashing and high winds. Spores germinate 12 hours after landing on host surfaces, and under optimum conditions, will penetrate and cause new infections within 24 hours. Spread from these types of spores (asexual conidia) is primarily limited to individual fields during a season. However, the formation of the sexual ascospores has been shown to spread the pathogen in wind currents to new fields at distances up to 10 miles away.

Management

Genetic Resistance

Genetic resistance can be useful, however there are no cultivars available with season-long resistance. Most resistance begins to break down shortly after flowering and pod formation. Alternative measures should be considered if conditions favor disease development after this time.

Cultural Practices

No single management tool will be successful. This disease is most effectively managed with the integration of several different strategies. Since only chickpeas are susceptible to A. rabiei, several cultural practices such as rotation with non-host crops, not growing chickpeas more frequently than every 3-4 years, and not planting new crops near previous blighted fields will all help to reduce inoculum levels and inhibit severe epidemics. Tillage practices like burial of infected residue and controlling volunteer chickpeas will also be beneficial.

Chemical / Biological Control

The fungicides Bravo, Headline, and Quadris reduce losses by allowing new growth to occur, but their use is not economical if disease pressure is low. The strobilurin fungicides Headline and Quadris are superior to Bravo, due to the greater residual activity, but many populations of the pathogen has recently been shown to have developed resistance to strobilurin fungicides. During the severe epidemic of Ascochyta blight in Nebraska in 2001, catastrophic losses were averted with the use of fungicides, yet reductions of 15-20% were still estimated from treated fields.

Links

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