Colonies of Ascochyta fabae on PDA white to ash-white with sparse to abundant pycnidia;reverse cream to light brown. Colonies more yellow on oat agar. Mycelium abundant, velvety, composed of hyaline to yellowish, smooth, branched, septate hyphae. Pycnidia separate partially immersed, yellow to brown, subglobose to globose, 200-250 µm with usually one papillate ostiole. Conidogenous cells hyaline, short subglobose to cylindrical, arising from innermost layer of cells surrounding pycnidial cavity. Conidia hyaline, straight or slightly curved, base slightly truncated or rounded, one- or sometimes two- or three-septate, 16-24 x 3.5-6.0 µm, not constricted at septa.
Symptoms of the disease are easily observed on leaves, stems and pods of plants in the field (see Symptoms). Where confirmation of the disease is required, surface-sterilized tissue from the edges of the lesions should be incubated on PDA to produce pycnidia containing one-septate spores. From older lesions, conidia can be obtained for microscopic examination by scraping the surface of lesions containing pycnidia into water on a glass slide.
If no structures are visible in leaf lesions, incubation of the leaves in a moist chamber overnight may allow distinction from chocolate spot caused by Botrytis fabae, which should produce characteristic conidiophores and conidia in lesions (Koike et al., 2006).
Seedborne infection can be detected by observing the characteristics of the colonies appearing after incubation of surface-sterilized seed on PDA (see Seed Health Tests).
Related invasive species
- Didymella fabae
Related Farm Practice
- Light
Ascochyta blight is the most severe disease of cool-season pulses (Davidson and Kimber, 2007). D. fabae (anamorph: Ascochyta fabae) attacks Vicia faba and can survive and reproduce in and spread from crop debris or be transported in infected seed. Introduction on infected seed occurred in Australia and Canada in the 1970s, and was probably the means for the pathogen’s original spread to countries outside southwestern Asia. Ascospores are disseminated by wind from the debris as primary inoculum and secondary cycles are initiated by conidia spread by rain splash from plant lesions. The fungus is host-specific in causing disease, but may be able to survive in non-host plants and reproduce on their debris. It is not treated as a phytosanitary risk or listed as an invasive pathogen by major organizations. Seed certification is the primary means of preventing its spread to new areas and the importation of new genotypes of the fungus to areas already infested.
Symptoms occur on leaves, stems and pods. Where seedlings have grown from infected seeds, lesions are more obvious on the upper parts of the stem and on the older leaves. Lesions on the leaves are usually circular, dark brown and initially about 1 mm diameter. After a short time, the lesions become larger and slightly sunken with a pale-brown to dark-grey centre surrounded by a broad, dark, chocolate-coloured margin. As the spots enlarge, they become more irregular in shape and coalesce to cover larger areas of the leaf. Some zonation may occur within the necrotic area of the lesions, which may cause confusion with lesions of chocolate spot caused by Botrytis fabae. A more general browning of the vascular tissue of the leaf may occur as the lesions develop. Prominent, dark pycnidia develop within the lesions, particularly as the leaves age or when conditions are moist. The pycnidia can vary in abundance and are sometimes concentrically arranged.
On the stems, the lesions are usually smaller at the early stages of infection, but elongate up the stem and become markedly sunken. Stem lesions are usually darker than those on leaves, and contain scattered pycnidia. When the lesions are deeply sunken, either the stems of the plants may break at the point of infection, causing the plants to lodge or, if infection occurs at an early stage, the stems may bend upwards producing a kink where the stems regrow vertically. At the seedling stage, when infection originates from the seed, the combination of stem and leaf infection may result in the death of the plant.
As the pods develop, lesions can be produced over the surface. They become very deep with dark brown centres containing abundant pycnidia. In damp conditions, the conidial masses produced are pale pink to yellow. Well-developed lesions may penetrate the pod wall and affect seed set or may blemish the developing seeds within the pod. However, seed staining does not always indicate infection by the pathogen, because other saprobic organisms may invade the damaged tissue of the pod. Colonies of D. fabae can also frequently be isolated from unstained seed during routine seed health tests in the laboratory (A Biddle, [address available from CABI], personal observation, 2000).
The fungus is highly specialized to Vicia faba (broad bean or faba bean), and inoculations on other legume species have been largely unsuccessful (Yu, 1947;Sepulveda, 1993), although Gaunt (1983) did find that infections could be obtained under certain specific conditions. Despite numerous attempts to cross-infect other legume species, only a very few reports of limited success have been made. Bondartzeva-Monteverde and Vassilievski (1941) found only atypical lesions formed by a broad bean isolate on species other than bean. Sprague (1929) and Beaumont (1950) found that isolates obtained from broad bean induced symptoms on peas (Pisum sativum), but the conditions and methods under which the tests were made were not clearly defined. Wallen and Galway (1977) were unable to obtain infection of peas in the greenhouse in Canada.
Hernandez-Bello et al. (2006) found species from legumes to be host-specific in causing disease, but the fungi could be isolated from inoculated, but asymptomatic, non-hosts;Ascochyta fabae was isolated from pea and lentil plants. The possibility exists, then, that this pathogen could survive in non-host plants and even reproduce on the debris. Autoclaved chickpea stems have been used to obtain teleomorph production from mated A. fabae isolates (Kaiser et al., 1997).
A. fabae has been reported as infecting Onobrychis viciifolia (sainfoin) in southwest Asia (Sharifnabi and Fatehi, 1996;Eken, 2003). The identifications based on morphology need to be tested by inoculation of isolates onto V. faba.