Surface hyphae dark-brown, septate, branched, shining, 14-20 µm broad, developing dark shiny appressoria at tips. Appressoria 250-300 µm, composed of several short branches. Intercellular hyphae pale-brown, branched, width varying. Ascomata pseudothecial, superficial or somewhat erumpent, black, setose, ostiolate, subglobose to obpyriform, uniloculate, 300-450 x 400-800 µ m. Wall 60-90 µ m thick, composed of four to six layers of oval to oblong cells, outer layers dark-brown to black. Setae septate, dark-brown, straight or flexuous, apex pale-brown. Asci cylindrical, short-stipitate, bitunicate, eight-spored, 230-300 x 24-30 µ m. Ascospores uniseriate, ellipsoidal to fusiform, pale-greenish, one-septate, 40-70 x 16-22 µ m, surrounded by a thin gelatinous sheath. Pseudoparaphyses hyaline, filiform, branched. In culture at 20°C, optimum for growth on potato dextrose agar, mycelium black with radiating hyphae and short aerial growth. For additional details, see Walker and Stovold (1986).
Look for a web of dark mycelium with large branched appressoria on the surface of stems, leaves and fruits. Infected soyabean (Glycine max) seed is small and light in weight (Walker and Stovold, 1986).
Related invasive species
- Arkoola nigra
Related Farm Practice
- Soil
A. nigra is an ascomycete known only from certain parts of Australia. It was identified after causing substantial losses on soyabeans (Glycine max) as cultivation of that crop was expanded, but its pathogenicity was found to extend to other wild and cultivated legumes, some strictly Australian and others native or naturalized in many other parts of the world (Walker and Stovold, 1986). There was evidence that it could attack non-legumes as well. The fungus was observed to survive in and on soyabean crop debris, and was capable of infecting seeds, but its natural source and life cycle are unknown. Although the fungus is unlikely to be carried by any crop materials exported from Australia, the significant disease it can cause under humid conditions must make it a concern for accidental introduction to other regions.
A. nigra produces an extensive, branching, dark-brown to black mycelium on the surface of soyabean (Glycine max) leaves, branches and pods. Circular to oval leaf spots, 1-10 mm in diameter, are grey to brown with a distinct thin, dark margin. Occurring on either side of the leaf, spots may coalesce and cause the leaf to tear easily, or a 'shot-hole' effect may appear when the centres of the spots fall out. If disease is severe, yellowing develops around and between the lesions. On pods and stems, the spots are oval-elongate and darker. Pods can be shrunken, containing discoloured seed below the lesion. Webs of dark mycelium can also appear on fallen parts and crop residue, as well as on the soil under and around them.
For additional details, see Walker and Stovold (1986) and Moore et al. (2006).
The fungus has only been observed on soyabeans (Glycine max) in the field, with the exception that one plant of Sida rhombifolia, a pan-tropic weed (USDA-ARS, 2009), was found to be infected (Walker and Stovold, 1986). Some of the other legumes (Fabaceae) in New South Wales, Australia were tested by inoculation. Most of these became infected, including both native and introduced species. Crotalaria pallida, an introduced forage legume, was heavily infected, as were the native species Kennedia rubicunda, Indigofera hirsuta and Vigna lanceolata (Walker and Stovold, 1986). The last two species have a pan-tropical distribution in the wild or under cultivation (USDA-ARS, 2009). Some native Glycine species were moderately affected (Walker and Stovold, 1986).