Diseases

Alternaria spp. (A. alternata [Fr.] Keissler, A. tenuissima [Kunze ex Nees et T. Nees: Fries] Wiltshire, A. infectoria Simmons, and other) . Black Point, Kernel Blight, Alternaria Glume Mold of Barley

Systematic position.

Deuteromycota, class Hyphomycetes, order Hyphomycetales, family Dematiaceae, genus Alternaria (teleomorph . Ascomycota, subdivision Pezizomycotina, class Dothideomycetes, order Pleosporales, family Pleosporaceae, genus Lewia).

Biological group.

Hemibiotroph.

Morphology and biology.

Alternaria species are considered to cause the Blackpoint of barley. This disease is characterized by brown discoloration of kernel coat at the germ end. Nevertheless, the pathogens can often be asymptotic inhabitants of seeds. Primary conidiophores of Alternaria spp. are olive, simple, sometimes branched; with 1-6 condiogenous loci; about 10-125 x 3 mkm; solitary or aggregated in small groups. Usually conidia are arranged in simple or branched chains. Conidia are light olive to dark brown, ellipsoid, ovoid, or obclavate (when a secondary conidiophore arises on a top of conidia); with 3-10 transverse and 1-5 longitudinal septae. Growing on natural substrata, conidia (conidial bodies) are 30-75 x 10-16 mkm, in culture they are 20-40 x 8-12 mkm in size. The use of artificial nutrient media allows to distinguish different species by three-dimensional sporulation pattern. A. alternata has relatively short and abundantly branched conidia chains. Branching is conspicuous in colonies after 2-3 days of incubation. Branches are usually produced due to lateral secondary conidiophores. Chains of A. infectoria are also branched, conidia often have long apical secondary conidiophores (20-60 mkm), with a few conidiogenous loci. Young chains of A. tenuissima are simple and long (8-15 and more conidia); 1-week-old chains can form several branches.

Distribution.

The Black Point of barley kernels is widespread and very common. A. tenuissima is the most usual Alternaria species on barley and other plants in different parts of the former USSR. A. alternata is also ubiquitous but rear pathogen. A. infectoria and allied species are prevalent in the European part of Russia, in Western Siberia, but they have not been found in the Far East.

Ecology.

The species are parasites of leaves and seeds or more rarely of stems and roots of plants from different families. Optimal conditions are high relative air humidity and temperature about 20-25.C.

Economic significance.

The prevalent opinion is that kernel infection by Alternaria spp. does not cause serious yield losses and does not damage quality of seeds and flour. Many Alternaria species can contaminate yield (including grain) by their metabolites that are toxic for plants, animals and human. The most dangerous species are A. alternata and A. tenuissima because of their ability to produce alternariol, tenuazonic acid, and other toxins, while there are no trustworthy cases of finding any known toxins in A. infectoria or allied species. Alternaria leaf blight may obviously reduce the yield. However, Alternaria species infect usually old leaves at the end of season or they accompany pathogens that are more aggressive.

Related references:

Gannibal Ph.B. 2004. Small-spored species of the genus Alternaria on grasses. Micologiya i fitopatologiya [Mycology and Phytopathology] 38(3): 19-28 (in Russian).
Rotem J. 1994. The genus Alternaria. Biology, epidemiology and pathogenicity. St. Paul: APS Press. 326 p.
Simmons E.G. 1967. Typification of Alternaria, Stemphylium, and Ulocladium. Mycology 59: 67-92.
Simmons E.G. 1986. Alternaria themes and variations (22.26). Mycotaxon 25(1): 287-308.
Simmons E.G. 1990. Alternaria themes and variations (27.53). Mycotaxon 37: 79-119.

© Gannibal Ph.B.