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Alternaria spot

Scientific Name
Alternaria brassicicola
Scientific Author
(Schwein.) Wiltshire
Taxonomy
(Ascomycota: Dothideomycetes: Pleosporales: Pleosporaceae)
Risk
Medium
Status
Exotic Disease Occurence in Thailand
Reliability
High
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Diagnostic Notes

Colonies amphigenous, effused, dark olivaceous brown to dark blackish-brown, velvety. Mycelium immersed, hyphae branched, septate, hyaline at first, later brown or olivaceous brown, intercellular and intracellular, smooth, 1,5-7,5 µm wide. Conidiophores arising singly or in groups of 2-12 or more, emerging through stomata, usually simple, erect or ascending,straight or curved, occasionally geniculate, more or less cylindrical but often slightly swollen at the base, septate, pale to mid olivaceous brown, smooth, up to 70 µm long, 5-8 µm wide. Conidia mostly in chains of up to 20 or more, chains sometimes branched, acropleurogenous, arising through small pores in the conidiophore wall, straight, nearly cylindrical, usually tapering slightly towards the apex or obclavate, the basal cell rounded, the beak usually almost non-existent, the apical cell being moreor less rectangular or resembling a truncated cone, occasionally better developed but then always short and thick, with 1-6 (-11) tranverse septa and usually few, but up to 6, longitudinal septa, often slightly constricted at the septa, pale to dark olivaceous brown, smooth or becoming slightly warted with age, 18-130 µm long, 8-30 µm wide in the broadest part, withthe beak 1/6 the length of the conidium and 6-8 µm wide. (Description taken from Ellis, 1971).


Stable isolates produce intensely brown-black, spreading colonies on PCA and V-8 agars.. Sporulation in stable isolates such as representative E.G.S. 42.002 is almost entirely from abundant, closely adjacent primary conidiophores that arise directly from the agar surface. Most conidiophores are simple, with a single apical conidiogenous site, ca 15-65 x 5-6 µm. Intermixed with these simple conidiophores are others of moderate length, up to ca 100-200 µm, which are variously branched and geniculate with conidiogenous sites at the tip of each branch or bend.

There are essentially no unusually long conidiogenous aerial hyphae in isolates as stable as E.G.S. 42.002 and others from Cruciferae. However, a very stable isolate from Digitalis purpurea (E.G.S. 35.1751) produces abundant long, suberect conidiogenous axes primarily in the light-deprived zones of PCA colonies. These aerial elements are 2-3 times as long as surface conidiophores and have numerous lateral and terminal conidiogenous branches. Conidia produced by this isolate from Digitalis are slightly smaller on average than those of typical A. brassicicola; otherwise they are not distinguishable microscopically from those commonly seen from cruciferous substrates.


Conidia are borne in long chains, usually with the initial few spores strikingly larger than subsequent secondary spores, which tend to decrease in size as the chain lengthens. Young unbranched chains may have 10-20 spore units. Usually, however, multiple branching of primary conidiophores and chains is a dominant character, resulting in loose tufts of 50-60 or more conidia.

Conidia in individual sporulation tufts (on PCA) are narrowly ovoid or ellipsoid in the larger initial units, then progressively broader ovoid, then smaller ovoid in terminal regions of the chain. Initial conidia have a size range of ca 50-60 x 12-17 µm and 6-7 transepta with 0-1 longiseptum in a few segments; intermediate ovoid spores ca 30-40 x 15-18 µm and 4-5 transepta with 0-2 longisepta in a few segments; smaller ovoid spores ca 10-25 x 6-10 µm and 1-3 transepta with no longisepta. No individual conidium has a true beak. The terminal conidium in any chain has a rounded apical cell. All other conidia, in both nature and culture, have an apical conidiogenous structure which usually is a 1-cell secondary conidiophore ca 5 x 5 µm, but which also frequently is a smaller, similar outgrowth lacking a septum that separates it from the spore body.

The mature spore body of conidia of all sizes is a medium yellowish tan that darkens to reddish brown in surface view; the pigment of the apical secondary conidiophore is a dilution of the same color. The spore wall, in edge rather than face view, and septa are of a similar but darker brown. Septa appear somewhat thickened in edge view, but are not typically embellisioid. Practically every transverse septum imposes a moderate degree of constriction on adjoining spore walls. The spore wall itself usually is quite smooth, but sometimes has punctulate or, in age, even more pronounced ornamentation. (Description taken from Simmons, 2007).

More Information
Specimen Contact
Suneerat Seemadua
Author
Seemadua, S., Athipunyakom, P., Beasley, D.R. & Shivas, R.G.
Created
31/01/2011 04:38 AEST
Last Updated
22/02/2011 16:17 AEST
Citation
Seemadua, S., Athipunyakom, P., Beasley, D.R. & Shivas, R.G. (2011) Alternaria spot (Alternaria brassicicola) Updated on 2/22/2011 4:17:39 PM Available online: PaDIL - http://www.padil.gov.au.
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Free for use under the Creative Commons Attribution 3.0 Australia License

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