LEAF SPOTS on both surfaces, frequently numerous, occasionally confluent, suborbicular or somewhat linear and parallel to veins, at first pale cinnamon brown, later becoming buff-coloured to pale greyish white, with a dark brown, narrow marginal zone, frequently surrounded by a chlorotic halo.
CAESPITULI amphigenous, hairy, dark brown to blackish, scattered on smaller spots, gregarious on larger ones.
MYCELIUM immersed in host substratum, ramified, composed of abundantly branched, septate, hyaline to subhyaline, 2-4 µm wide hyphae, at first intercellular, then partly intracellular as leaf tissue becomes necrotic.
CONIDIOPHORES (cp) arising from outer cells of stromata, macronematous, more or less erect, robust, flexuous, frequently geniculate, cylindrical, attenuating very slightly toward the obtuse apex, rarely branched, thick-walled, smooth, cicatrized, usually septate, mid to dark brown, often paler distally, up to 250 µm long and 7-12 µm wide.
Conidiophore scars slightly protuberant, thickened, minutely fimbriate, 1-2 µm diam, long, 1.5-2.5 µm thick at the base.
CONIDIA (c) holoblastic, solitary, or, infrequently, with a second produced above the first, straight or slightly curved, oblong to cylindrical, or somewhat ovoid with straight sides or broadly hourglass-shaped, obtuse at each end, densely verruculose, 1-5- (mostly 2-3-) septate, up to 80 µm long (mostly 35-60 µm) x 22 µm wide (mostly 15-18 µm), with a thickened, non or slightly protruding, fimbriate basal hilum.
PLANT HOST AND DISTRIBUTION. The plant hosts of H. gracile are different species of the genus Iris, especially I. germanica L. and its cultivars (McKemy and Morgan-Jones 1990).
Heterosporium gracile has a worldwide distribution (Brooks 1953; McKemy and Morgan-Jones 1990).
NOTES. The teleomorph of H. gracile is Mycosphaerella macrocarpa (Kleb.) Jorst.
Heterosporium gracile is the causal agent of leaf blotch or leaf spots of rhizomatous and bulbous Iris species and cultivars. It has also been reported from other members of the Iridaceae (McKemy and Morgan-Jones 1990). There are reports of its occurrence on Gladiolus x hortulanus L.H. Bailey, Freesia x hybrida L.H. Bailey, and Narcissus species (Amaryllidaceae).
The fungus spreads epidemically in wet weather and may destroy most of the foliage. Heterosporium gracile infects its plant hosts through the stromata, through which the conidiophores subsequently protrude.
McKemy and Morgan-Jones (1990) discussed the taxonomic status of the generic name Heterosporium Klotzsch and accepted Heterosporium as an entity separate from the genus Cladosporium Link sensu stricto.
Brooks F. T. 1953. Plant diseases. Geoffrey Cumberlege. Oxford University Press. London, new York, Toronto.
McKemy J. M., Morgan-Jones G. 1990. Studies in the genus Cladosporium sensu lato. II. Concerning Heterosporium gracile, the causal organism of leaf spot disease of Iris species. Mycotaxon 39, 425-440.