Fungous diseases of plants . is so slightthat it cannot be considered of much economic importance. The affected areas of the leaf are soconspicuous, however, as to attractthe attention of all interested inparasitic fungi (Fig. 81). Thefungus occurs upon a number ofspecies of Acer, the first evidencesof the spot being manifest by yel-low, thickened areas soon after theleaves have attained full size. Across section shows that beneaththe cuticle there are produced ingreat quantity on short conidio-phores arising from a stromatic tissue unicellular, curved conidia,and these conidia serve to spread
Fungous diseases of plants . is so slightthat it cannot be considered of much economic importance. The affected areas of the leaf are soconspicuous, however, as to attractthe attention of all interested inparasitic fungi (Fig. 81). Thefungus occurs upon a number ofspecies of Acer, the first evidencesof the spot being manifest by yel-low, thickened areas soon after theleaves have attained full size. Across section shows that beneaththe cuticle there are produced ingreat quantity on short conidio-phores arising from a stromatic tissue unicellular, curved conidia,and these conidia serve to spread the fungus, it is believed, duringthe same season. This stage is referred to the form genus Melas-mia. The tough blackened structures, which appear in the affectedspots as the season advances, consist in reality of sclerotioidalmasses of fungous tissue, black without but white within, penetrat-ing all medullary parts of the leaf. These areas are much thickerthan the normal leaf. After the fall of the leaf further growth or. Fic,. 81. The Black Spot of Maple ASCOMYCETES 209 differentiation takes place in the sclerotial areas, so that there isfinally developed by the next spring rather unlimited, complexapothecia, often cm. broad, which rupture by irregular fissuresalong the ridges of the wrinkled surface. The asci are club-shaped,and bear eight needle-shaped spores. Numerous paraphyses withincurved or hooked tips are present. The asci are 120—130 X 9—10 fjL. At maturity the large spores (65-80 x At) are ejectedforcibly from the ascus, doubtless distributed by the wind, andthey are provided with a mucilaginous membrane which, accord-ing to Klebahn, serves for adherence to the host. Artificial infec-tion with ascospores has been effected, and after such infectionthe pycnidial stage may be produced within about eight weeks. Among other common and conspicuous species of Rhytismaof wide distribution are Rhytisma Salicinum (Pers.) Fr. occur-ring on various species of Sa
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