A Further Study of the Morphology and Life History of the Rose Black Spot Fungus

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Abstract
Summary A description of the main features to be noted in a study of Diplocarpon on rose would include the following. Infection is most noticeable as black spots on the leaves, though the canes of certain varieties are subject to attack. Infection occurs directly through the cuticle on either side of the leaf. The superficial, primary mycelium is subcuticular and is composed of colorless uninucleate cells, the hyphae forming a network and tending to be associated in fascicles composed of several parallel filaments radiating from the point of infection. The internal mycelium is intercellular. Haustoria, especially conspicuous in epidermal and palisade cells, are simple uninucleate structures, usually with a conspicuous thick cup-shaped stalk and a sheath sometimes fairly distinct. The black spot is due to disorganization of host cells and appears one or two weeks after inoculation. Summer acervuli, usually on the upper side of the leaf, occasionally on the lower, are subcuticular and contain two-celled colorless conidia arising from short inconspicuous cells of the thin basal stroma. The conidia, 18–25 × 5–6 μ, are usually constricted at the septum and germinate readily in water or on agar media. Spermogonia or microacervuli develop on the black spots on old leaves in March and April. They are subcuticular and usually on the upper side of the leaf. The spermatia are uninucleate, 2–3 μ long, and resemble very small spores. They are cut off from two-celled stalks simulating the conidia except that they are smaller and taper upward. Sometimes normal two-celled conidia are present in spermogonia. Internal deep-seated acervuli develop on old leaves early in April in a stroma located between the upper epidermis and the palisade layer. They are capped by a mass of thick-walled brown cells. They may rarely develop in the spongy parenchyma on the lower side of the leaf. The first conidiophores are filamentous and usually three or four cells long, the upper two cells being cut off as a true two-celled conidium. In old fruit bodies the conidia have a single stalk cell or arise directly from the sporogenous tissue cells, two or three spores often arising from the same cell, without having individual stalks. The conidia are colorless, 20–25 × 5–6 μ, the upper cell usually thicker, not constricted noticeably at the septum. They are extruded in a whitish mass as the fruit body breaks open. The conidia germinate readily in water or on agar media.