Ultrastructure of Conidia of the Plant Pathogenic Fungus Entomosporium Mespili

Entomosporium mespili has emerged as a significant pathogen of red tip (Photinia × fraseri), a popular and widely grown ornamental in the southeastern United States. This fungal pathogen produces its distinctive multi-celled, insect-like asexual spores or conidia (Fig. 1) in structures known as acervuli (Fig. 2) that rupture the surfaces of infected leaves. This study examines the fine structure of these conidia using a combination of scanning and transmission electron microscopy. In the case of transmission electron microscopy, conidia were prepared for study using either plunge freezing or high pressure freezing followed by freeze substitution.Each mature conidium of E. mespili consists of four to six cells (Fig. 1). These include one apical cell and one basal cell and two to four small lateral cells attached to the basal cell. The apical and lateral cells each possess a long, slender appendage. Excluding these appendages, the length of a mature conidium is usually 20-24μm while the diameters of the apical and basal cells are 8-11μm and 6-9μm respectively.

The Alternaria brassicae – Nectria inventa host–parasite interface

The Verticillium state of Nectria inventa is a destructive parasite of Alternaria brassicae. Tropic growth of parasite hyphae towards hyphae and conidia of A. brassicae occurs in the vicinity of the host. Upon contact, the parasite hyphae often form appressorium-like bodies on the host cells and produce fibrous adhesive material at the host–parasite interface. Conidia are penetrated more commonly than hyphae. Penetration of the septa in hyphae results in a separation of cells. Penetration of a mature conidium also occurs commonly at a septum. The presence of a large hole in the wall of the host cell and the meshwork of material at the penetration site suggest that enzymatic breakdown of host cell wall occurs. Juvenile conidia are penetrated usually at the basal pore.

THE DEVELOPMENT AND STRUCTURE OF THE CONIDIA OF ERYSIPHE POLYGONI DC. AND THEIR GERMINATION AT LOW HUMIDITY

The germination of the conidia of Erysiphe Polygoni DC. takes place through a range of relative humidity from approximately zero to 100% and, therefore, independently of the moisture content of the surrounding atmosphere. In germinating thus, they differ from the spores of some other erysiphaceous fungi and of non-erysiphaceous fungi in general.In E. Polygoni, the conidium is cut off from the conidiophore by a ring of wall material which is added to inwardly until a perforate disk is formed. Later, the pore is closed and the mature conidium remains attached to its conidiophore only by a minute papilla. The conidia have never been observed to germinate in situ, and they are passively discharged.The conidium wall is relatively impervious to water, stain passing into the spore only at the papillate end. Assuming the wall to be relatively impervious to gases also, an explanation is offered for the mechanism of germination of the conidia when they are dislodged from their conidiophores and allowed to fall on dry slides. The papilla provides a permeable spot in the spore wall. It is not exposed until after the spore has been detached. Upon exposure to air, the papilla allows carbon dioxide to pass out from the protoplast and oxygen to pass in, causing respiration and other germination processes to begin.Evidence in support of this suggestion is presented. When freshly detached conidia were held in an atmosphere containing 10% carbon dioxide, germination was checked. These conidia germinated perfectly when removed from the carbon dioxide. Germination was similarly checked by holding the spores in an atmosphere of nitrogen.No shrinkage of the conidia during germination was observed, but shrivelling and collapse take place when death is imminent.