Microscopy studies of powdery mildew on summer squash

Powdery mildews are characterized by the appearance of spots or patches of a white to grayish, powdery, mildewy growth on plant tissues, entire leaves or other organs. Ervsiphe cichoracearum, the powdery mildew of cucurbits is among the most serious parasites, and the most common. The conidia are formed similar to the process described for Ervsiphe graminis by Cole and Samson. Theconidial chains mature basipetally from a short, conidiophore mother-cell at the base of the fertile hypha which arises holoblastically from the conidiophore. During early development it probably elongates by polar-tip growth like a vegetative hypha. A septum forms just above the conidiophore apex. Additional septa develop in acropetal succession. However, the conidia of E. cichoracearum are more doliform than condia from E. graminis. The purpose of these investigations was to use scanning electron microscopy (SEM) to demonstrate the nature of hyphal growth and conidial formation of E. cichoracearum on field-grown squash leaves.

New species of Stephanoascus with Sporothrix anamorphs

Two new folicolous fungal epiphytes, Stephanoascus flocculosus and Stephanoascus rugulosus, with distinctive conidial anamorphs, Sporothrix flocculosa and Sporothrix rugulosa, respectively, are described and illustrated. Both Stephanoascus species have two-celled asci and galeate ascospores. These two species are distinguished from previously described Stephanoascus species by asci with long, cylindrical appendages. Their anamorphs can be distinguished by colony morphology and conidial characteristics. Sporothrix flocculosa produces powdery to somewhat tufted to plumose colonies on malt–yeast extract agar, while colonies of S. rugulosa are yeast-like and wrinkled. Sporothrix flocculosa produces subglobose to obovoid, rough-walled secondary conidia, while S. rugulosa produces ellipsoidal to narrowly obovoid secondary conidia that are smooth or rarely slightly warted. The conidiogenous cell of S. flocculosa is produced directly on the vegetative hypha; the conidiogenous cell of S. rugulosa is produced on a conical hyphal projection.

THE DEVELOPMENT OF BASAL BODIES AND FLAGELLA IN ALLOMYCES ARBUSCULUS

The development of basal bodies and flagella in the water mold Allomyces arbusculus has been studied with the electron microscope. A small pre-existing centriole, about 160 mµ in length, was found in an inpocketing of the nuclear membrane in the vegetative hypha. Thus, formation of a basal body does not occur de novo. When the hyphal tip started to differentiate into gametangia, the centrioles were found to exist in pairs. One of the members of the pair then grew distally to more than three times its original length, whereas the other remained the same size. The larger centriole would correspond to the basal body of a future gamete. Gametogenesis was usually induced by transferring a "ripe" culture to distilled water. Shortly after this was done, a few vesicles were pinched off from the cell membrane of the gametangium and came in contact with the basal body. Apparently, they fused and formed a large primary vesicle. The flagellum then started to grow by invaginating into it. Flagellar fibers were evident from the very beginning. As the flagellum grew so did the vesicle by fusion with secondary vesicles, thus coming to form the flagellar sheath. The different stages of flagellar morphogenesis are described and the possible interrelationships with other processes are discussed.

ASCOCARPIC DEVELOPMENT IN DIPOROTHECA RHIZOPHILA

In Diporotheca rhizophila Gordon & Shaw upright unicellular perithecial initials are produced on the superficial mycelium covering the host's roots. By septation the initial becomes a three-celled primordium. Two cells, produced laterally from the penultimate cell of the primordium, by division in two planes, form the perithecial wall. During final maturation two to three divisions in the third plane make the wall three to five cells thick. The centrum originates from the apical cell of the primordium. Dikaryotization of an immature perithecium involves the production of one to many hyaline receptive hyphae from the dark, pigmented, peridial cells. These receptive hyphae fuse with superficial vegetative hyphae, and a nucleus from a cell of the vegetative hypha apparently migrates into the centrum. No structure within the centrum of the perithecium could be interpreted as an ascogonium. The same hyaline, pseudoparenchymatous filaments which form the ascal generative cells in the basal and central areas of the centrum give rise to periphyses within the perithecial neck.