Studies on Graminicolous Species of Phyllachora Fckl. II.. Invasion of the host and development of the fungus

Abstract

Infection of grasses by species of Phyllachora Fckl. has been observed, and a detailed examination of the life cycle of two species of this genus has been made on hosts artificially inoculated while growing under glass-house conditions.

Gemiiiatiiig ascospores of P. ischaemi and P. parilis prodced appressoria on the leaves of their respective hosts, Ischaemum australe and Paspalurn orbiculare. From each appressorium an infection peg penetrated into the lumen of an epidermal cell and expanded into a normal hypha. Some branches of this hypha invaded adjacent epidermal cells, thus laying the foundations of the clypeus, while other branches invaded the underlying mesophyll cells. At first all hyphae were intracellular and passed from cell to cell by means of fine infection hyphae produced by appressorium-like swellings of the hyphae appressed to the cell wall. Intercellular mycelium was found at a later stage when hyphae were forming perithecium initials.

The observation that the clypeus developed independently of the perithecium dispels some existing confusion about its origin. The clypeus developed in the epidermal cells of the host and not as an outgrowth of the ostiolar region of the perithecium.

The perithecium initial developed deep in the mesophyll, and in the case of Phyllachora parilis was preceded by the formation of a subclypeal pycnidium containing filiform spores. In each case, the perithecium expanded until its ostiolar region came into close contact with the clypeus. The ostiole then developed right through the ciypeus, and its development is believed to be lysigenous. The mouth of the ostiole remained closed by a membrane which appeared to be the undissolved cuticle.

It was noted that asci of all species examined possessed an ascus crown, a structure not previously observed in species of this genus.

It has been found that the anatomy of the host can determine the form of some structures of Phyllachora spp. Clypeus thickness is governed by the size of the epidermal cells, while its radial expansion is checked by the mechanical tissue associated with vascular bundles. Similarly, perithecium size and shape are influenced by the amount of mechanical tissue in a leaf.

The time for P. ischaemi to complete its life cycle was influenced by seasonal conditions. Colonies arising from infections in April 1961 discharged ascospores in 32 days, whereas infections made 1 month later did not produce sporulating colonies until 54-58 days later. The full life cycle of P. parilis took 62-77 days when inoculations were made in May 196 1.