The effect of barley yellow dwarf virus (BYDV) infection on grain yield of wheat, cv. Olympic, was determined over 3 years at three sites in Victoria in field plots inoculated with infective Rhopalosiphum padi (L.). Inoculation before tillering lowered grain yields by 9-79 %, whereas inoculation at early stem extension lowered yields by only 6-9 %. There was a linear relationship between the percentage of plants infected with BYDV at an early stage of growth and grain yield at all sites. Other components of yield affected included numbers of tillers, numbers of heads with sterile terminal spikelets, grain weight per head and weight of individual grains.

SUMMARY Histochemical tests, light and electron microscopy have been used to investigate the structure of the dormant spore of Gigaspora margarita (Endogonaceae). There is a plug in the neck of the mature spore consisting of a dense matrix of highly osmiophilic material bounded by wall‐like depositions. The complex spore wall is composed of four prominent layers, an inner and outer layer that contain polysaccharide, protein and lipid; a laminated chitinous layer and an ill‐defined‘cementing’layer. The cytoplasm is composed of amorphous material and a variety of granules and particles that form a fine network between large oil droplets. In certain areas the cytoplasm is concentrated and forms two dense regions named the 'synthesizing’and‘nuclear’centres which contain different proportions of various organelles. Two unusual types of organelles found in the cytoplasm of G. margarita spores have been previously described in honey‐coloured, sessile spores of Acaulospora laevis.

A local isolate of ryegrass mosaic potyvirus (RMV) was identified from a pasture in Leongatha, south-east Victoria. The virus was mechanically inoculated to ryegrass and oats but could not be detected by ISEM in inoculated and symptomless wheat, cocksfoot or barley. Symptoms associated with infection varied between a white mosaic and a brown necrosis. The presence or absence of RMV did not significantly affect the total herbage yields of field plots. However, infection with RMV resulted in reductions in dry weight of 5-50% in the ryegrass component of total herbage yields; the reduction in ryegrass was compensated by increased weed growth. The extent of the reductions in ryegrass associated with RMV infection varied with both species and cultivar. Both the host range study and the herbage yields from the field trial showed that the Victorian isolate of RMV differed from isolates reported overseas.

SUMMARY Light and electron microscopy have been used to study the emergence through the spore wall and subsequent growth and development of the germ‐tube of Gigaspora margarita. Spore‐wall penetration by the germ‐tube is unusual as two different mechanisms seem to be operating, one for the inner spore‐wall layers and another for the outer layer. As the germ‐tube develops a primary wall layer is formed, and this can be traced back to its origin as a layer continuous with the thickened region of the innermost spore‐wall layer. In the maturing germ‐tube a highly osmiophilic secondary wall is deposited and this may be the wall layer that persists when the fungus penetrates a host root and sets up a mycorrhizal association. In common with other fungi that have been studied, the ultrastructural organization found in G. margarita can be categorized into approximately three zones: the apical zone, the subapical zone and the zone of vacuolation. Other features of the cytoplasmic organization are also discussed including the possible significance of bacteria‐like organisms, membrane‐bound crystals, osmiophilic granules and glycogen particles. Features of the culture‐grown germ‐tubes are also compared with those of host‐associated stages of the fungus reported in the literature.