The host range of Mastreviruses is largely confined to the Gramineae, except for bean yellow dwarf and tobacco yellow dwarf viruses. Unlike the whitefly-transmitted geminiviruses, however, grass geminiviruses generally have wide, often overlapping, host ranges. Both annual and perennial grasses as well as cereal crops are infected. In part this reflects the wide feeding range of their insect vectors (Rose, 1978; Damsteegt, 1983). For example, abundant oviposition and nymphal development of C. mbila is supported on several grass spp. in the genera Aegilops, Andropogon, Avena, Bothriochloa, Digitaria, Echinochloa, Eleusine, Hyparrhenia, Panicum, Schizachrium, Sorghastrum, Sorghum, Trichachne, Trichloris, and Zea (Damsteegt, 1983).
DSV was originally isolated from Digitaria sanguinalis (D. setigera; see Pinner & Markham, 1990b) (Dollet et al., 1986). MiSV has been isolated from Miscanthus saccifloris (Yamashita et al., 1985). WDV infects a variety of grasses and most cereal crops including Triticum, Avena, and Hordeum (Vacke, 1972). However, the host ranges of DSV, MiSV, and WDV have not yet been fully explored.
The most detailed host range studies of Mastreviruses have been performed on MSV and CSMV and illustrate the extensive host ranges to be found for Mastreviruses.
In southern Africa, streak disease has been recorded in wild and cultivated grass species in the following tribes: Andropogoneae (Cymbopogon, Imperata, Rottboelia); Eragrosteae (Dactyloctenium, Diplachne, Eleusine, Eragrostis, Leptochloa, Setaria); Paniceae (Digitaria, Panicum, Paspalum); Sporoboleae (Sporobolus); Zoysieae (Tragus); Maydeae (Zea, Euchlaena); Hordeae (Hordeum) and Aveneae (Avena) (Storey, 1925b; Storey & McClean, 1930; McClean, 1947). Each grass variety studied was supposed to have a strain of MSV that was specialized to it ("host-adapted") and either non-virulent or only weakly virulent to other plant species (Storey & McClean, 1930; Bock, 1982).
While for most of these hosts, the identity of the infecting virus or strain of virus has not been rigorously tested, McClean (1947) distinguished between five different viruses. These comprised a severe ("A-type") and a mild ("B-type") strain of MSV, a sugarcane streak virus, a Sporobolus virus, and a virus that was apparently common to Eleusine indica and Paspalum notatum.
Damsteegt (1983) found that for the A-type MSV alone, 54 of 138 grass accessions from around the world were susceptible upon experimental transmission using C. mbila. The susceptible hosts (a total of 33 genera and 54 species) came from the tribes Andropogoneae, Aveneae, Chlorideae, Festuceae, Glycerieae, Hordeae, and Paniceae. Within each genus tested, not all species were susceptible and some of the susceptible species showed a low infection incidence. This was possibly due to plant heterogeneity and/or vector feeding preferences (Damsteegt, 1983). In addition, MSV type-A infected all cereal crops tested (barley, rye, wheat, oats, maize, and rice), as well as several subspecies of Zea mays and various Tripsacum accessions (Damsteegt, 1983).
In Mauritius, natural streak infections have been recorded on Coix, Cenchrus, Brachiaria, Panicum, Paspalum, and Digitaria spp. as well as on Z. mays and Saccharum hybrids (Autrey & Ricaud, 1983). Distinct, host-adapted strains were obtained from these grasses, and Autrey & Ricaud (1983) considered that only the viruses found on Coix lachryma-jobi, Cenchrus echinatus, Brachiaria reptens, and B. eruciformis to be epidemiologically relevant to MSV infection of cultivated maize in Mauritius.
A study has been made of the host ranges of CSMV and related Australian grass geminiviruses (Greber, 1989). Of 25 grasses and cultivated cereals tested, the type strain of CSMV alone infected species from 15 genera, either naturally or experimentally. Oats, barley, wheat, maize, and the grasses Dactyloctenium aegypticum and Leptochloa filiformis were infected by all of the viruses examined (Greber, 1989). Host range and ease of host infection varied greatly between the viruses tested. Microlaena stipoides was an exclusive host for CSMV-M, a strain of CSMV. Bromus catharticus was exclusive for the B. catharticus geminivirus, while Paspalum spp. were readily infected only by isolates of paspalum striate mosaic virus (PSMV) (Greber, 1989).
Even cereal geminiviruses recognized as being distinct from one another and coming from widely different parts of the world often show overlapping host ranges. Thus, WDV, MSV, and CSMV all naturally infect Hordeum vulgare and Avena sativa, the latter being recognized as the principal host of WDV (Lindsten et al., 1980; Damsteegt, 1983; Greber, 1989). Likewise, CSMV, PSMV and MSV all naturally infect Zea mays, the principal host of MSV (Greber, 1989; Damsteegt, 1983). Besides crop plant hosts, CSMV and MSV share a number of wild grass hosts, e.g. Chloris gayana, Eleusine indica, Lolium multiflorum, Paspalum conjugatum, and Setaria italica (Greber, 1989; McClean, 1947; Damsteegt, 1983; Pinner et al., 1988).
Recently, an artificial inoculation method termed "agroinfection" has been developed (Grimsley et al., 1987). Using this technique, whole geminivirus genomes can be cloned into the Ti plasmid of Agrobacterium tumefaciens and inoculated into meristematic host tissue. Apparently normal infection ensues, and normal virus particles can be recovered in quantity (Grimsley et al., 1988). By this means DSV was successfully agroinoculated into maize and A. sativa, as well as into D. setigera (initially identified incorrectly as D. sanguinalis) (Donson et al., 1988), thereby apparently extending its potential host range.