0uo;Victoria’ (98%) was significantlyhigher than in ‘T11’ (58%), which was higher than‘Opal’ (31%). Results suggested that ‘Victoria’was more susceptible to infection by all 3 viruses than traditionally growncultivars and recently released or experimental cultivars. Planting with elitematerial and roguing of infected plants has been ineffective as a viruscontrol strategy with this cultivar.

The effect of Hop latent virus (HpLV), Hop mosaic virus (HpMV), and Prunus necrotic ringspot virus [PNRSV (apple, A, and intermediate, I, serotypes)], on the survival of softwood cuttings, the vigour (height) of early season growth, cone yield, and the levels of brewing organic acids in mature plants, was assessed in four hop (Humulus lupulus) cultivars in Tasmania, Australia. Virus infections were associated with an increase in the mortality of softwood cuttings following propagation. In all cultivars, height of early growth was a poor indicator of the effect of viruses on cone yield and levels of brewing organic acids (alpha and beta acids). In cv. Nugget, infection by the virus combinations studied was not associated with reductions in cone yield, however plants infected by PNRSV-I, in 2000, had 11% lower alpha acids and 7% lower beta acids. In Opal, infection by HpLV and HpMV were the most deleterious to cone yield, however the effect of HpMV was ameliorated when in combination with PNRSV-I. Reductions in alpha and beta acid content were attributable only to mixed infections of HpLV + HpMV in combination with either serotype of PNRSV. In Pride of Ringwood, yield loss was mostly attributable to HpMV and to a lesser extent, HpLV. Some ameliorations in cone yield loss occurred in plants containing a mixed infection between HpMV and HpLV or either of the ilarvirus serotypes. Both of the ilarviruses and HpMV caused reductions in alpha acid content. In Victoria, cone yield loss was mostly attributable to combinations of viruses such as HpLV + PNRSV-I and HpLV + HpMV. The deleterious effect of HpLV + HpMV was ameliorated by PNRSV-A and to a lesser extent, PNRSV-I. Infection by the virus combinations studied did not significantly affect alpha and beta acid levels in either year. Results suggested the effect of viruses and their combinations differed between cultivars and varied between seasons. This information, when combined with knowledge of the rates of virus re-infection, can be used to recommend control strategies for the Australian hop industry.

In the garden pea (Pisum sativum L.), the mutant allele le blocks the conversion of GAZ0 to GAl (Ingram et al., 1984), reducing the endogenous level of GAl (Ross et al., 1992; Smith et al., 1992). This is responsible for the dwarf phenotype of le plants. It has been suggested that le codes for an altered form of GA20 3B-hydroxylase, the enzyme that catalyzes the step GAZo to GAl (Ross et al., 1989; MacMillan, 1990). Recently, however, Smith (1992) questioned this proposa1 in a model addressing severa1 important aspects of GA biosynthesis in pea. According to this model the apical bud itself does not metabolize GAZo to a significant extent. Instead, it is suggested that GAl production is confined largely to young stem tissue, where it is regulated by an inhibitor of GA2o 30-hydroxylation produced by the apical bud (and possibly by young, expanding leaflets). Smith (1992) proposed that in le plants the inhibitor is constitutively produced, thereby blocking GA1 production. Smiths model is based on the metabolism of [13C,3H]GAz0 by excised leaves, stems, and apical buds. The metabolites were chromatographed by HPLC as the free acids (Smith, 1992). In extracts from apical buds and leaves the level of radioactivity co-eluting with GAl (GA1-like) was very low. This level was higher in extracts from stems, but in a11 cases was not affected by the Le/le gene difference.We have investigated the metabolism of [13C,3H]GAz0 in plants either left intact or with their apical buds removed (decapitated). Furthermore, we have reproduced the experimental conditions used by Smith (1992) and have analyzed the resulting extracts by HPLC as the free acids and methyl esters, and by GC-MS. Our results do not support the in- MATERIALS A N D METHODSThe lines used were the isolines 205+ (Le, tall) and 205-(le, dwarf), developed from cross L2 X L77 at Hobart (Tasmania, Australia). The plants were grown, two per pot, in a heated glasshouse under an 18-h photoperiod (Reid and Potts, 1986). Experiments were conducted when the plants had expanded to five to seven leaves (including the scale leaves).The substrate in a11 experiments was [17-13C,3H~]GA~~ (1.27 X 10" Bq mmol-'; Ingram et al., 1984). Two experimental systems were used. First, substrate (2 r g in 10 PL of ethanol) was applied to the second uppermost fully expanded leaf of 12 tall and 12 dwarf plants. Immediately afterward, six tall and six dwarf plants were decapitated by excising just below the node situated three nodes up from the treated leaf. This left three intemodes and two leaves above the treated leaf in decapitated plants. After 2 d, the uppermost two of these intemodes were harvested together (stem portion), as were the stipules, petiole, and leaflets of the uppermost leaf (leaf portion). Intact plants were harvested by first excising the apical portion as before and then harvesting the next two intemodes down and (separately) the associated leaf, as for decapitated plants. A11 excised portions were then immersed in cold (-20°C) methanol.Second, the technique of Smith ...

Coat protein (CP) sequences of 17 Ilarvirus isolates were obtained from hops at three farms in Tasmania, Australia. Phylogenetic analysis of these sequences and additional database sequences indicated several Apple mosaic virus (ApMV) isolate clusters distinct from Prunus necrotic ringspot virus (PNRSV): one containing isolates from apple; one containing a single isolate from almond; a third containing Australian hop isolates of the 'apple' serotype and a German isolate of unknown origin; and a fourth containing Australian hop isolates of the 'intermediate' serotype. Isolates from hop, pear and prune from the Czech Republic either formed a fifth grouping, or were divergent members of the 'intermediate' serotype group. Deduced amino acid (aa) residue differences between the coat proteins of the two hop isolate serotype groups were highlighted as possible regions of serological differentiation. No evidence for coinfection of plants with both serotypes was found. Tests of ApMV-infected hop buds using the Shirofugen flowering cherry assay revealed a possible differentiation of the two strains based on hypersensitivity. Because of serological similarities to PNRSV, these viruses have commonly been reported as strains of PNRSV. However, this study shows ilarviruses from Australian hops are strains of ApMV, but distinct from those infecting Malus spp.

SummaryThe transmission of Apple mosaic virus (ApMV; hop, H and intermediate, I serotypes) in Australian hop cultivars was assessed in glasshouse and field trials. Under field conditions, the rate of ApMV transmission was halved when contact between neighboring plants was prevented by early season applications of paraquat to restrict basal shoot growth. However, in a separate field trial the presence of root grafts between hop plants, which may contribute to virus transmission, was also suggested. In glasshouse trials, ApMV was transmitted successfully to hop by the mechanical inoculation of infective sap, simulated pruning, foliar contact, and root grafting, but not by root contact. The rate of mechanical transmission of ApMV to the hop cultivar ‘Victoria’ was greater than to other hop cultivars commonly grown in Australia. However, success of mechanical transmission of ApMV also appeared to be influenced by the cultivar from which inoculum was obtained. ApMV was detected throughout the year in all tissues, in chronically infected field grown plants of cultivar ‘Victoria’, suggesting a uniform virus distribution. The reliability of ApMV detection by serology did not decline in ‘Victoria’ plants later in the growing season as occurred in other cultivars.