Apple mosaic virus (ApMV; family Bromoviridae, genus Ilarvirus) is one of the oldest and most economically important viruses of apples (Malus × domestica Borkh.) (1,3). Yield losses may vary from negligible to as much as 50%, depending on the affected cultivar. Although ApMV is found worldwide and occurs naturally in more than 65 plant species (1), it has not been reported to occur in Alaska. In July 2011, noticeably bright yellow mosaic leaves were observed on apple ‘Valentine’ and its rootstalk ‘Ranetka’ from an apple orchard in Wasilla, AK. Leaves were collected and assayed by reverse transcription (RT)-PCR using ApMV-specific primers (2) and total RNA extracted with buffer modifications to RNeasy Plant Mini Kit (Qiagen, Valencia, CA). Briefly, 50 mg of leaf tissue was ground in liquid nitrogen and 450 μl of SE buffer (0.14 M NaCl, 2 mM KCl, 2 mM KH2PO4, 8 mM Na2HPO4·2H2O [pH 7.4], 0.05% vol/vol Tween-20, 2% wt/vol polyvinylpyrrolidone 40, 0.2% wt/vol ovalbumin, 0.5% wt/vol bovine serum albumin, and 0.05% wt/vol sodium azide) was added, and after vigorous vortexing, 80 μl of the mixture was added to 400 μl of RLT buffer supplied by the kit and then processed as directed by the manufacturer (4). Direct sequencing of the predicted ~260-bp PCR product resulted in 97 to 98% nucleotide identities to ApMV accessions in GenBank when analyzed by BLAST. To determine the distribution and incidence of infection in the Wasilla orchard, all 118 apple trees (99 cultivars) were then sampled and assayed serologically by double-antibody sandwich-ELISA with ApMV antiserum according to the manufacturer's protocol (Agdia, Inc., Elkhart, IN). Apple ‘Geneva Early’ and the same ‘Valentine’ tree and its rootstock tested positive for ApMV by ELISA and RT-PCR. Strong diagnostic ApMV symptoms were not apparent on the infected ‘Geneva Early’, which is typical for most commercially grown apples. No leaves were available on the ‘Ranetka' rootstock of ApMV-infected ‘Geneva Early’ for virus indexing. An additional 21 apple trees with no symptoms from an orchard in Talkeetna, AK tested negative to ApMV by ELISA. Limited natural spread of ApMV to other plants may be by pollen and seed transmission. The most prevalent mode of transmission is from ApMV-infected rootstock and grafts. It is important to obtain new propagation plant material from certified virus tested nurseries and to avoid grafting plant material containing ApMV. To my knowledge, this is the first report of ApMV in Alaska.
References: (1) R. W. Fulton. No. 83. CMI/AAB Descriptions of Plant Viruses. 1972. (2) W. Menzel et al. J. Virol. Methods 99:81, 2002. (3) M. J. Roossinck et al. Virus Taxonomy. Eight Report of the International Committee on Taxonomy of Viruses, 1049, 2005. (4) J. Thompson et al. J. Virol. Methods 111:85, 2003.