T. A. Mekuria, Department of Plant Pathology, Washington State University, IAREC, Prosser, 99350;
T. J. Smith, Washington State University County Extension, Wenatchee, 98801;
E. Beers, Department of Entomology, Washington State University, TFREC, Wenatchee, 98801;
G. W. Watson, Plant Pest Diagnostic Center, California Department of Food and Agriculture, Sacramento, 95832; and
K. C. Eastwell, Department of Plant Pathology, Washington State University, IAREC, Prosser, 99350
Little cherry virus 2 (LChV2; genus Ampelovirus, family Closteroviridae) is associated with Little Cherry Disease (LCD), one of the most economically destructive diseases of sweet cherry (Prunus avium (L.)) in North America (1). Since 2010, incidence of LCD associated with LChV2 confirmed by reverse transcription (RT)-PCR assays has increased in orchards of Washington State. LChV2 was known to be transmitted by the apple mealybug (Phenacoccus aceris (Signoret)) (3). However, the introduction of Allotropus utilis, a parasitoid platygastrid wasp (2) for biological control, contributed to keeping insect populations below the economic threshhold. In recent years, the population of grape mealybug (Pseudococcus maritimus (Ehrhorn)) increased in cherry orchards of Washington State (Beers, personal observation). Since grape mealybug is reported to transmit Grapevine leafroll associated virus 3 (Ampelovirus) in grapevine (4), this study investigated whether this insect would also transmit LChV2. A colony of grape mealybugs on Myrobalan plum (Prunus cerasifera Ehrh.) trees was identified visually and morphologically from slide mounts. In a growth chamber, first and second instar crawlers were fed on fresh cut shoots of sweet cherry infected with a North American strain (LC5) of LChV2. After an acquisition period of 7 days, 50 crawlers were transferred to each young potted sweet cherry trees, cv. Bing, confirmed free from LChV2 by RT-PCR. This process was repeated in two trials to yield a total of 21 potted trees exposed to grape mealybug. One additional tree was left uninfested as a negative control. After 1 week, the trees were treated with pesticide to eliminate the mealybugs. Two to four months after the inoculation period, leaves were collected from each of the recipient trees and tested by RT-PCR for the presence of LChV2. To reduce the possibility of virus contamination from residual mealybug debris on leaf surfaces, the trees were allowed to defoliate naturally. After a 3-month dormant period, the new foliage that emerged was then tested. Two sets of primers: LC26L (GCAGTACGTTCGATAAGAG) and LC26R (AACCACTTGATAGTGTCCT) (1); and LC2.13007F (GTTCGAAAGTGTTTCTTGA) and LC2.14545R (CATTATYTTACTAATGGTATGAC) (this study) were used to amplify a partial segment of the replicase gene (409 bp) and the complete (1,080 bp) coat protein gene of LChV2, respectively. Of 21 trees tested, 18 yielded positive results for LChV2. The reaction products from six randomly selected trees were cloned and the virus identity was verified by sequencing. The sequences of RT-PCR amplicons from both primer pairs showed ≥99% identity to LChV2, strain LC5 (GenBank Accession No. AF416335). The result confirmed that P. maritimus transmits LChV2, a significant finding for this cherry production region. Grape mealybug is of increasing concern in the tree fruit industry because it is difficult to control in established orchards. The presence of infested orchards that serve as reservoirs of both LCD and this insect vector present a challenge for management. To the best of our knowledge this is the first report to show transmission of LChV2 by grape mealybug.
References: (1) K. C. Eastwell and M. G. Bernardy. Phytopathology 91:268, 2001. (2) C. F. W. Muesbeck. Can Entomol. 71:158, 1939. (3) J. R. D. Raine et al. Can. J. Plant Pathol. 8:6, 1986. (4) R. Sforza et al. Eur. J. Plant Pathol. 109:975, 2003.