Cardamom mosaic virus

The virions belonging to Macluravirus genus, are non-enveloped, flexuous and, filamentous with size measuring 650-660 nm lengthwise and 10-12 nm diameter and pinwheel type inclusion bodies relating it to the family of potyviruses[8] . The CdMV genomes are single-stranded linear, positive-sense RNA as reported by Jacob and Usha (2001)[3]. As a result of amplification, cloning, sequencing and gene expression, it can be reported that the CdMV contains ~8.5kb of genome, presence coding regions for partial Cytoplasmic Inclusion, CdMV encoded proteins: complete Nuclear Inclusion b (NIb) gene in the genome which is the RNA dependent RNA polymerase (RdRp), Nuclear Inclusion a (NIa) Protease (Nia 1 protease) and a complete 6K2 Viral Protein Genome-Linked. The coat protein of potyvirus is a suitable carrier to display the epitopes of the pathogens. The sequencing of coat proteins and 3’ UTR regions of different isolated strains of CdMV from different Cardamom growing geographical regions in India revealed that there are three different strains causing the CdMV disease on the basis of severity of symptoms on young and matured leaves, transmission efficiency, effect on plant height and leaf area[9]. The strains varied in the N-terminal region of coat protein in their sequence.

There is a long association between the infection of Macluravirus and spice crops. They have a narrow host range infecting families Amaranthaceae, Asteraceae, Iridaceae, Dioscoreaceae, Amaryllidaceae, Moraceae, Aizoaceae, Ranunculaceae and Zingiberaceae. For CdMV, the natural hosts are small cardamom (Elettaria cardamomum) along with their other hosts that belong to the Zingiberaceae family-Amomum connecarpum, A. invollucraltum, A. microstephanum, A. muricatum, A. pterocarpum, A. subulatum, Alpinia neutans, Alpinia mutica, Curcuma neilgherrensis, Hedychium flavescens, Zingiber cernuum, and Maranta arundinacea of Marantaceae family[10]

The expression of symptoms associated with the disease varies according to the varieties grown, growing region and strain variations; these include mild to severe mosaic, overall chlorosis, chlorotic flecks and necrotic and ring spots. The virus infection is systemic in nature that gradually spreads to all tillers in a clump and can infect plants of all stages.

The first visible symptom of the disease is seen on the youngest leaves of the affected tiller that appear as spindle shaped slender chlorotic flecks (2-5 mm in length). The flecks later develop into pale green discontinuous stripes that run parallel to the vein form the midrib to leaf margin. The mosaic symptoms are masked as the leaves mature. The next emerging leaves of the infected plant will show the characteristic mosaic symptoms over the entire lamina. The mosaic type mottling is often seen on the leaf sheaths and young shoots. The mature leaves formed before infection do not develop symptoms. As the disease advances, the size of the leaves eventually creating a negative effect in the plant's vigor and causing stunting of the clump with a few slender tillers and shorter panicles. The plants infected by mosaic or Katte can survive for many years and act as the source of inoculum.[9][11]

Virus Transmission and Spread

The Katte virus is transmitted through the aphid vector Pentalonia caladii (formerly P. nigronervosa f. caladii). They are also transmitted by infected rhizomes, infected clones, seedlings raised in the vicinity of infected plantations, volunteer plants and a few of the infected zingiberacae.[7][12][13]. Along with P. caladii, various other species of aphid species have been reported to transmit the virus[10][14]. The virus can be transmitted by both the nymphal and the adult stages, but the efficiency increases with age of the vector; alate and apterous form are the most efficient[14]

In plantations, the aphids are prevalent throughout the year, although a decline occurs during monsoon. The migrating population was found to be the maximum during January-February, while the populations of alate vectors were found higher during November- May[2]. The primary spread in the plantation occurs due to active viruliferous alate forms of the vector at random with a radius of 400-600 m from the virus source[4]. The secondary spread is internal with low rate of spread and as the apterate adults become active, there comes a centrifugal influx of the primary source. The secondary spread was found to be gradient within 40m radius from the initial source in India while in Guatemala, the rate of disease spread was very fast (83% within 6 months of plantation).[10]

In the field, Katte virus incubated from 20 to 114 days at different months and their expression is influenced by growth of the plants. When the plants are in active phase of growth (May-November) the symptoms start expressing and develop on young seedlings (3-4 leaves stage) within 15-20 days of incubation, while, the expression is slowed due to prolonged incubation period (December – March) and it expressed in the adults only after 30-40 days of incubation during active times (90-120 days during winter months)[13]. Although, the aphid population increased during November-May in the field and reduced during the monsoon, they were found throughout the year.