Introduction
Positive sense single stranded RNA [(+)ssRNA] are the most prevalent genomic type among plant viruses. The plus sense RNA viruses do not have envelopes and have monopartite, bipartite or tripartite genomes.
These viruses’ genome may act as its own mRNA during replication cycle. After the viral particle enters the cell, (+)ssRNA molecule’s uncoating occurs and translation of viral genes into proteins immediately begin on the host’s ribosomes. After the proteins are generated, they are post-translationaly processed, if necessary, by self encoded proteinases. The viral RNA-dependent RNA polymerase and methyltransferase/helicase proteins then recruit host factors to form both subgenomic RNAs and progeny genomes. Coat proteins then envelope the progeny genomes to generate new viral particles.
Potyvirus
Potyviridae –
| Organism | Disease | Transmission | Management |
|---|---|---|---|
| Potyvirus – Plum pox virus Synonyms plum pox virus, PPV; plum pox virus PPV; Plum pox potyvirus Acronym: PPV |
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| PPV, as a member of the genus Potyvirus, has a genome composed of a single stranded RNA molecule of near 10,000 nucleotides. Its expression strategy, similar to that of the rest of potyviruses, includes translation of a unique long open reading frame (ORF) further processed to yield viral products. | Sharka (pox in Bulgarian) or plum pox disease, is considered one of the most devastating diseases of stone fruits. The disease causes reduced quality and premature dropping of fruits. While yellow or brown blotches or rings on the leaves or fruit of Prunus plants are characteristics of plum pox, other symptoms include severely deformed and bumpy fruit; leaf distortion and drop; and deformed, discolored seeds. Sometimes, infected trees display no symptoms on the leaves or fruit at all. In Chile and Pennsylvania, scientists discovered numerous, symptom-free orchards infected with PPV after confirming presence of virus with laboratory tests. | The introduction of infected plant propagation material is considered the most important means of long distance spread of PPV. In addition, the virus is non-persistently transmitted by a number of aphid species existing in each region. At least 20 different aphid species (including Myzus persicae, Aphis spiraecola, A. gossypii, and A. fabae) can transmit PPV throughout an orchard and to other trees in nearby orchards, forests, and residential areas. | Virus control in countries where the disease is not present are based on quarantine measures to avoid its introduction. In countries where the level of infection is low and infected trees are restricted to limited areas, eradication has made it possible to maintain a low level of infection and more rarely to eliminate the disease. Conventional breeding for sharka resistance has been hampered by the scarcity of resistant cultivars. The cultivar Honey Sweet plum now provides a unique source of germplasm for future breeding programs worldwide. |
| Potyvirus – Maize dwarf mosaic virus Synonyms maize dwarf mosaic potyvirus Misnomers: maize dwarf mosaic potyvirus MDMV; maize dwarf mosaic virus MDMV; Potyvirus Maize dwarf mosaic virus Acronym: MDMV |
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| MDMV has a single-stranded RNA of + polarity. The RNA is about 9,500 bp long and carries a VPg (viral genome-linked protein), which is covalently bound to its 50-end, and a poly(A) tail at its 30-end. The genome contains a long open reading frame (ORF) translated into a large 338 kDa polyprotein that is co- and/or post-translationally cleaved into 10 final protein products. This pathogen consists of 4 strains: C, D, E, and F. | MDMV strains are known to infect maize, sorghum and Johnson grass. Symptoms appear six weeks after aphids feed and transmit this virus. Generally, infected plants develop distinct chlorotic mosaics, mottles or streaks on green tissues. Yield loss is caused by stunting and halting of ear formation and development. | Aphids (both adults and nymphs) transmit the virus during feeding. The virus is acquired by the aphid within seconds of feeding and a latent period is not required for transmission to new host plants. Aphids do not retain the virus after molting. The virus overwinters in alternate hosts. Can also be transmitted to a lesser extent through seed and mechanically through plant damage. Mechanical transmission is considered to be a problem predominantly in greenhouses. Y | Resistant varieties are widely available and offer the best means of disease management. Use of insecticides, biological control agents and cultural practices (management of aphid refuges) that reduce aphid populations will reduce transmission of viruses. Excessive use of nitrogen fertilizer favors aphid reproduction. Application of nitrogen above recommended levels should therefore be avoided. Removal of volunteer plants and management of alternate hosts, such as Johnson grass and graminaceous weed species, will reduce sources of inoculum. Use of certified, virus-free seed will limit seed-borne transmission of the MDMV and SCMV. |
| Potyvirus – Sugarcane mosaic virus Synonyms sugarcane mosaic virus, SCMV; sugarcane mosaic virus SCMV; sugarcane mosaic potyvirus Acronym: SCMV |
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| One of the most widely distributed and important pathogens of sugarcane worldwide; consist of a large number of strains, differing from each other in certain biological and antigenic properties. The capsid protein is approximately 37 kDa in size. Amplification product (approximately 900 bp) can be obtained from the collected and inoculated plants but not from healthy plants which may confirm the presence of SCMV in the symptom-expressing plants. | Mosaic is identified primarily by its leaf symptoms. Symptoms may vary in intensity with the cane variety, growing conditions, and the strain of the virus. The most distinctive symptom is a pattern of contrasting shades of green, often islands of normal green on a background of paler green or yellowish chlorotic areas on the leaf blade. The infection may be accompanied by varying degrees of leaf reddening or necrosis. | There are three principal modes of spread of SCMV: (1) by aphid vectors, (2) by infected seed cane and (3) by mechanical inoculation. Only aphid vectors and infected seed cane are important in the field. Mechanical transmission, for the most part, is important only in greenhouse and laboratory research. | The use of resistant varieties is the most effective method of control. Planting mosaic-free seed cane is essential. Management practices targeting insect vectors and control methods aimed at eradication have not been very effective. Also, the practice of destroying diseased plants is not considered feasible if the infection level exceeds 5%. Control of mosaic through heat treatment of cuttings is partially effective. It has been noted that some sugarcane plants recover from mosaic. |
| Potyvirus – Soybean mosaic virus Synonyms soybean mosaic virus, SbMV; soybean mosaic virus SMV Acronym: SbMV |
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| Genome is a single-stranded, positive sense RNA molecule that is approximately 10 kb in length and contains a single open reading frame. It encodes a large polyprotein that is co- and post-translationally cleaved into 11 final protein products. SMV is found in all soybean-growing regions of the world. Data suggest a high genetic diversity of SMV. | Most infected cultivars are slightly stunted with fewer pods that are sometimes dwarfed and flattened, without hairs, and without seeds. Leaves have a mosaic of light and dark green areas that may later become raised or blistered, particularly along the main veins. Primary leaves of some cultivars may show necrotic local lesions, which merge, into veinal necrosis followed by yellowing and leaf abscission. Seeds from infected plants may be mottled brown or black. Not all mottled seeds contain virus and not all seeds from virus-infected plants are mottled. | The virus is transmitted by aphids and also seed-borne at a rate less than 5% in most varieties. SMV is sap and graft-transmissible. At least 32 aphid species, belonging to 15 different genera, transmit the SMV in a nonpersistent manner. Infected plants are primary inoculum sources for SMV. | At least three resistance genes to SMV have been identified. At the present time, the use of SMV-free seed and avoiding late planting of soybean are the best control measures to preclude loss induced by SMV. Serological seed indexing techniques and/or grow-out tests can be used for virus detection in seed lots. Rouging, in addition to being generally impractical in the field, may not be very effective because of the tendency for symptoms in soybean to be masked above 30°C. |
| Potyvirus – Potato virus Y Synonyms: potato virus Y, PVY; potato virus Y PVY; potato virus Y PVY-H Acronym: PVY |
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| Potato virus Y (PVY), the type member of the genus Potyvirus, is a major pathogen of solanaceous crops, such as potato, tobacco and pepper. Isolates of PVY largely differ by their pathogenicity properties in different host species and cultivars. | Symptoms vary. Severe symptoms include dark brown, dead areas in the blade of nearly mature leaflets. Leaves formed after the onset of PVY exhibit mild wrinkling, slight distortion, and mild mottling. Leaflets of plants infected for some time are rolled downward with curved petioles (leafstalks), giving the plant a drooping appearance. | PVY is transmitted in the nonpersistent manner by many aphid species. Aphids can acquire the virus in less than 60 sec. from an infected plant and transmit it to a healthy plant in less than 60 sec. The virus may be retained by the aphid for longer than 24 hours. PVY can also be transmitted mechanically. The virus does not appear to be seed transmitted. | Strategies for controlling PVY include the following: eradication of all wild reservoir hosts in and around fields; early planting to avoid high aphid populations later in the season; rouging; monitoring aphid populations early in the season. |
| Potyvirus – Zucchini yellow mosaic virus Synonyms: zucchini yellow mosaic virus, ZYMV; zucchini yellow mosaic virus ZYMV Acronym: ZYMV Misnomer: Zucchini yellow mosaic potyvirus ZYMV |
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| A genome of 9,593 nt is arranged as a single open reading frame encoding a polyprotein precursor that is processed into 10 putative proteins. Of these, the coat protein (CP) is involved in the encapsidation of viral RNA, vector transmission, and the regulation of viral RNA amplification and cell-to-cell and systemic movement. Virions are flexuous filaments of 680–730 nm in length and 11–13 nm in diameter, composed of about 2,000 subunits of a single 31-kDa protein. | A major pathogen of members of the family Cucurbitaceae. The main cultivated host species include rockmelon, cucumber, zucchini, pumpkin, watermelon, etc. Symptoms include yellowing, stunting, leaf deformations, and misshaped and discoloured fruits, which often renders the fruits unmarketable, drastically reducing agricultural yields. The leaf symptoms appear as severe mosaic, deformation, blistering, and reduced size. | Both cucurbit and non-cucurbit aphids transmit ZYMV in the field and once the virus is introduced to a cucurbit planting its spread within the field is generally very rapid. ZYMV is seed-borne at low levels in zucchini and squash. | Phytosanitary, cultural and biological control measures include: removal of potential virus and aphid sources among weeds, removing old crops, and avoiding overlapping and side-by-side plantings; deploying reflective or other plastic mulches to deter aphid landings and crop covers to prevent early aphid attaks; and using cross protection with mild ZYMV strains. In addition, host resistance genes against ZYMV have been described in some cucurbits. However, resistance breaking strains occur so resistance should never be used alone. |
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Bymovirus
Potyviridae –