Global status of tospovirus epidemics in diverse cropping systems: successes achieved and challenges ahead.

The plant virus family Flexiviridae includes the definitive genera Potexvirus, Mandarivirus, Allexivirus, Carlavirus, Foveavirus, Capillovirus, Vitivirus, Trichovirus, the putative genus Citrivirus, and some unassigned species. Its establishment was based on similarities in virion morphology, common features in genome type and organization, and strong phylogenetic relationships between replicational and structural proteins. In this review, we provide a brief account of the main biological and molecular properties of the members of the family, with special emphasis on the relationships within and among the genera. In phylogenetic analyses the potexvirus-like replicases were more closely related to tymoviruses than to carlaviruses. We postulate a common evolutionary ancestor for the family Tymoviridae and the two distinct evolutionary clusters of the Flexiviridae, i.e., a plant virus with a polyadenylated genome, filamentous virions, and a triple gene block of movement proteins. Subsequent recombina...

Family Flexiviridae: a case study in virion and genome plasticity.

The plant virus family Flexiviridae includes the definitive genera Potexvirus, Mandarivirus, Allexivirus, Carlavirus, Foveavirus, Capillovirus, Vitivirus, Trichovirus, the putative genus Citrivirus, and some unassigned species. Its establishment was based on similarities in virion morphology, common features in genome type and organization, and strong phylogenetic relationships between replicational and structural proteins. In this review, we provide a brief account of the main biological and molecular properties of the members of the family, with special emphasis on the relationships within and among the genera. In phylogenetic analyses the potexvirus-like replicases were more closely related to tymoviruses than to carlaviruses. We postulate a common evolutionary ancestor for the family Tymoviridae and the two distinct evolutionary clusters of the Flexiviridae, i.e., a plant virus with a polyadenylated genome, filamentous virions, and a triple gene block of movement proteins. Subsequent recombination and gene loss would then have generated a very diverse group of plant and fungal viruses.

Family Flexiviridae: A Case Study in Virion and

The helper component-proteinase (HC-Pro) protein of potyviruses is a suppressor of gene silencing and has been shown to elicit plant developmental-defect-like symptoms. In Zucchini yellow mosaic virus (ZYMV), a mutation in the highly conserved FR180NK box of HC-Pro to FI180NK causes attenuation of these symptoms. At 5 days postinoculation and before symptoms appear, virus accumulation, HC-Pro protein levels, and viral short interfering RNA (siRNA) levels are similar for the severe (FRNK) and attenuated (FINK) strains. At this stage, ZYMV(FRNK) caused greater accumulation of most microRNAs (miRNAs), and especially of their complementary miRNA "passenger" strands (miRNA*s), in systemically infected leaves than the attenuated ZYMV(FINK) did. HC-Pro(FRNK) specifically bound artificial siRNA and miRNA/miRNA* duplexes with a much higher affinity than the mutated HC-Pro(FINK). Further analysis of the mutant and wild-type HC-Pro proteins revealed that suppressor activity of the ZYMV HC(FINK) mutant was not diminished. However, the FINK mutation caused a loss of HC-Pro suppressor function in other potyviruses. Replacement of the second positively charged amino acid in the ZYMV FRNK box to result in FRNA also caused symptom attenuation and reduced small RNA duplex-binding affinity without loss of suppressor activity. Our data suggest that the highly conserved FRNK box in the HC-Pro of potyviruses is a probable point of contact with siRNA and miRNA duplexes. The interaction of the FRNK box with populations of miRNAs directly influences their accumulation levels and regulatory functions, resulting in symptom development.

The Conserved FRNK Box in HC-Pro, a Plant Viral Suppressor of Gene Silencing, Is Required for Small RNA Binding and Mediates Symptom Development

Rapid food price rises have highlighted serious concerns about food security globally and have had a huge impact on achieving Millennium Development Goal 1. Since 2007, an estimated 100 million more people have fallen into absolute poverty. Most live in developing countries where low incomes (less than $1 per day) make it difficult to access food. Access to sufficient food for dietary needs and food preferences defines food security. However, whilst price rises have brought food security into sharp focus, underlying problems need to be addressed. Over the last three to four decades, there has been chronic under-investment in agriculture at all levels. Development aid to agriculture has declined and often in-country policies do not support the sector. Low crop yields are common in many developing countries and improved productivity is vital to reducing rural poverty and increasing food security. Whilst the causes of low productivity are complex, one major contributory factor is crop losses due to plant health problems. Often accurate information on the extent of these losses is missing but estimates of 30–40% loss annually from “field to fork” are common. Any future solution regarding improved global food security must address these losses and that means improving plant health. Two trans-boundary diseases, wheat stem rust race Ug99 and Coffee Wilt Disease of Coffea are highlighted. CABI has a number of plant health initiatives and one radical approach (Global Plant Clinic) involves partnership with in-country services to deliver plant health advice to farmers at the point of demand. Such innovations are entirely consistent with a proposed new “Green Revolution” which would need to be “knowledge intensive”.

The importance of plant health to food security.

Sugarcane is a major commercial crop grown in tropical and subtropical regions of the country. During the last 100 years, the country has witnessed epidemics of various diseases like red rot, smut, wilt, rust, leaf scald and yellow leaf (YLD). The damage caused to sugarcane during each epidemic would vary depend upon the nature of disease and spread of the affected varieties. Many sugarcane varieties were replaced due to their breakdown to a new disease or to a new pathogenic strain. Still, red rot remains to be the major disease of sugarcane in the country and wilt and smut are the other major diseases seriously affecting cane production in India. In addition, the new disease YLD causes serious damage to cane productivity in many states. Severity of grassy shoot disease, pokkah boeng and rust is also growing in many parts of the country. Propagation of sugarcane through vegetative cuttings favours spread of diseases through planting materials. Primary transmission of different diseases through seed canes poses serious threat to sugarcane growth and performance. Disease resistant varieties play a crucial role in managing many of the diseases in sugarcane and several varieties have been developed to mange the diseases in the past. In addition, different agronomical approaches and physical method like heat therapy are being effectively used to manage the diseases in sugarcane. Recently, multiplication of sugarcane through tissue culture is being advocated to produce virus and phytoplasma disease-free planting materials. Among serological methods, ELISA and tissue-blot technique have become very popular and largely used for routine detection of bacterial, phytoplasmal and viral pathogens in sugarcane. Molecular techniques like polymerase chain reaction (PCR), reverse-transcriptase (RT)-PCR and nested PCR assays are most sensitive than the serological techniques especially to detect sugarcane viruses and phytoplasmas which occur at low concentrations. Use of disease resistant varieties along with healthy seed nursery programmes would form the basis to successfully manage the diseases in sugarcane.

Disease Scenario and Management of Major Sugarcane Diseases in India

The presence of phytoplasmas and their associated diseases is an emerging threat to vegetable production which leads to severe yield losses worldwide. Phytoplasmas are phloem-limited pleomorphic bacteria lacking the cell wall, mainly transmitted through leafhoppers but also by plant propagation materials and seeds. Phytoplasma diseases of vegetable crops are characterized by symptoms such as little leaves, phyllody, flower virescence, big buds, and witches' brooms. Phytoplasmas enclosed in at least sixteen different ribosomal groups infecting vegetable crops have been reported thus far across the world. The aster yellows phytoplasma group (16SrI) is presently the prevalent, followed by the peanut witches' broom (16SrII). Wide and overlapping crop and non-crop host ranges of phytoplasmas, polyphagous insect vectors, limited availability of resistance sources and unavailability of environmentally safe chemical control measures lead to an arduous effort in the management of these diseases. The most feasible control of vegetable phytoplasma diseases is a consequence of the development and implementation of integrated disease management programs. The availability of molecular tools for phytoplasma identification at the strain level greatly facilitated this kind of approach. It is moreover essential to understand the molecular basis of phytoplasma-vector interaction, epidemiology and other factors involved in disease development in order to reduce the disease outbreaks. Information on the knowledge about the most widespread phytoplasma diseases in vegetable crops is reviewed here in a comprehensive manner.

https://krishi.icar.gov.in/jspui/bitstream/123456789/41130/1/fmicb-10-01349%20%281%29.pdf

Global Status of Phytoplasma Diseases in Vegetable Crops

Phytoplasmas have been found associated with 129 plant species in India including vegetables, legumes, spices, medicinal plants and ornamental plants, cash and oil crops, palms, fruit trees and weeds, where ten 16Sr groups of phytoplasmas have been identified so far. Sesame phyllody, brinjal little leaf, sugarcane grassy shoot, sandal spike, coconut root wilt, areca nut yellow leaf and many diseases in ornamental plants are causing the most severe economic losses in the country. Identified phytoplasmas are related to ‘Candidatus Phytoplasma asteris’, ‘Ca. P. pruni’, ‘Ca. P. ziziphi’, ‘Ca. P. trifolii’, ‘Ca. P. solani’, ‘Ca. P. cynodontis’, ‘Ca. P. oryzae’, ‘Ca. P. phoenicium’, ‘Ca. P. australasia’ and ‘Ca. P. pini’. Among them ‘Ca. P. asteris’-related strains (aster yellows, 16SrI group) are the prevalent phytoplasmas identified associated with 62 diseases followed by phytoplasmas classified in 16SrII, 16SrVI, 16SrXIV and 16SrXI groups. At least seven insect vectors, belonging to leafhoppers and planthoppers species, have been reported as putative or natural vectors for important phytoplasma diseases. The majority of phytoplasma disease reports are from north and south part of India. Little attempt has been made to genomics, epidemiology, host phytoplasma interaction and management aspects of these diseases. Presently the suggested effective management practices in India are growing resistant varieties, control of insect vectors, weed species as alternative hosts and use of healthy planting materials.

A century progress of research on phytoplasma diseases in India.

Soybean mosaic virus.

Characterization, diagnosis and management of plant viruses , Characterization, diagnosis and management of plant viruses , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Govind Pratap Rao

Papers

Disease Scenario and Management of Major Sugarcane Diseases in India

Global Status of Phytoplasma Diseases in Vegetable Crops

A century progress of research on phytoplasma diseases in India.

Soybean mosaic virus.

Characterization, Diagnosis And Management of Plant Viruses