Emerging geminivirus problems: A serious threat to crop production

Geminiviruses form the second largest family of plant viruses, the Geminiviridae, represented by four genera: Mastrevirus, Curtovirus, Topocuvirus and Begomovirus. During the last two decades these viruses have emerged as devastating pathogens, particularly in the tropics and subtropics, causing huge economic losses and threatening crop production. Epidemics caused by re‐emerging and newly emerging geminiviruses are becoming frequent even in regions that were earlier free from these viruses. Compared to mastreviruses and curtoviruses, begomoviruses have emerged as more serious problems in a variety of crops, for example, cassava, cotton, grain legumes and vegetables. Major contributory factors for the emergence and spread of new geminivirus diseases are the evolution of variants of the viruses, the appearance of the whitefly ‘B’ biotype and the increase in the vector population. Variability in geminiviruses has arisen through mutations, recombination and pseudorecombination. Genomic recombination in geminiviruses, not only between the variants of the same virus but also between species and even between genera, has resulted in rapid diversification. From the disease point of view, most virulent variants have developed through recombination of viral genomes such as those associated with cassava mosaic, cotton leaf curl, and tomato leaf curl diseases. Heterologous recombinants containing parts of the host genome and/or sequences from satellite‐like molecules associated with monopartite begomoviruses provide unlimited evolutionary opportunities. Human activity has also played an important role in the emergence of serious geminivirus diseases across the globe, like the changes in cropping systems, the introduction of new crops, the movement of infected planting materials and the introduction of host susceptibility genes through the exchange of germplasm.     

Studies on resistance of pea to pea seed borne mosaic virus and new pathotypes

Studies were carried out to search for virulent pathotypes of pea seed borne mosaic virus (PSbMV) on pea and to explore new sources of resistance in French and Indian pea collections. A virulent pathotype, PSbMV-Pi, capable of partially overcoming the recessively resistant gene sbm 1, was identified for the first time in an Indian pea line. PSbMV-Pi did not produce visible symptoms on sbm 1 lines, on which it had a reduced multiplication and could no longer be detected by ELISA seven wk after inoculation. However, it multiplied normally on the susceptible cultivars and could be differentiated from other strains on a set of strain differentials. Also, another strain, PSbMV-Pv, of the virulent pathotype, that appeared to multiply slightly better than PSbMV-Pi on sbm 1 lines, was recovered from the local strain of PSbMV.
Five new sources of resistance to PSbMV were screened from the Indian pea collection by a rapid screening based on the assumption that, in germplasm collections, the virus is generalised to all susceptible lines by aphid vectors. The initial step of testing a few testas of each germplasm line in ELISA and the rejection of positive lines eliminated 85% of the germplasm before further testing in the field. The inheritance studies on four of the five resistant lines lead to reidentification of the sbm 1 gene. The sbm 1 lines behaved fairly well under heavy inoculum pressure in the pea fields in 1989.     

Viruses occurring in white clover (Trifolium repens L.) from permanent pastures in Britain

The uptake of captan from aqueous solution by conidia of Neurospora crassa can be markedly reduced by pretreatment of the cells with thiol reagents such as iodoacetic acid (IOA). The nature of this reaction has been investigated and it is shown that IOA largely reacts with the soluble thiol pool of the spores. Captan, however, reacts with both soluble and insoluble thiols and it is suggested that whilst the former is a detoxication process the latter may provide the key to its toxic action. Using glutathione as a model soluble thiol the molar ratio and pH dependence of the captan detoxication process has been determined and compared with the cellular reactions. Assay of ³⁵S-labelled captan has shown that captan is almost completely decomposed by the spores and that one-third of the accumulated captan is converted to sulphur compounds fixed to insoluble cell entities.     

Preliminary studies on sap-transmissible viruses of red clover (Trifolium pratense L.) in England and Wales

Red clover plants, collected from nine widely separated permanent pastures in England and Wales, were tested for sap-transmissible viruses. Viruses were identified by the symptoms they caused in test plants, by electron microscopy, and by serological tests. Of the 265 plants tested 14% were infected. Only pea mosaic virus was common and widespread; it was found in 8% of the plants, and in seven of the fields. Other viruses isolated were arabis mosaic, bean yellow mosaic, red clover mottle, and red clover vein mosaic; only red clover mottle virus produced diagnostic symptoms in red clover. No viruses were detected in seedlings grown from seed from eighty-nine commercial seed crops. Attempts to transmit red clover mottle virus by the Collembolan Sminthurus viridis L., which is common on red clover, failed.     

Comparative epitope profiles of the particle proteins of whitefly-transmitted geminiviruses from nine crop legumes in India

Geminiviruses associated with yellow or golden mosaic diseases of legume crops in two regions of India were compared by testing their reactivity with 27 monoclonal antibodies (MAbs) prepared to the particles of African cassava mosaic (ACMV) or Indian cassava mosaic (ICMV) viruses. The viruses fell into two main groups. Group 1 comprised isolates of dolichos yellow mosaic virus; these reacted with three or four ACMV MAbs and four ICMV MAbs. Group 2 comprised isolates of horsegram yellow mosaic virus, together with isolates from blackgram, cowpea, French bean, pigeonpea, soybean, Indigofera hirsuta and probably also isolates from mungbean. These reacted with three or four ACMV MAbs but with few or no ICMV MAbs. Isolates within each group differed slightly in epitope profile, depending on the source species (Group 2) or geographical origin (Groups 1 and 2). Isolates from lima bean resembled those in Group 2 but had some antigenic differences, and their status is uncertain. The poor detectability of geminivirus isolates in mungbean may reflect a low virus concentration in this species.     

Molecular characterisation of papaya ringspot potyvirus isolates from India

Papaya ringspot potyvirus (PRSV) causes major diseases of papaya and cucurbits in the Indian subcontinent. Based on biological properties, PRSV isolates are classified as either papaya infecting (P), or non-papaya infecting (W) types. To characterise the P and W isolates from India at the molecular level, c. 1.7 Kb of the 3′-terminal regions comprising a part of the nuclear inclusion b (Nib) gene, the complete capsid protein (CP) gene and the untranslated region (UTR) of both the P and W isolates were cloned and sequenced. Comparative sequence analyses showed that the 3′-UTRs in isolates P and W were 209 nucleotides in length excluding the poly (A) tail, and shared 96% identity. The CP genes of the two isolates were also similar, with 87% nucleotide identity and 93% amino acid identity. The amino acid differences between the CP genes were mostly confined to the amino terminus. The DAG triplet associated with aphid transmissibility was present in the CP of isolate W, but it was replaced by DAD in the P isolate. The partially sequenced Nib genes were also 90% identical, but isolate W contained an additional amino acid (threonine) just upstream of the cleavage site (Q/S) between Nib and CP. This is the first reported comparison of the molecular characterisation of PRSV-P and W isolates from the Indian subcontinent.