No abstract

Management of geminiviruses is a worldwide challenge because of the widespread distribution of economically important diseases caused by these viruses. Regardless of the type of agriculture, management is most effective with an integrated pest management (IPM) approach that involves measures before, during, and after the growing season. This includes starting with resistant cultivars and virus- and vector-free transplants and propagative plants. For high value vegetables, protected culture (e.g., greenhouses and screenhouses) allows for effective management but is limited owing to high cost. Protection of young plants in open fields is provided by row covers, but other measures are typically required. Measures that are used for crops in open fields include roguing infected plants and insect vector management. Application of insecticide to manage vectors (whiteflies and leafhoppers) is the most widely used measure but can cause undesirable environmental and human health issues. For annual crops, these measures can be more effective when combined with host-free periods of two to three months. Finally, given the great diversity of the viruses, their insect vectors, and the crops affected, IPM approaches need to be based on the biology and ecology of the virus and vector and the crop production system. Here, we present the general measures that can be used in an IPM program for geminivirus diseases, specific case studies, and future challenges.

Information on the distribution and prevalence of the economically destructive Begomovirus species and recombinant forms infecting fresh-market and processing tomato crops in Brazil is crucial in guiding breeding programs and also to understand the evolutionary mechanisms associated with the upsurge of so many species and quasi-species comprising this unique disease complex. An extensive survey was carried out over 3 years (between 2002 and 2004) aiming to study the diversity of begomoviruses in tomato plants, predominantly collected in central Brazil. Polymerase chain reaction (PCR) with degenerated primers was used to detect the begomoviruses in tomato leaf samples showing virus-like symptoms in commercial fields. Seven hundred and seventeen out of 2,295 samples were found to be PCR positive for a begomovirus infection. High quality sequences were obtained from a fragment encompassing the 5' region of the coat protein (CP) gene and a segment of the intergenic region for 295 isolates from distinct geographic regions. Comparison analyses with those available in public databases enabled preliminary classification of the isolates into four previously described and/or proposed species: Tomato severe rugose virus (61%), Tomato golden vein virus (29.8%), Tomato mottle leaf curl virus (7.1%), Tomato yellow vein streak virus (0.7%), and two putative new species (1.4% of isolates). Within the prevailing species, we noted a relatively low degree of diversity, possibly indicating the existence of recent population founder effects and/or recent selective sweeps.

The development of tomato spotted wilt tospovirus (TSWV) infection in the midgut and salivary glands of transmitting and non-transmitting thrips, Frankliniella occidentalis, was studied to elucidate tissue tropism and the virus pathway within the body of this vector. Immunohistological techniques used in this study showed that the midgut, foregut and salivary glands were the only organs in which virus accumulated. The first signals of infection, observed as randomly distributed fluorescent granular spots, were found in the epithelial cells of the midgut, mainly restricted to the anterior region. The virus subsequently spread to the circular and longitudinal midgut muscle tissues, a process which occurred late in the larval stage. In the adult stage, the infection occurred in the visceral muscle tissues, covering the whole midgut and foregut, and was abolished in the midgut epithelium. The infection of the salivary glands was first observed 72 h post-acquisition, and simultaneously in the ligaments connecting the midgut with these glands. The salivary glands of transmitting individuals appeared heavily or completely infected, while no or only a low level of infection was found in the glands of non-transmitting individuals. Moreover, the development of an age-dependent midgut barrier against virus infection was observed in second instar larvae and adults. The results show that the establishment of TSWV infection in the various tissues and the potential of transmission seems to be regulated by different barriers and processes related to the metamorphosis of thrips.