Epidemiology and management of rice tungro disease

Since the mid 1960s, rice tungro virus disease has had a serious economic impact on rice production in South-East Asia. Although the frequency and intensity of outbreaks have declined since their peak in the late 1960s and early 1970s, occasional major epidemics do sill occur, often associated with changes in agronomic practices. Epidemiology and Management of Rice Tungro Disease is the outcome of a workshop held in Malaysia where researchers including representatives from five major rice producing countries met to discuss the present status of the disease and the progress of research into its management. The publication outlines current options for integrated management of tungro and critically reassesses vector control strategies. The book will be of interest to scientists and officials from national agricultural departments who have to make decisions on the management of this important disease

Tungro disease dynamics

Rice tungro disease has a complex epidemiology and tungro epidemics are influenced by many interacting factors. Although most of these factors have been known for some time, only recently have there been significant advances in understanding how they operate and combine to cause epidemics. This advance has been mainly due to the adoption of more holistic approaches such as the integration of research on the leafhopper vector and the virus in studying tungro dynamics. The availability of new diagnostic tools for the rapid identification of tungro viruses in rice plants and in leafhopper vectors has also contributed to the new knowledge. Nevertheless, forecasting tungro disease outbreaks remains an imprecise science. This chapter describes the population dynamics and dispersal characteristics of the leafhopper vectors of tungro viruses and shows how these influence tungro disease development. It examines the spread of tungro disease within and between rice fields and illustrates how modelling approaches can enhance understanding of tungro dynamics. Finally, key risk factors for tungro disease and their relevance for the development of improved management strategies are discussed

Insect-screened cultivation to reduce the invasion of tomato crops by Bemisia tabaci: modelling the impact on virus disease and vector

1: In two experiments carried out in Guadeloupe, barriers were used to reduce the entry of the virus vector Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) to tomato plots. The barriers erected around the crop were of insect-proof cloth fences (<50 mesh), 1.5 m in height, in the first experiment with a deltamethrin-treated, insect-attracting strip facing inwards, and, in the second, with the barrier but no insecticide-treated strip. 2: A mathematical model of epidemic development was fitted to the symptom data from the treated and control (unprotected) tomato plots. There were two viruses present, tomato yellow leaf curl and potato yellow mosaic; specific detection confirmed that symptoms gave an accurate indication of infection and that the two virus diseases had similar progress curves. 3: Parameter estimates obtained by model-fitting suggested that the barriers reduced vector immigration by approximately 12-fold but that B. tabaci retention within the plots was also increased slightly despite the mortality caused by the insecticide-treated strips. Disease establishment was delayed by approximately 2 weeks. The results obtained in the second experiment involving barriers deployed without insecticide-treated strips could be explained by a large increase in B. tabaci retention within the barriers resulting in more rapid virus disease progress than in controls. The results of mathematical modelling indicate that partial insect barriers can be worse than none because sufficient whiteflies can enter to establish a population and, at the same time, large numbers are retained in the barrier plot, with the net effect being a more rapid population increase than in the absence of barriers

Development of silverleaf assay, protein and nucleic acid-based diagnostic techniques for the quick and reliable detection and monitoring of biotype B of the whitefly, Bemisia tabaci (Gennadius)

The aim of this study was to develop and optimize silverleaf bioassay, esterase analysis and PCR-based techniques to distinguish quickly and reliably biotype B of the whitefly, Bemisia tabaci (Gennadius), from Indian indigenous biotypes. Zucchini and squash readily develop silverleaf symptoms upon feeding by the B biotype, but they are not readily available in Indian markets. A local pumpkin variety ‘Big’ was, therefore, used in silverleaf assay, which developed symptoms similar to those on zucchini and squash and can be used reliably to detect B biotype. Analysis of non-specific esterases of B and the indigenous biotypes indicated both quantitative and qualitative differences in esterase patterns. Two high molecular weight bands were unique to B biotype and they occurred in abundance. These esterases were used to develop quick and field-based novel detection methods for differentiating B from the indigenous biotypes. Development of these simple and cost-effective protocols has wider application as they can be potentially used to identify other agricultural pests. Mitochondrial cytochrome oxidase I gene sequences and randomly amplified polymorphic DNA (RAPD) polymorphisms, generated using the primer OpB11, were also found useful for detecting B. tabaci biotypes. A B biotype-specific RAPD band of 800 bp was sequenced, which was used to a develop sequence characterized amplified region (SCAR) marker. The SCAR marker involved the development of B biotype-specific primers that amplified 550 bp PCR products only from B biotype genomic DNA. Silverleaf assay, esterases, RAPDs or a SCAR marker were used in combination to analyse whitefly samples collected from selected locations in India, and it was found that any of these techniques can be used singly or in combination to detect B biotype reliably. The B biotype was found in southern parts of India but not in the north in 2004–06