Roguing

Abstract: Most plant viruses are vectored by arthropods, often Homopteran insects. Four general classes of plant-virus diseases have been recognized; they are distinguished by the transmission characteristics and the nature of the interaction of the virus with the vector. For nonpersistently transmitted viruses, the virus is usually restricted to the stylet of the insect. For persistently transmitted viruses, the virus is ingested, passes through the gut wall into the haemolymph, and then moves to the salivary glands where it can potentially be transmitted to other plants. Persistently transmitted viruses, have two subclasses termed circulaive if there is no multiplication in the insect vector and propagative if there is. A fourth class, semipersistent, which is intermediate between nonpersistent and persistent, is generally recognized; in this class the virus moves to the foregut of the insect. A model has been developed in which an SEIR-type epidemic for the host plant is linked with the insect vector population (an SEI model with vertical transmission) to describe the transmission process. This model was used to compare the transmission characteristics of the four virus classes directly, and to explore the consequences for epidemic development and possible control options. Depending on the assumptions made about migration, it was possible to obtain an expression for the basic reproductive number, R 0 . Expressions were also obtained for equilibrium values for the host and vector population classes; and a numerical analysis indicated that these equilibria were stable for known or reasonable estimates of the parameter values. The basic reproductive number, R 0 , was used to examine the relative contributions of key parameters in distinguishing the four virus disease classes using parameter values and ranges taken directly from the literature or estimated indirectly. Pairwise plots of parameter values which satisfied the threshold criterion R 0 = 1 clearly separated the propagative class from the other categories. On holding the other parameters constant, a much larger vector population or vector activity was required to satisfy the epidemic threshold for propagative viruses. Similar conclusions were reached from plots of the healthy host and viruliferous vector populations against key parameters. The model framework was used to analyse the effectiveness of roguing (the removal and destruction) of diseased plants and/or reduction of the vector-population size, for example, by insecticide treatment or vegetation management. Roguing would only be effective for nonpersistently transmitted viruses at relatively low vector-population densities. Roguing would usually only be needed for propagative viruses at very high population densities. There would be a clear advantage in reducing the vector-population density for propagative viruses, and control measures aimed at reducing populations would only be effective for these viruses.

Abstract: The dynamics of a virus disease in a perennial plant population are modelled. The population is divided into healthy, latently infected, infectious and post-infectious plants and linked differential equations describe the dynamics of each category. Qualitative analysis of this model shows stable dynamics and threshold conditions for disease persistence. Stable equilibria are reached after several years. The dynamics of the model are highly sensitive to changes in contact rate and infectious period. Disease management by roguing (removal) of infected plants and replanting with healthy ones is investigated. Roguing only in the post-infectious category confers no advantage. At low contact rates, roguing only when plants become infectious is sufficient to eradicate the disease (...)

Abstract: Viral diseases are a key constraint in the production of staple food crops in lesser developed countries. New and improved disease control methods are developed and implemented without consideration of the selective pressure they impose on the virus. In this paper, we analyse the evolution of within-plant virus titre as a response to the implementation of a range of disease control methods. We show that the development of new and improved disease control methods for viral diseases of vegetatively propagated staple food crops ought to take the evolutionary responses of the virus into consideration. Not doing so leads to a risk of failure, which can result in considerable economic losses and increased poverty. Specifically in vitro propagation, diagnostics and breeding methods carry a risk of failure due to the selection for virus strains that build up a high within-plant virus titre. For vegetatively propagated crops, sanitation by roguing has a low risk of failure owing to its combination of selecting for low virus titre strains as well as increasing healthy crop density.

Abstract: Only a small proportion of Bemisia tabaci collected in totally infected cassava fields at a site in Cote d'lvoire transmitted African cassava mosaic virus (ACMV) to test plants. Nevertheless, the monthly increase in disease incidence in an experimental planting was directly related to numbers of adult whiteflies counted on plants 6 weeks earlier. In plots at different spacings, the greatest incidence of ACMV expressed as a percentage of the total stand occurred at the lowest plant density. Much speread into the spacing trial and into two other experimental plantings occurred from outside sources and followed downwind gradients. By contrast, spread from ACMV-infected sources within plantings was limited. It occurred in all directions but over distances of only a few metres. These contrasting patterns of spread are attributed to the differentbehavior of B. tabaci above and within the crop canopy. It is concluded that contamination of cassava fields in the coastal forest area of Cote d'Ivoire is due mainly to rapid spread from outside sources which leads to internal foci that contribute to some further, although limited, spread. These findings are discussed in relation to possible control strategies based on the release of healthy cuttings, dense planting and subsequent roguing. Such measures are unlikely to be effectve in the coastal forest region of Cote d'Ivoire and adjacent countriesunless varieties are grown with greater resistance to infection than those currently used.