How can control of invading plant pathogens increase their rate of spread? How can we prevent it?

Published online
05 Nov 2020
Content type

Sharp, R.

Publication language
Africa South of Sahara & Central Africa & Uganda


This paper presents the results of studies conducted to determine the effect of the control of invading plant pathogens in increasing their rate of spread and some methods to prevent this spread. This effect was studied in the context of the East African cassava mosaic virus-Ugandan variant (EACMV-UG) pandemic from 1997 that spread across central Africa where the endemic African cassava mosaic virus (ACMV) was also present. This disease was spread in one of two ways: by a whitefly vector, Bemisia Tabaci; and, through the trade of infected planting material. A whole array of methods to control this disease existed, such as the use of resistant cultivars, insecticides, roguing, removal of infected planting material and clean cutting initiatives. A model of the spread of EACMV-UG was developed and 2 scenarios were tested: one where it spreads into a region that is also infected by the endemic ACMV; and another in which it spreads into a region with no competitive endemic strains. The business-as-usual control levels was then intensified to investigate what the effect of control was under these 2 scenarios. It was found that, while all forms of control slowed the spread of disease when there were no competing endemic strains, many increased the rate of spread when ACMV was present. The only form of control that reduced the rate of spread was by restricting the trade of infected planting material. The case where control was applied only when it reached a certain level in a given region was then investigated. This was the more likely scenario in practice as control would only usually be intensified once the disease has reached detectable levels. When control was applied in this way, the increase in the rate of spread of the epidemic was no longer observed. Some recommendations to optimally control invasive pathogens was given, i.e. if it is known that the invasive strain is not in competition with competing endemic strains, then intensifying control ahead of the wave of advance slows the spread of the invader. If, however, the invasive strain is in competition with competing endemic strains, it is recommended that the only control measure that should be applied ahead of the wave are trade restrictions and to only intensify other forms of control once the wave front has passed.

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