For example, an epidemic on coca in Peru in the late 1980s was linked to F. oxysporum f.sp. erythroxyli (Arévalo et al. 2000). That natural epidemic can be viewed to some extent as a large-scale field trial that is informative in considering several questions: Did the epidemic inflict sufficient damage to reduce crop production? Did it cause recurrent or permanent effects on coca in the affected areas? If the effects of this natural epidemic are an indication, how effective would it be if the same pathogen were applied as a mycoherbicide to curtail the production of illicit-drug crops?

A wilt of coca (E. coca) caused by F. oxysporum resulted in extensive losses to the coca crop in the Huallaga Valley, the main coca-producing region in Peru (Arévalo et al. 2000). Although definitive reports of the presence of the disease go back at least as far as 1932, its incidence increased sharply in the 1980s, at the time of the increase in coca production, the subsequent increase in use of agricultural chemicals, and the reduction in cultivation time (Nelson et al. 1997). The pathogen was reported as F. oxysporum f.sp. erythroxyli (Nelson et al. 1997; Arévalo et al. 2000) and one of its strains was found to be identical to a strain of the pathogen studied in Hawaii by Sands et al. (1997) and Nelson et al. (1997).

The most recent epidemic of F. oxysporum f.sp. erythroxyli wilt of coca in the Huallaga Valley of Peru began in 1987. It was estimated that 52% of the coca crops were affected during 1992-1994. To describe the impacts of the disease, Arévalo et al. (2000) quantified its incidence and development in 11 coca fields in five regions in the valley. The coca plants sampled were 14-93 months old. Overall, the coca leaf yields were reduced by 74%. Disease incidence (the proportion of sampled plants that were diseased) ranged from 54% of 32-monthold plants to 79% of 93-month-old plants. The level of disease, measured as the area under the disease-progress curve, ranged from 5.67 on 39-month-old plants to 28.2 on 93-month-old plants (that is, the older plants were more diseased). Thus, the Peruvian F. oxysporum f.sp. erythroxyli epidemics of the 1980s and 1990s were highly devastating to coca production in the Huallaga Valley.

No formal followup studies of the disease have been performed, but anecdotal observations suggest that the coca wilt disease persists and affects coca leaf production in some portions of the valley. In some areas, however, cultivation of coca has been moved to new fields to escape the disease (personal communication, E. Arévalo, Instituto de Cultivos Tropicales, November 16, 2010, to B. Bailey, USDA). Hence, the epidemic, even while inflicting severe losses, has not deterred coca production, which has continued even in the presence of sporadic disease outbreaks.

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