effects on target plants and nontarget organisms; and research and development needs.
To address its task, the committee reviewed publications and other publicly available information on the three proposed mycoherbicides and relevant publications on related fungi and other mycoherbicides that have been developed against undesirable plant species (weeds). The publications were identified through literature searches and by consulting the ONDCP, the U.S. Department of State, the U.S. Department of Agriculture, and the UN Office on Drugs and Crime. The available studies on the proposed mycoherbicides were few, were not all peer-reviewed, and were primarily greenhouse, growth-chamber, and small field studies conducted under controlled conditions. Those limitations made it difficult for the committee to draw conclusions or to make predictions about the performance of the proposed mycoherbicides in larger field settings and under natural conditions.
On the basis of its review, the committee concluded that the available data are insufficient to determine the effectiveness of the specific fungi proposed as mycoherbicides to combat illicit-drug crops or to determine their potential effects on nontarget plants, microorganisms, animals, humans, or the environment. The questions normally asked before a fungal pathogen is registered as a mycoherbicide in the United States have not been adequately addressed. The committee offers the following assessment of what can and cannot be determined at the present time regarding each of the issues raised in the statement of task.
The degree of control that might be provided by the proposed mycoherbicides, the mechanisms by which they cause disease, and how control of the target plants could be maximized have not been established. Although each of the proposed mycoherbicides has been shown to cause disease in its target plant, disease severity was inconsistent and depended on biotic factors (such as age of plants and strain of fungus) and abiotic factors (such as moisture level, temperature, and ultraviolet radiation). For example, the cannabis mycoherbicide caused plant death in one study but low to moderate disease severity in a second study. For the coca mycoherbicide, published mortality ranged from 35% to 94%, but the background incidence of disease and background mortality in noninoculated plants also were high, sometimes approximating those observed in inoculated plants. Some varieties of cannabis were found to be resistant to the mycoherbicide and exhibited no effects or less severe disease.
In studies of the opium poppy mycoherbicide, a range (6-100%) of leaf necrosis in greenhouse and growth-chamber experiments was reported. Disease severity depended on the type of inoculum used (sexual or asexual spores), the age or growth stage of the plants, and environmental conditions, particularly the dew period and temperature after inoculum application. Plants in early development stages were killed or suffered foliar damage; at more mature stages,