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Antibiotic use in plant disease control
Patricia S McManus, PhD
University of Wisconsin-Madison, Dept of Plant Pathology, Madison, Wisconsin, USA
A wide range of food crops and ornamental plants are susceptible to diseases caused by bacteria. Bacterial plant
diseases are notoriously difficult to control and often result in sudden, devastating financial losses to farmers.
In the 1950s, soon after the introduction of antibiotics into human medicine, the potential for these "miracle
drugs" to control plant diseases was recognized. Unfortunately, just as the emergence of antibiotic resistance
sullied the miracle in clinical settings, resistance has also limited the value of antibiotics in crop protection.
In recent years, antibiotic use on plants, and its potential impact on human health, has been debated in several
countries.
Practical and Political Aspects
In the United States, streptomycin is registered for use on twelve fruit, vegetable and ornamental plant species;
oxytetracycline is registered for use on four fruit crops (Table 1). Both antibiotics are applied primarily for
the control of bacterial diseases, although streptomycin is also used, to a limited extent, to control diseases
caused by water molds, and oxytetracyline has been used to control certain diseases caused by phytoplasmas (mycoplasma-like
organisms that infect plants). Tree fruits account for the majority of antibiotic use on plants in the US. In 1995,
approximately 25,000 pounds of streptomycin and 13,700 pounds of oxytetracycline were applied to fruit trees in
the major tree-fruit states (1). Antibiotics were applied to apple (20%), pear (35% to 40% ) and peach (4%) acreage.
Although the diversity and quantity of antibiotics used for plant disease control is meager, less than 0.1% of
total antibiotic use in the US, compared to medical and veterinary uses, antibiotic-resistant plant pathogens have
developed.
Streptomycin resistance occurs in plant pathogens (Table 2). Surveys have not revealed oxytetracycline resistance
in plant pathogenic bacteria but have identified tetracycline-resistance determinants in nonpathogenic orchard
bacteria (2). Chiou and Jones have described two genetically distinct types of streptomycin resistance: a point
mutation in the chromosomal gene rpsL
which prevents streptomycin from binding to its ribosomal target (MIC >1,000 mg/ml); or inactivation of streptomycin
by phosphotransferase, an enzyme encoded by strA and strB (MIC 500-750 mg/ml) (3). The genes strA and strB usually reside on mobile genetic elements and have been identified in at
least 17 environmental and clinical bacteria populating diverse niches.
Because antibiotics are among the most expensive pesticides used by fruit and vegetable growers, and their biological
efficacy is limited, many growers use weather-based disease prediction systems to ensure that antibiotics are applied
only when they are likely to be most effective. Growers can also limit antibiotic use by planting disease resistant
varieties and, in some cases, using biological control (applying saprophytic bacteria that are antagonistic to
pathogenic bacteria). Despite these efforts to reduce growers dependency on antibiotics, these chemicals remain
an integral part of disease management, especially for apple, pear, nectarine and peach production.
Antibiotic use on crops and ornamental plants in the US is regulated by the Environmental Protection Agency. Product
labels and supplemental literature clearly state what type of clothing, boots, gloves and respirators must be worn
by mixers, applicators and persons entering a treated area after antibiotics have been applied. These documents
are legally binding and it is a violation of federal law to use an antibiotic in a manner inconsistent with its
labeling. In addition to federal laws, states have pesticide laws and help enforce the federal mandates. Thus,
although the application of antibiotics to plants is markedly different from clinical use and may appear to occur
under uncontrolled conditions (i.e., the open environment), it is a highly regulated activity; farmers are bound
by stringent measures to protect the health of workers and the environment.
Given these seemingly rigid regulations, does antibiotic use on plants pose a human health risk? One consumer advocacy
group has argued that applying antibiotics to crops is an imprudent luxury that may eventually lead to the demise
of lifesaving drugs (5). Growers, however, defend their practice as being so limited in scope that it is inconsequential
to human and environmental health. Unfortunately, both sides lack sound, quantitative data to defend their positions.
For now, this leaves us with a contentious debate based on circumstantial evidence. On the one hand, fruit and
vegetable producers have sizable economic interests, including their livelihoods, at stake when dealing with bacterial
diseases. The amount of antibiotics used in plant disease control is minuscule compared to total use and no apparent
human health issues have arisen after four decades of use. On the other hand, medical experts have witnessed the
failure of one antibiotic after another in clinical settings which, at least superficially, appear to be much more
confined and strictly controlled than farm settings.
Special Aspects of Plant Antibiotic Use
Although antibiotic use on plants is minor relative to total use, application of antibiotics in the agroecosystem
presents unique circumstances that could impact the build-up and persistence of resistance genes in the environment.
First, antibiotics are applied over physically large expanses. In regions of dense apple, pear, nectarine or peach
production, antibiotics are applied to hundreds of hectares of nearly contiguous orchards. Moreover, the past decade
has seen a dramatic increase in the planting of apple varieties and rootstocks that are susceptible to the devastating
bacterial disease, fire blight. This has created a situation analogous to clinical settings where immune-compromised
patients are housed in crowded conditions--settings associated with the proliferation and spread of antibiotic-resistance
genes.
Second, the purity of antibiotics used in crop protection is unknown. Reagent and veterinary grade antibiotics
have been found to contain antibiotic resistance genes from the producing Streptomyces
spp (6). Plant-grade antibiotics are unlikely to be purer
than those used for treating animals and may themselves be an origin of antibiotic resistance genes in agroecosystems.
The genes that were amplified from antibiotics, otrA and aphE, are different from the resistance genes
strA and strB that have been described in plant-associated bacteria (7). Thus, it may
be that plant-grade antibiotics are a potential origin of resistance genes in the environment, but are not necessarily
present and active in plant pathogenic bacteria.
The Challenge for Granting Agencies
The evolution of antibiotic resistant bacteria is outpacing the discovery of new antibiotics. Fruit and vegetable
growers struggle to maintain the registration and efficacy of the only two antibiotics at their disposal. This
political battle follows the Food Quality Protection Act of 1996, a pesticide law that threatens the registration
of several pesticides on which fruit and vegetable growers depend to stay in business. Thus, the stakes are high
for both human medicine and food production. Knowledge of the origins and acquisition of antibiotic resistance
genes in the environment is central to developing strategies to retain the efficacy of antibiotics to control diseases
of humans, animals and plants. But how will this knowledge develop? There is no shortage of scientific expertise
in the field of antibiotic resistance. Rather, the gap appears to be in joining experts from different disciplines
and then persuading granting agencies that have traditionally funded either medical or agricultural research to
recognize antibiotic resistance for the global and multidisciplinary phenomenon that it is.
References
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- Center for Science in the Public Interest. 1998. Protecting the Crown Jewels of
Medicine: A Strategic Plan to Reduce the Spread of Antibiotic Resistance. Washington, DC: CSPI.
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