Every year we receive inquiries from gardeners and the general public about apples covered in black powdery spots, wondering if those fruits are safe to eat. When presenting at extension programs or events, we show some of the dramatic symptoms viruses can cause in pumpkins, zucchinis, and other cucurbit fruit, and receive the question, "Could I catch the virus that made that plant sick?"
In most cases, the answer is no. The fungi, bacteria, viruses, and nematodes that cause disease in plants are very different from those that cause disease in humans and other animals. However, some plant pathogens may be able to infect humans as well as plants, and those that do tend to be "opportunistic pathogens," especially on a segment of the population at risk. For example, people with suppressed or compromised immune systems, taking certain medications or suffering from medical conditions or other causes that may cause the human immune system to be weak (immunosuppressed).
Eating or touching infected plants or their parts would not likely infect us with the same pathogen that is making the plant sick. Though, consider that produce from infected plants often has a flavor or texture very different from healthy fruit, so eating it may not be desirable anyway. Unless the disease is merely a superficial spot (such as sooty blotch and flyspeck on an apple), it may be best to avoid diseased produce. Canning of symptomatic produce is not recommended. There are chances the acidity of the final product may change, resulting in spoilage or increased risk of undesirable conditions that may encourage microorganisms potentially harmful to humans to thrive in this new environment.
Some examples of microorganisms that are reported causing problems in humans and plants include some bacteria, fungi but also their products (toxins, etc). An example of this is the bacterium Pseudomonas aeruginosa, which can cause a weak, soft rot of plants such as lettuce. In people with compromised immune systems, this bacterium is known to infect the urinary tract, lungs, blood, and burns and other wounds. It is especially common in hospitalized patients whose immune systems are compromised by severe burns, cancer, AIDS, or cystic fibrosis. For most of us (and for most healthy plants), P. aeruginosa is not a concern.
Some fungi that live on decaying plants can cause disease in humans. One example is Sporothrix schenckii, a fungus that frequently lives on dead rose thorns. This fungus can cause sporotrichosis, also called "rose-picker's disease", if it gets into a person's skin (such as through a scratch) and into the lymph system, or if a person inhales its spores. Symptoms of this disease in humans can include problems with the lungs, eyes, central nervous system, bones and joints. For more interesting information on this fungus, including a gory picture of infected skin, please see (if you can handle the hard images of diseased skin) http://botit.botany.wisc.edu/toms_fungi/feb2003.html
Additionally, some plant pathogenic fungi produce compounds that can be toxic to people, although the pathogen itself does not infect people. For example, some fungi that cause ear rots on corn, such as Fusarium, produce "mycotoxins" (toxins produced by fungi). The mycotoxins produced by Fusarium include fumonisins, zearalenone, and the aptly-named vomitoxin. Effects of mycotoxins in livestock that are fed contaminated grain can include development and reproductive problems, vomiting, general lethargy, and death, depending on the particular mycotoxin present and the level of contamination. Aspergillus flavus is a common contaminant of grain and peanuts, and it produces mycotoxins called aflatoxins. At very high levels (acute exposure), aflatoxins can cause vomiting, pain, convulsions, and death. At lower levels of longer duration (chronic exposure), they can lead to cancer. Mycotoxins are generally an issue only on grain, not on common garden produce, and grain for human consumption is well monitored for their presence.
In general, pathogens that infect plants do not specialize in infecting people. You are not likely to catch a disease from working with diseased plants in your garden, but it is a potential risk (depending on the infection), and consideration should be taken. Garden produce from a sick plant is generally safe to eat, although it may not be desirable. Avoid eating moldy or rotten produce, though, as some fungi and bacteria can produce toxic compounds.
Originally prepared by Christine Engelbrecht, updated by Lina Rodriguez Salamanca
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