Infectious disease is on our minds these days, but let us at least step away from our human infectious disease waking nightmare, to talk about infectious plant diseases (see also https://bygl.osu.edu/index.php/node/1450). Yes, as horticulturists know, plants have pathogenic fungi, bacteria, oomycetes, nematodes, phytoplasmas, plants (such as mistletoes), and even viruses, such as the tobacco mosaic virus and the impatiens necrotic spot virus, that cause plant diseases. With that in mind, let us discuss several curious aspects of infectious plant diseases and beyond.
As often is apropos, let us start with Shakespeare, from Sonnet 35: “Roses have thorns, and silver fountains mud;/Clouds and eclipses stain both moon and sun,/And loathsome canker lives in sweetest bud,/All [plants] make faults.” He was right that all “plants” (actually he said “men”) make faults; no plant or animal is without its fair share of diseases and disorders.
The Bard was not quite right about “thorns” at least to a botanist, who define roses’ thorn-like structures as “prickles”, outgrowths of the epidermis of stems, containing no vascular tissue. Thorns are modified stems arising from the axil of a leaf. In a way of course this distinction may be described as “full of sound and fury, signifying nothing” in light of the reality of “if you prick us do we not bleed” , to mix a few Shakespearean metaphors. Nor considering modern terminology would be consider buds to have “cankers”, since this term is generally reserved for symptoms of disease on stems.
But that is beside the point, so to speak. I spoke of this recently to a group in Tiffin (back in ancient days when we all mingled in groups). It was part of an exercise I often do about infectious plant diseases. As with human diseases, when we talk about infections we talk about host-parasite relationships inside host cells and about the range of susceptible host organisms affected by a particular pathogen. These are topics we are all becoming attuned to in the news these days as we talk about the COVID-19 disease, the CDC, and pandemics.
In the case of plant diseases, of course, we talk about plant disease clinics, rather than hospitals dealing with infectious diseases of humans. With that in mind, I posed the question to the group: are there any plant pathogens that have a host range so wide that they will also infect humans? Host range is something we talk about all the time with plant diseases since it helps us understand the risk of a particular disease to a particular crop. For example, plum black knot, caused by the fungus, Apiosporina morbosum, only occurs on plums and related plants in the genus Prunus, such as cherry and peach and almond.
Bacterial fireblight, caused by the bacterium Erwinia amylovora, occurs only on plants in the rose family, such as apple (Malus), pear (Pyrus), and rose (Rosa), and so on. It is a useful way to assess risk when diagnosing plant diseases. Most plant pathogens have a narrow host range, occurring only on one species, one genus, or one family of plants. Other plant pathogens have a wide host range, such as the Botrytis gray mold fungus, with hundreds of different host plants.
So, is there any pathogen with so wide a host range it affects both plants and animals such as humans? I tend to say – no. The closest is certain minor flower and leaf pathogens such as the Alternaria fungus that are one of the reasons doctors prohibit well-wishers from bringing flowers into ICU units. These fungi, however, do not infect humans; they simply clog up respiratory tissue with spores; cells of humans are not infected by Alternaria. So that is not a pathogen that has a host range so wide that it affects both humans and plants.
Also, toxins, such as those produced by, for example, the ergot of rye fungus, Claviceps purpurea, though they affect humans with gangrenous and hallucinogenic ergotism, do not infect human cells, though they certainly can affect humans. The ergot fungus is a plant pathogen, but not a human pathogen. Sometimes people mention allergens, but they do not infect humans (or plants); they simply cause an allergic reaction for humans, not an infection.
One possible caveat to all this: how about our growing awareness of immune-compromised people, something the public became more aware of due to diseases such as AIDS or with the use of drugs to deliberately suppress the immune system when transplants are needed? There may be some case to be made that certain mold fungi that cause plant diseases can cause human infections of severely immune-compromised humans, but this is still at the edge of our scientific understanding thus far. Rest assured, though, you will never get Dutch elm disease and elm trees will never catch the common cold! Their host ranges are simply not that broad.
Which brings us to this example to make the point: rose gardener’s disease. The fungus that causes this disease of humans is Sporothrix schenckii. Rose gardener’s disease qualifies as a human disease, as the spores of this fungus infect human skin, and rarely more seriously if inhaled, leading to systemic infections. This fungal pathogen of humans occurs in soil, on sphagnum peat, and on rose “thorns”, which if you are pricked and the pathogen is present may cause you to become infected. So, is this not a case of both a human and a plant disease?
No, Sporothrix schenckii is not a plant pathogen, it does not infect rose prickles or any other plant tissue, it is simply an occasional fellow traveler if splashed from soil onto prickles. So at least for plants, this fungus does not join with plant pathogens that cause plant diseases, such as rose black spot, bacterial crown gall, or rose rosette virus disease. Never the twain shall meet on us humans.
These kinds of distinctions can aid in our critical thinking as we learn more about infectious disease, certainly our topic du jour, and for many future days, weeks, months and years. Pathogens and disease are ever with us: it is just one way that organisms (naturally) interact. Natural, but often to our despair and always a topic of our desire to manage.
By the way, the rose-gardener’s disease can be quite serious if left untreated by antibiotics prescribed by a human (not a plant) doctor. Especially if the person pricked by that rose has a compromised immune system. So, if you develop sporotrichosis from Sporothrix schenckii, and have symptoms of discolored lesions where prickles penetrated the skin, check with your physician.
BTW: I have not included any images of rose gardener’s disease for two reasons: 1.) I do not have any of my own and try not to use too many images that are not mine; and 2.) They are rather gruesome. You can google them, though, if you are curious. So, let us close with a rosier image.