What exactly is a virus? It’s stranger than you might think.
About 85 percent of plant diseases are caused by fungal pathogens. These are thread-like organisms that form cottony masses, referred to as mycelium, that you see growing on expired bread.
Fungi produce billions of spores that act like seeds. They are quite small but visible to the naked eye. Spores of rust disease erupt from infected leaf tissue like orange powder. Under the right conditions, a fungal thread, or hyphae, will germinate to penetrate plant tissue.
The vast majority of fungi are beneficial saprophytes that feed on dead organic matter, breaking down leaf litter, logs and other elements in the garden. Fungi that feed on living hosts, such as our plants, are parasitic but don’t always kill the host. They may weaken it or simply make it look bad. Many leaf spot diseases are relatively harmless.
Fungicides prevent infection by coating the foliage to stop fungal growth when spores make contact. Some fungicides are absorbed in plant tissue.
A small percentage of plant disease is caused by bacterial pathogens. Bacteria are single-cell organisms that are smaller than fungal spores. A bacterial cell is 10 times smaller than plant and animal cells. In fact, some estimates suggest our bodies contain 10 times more bacterial cells than human cells. We are essentially 90 percent bacteria. Think about that.
Fungal spores require 10-times magnification to see their shape and size. Bacterial cells would require as much as 1,000 times. In plants, bacteria appear as sticky sap, often referred to as bacterial ooze.
Parasitic bacteria infect plants in a variety of ways, whether through open wounds or natural leaf openings. They can be splashed onto plants or introduced by tools. Insects carry many bacterial pathogens. Referred to as vectors, the insect may be harmlessly infected with the bacteria but pass it onto a plant host when it feeds.
Citrus greening is serious disease with no cure. A small insect called a psyllid carries the bacteria. The psyllid has a straw-like mouth part that pierces plant tissue to suck out the nutrients. The feeding doesn’t cause much damage, but the act of feeding injects bacteria into the plant. From an evolutionary point of view, the bacteria hitch a free ride to a susceptible host. The insect benefits from this relationship by weakening the host with a disease, reducing the plant’s defenses.
Many bacterial diseases are controlled with copper-containing fungicides such as Bordeaux mixture, a combination of copper sulfate and lime.
A virus, however, is far different than fungi and bacteria. It isn’t a cell. It’s a particle. It would require an electron microscope to see one. They’re not really considered living organisms. They’ve been considered a biological chemical. They are solely obligate parasites requiring a living host to exist.
Simply put, a virus is a small amount of genetic material, such as DNA, wrapped in a protein coating. It doesn’t feed on cells in the same way fungi or bacteria do. A virus has one mission: to make more of itself. It does this by hijacking a host cell.
The virus infiltrates the cell’s defense system to release its DNA. These short scripts of code reprogram the host’s nucleus, the captain of the cell, to start producing viral DNA. The hijacked cell becomes a viral particle factory, spilling the newly manufactured viruses to neighboring cells.
In plants, a viral disease is permanent. There are no products that prevent or cure a plant virus. Fortunately, most plant viruses are relatively harmless.
A common symptom is variegation, the striping or mottling of foliage. In the Lowcountry, this can be seen on camellia flowers and foliage.
Most plants that exhibit variegation are a result of genetic mutation rather than a virus, although there have been practices that intentionally use viruses to achieve striping effects.
Viral disease in plants is primarily controlled by prevention.