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A plant pathogen is a disease-causing organism which attacks plants. Plant pathogens are of interest for a number of reasons, ranging from concerns about fragile ecosystems to the desire to protect the food supply. There are numerous types of pathogens which can target plants, including viruses, bacteria, protozoans, parasites, worms, and archaea. People who study plant pathogens are known as phytopathologists or plant pathologists.
Plant pathogens can attack in a number of different ways. Some colonize the tissue in the plant, others settle on the surface of the plant, and others may go for specific areas such as the roots, stems, and leaves. Pathogens commonly cause problems like tissue death, browning, a decrease in fruiting, problems with setting flowers, and so forth. In extreme cases, they can kill the host plant.
Like pathogens which attack humans, plant pathogens are very diverse. A plant pathogen may have evolved to attack a specific genus or species of plants, or to more broadly infect most plants. Some plant pathogens take advantage of specific biological processes which can occur within host species, while others settle for using plants as a home because they have valuable resources such as nutrients.
In agriculture, plant pathogens cause serious damage each year. Once a plant has been attacked by a plant pathogen, its products usually cannot be sold. The investment in the plant is rendered useless by the pathogen, and usually great expense is involved in eradicating pathogens from the field and addressing other fallout from the infestation. In some cases, a field becomes so diseased that it must be allowed to lie fallow for several years to recover before attempts are made to grow crops there again.
A plant pathogen can also be of concern when it is accidentally imported or deliberately introduced to a vulnerable ecosystem. Some island nations are especially concerned with this, as they have unique flora which could be decimated by an introduced pathogen from the mainland. Pathogens can also be a problem when they spread between districts; for example, a pathogen which attacks grapes and is limited to Europe can pose a major threat if it reaches wineries in Australia, which may not be prepared to manage the pathogen.
Ecologists are interested in the study of pathogens which can infect plants, looking at their effects on various species and ecosystems. When plants experience dieoffs and other problems, a plant pathogen is often the cause, and a pathologist may be called in to find out which pathogen is responsible, and develop a plan for managing it.
@KoiwiGal - Getting the right variety is only the start of it. It's quite easy to keep any plant disease free as long as you do the prep for it, and know what you're doing.
For example, you can't continue to plant the same variety of plant in the same place multiple years in a row. No matter how well your tomatoes did in that bed last year, they probably put some kind of pathogen into the soil. That will only get worse if you don't swap them with something radically different, like a bean.
Other kinds of plants, like strawberries, are only meant to be used for a couple of years. Strawberries are all very inbred and prone to disease. Once they've fruited once or twice the berries are going to get smaller and less abundant because the diseases will increase until the plant is basically useless.
It's annoying, but that's the way it goes.
@croydon - That's a good idea for anyone who is planning to put in rose beds. Roses are so sensitive that if you are planning to have them, you're better off getting varieties that are as hardy as possible for your environment.
Buying one or two of each type as a sample and trialing them before buying any more is much better than just getting whatever is on sale.
My grandmother used to spend hundreds on her roses, because they were always coming down with something. Not only that, she would have to spend hours out there trying to perk them up.
You'll have more time and money if you get the right variety in the first place.
One of the things I really admire about my local botanic gardens is that they are making an effort to become completely organic. This is much more difficult than it sounds, because often when people go organic in another kind of horticultural venture, they can just accept that the plants will look a bit moth eaten, but that the product will taste or look roughly the same as non-organic produce.
But, the garden has to look good all the time, of course.
One of the things they are doing is replacing their roses with ones that have a much higher disease tolerance. They are trialing a bunch of new roses in a bed near the main garden so that they can test how they do.
That way they know which ones to order in and which ones to replace.