Humans have an immune system that keeps them healthy. Do plants have something similar that helps them survive? Most definitely yes! How plants defend themselves against diseases is now being studied at nearly all of the major universities with great strides being made on the subject. In general plant defenses can be divided into two main categories. The first is simple structural defenses such as a plant with waxy leaves that pathogens canŐt attach to very well. The second is induced defenses. These defenses are initiated after the plant is under attack, more like a humanŐs immune system.

Structurally some plants have attributes that help keep them free of invading pathogens. Waxy or tough leaves make it hard for bacteria or fungi to get into the plant. In addition, many bacteria and fungi rely on water to live, move and reproduce. If the plantŐs leaves shed water quickly, it is less likely that a pathogen will be able to get a foothold. Some plants take this concept even a step farther and have spines, thorns, fuzz, or prickles to make its outer surface less friendly.

The defenses a plant initiates after it is under attack is far more complicated. A key strategy is to simply try to exclude the pathogen. Plants have several methods to accomplish this. When a plant recognizes that it is under attack it begins to strengthen its cell walls. Stronger cell walls may act as a barrier against invaders. But it gets even more interesting that that! Some plants actually produce materials that trap the invading pathogen and prevent it from spreading! Some plants produce compounds in the cell wall that actually degrade the enzymes that some pathogens produce to break down the plant and gain access. Other plants stop the flow of nutrients and water to the infected part of the plant and allow that one part of the plant to die, and hopefully save the rest. Some plants will drop the leaves that are diseased to try to get rid of them. Trees often deposit large amounts of gum around diseased areas in an attempt to keep the pathogen isolated. A phenomenon known as the hypersensitive response is when plants actually kill their own cells in the area of attack. If a pathogen requires a live host, killing off a few cells to save the rest of the plant sounds like a great strategy. Along with the hypersensitive response comes a cascade of other events that provide even more protection to the plant.

When a disease-causing organism attacks a plant, it produces several types of antimicrobial compounds to ward off the problem. Generally speaking, these compounds include Phytoalexins and PR (Pathogenesis Related) Proteins. They are systemic, meaning that they are present not only at the site of infection, but all over the plant. Pytoalexins are similar to antibodies are produced in response to infection and are directly toxic to the invading organism. Interestingly, some pathogens have developed ways around this by actually secreting substances that suppress phytoalexin production. There are numerous types of PR proteins. Some are toxic to invading pathogens. Some inhibit fungi from producing, releasing, or germinating spores. Some actually breakdown the cells walls of fungi and bacteria.

Lastly plants have a host of less obvious responses to pathogens including growing more vigorously, increasing photosynthesis, increasing root-mass, speeding up photosynthesis and profuse blooming. You might wonder why a plant would do this. The answer is left up to hypothesis. It seems logical that if a plant is going to have to strengthen its cell walls and produce antimicrobial compounds it may need an extra energy supply to do the job. Maybe that is the reason for some of these processes. Or maybe it is as simple as the plant trying to outgrow the problem. If the plant can grow faster than a disease can destroy it, then it wins the game of life. The mystery of why challenged plants love to reproduce is an interesting one. Most obvious is the fact that if a plant is facing destruction, it is to its benefit to reproduce profusely and carry on the species. This is the basis behind the old gardeners trick to prompt blooms from reluctant plants by hitting them with a rolled up newspaper. This "scares" the plant and causes it to send energy toward reproduction, hence flowers! It is as if the plant thinks it is going to die and had better get on with reproducing while it still can. It is amazing how intricately the threads of nature and life are woven.

So, now that you have had your lesson in plant pathology, can you and I actually harness or enhance all of these amazing abilities of plants to defend themselves, or is it just interesting science? Prepare for the future Đ Scientists at Cornell University discovered a protein that is produced by the bacteria that causes Fireblight in apple trees! This protein already occurs in nature, is not harmful, and does not cause any diseases. It simply tricks the plant into thinking that there is a pathogen present and that it should mount a defense, similar to a vaccination. The protein causes this reaction without stressing or injuring the plant. The result is a healthier plant with far more flowers! This product is now available commercially under the name Messenger¨. Already gardeners are discovering this inventive new technology. The American Rose Society has endorsed it as well as the National Home Gardening Club and the National Gardening Association. Messenger® can be purchased directly from the manufacturer EDEN BIOSCIENCE www.messenger.info, or from fine garden centers in some areas. Give it a try. See what it's like to harness nature!