Do you remember the fire safety lesson from elementary school where they told you that three things had to be in place for fire to occur? [The answer is fuel, heat, and oxygen.] Take any one of those components out of the equation and viola! No fire. Well, plant diseases work much the same way.
The three sides of plant disease
For a disease to take hold in your garden, the environment has to be right, there has to be a host, and, of course, the disease (or pathogen) has to be there. Alter any one of those three and any diseases that do occur will be much less severe and less frequent. Remove one of those three and there won’t be a disease at all. Since prevention is far easier than treatment, the disease triangle is a handy tool for you to use in the garden. Another way to look at these factors is with a Venn diagram. When there is enough overlap between the three conditions, disease can occur.
How healthy plants defend themselves
You may be surprised to learn that many pathogens are already present in your garden. New arrivals are unusual and notable. That’s why quarantines are used. So, if all the disease-carrying pathogens are already present, why doesn’t everything just die a horrible death the moment it appears? Like us, plants have evolved to protect themselves, though not in the same way. Unlike us, plants do not have immune systems. What they do have is chemical warfare that has developed in tandem with pathogens. Plants use these steps to protect themselves, when they are able:
(And all this time, you probably thought that plants were relatively passive, right?)
Symptoms & disease identification
Symptoms of plant disease include wilting, stunting, deformed leaves or other growths, cankers, and chlorosis, just to name a few. If you notice disease symptoms, use that information to figure out what is making your plants sick. Feel free to share questions and photos in the Comments sections, plus there are several online resources and you can always contact your local Master Gardeners for advice. Once you know what your plants are up against, you can use the disease triangle to break the cycle.
Pathogens are disease-carrying bacteria, viruses, or other microorganisms. Many times, if they are not already present, these pathogens catch a ride into a landscape on flying insects, tools, shoes, and newly acquired plants. Chewing insects may simply leave behind a point of entry for disease, or they may be infected themselves, transferring viruses or bacteria to the host plant as they feed. You can interrupt the pathogen side of the triangle with these tips:
You can control common environmental factors with these good cultural practices:
The host is the plant that gets sick. Some diseases affect only a single host, while others can infect many different types of plants. This is where knowing more about your plants can really help. [Did you know that lilacs grown near apple trees are more likely to get bacterial blight? Or that cedar apple rust can only occur when apples are grown within a couple of miles of Eastern red cedar trees?] These tips can help prevent disease by helping the host plant:
Many botanists have added time as a factor, converting the disease triangle into a pyramid. They do this because time can be a critical factor in disease development. Diseases take time to infect a plant and to reproduce. Water sitting on a leaf for a few minutes may do nothing, while several hours of surface water on the same leaf may be deadly. Regularly monitoring plants for signs of disease can put time in your favor.
Keeping plants healthy overall helps them fight pests and diseases the way they have evolved to do so. Plants experiencing heat or water-stress, nutrient deficiencies, inappropriate or nonexistent pruning, or physical damage are less able to defend themselves.
Each microclimate has it own set of problems. Learning about your plants and what they are facing, puts you in control of the disease triangle.
You can grow a surprising amount of food in your own yard. Ask me how!