well if we're going to start talking about plant diseases the first thing that we really need to do is Define what we mean by a plant disease and the way that I like to do that is in the context of what I call the disease triangle this is basically a conceptualization that was developed by plant pathologist to describe the three important components that you need to have a plant disease the first is what's called a susceptible host basically you need some sort of plant that can come down with the disease and by the time that we're done with this talk you're going to realize that basically any plant can get a disease of one kind or another the other thing that you need as part of the disease triangle is something called a pathogen and what we're basically talking about here are microorganisms that can cause disease and we're going to be spending a lot of time during this presentation talking about the different types of pathogens and the diseases that they cause finally the third important component in the disease triangle is something called a favorable environment basically you need weather conditions that are favorable to allow the interaction action between the pathogen and the susceptible host to occur and often times what we're really talking about in terms of favorable environment is a lot of moisture because many of the disease causing organisms tend to like wet conditions so it's really this interaction between these three components the susceptible host the pathogen and the favorable environment which leads to what we call disease and what we're really looking at when we see disease is some sort of abnormality in the plant and we're going to talk a lot about what we call symptoms and these are basically abnormalities that that can be caused by different types of diseases now while I think this triangle is very useful for thinking about what a disease is the other thing that I think the triangle is very useful for is thinking about Disease Control a lot of the techniques that we use for managing disease basically are designed to eliminate or at least reduce the importance of one or more of the corners on this triangle so let's look at susceptible host what might we do in order to reduce the importance of this particular corner of the disease triangle think about that for a second well one of the things we can do is use what are called resistant varieties and you've probably heard of these before particularly if you're something like a vegetable or a fruit grower you can buy varieties of your favorite vegetable or your favorite fruit tree that have been bred for resistance they're less likely to get severe levels of disease than other types of varieties and what scientists have found is if they look across the populations of all individuals of a particular type of plant they see variability plants are just as variable as we are as people and you can find some variants of plants that are less likely to get disease than others and they take these they interbreed them and then they come up with a named variety that they can sell as having disease resistance now that's one technique to use to make that susceptible host Corner less important in the Triangle the other thing sometimes that we have to recommend is simply not to grow a particular plant because again in certain situations it's just a losing battle and you're better off growing some other plant rather than the one that you really rather grow but is really prone to disease okay let's move over on to the pathogen corner of the triangle and let's think about that for a second how can you get rid of pathogens now I know by the end of the talk you're going to think that pathogens are everywhere you're never going to get away from them but there are methods that you can use to make this corner less important in terms of disease and think about that for a second well one of the things that you can do is something that you do every fall or you should be doing every fall and that's good fall cleanup removing Le debris cutting back old plants that have died back removing that material from your yard all of that is really impacting the level of pathogens in your landscape because pathogens often times survive in Old plant debris and so one way to get rid of them is to get rid of that debris and that's a very critical management tool for managing a lot of different types of diseases that you're likely to see finally in terms of favorable environment think about that we've got Mother Nature kind of doing what she wants to do how can we possibly modify that corner of the triangle think about that for a second well with a favorable environment one of the things we do have a lot of control over is how much we water in a garden we have to be very careful not to overwater because that tends to provide a very moist environment that's very favorable for all kinds of disease causing organisms the other environmental factor that we tend to have a lot of control over is relative humidity and that's really a function of plant spacing so if we have a lot of plants packed tightly in a garden the way that I do in my garden at home then we tend to trap a lot of humid air around plants and you have to keep in mind plants are natural humidity generators they do something called transpiration so they naturally lose moisture through their leaves that creates a little humid envelope around the leaf surface so anything you can do to space plants farther apart to allow better air flow that tends to remove that humid air and that's a function of either if you've got an existing Garden thinning plants or if you are establishing a new Garden making making sure your plants are spaced far apart to get that good air flow