Most of the diseases that afflict our gardens are caused by fungi, although there are some that are caused by bacteria and viruses. Fungi are found in every nook and cranny in the world. It is a microscopic world, but they often manifest themselves in very visible ways. Mushrooms are one, and diseases in our garden are another.
Dealing with diseases in the garden is no different than bugs. Identification is the key. Even more important to me though is a knowledge of the Disease Triangle. It is a very concise and easily understood visualization of the disease process. It also helps in formulating prevention strategies.
The Triangle illustrates that there are three components of disease: the pathogen, the plant host, and the environment. Without all three present, there is no disease possible. For example, the fungi responsible for black spot on roses is Diplocarpon rosae.
It is ubiquitous but unless it comes upon both the susceptible plant host, and the right environmental conditions at the same time there will be no disease; therefore, strategies for the prevention of black spot center on preventing the concurrence of the three components of the Disease Triangle.
It is often recommended that leaves with black spot be cleaned from rose beds. This is an attempt to remove the pathogen component. Although it can help lessen the disease pressure, Diplocarpon rosae is so ubiquitous in our region, given a susceptible rose and the right conditions, black spot is going to happen.
That is why the Knockout Rose has become such a hit. It is a rose that is resistant to black spot so the gardener gets the beauty of a rose without the work associated with susceptible roses. Universities and nurseries are putting a lot of energy into this angle of disease prevention. Ohio State has done a lot of work out in Wooster evaluating crabapple varieties for their resistance to the three common diseases of crabs: leaf scab, frog-eye leafspot, and fireblight. The problem is that disease causing organisms often develop new strains and sometimes what was once resistant is now susceptible.
That still leaves the environmental conditions angle, and the old advice to water in the morning is because most disease causing agents need moisture in order to germinate and penetrate the host tissue. Black spot is not a problem for rose growers in California. The growing season is dry, and unless there is overhead irrigation, the right environmental conditions are not present.
The story is different for us here in Ohio. Even if we don’t use overhead irrigation, God provides plenty of it, so often fungicidal sprays are required if you don’t want black spot and you want to grow a rose that is not resistant to it.
The thing the gardener should know about fungicides is that they don’t wipe out the target fungus nor cure any plant tissue that is already infected. They should be thought of more as a protective covering over the plant tissue. That is why thorough coverage is essential if your fungicide application is going to be effective.
If the fungicide doesn’t already contain it, a spreader/sticker should be added. It does just what the name implies, helping to “wet” the leaf completely and adhere to it better. Because most fungicides are broken down by sunlight and rain, repeat applications are necessary. It is important to read the fungicide label for proper application instructions. Know also that repeated applications of the same fungicide can lead to resistance on the part of the pathogen.
Fungicides will not help control diseases caused by virus, bacteria, or other microbial agents. Fireblight on crabapples is caused by a bacteria. Although there are sprays available for bacterial diseases they are not widely available. The best thing to do for fireblight is to prune out the diseased portions. Often times this may mean going well below the outward sign of infection.
Make sure you sterilize your pruners or cutting instrument afterwards, otherwise you can spread the infection. This is true when working with any diseased plant. Rubbing alcohol or bleach is often recommended, but more than a quick dip is required. That’s why I use a propane torch. That’s how they sterilize their instruments in labs. Just remember to turn off the torch before setting it back down in the five gallon bucket you might be carrying it in. Experience is a good teacher.
Over the past few weeks, the principles of pest control I’ve been talking about are taken from training in Integrated Pest Management. A better term for it would be Informed Pest Management, but whatever it is called IPM is a better approach to pest control than the old approach of “It’s Friday, so I must spray Sevin again.”
It is not an organic approach but aims to reduce their use. IPM also acknowledges the negatives of pesticide use and looks to mitigate them by making sure that they are used effectively. It is also an economically sound practice. Pesticides are expensive and their indiscriminate and ineffective use can be costly.
I have only touched on a few of the principles of Integrated Pest Management, but tonight at the downtown Mansfield library at 6:30 p.m., Master Gardener Alan Steiner will be talking more about it. His program will talk about best gardening practices , selecting the most pest and disease resistant plant, using appropriate cultural practices, determining when a pest warrants using a control. There will also be a discussion of organic vs. man-made chemical gardening.
If you can’t make it, look on the internet for more information. One of the best sites for information about the principles and practice of IPM is Cornell’s site.
Finally, weather’s looking good this weekend. Make some time for getting dirty. Guaranteed to make you and your garden happy.
Since his days as forced labor in his mother’s garden, John has learned to enjoy gardening and has actually made a career out of it. From the backyards of Columbus to the rooftops of Manhattan, John has soiled his hands. Former Head Gardener at Kingwood Center, John still gets his hands dirty in the local community. You can contact him at jmakley@centurylink.net.