By Mark Gleason
Plant Pathologist
Iowa State University Extension

As the ground thaws out, gardeners are starting to get excited about the 2008 growing season. What kind of perverse spoilsport would bring up plant diseases at such a hopeful moment? 

Well, me.  I get paid to remind gardeners about the dark side of the garden force: the microorganisms that attack your plants. 

Most people who aren’t grossed out by plant diseases are at least puzzled about them.  But reviewing just a few simple ideas can help gardeners get ready for 2008’s disease challenges.

Plant problems can come from either stresses (such as overwatering or under-fertilization) or microorganisms. Every contagious plant disease involves an interaction between three players: the host plant (the victim), the pathogen (the assailant), and the environment.

Plants get sick only when a pathogen comes in contact with a plant that is susceptible to it, and then only when the environment allows the pathogen to attack. Integrated pest management (IPM) emphasizes how each of these three players can be manipulated to reduce the risk of a contagious disease outbreak. 

Even though there are thousands of different species of plant pathogens – fungi, bacteria, nematodes, viruses and even parasitic plants – deciding how to manage them is simpler than it might seem. Luckily, each type of host plant – hosta, daylily, astilbe and what have you – is attacked by a relatively short list of pathogens. So, once you know the name of your host plant, you have already taken a huge step forward in reducing the diagnostic challenge. 

Another plus is that pathogens come in only a few types. For example, some are root-rotters, some are leaf-spotters and some cause plants to wilt or turn yellow. The thousands of species really only have a limited bag of tricks.

For example, although many species of powdery mildew fungi attack garden perennials, almost all powdery mildew species behave similarly. So once you understand how to manage powdery mildew on dahlia, the same general IPM approach is likely to work on delphinium – even though the species of powdery mildew fungi usually differ on plants from different plant families. By learning the basic plan of attack used by each type of pathogen, you will have a head start on deciding how to prevent diseases on the plants you grow.

Diagnosing the cause of a plant disease puts you on the road to managing it successfully. Once you know what is causing the symptoms on your plants, you can zero in on the best ways to control the disease – and take steps to avoid it in the future. A few good reference books can help you diagnose disease problems by matching what you see on your plants with the book’s photos and text descriptions. The Internet can be helpful, too, especially sites from state Extension Services. 

Extension specialists, Master Gardeners and plant diagnostic clinics in Iowa have a wealth of expertise in diagnosis, as well as treatment and prevention advice. It’s worth your while to get acquainted with the garden plant disease experts in your area, in order to learn how they can help you. Iowa State University’s Plant and Insect Diagnostic Clinic (phone: (515) 294-0581 and e-mail: sickplant@iastate.edu)  is a wonderful resource for helping you figure out sick-plant problems.

But you don’t have to be a scientist to be good at diagnosis. Armed with a few well-illustrated resources, anyone who loves plants and is a careful observer of clues can become an excellent disease troubleshooter.

Being a gardener means making decisions that affect the risk of disease outbreaks. Which varieties you choose, the cultural practices you use to grow them and which (if any) chemical or biological control products you apply can all affect the appearance of your plants.

Each decision a grower makes can influence the other decisions. For example, deciding to grow a cultivar that is very resistant to major diseases can effectively reduce the aesthetic and financial costs of disease. On the other hand, growing a particular cultivar that you really like, but that is especially disease-prone, means that you may choose to put extra effort into protecting it from diseases.

How do you decide what disease management strategies make the most sense for you?  Making sensible choices seems challenging when you are growing many different species of garden plants, each with its own growth characteristics and risks. To avoid getting lost in the details, it’s helpful to rely on some basic disease management principles. 

IPM is a strategy for harmonizing management practices – including cultural, genetic resistance, biological, mechanical and chemical options - to keep diseases, insects, weeds and other pests in check, with minimal harm to the environment and your checkbook. An IPM plan should make sense for your particular gardening situation. Likewise, disease management plans need fit into strategies for controlling insects and other pests, and for meeting the grower’s goals.

For home gardeners, simple things can have a huge impact on suppressing diseases. Just by watering plants at the soil line rather than overhead will help to discourage leaf diseases.  Rotating garden vegetables, to avoid replanting the same sorts in the same places each year, can also give you an advantage. And disease-resistant varieties are the cheapest and best disease insurance you can buy.

Remembering just a few simple IPM ideas can make for happier gardeners and frustrated pathogens in 2008. With these ideas and some luck, your early-spring gardening fantasies could turn into reality. 

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