How To: Control plant disease with sanitation

We prune. We rake. We cut down dead limbs. Then, we haul it all away. Mostly, we do this to achieve a certain look. However, landscape cleanup may have benefits beyond aesthetic value. It may actually help control some diseases.

Many grounds-care professionals rely on fungicides or other chemicals to control plant diseases in landscapes. However, pesticides are not always practical solutions. For instance, pesticides often have restrictions that limit when or where you can apply them. Also, chemicals alone simply cannot solve all of your disease problems. Therefore, it is a good strategy to combine several practices to manage disease most effectively. This approach is called integrated pest management, or IPM. IPM is a term you have probably heard. It refers to pest management that combines all the best control strategies to reduce pest problems. Among other things, this means using cultural practices such as sanitation to reduce the severity of diseases

Why sanitation works The theory behind sanitation. Before disease occurs, infection must take place; and before infection, the plant must come in contact with the pathogen (the organism that causes the disease). Plant pathologists have developed the concept of the disease cycle to show both the continuous nature of diseases and how each step depends on the previous step. Loosely, you can divide the disease cycle into five steps: 1. Contact: The pathogen--in some form--comes in contact with the plant. 2. Entry: The pathogen enters the plant. 3. Infection: The pathogen begins causing disease in the host plant. 4. Dormancy: The pathogen survives harsh conditions in a dormant state. 5. Spread: The pathogen moves to other plants.

When we use sanitation to control diseases, we try to break this cycle after Step 3 to prevent Step 1 from recurring. Sanitation works because you remove the part of the plant that contains the pathogen (or inoculum) before it spreads and infects another plant or another part of the same plant. Typically, you think of sanitation as the pruning and cleanup that grounds managers perform in winter. However, many diseases can spread any time of the year that conditions are favorable, not just after winter. Therefore, the removal of some diseases whenever they appear helps reduce the chance of diseases spreading during the growing season. Correct diagnosis will tell you if you are dealing with such a disease.

The practice of sanitation. Fortunately, the concept of disease-limiting sanitation is one you can easily put into practice by cutting out dead limbs, removing dying plants and raking up infected leaves. There are, however, some key points to keep in mind to ensure success.

Remove diseased portions completely. If you remove only the part of the plant that looks ill or dead, you may not be removing all of the disease. The pathogen may already have grown into the tissue beyond the obvious affected area.

Fireblight is an excellent example of this. When you prune out a fireblight-infected limb, you must prune out dead or cankerous areas 6 to 12 inches below the visible diseased area to make certain you have removed all of the inoculum.

Another example is black knot, a disease of plum and cherry. You must cut several inches below the large swellings to eliminate all of the fungus and prevent it from growing down the branch and causing more disease.

Know which diseases respond to sanitation. You easily can keep some diseases in check with sanitation to remove inoculum but with others you may find that the disease continues to reappear. This is because some diseases tend to be local or isolated problems that spread slowly, while others are widespread in your area. Thus, more spatially limited problems (many viruses, soil nematodes, some systemic wilt diseases and root rots) often are relatively easy to keep in check with sanitation and other cultural practices.

Conversely, sanitation has a less obvious effect on diseases that exist on many nearby properties and are spread through the air (for example, cedar-apple rust and sycamore anthracnose). Such diseases tend to be foliar, and your sanitation practices may not prevent the disease from recurring because of outside inoculum sources (although they may reduce the severity of disease).

Base sanitation practices on correct diagnoses. Sanitation is an all-purpose disease control that works by reducing the amount of inoculum present. However, knowing if, and when, to prune is essential. You should only prune out certain diseases--for example, pine-pitch canker--in the winter. Conversely, you should prune fireblight as soon as you notice the disease, but only when the weather is dry. (Dormant-season pruning of fireblight cankers also is recommended.) Powdery mildews of oaks, however, will not be affected by pruning at all. Instead, raking up leaves during the fall removes much of the inoculum.

Determining the appropriate control for specific diseases requires first that you correctly diagnose the disease. Knowing which disease is the problem allows you to make correct management and disposal decisions based on the life cycle of the pathogen. For example, it does no good to eliminate tree leaves to prevent vascular diseases. Worse, pruning at the wrong time of year or in wet conditions could do more harm than good. Therefore, it is worthwhile to have the disease correctly identified and get some specific management recommendations from an expert. Most diagnostic services provide control information including sanitation practices, if applicable. Most states have plant-disease diagnostic services available. Contact your county extension service or check the phone book.

What to remove The basis of sanitation for disease control is that if you remove the part of the plant that contains the pathogen, you reduce the chances of disease spreading. For foliar diseases, the inoculum is in leaves. Fungi that attack fruits of the plant often produce spores from the infected fruit. The pathogens of canker diseases reside in diseased twig, branches or trunks. Thus, the most commonly used sanitation practices to rid an area or plant of inoculum are raking, plant removal and pruning.

Because many pathogens need wounds and water to infect a plant, prune out infections when tissues are dry. Winter may be the best time in some regions, but it may be the wettest season in other areas. When pruning out diseases, sterilize your pruning shears between cuts with a 70-percent alcohol or 10-percent household-bleach solution. Bleach is corrosive, so be sure to wash your shears before storing them.

Occasionally, you must remove an entire plant to prevent the spread of the disease. Examples are pine wilt (caused by the pinewood nematode), Phytophthora blight of periwinkle and Hypoxylon canker of oak. In such cases, if you can't control the disease, it becomes necessary to sacrifice ailing plants to prevent the disease from spreading to nearby healthy specimens.

Another practice counted as a type of sanitation is the removal of alternate host plants. Certain fungal pathogens, rusts in particular, require two different host species to complete their life cycle. Eliminating just one of the hosts should, theoretically, eliminate the disease. Based on this concept, you often hear recommendations to remove cedars (Juniperus spp.), the alternate hosts of cedar-apple rust and cedar-quince rust, that are growing in the vicinity of susceptible plants such crabapples or hawthorns. Unfortunately, it often is impossible in landscape situations to remove all such plants within the recommended minimum radius of several hundred yards. Nevertheless, it makes sense to eliminate what cedars you can--it may at least reduce the severity of rust outbreaks.

What to do with diseased material Now that you have cut all of the dying limbs down or raked your infected leaves into piles, you have two options: get rid of the waste or reuse it. Getting rid of it includes burial, burning or sending it to a landfill. All of these methods remove the inoculum and therefore will successfully reduce the incidence of many diseases.

Today, however, many people are interested in reusing waste materials. Studies show that at least 20 percent of the material sent to landfills is organic yard waste, and this has prompted greater emphasis on steps that reduce the amount of yard waste going to landfills. Therefore, many individuals and communities use this material as mulch or compost.

With most landscape waste, these solutions work well. However, should these methods include diseased plant material? Compost piles, if you manage them properly, reach temperatures of about 160 degrees F. At this temperature, many pathogens die within 3 weeks. It is important to bury (within the compost pile) any diseased material you add. Mixing the compost every few days--which you should do anyway--also helps keep diseased material buried and decomposing.

Composting works well for herbaceous material and leaves but is not usually recommended for woody plant tissues because they take so long to degrade. If you do add wood, be sure to chop it into 1.5-inch or smaller pieces. Do not compost plants with soilborne or vascular (wilt) diseases. Compost piles may not get hot enough to destroy these organisms and, when you use that compost, you could introduce the pathogen into your soil. Because soilborne and vascular diseases are so difficult to treat, you should avoid spreading the pathogens that cause these diseases whenever possible.

Use equal amounts of green (for example, grass clippings) and dry (such as dead leaves) plant material together when composting. This type of mix provides the right carbon-to-nitrogen ratio to compost effectively. Research shows that compost bins need to be at least 36 inches in each dimension to reach high-enough temperatures, which is the most critical factor for reducing inoculum. Contact your local county extension office for more information about composting.

Mulching with most types of woody material should be acceptable, unless the tree from which the wood came was infected with a vascular disease. Some vascular diseases, such as Verticillium wilt, enter the plant from the soil and infect vascular tissues. Preliminary research from Minnesota demonstrates that the Verticillium fungus can infect eggplants growing in, or mulched with, fresh wood chips from maple trees infected with Verticillium wilt. Although we need more research to confirm that mulches from vascular-infected trees can spread disease to other landscape plants, this research suggests that it may be possible. Ongoing studies in this area will help clarify the results and determine if this is a concern in landscape operations.

Sanitation should be a common component of your disease-control practices. Along with proper plant selection and care, and the judicious use of pesticides, sanitation should play a prominent role in any integrated approach to plant-disease control.

Betsy Hudgins is an extension plant pathologist at Oklahoma State University (Stillwater, Okla.).

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