Superintendent Magazine - July, 2010

COLUMNS

The Control Center: Bacterial Wilt

By John C. Fech and Roch E. Gaussoin
PHOTOS BY PETE DERNOEDEN,UNIVERSITY OF MARYLAND

Typical bacterial wilt symptoms.

Most turf diseases are caused by pathogenic fungi that degrade the quality of the leaves and stems or compromise the root system and capacity to absorb water and nutrients. Other maladies are caused by abiotic factors such as compaction, inadequate soil drainage and winter injury.

Bacterial wilt, caused by Xanthomonas campestris and Xanthomonas pv. Graminis, is the only known bacterial disease of turf in the U.S. Primarily a disease of annual bluegrass, bacterial wilt has a wide geographic range, but appears to be most problematic in the northeastern U.S. Infection of C-15 Toronto creeping bentgrass greens from previous decades is also well-documented. Because of its capacity for rapid spread through a putting green and its devastating effects on the conductive vessels of the turfgrass plant, bacterial wilt is a major concern.

Season of infection

Bacterial wilt is favored by periods of heavy rainfall followed by warm, sunny days with cool to moderately warm nights. Heavy irrigation volumes add to the likelihood of infection. As a result, late spring/early summer and early fall are favored times of the year for bacterial wilt. The disease may subside during sunny, dry weather, but commonly
redevelops if rainy weather patterns return.

Symptoms

This disease’s name is quite apt. The leaves on individual plants lose turgor and wilt from the tip back to the stem. Within a few days, the entire leaf becomes wilted, turns off-color and eventually shrivels. Early in the infection process, the crowns and roots remain firm, white in color and generally unaffected. As the disease progresses, softening and degradation of the whole plant occurs. As a result, shallow depressions often develop, compromising the playability of the putting surface. Viewed from above, whitish-tan, dime-sized spots are evident.

Bacterial wilt is not easily diagnosed as it is easily confused with other maladies, such as smut and anthracnose. This is especially true once initial infection points combine with each other to form small, irregular patches. It can be further confusing to the turf manager when anthracnose is present in addition to bacterial wilt.

Similar to other turf diseases, it can spread through a green rapidly. It is common for a green to appear mottled when infected, as disease-tolerant grass plants remain unaffected adjacent to wilted plants. In addition to its proliferation during rainy weather, it tends to be more problematic on waterlogged soils and shaded or poorly drained greens. Anecdotal accounts also lend credence to the notion that greens that receive low air circulation are more susceptible to infection.

Typical effects of bacterial wilt infection.

Life cycle

Bacterial wilt overwinters in thatch and the remnants of diseased plant material. It is moved around the golf course via rain splash. Physical dissemination methods, such as carrying the inoculum on shoes or various pieces of turf equipment, may be secondarily involved.

Wounds from mowing and verticutting offer good entry points for the disease. Once inside the plant, bacterial wilt can quickly move from the leaves to initially occupy the water-conducting vessels, later moving to the crown and roots. Once the process of infection begins, the result is wilting and death of infected plants.

Cultural control

Because the pathogenicity of bacterial wilt tends to be enhanced as a result of abrasion or golf maintenance operations that bruise or cut tissues, consideration of common cultural practices is recommended. Verticutting, cultivation and sand topdressing have the capacity to wound turf plants, creating additional disease entry points to existing stomates and hydathodes. These practices are performed for the benefits of improved rooting, increased drainage and thatch management; each also have negative side effects associated with them, such as the increased potential for desiccation and possible creation of soil layers. The potential for increased bacterial wilt infection is another possible consequence. As long as the disease is active, minimizing any operation that injures the blades or crowns of the grass plant should be considered. Obviously, mowing must continue, perhaps on a less-frequent basis.

Adequate air movement is important when managing bacterial wilt.

Disease severity usually decreases when mowing height is increased and frequency is reduced. Unfortunately, these changes result in slower speed and playability of putting greens, producing an undesirable trade-off. Rolling might be considered as a less injurious substitute to mowing. Changing from grooved to solid rollers also may help limit infection. As much as is possible, avoid mowing wet greens, as mowing when the grass blades are dry, most likely in the afternoons instead of the morning hours, may slow the spread of the disease. Of course, mowing in the afternoon may interrupt golf play to an unacceptable level, which is something else to consider.

Another possible cultural control strategy involving mowing should be considered when infection is present on only a few greens on the course. If this is the case, a mower that is used only on those greens can be helpful in restricting the transmission of the pathogen to other greens. As a preventative measure, this mower should be cleaned with a 10 percent bleach solution after each use, and then rinsed with clean water.

Nitrogen fertility can help to mask the damage and may hasten recovery. Foliar or soluble applications of nitrogen at 0.1 to 0.125 actual N per 1,000 square feet are recommended.

Bacterial wilt is increased on shaded greens and when air circulation is restricted. Consider a tree removal program or the use of fans to decrease incidence of wilt. Poor surface drainage or consistently waterlogged regions of the putting surface should be addressed. Deep-tine aerification or repair of crushed or damaged drain tile or retrofitting new tile into an existing green may suppress bacterial wilt infection and will also increase green health and function.

Chemical control

The use of fungicides on a bacterial infection is, more often than not, unsuccessful. Some fungicides may have limited bacterial suppression while others will have none. Copper hydroxide, found in products like Junction, Kocide and others, has been reported to provide moderate to good short-term control. Contrary to the traditional 14 to 28-day spray interval common for many fungal pathogens, anecdotal evidence indicates copper hydroxide should be sprayed on five to seven-day intervals in at least 5 gallons of water per 1,000 square feet to be effective. Copper-hydroxide may cause discoloration and should not be mixed with other chemicals, including foliar and soluble fertilizers, unless allowed on the product label.

John Fech is an extension educator specializing in turf and ornamentals at the University of Nebraska-Lincoln. Roch Gaussoin is a professor of horticulture and extension turf specialist at the University of Nebraska-Lincoln.