Bacterial diseases can contribute to turfgrass decline, although they are often overshadowed by fungal diseases. Research on bacterial turfgrass diseases has increased, but their etiology remains poorly understood.
Bacterial wilt and bacterial decline are two notable diseases affecting turfgrasses. Symptoms of bacterial diseases have been debated due to unreliable inoculation protocols.
Credit: PACE Turf
Causal agents
Xanthomonas translucens causes bacterial wilt on annual bluegrass
Acidivorax avenae causes bacterial decline on creeping bentgrass
Primary Hosts
Creeping bentgrass, annual bluegrass, and perennial ryegrass.
Disease Cycle
Bacteria, including plant-pathogenic types, are found in soil, foliage, and organic debris. They enter plants through injuries or natural openings. Pathogenic bacteria produce substances that harm plants, such as gibberellic acid, which causes excessive growth in etiolated turfgrass. Some bacteria survive in debris or infected plants.
Bacterial wilt occurs during rainy weather, while bacterial decline is common in hot, dry periods. These bacteria disrupt water and nutrient flow, causing wilt and plant death. X. translucens can spread with mowing equipment, while A. avenae does not appear to spread via mowers.
Epidemiology
Moisture facilitates the movement of bacteria in turfgrass, with films of moisture on leaf surfaces aiding bacterial transportation. Low-cut turf, such as putting greens, is more susceptible to bacterial diseases like etiolation, decline, and wilt due to closer proximity to the soil surface.
The timing of disease occurrence varies. Bacterial wilt occurs during warm to cool, wet periods in spring and autumn, with possible activity during favorable summer conditions. Bacterial decline is prevalent during high heat and drought, intensifying disease symptoms and adding stress to the turfgrass.
Symptoms
Bacterial wilt
When bacterial wilt occurs on Poa annua and Lolium perenne, infected plants exhibit wilting, yellowing, and a lime green or bluish-green appearance. Reddish-brown spots develop, fading to light brown or bleached tan as the turf dies. Stem bases may appear discolored and water-soaked. Younger leaves can also exhibit etiolation.
Credit: John Kaminski/Penn State University
Bacterial decline
When bacterial decline occurs on Agrostis stolonifera, etiolation symptoms progress to general decline, with browning and twisting of leaves, and dieback to the leaf sheath. No water-soaked lesions appear on leaf tissue. Extensive colonization of xylem vessels disrupts water movement.
Signs
One of the signs of bacterial diseases in turfgrass is the observation of bacterial streaming from the xylem vessels (see video below). This can be observed when cutting a stem or root and observing a slimy ooze or exudate coming out from the cut surface. Bacterial streaming is a result of bacteria multiplying and colonizing the xylem, which are the water-conducting vessels of the plant.
Management
Bacterial diseases are difficult to control as no legal antibiotic options are available. Extreme cultural practices not always suitable for golf course turfgrass are often required.
Cultural
Cultural management strategies for bacterial diseases in turfgrass include
- minimizing leaf wetness
- adjusting mowing practices
- timing mowing when turf is dry
- avoiding abrasive practices
- selecting resistant cultivars
- optimizing fertilization and irrigation
- adjusting plant growth regulator programs
These practices aim to reduce stress and create unfavorable conditions for bacterial growth.
Chemical
Control options for etiolation caused by bacterial pathogens vary depending on the specific pathogen involved. Adjustments in plant growth regulator (PGR) programs have been shown to be effective in reducing symptoms of etiolation outbreaks. For infections caused by A. avenae, reducing or eliminating the application of trinexapac-ethyl, a late gibberellic acid inhibitor, can help reduce etiolation and bacterial decline. Switching to early gibberellic acid inhibitors, such as paclobutrazol or flurprimidol, can also be effective in reducing etiolation caused by certain bacterial pathogens.
It is important to note that the response to PGRs may vary among bacterial pathogens due to their gibberellic acid production properties. A. avenae bypasses the regulation pathway of trinexapac-ethyl, while X. translucens has not been shown to produce gibberellic acid. Trinexapac-ethyl has been found to reduce etiolation caused by X. translucens. Certain demethylation inhibitor fungicides, which have gibberellic acid-regulating activity similar to paclobutrazol and flurprimidol, have also shown efficacy in reducing etiolation.
Chemical control measures for bacterial etiolation are limited because many fungicides do not have activity against bacterial pathogens. However, some fungicides have proven effective when applied preventively. Certain demethylation inhibitor fungicides and products containing acibenzolar-S-methyl have shown efficacy in reducing etiolation caused by bacterial pathogens when applied preventively. Copper hydroxide has been effective in reducing bacterial pathogens on plant surfaces, but concerns about phytotoxicity have been reported with frequent use on closely mown turfgrass.