Cover Story

By Wakar Uddin, Ph.D.

Basal rot anthracnose is a destructive disease of annual bluegrass (Poa annua L.) and creeping bentgrass (Agrostis stolonifera L.) in North America and western Europe. Anthracnose in turfgrass had been previously classified as senectopathic problem associated with advanced senescence of plant tissue where infection is usually initiated (1). However, incidence and severity of the disease in golf course putting greens have dramatically increased in recent years (2, 3). The reason for the increased problem in putting greens is unclear. It is likely that certain cultural practices and changes in pathogen population may have, at least in part, contributed to the problem.

Two phases of the disease are commonly recognized – the foliar blight and the basal rot (5). The foliar blight phase develops during warm periods in the summer, and the basal rot phase may occur at any time of the year depending on the geographic area. In close-cut annual bluegrass, small orange and yellow spots may appear in the winter or spring, and infection of crown results in yellowing followed by death of the plants. Infected plants later develop acervuli and melanized setae and profuse production of these structures may be found on crown and stem tissues or between the sheaths of the dying and deteriorating (Fig. 1). Nutritional deficiencies and environmental stresses are also important factors influencing anthracnose basal rot development.

Fig. 1. Severe infection of crown, roots, and stem tissue of annual bluegrass by Colletotrichum cereale.

Turfgrass cultural practices that cause mechanical injury to plant tissue, particularly crown, stolon, and stem tissue, provide infection courts for the anthracnose pathogen for efficient infection. Vertical mowing of putting greens is commonly used to reduce grain, puffiness, excessive thatch, and non-uniform shoot density of putting green turf for better turf quality and faster putting speeds (6). Vertical mowing could cause injury to crowns, roots, and stolons, depending on the intensity of the operation. Maintenance of a proper mowing height is also important in putting green management. Golf course superintendents usually prefer maintaining a relatively low mowing height to achieve ideal green speed (4). However, low mowing heights may also contribute to significant plant injury that is likely to provide a greater number of infection courts for the pathogen. The injury could even be much more severe in scalped areas of the green where the mowing height is even lower. Therefore, vertical mowing and low mowing height are potentially serious factors governing basal rot anthracnose development. This study was undertaken to determine the effects of vertical mowing and mowing height on the severity of anthracnose basal rot in mixed turf of creeping bentgrass and annual bluegrass.

Mowing practices – the horizontal and vertical cuts

Two important cultural practices, horizontal and vertical mowing, cause significant injury to plant tissue which can serve as the infection court for the anthracnose basal rot pathogen. The effects of the two mowing factors were evaluated on annual bluegrass, a putting green maintained under low-nitrogen fertility (0.25 lb. single application in early May) with irrigation regime to maintain low soil moisture. The vertical mowing done at three different depths: Deep (0.20 inch), shallow (0.13 inch), and none (control). The horizontal mowing was applied at three heights of cut: High (0.17 inch), medium (0.12 inch), and low (0.08 inch).

Following the mowing, the turf in the putting green was inoculated with Colletotrichum cereale which was originally isolated from the symptomatic annual bluegrass turf at the Valentine Turfgrass Research Center at the Pennsylvania State University. A spore suspension of C. cereale (10 4 conidia per ml aqueous suspension) was applied to the turf with a hand-held sprayer. Inoculation was conducted in the late afternoon (prior to sunset), and the turf was covered with a 6-mil plastic sheet over the three subsequent nights (cover removed each morning). Severity of anthracnose basal rot (Index 0-10; 0=turf asymptomatic; 10=>90% turf area symptomatic) was assessed twice. Symptomatic plants were randomly collected from the plots and evaluated for presence of acervuli, conidia, and setae on the affected tissues. Disease severity data were subject to analysis of variance to test the effects the mowing practices on anthracnose basal rot.

How the horizontal and vertical cuts affect anthracnose development

An assessment of basal rot anthracnose three weeks after inoculation indicates that mowing height and vertical mowing influenced the development of basal rot anthracnose in annual bluegrass. In the first disease assessment, the effect of mowing height and vertical mowing on disease severity were significant (P≤0.05). Under the high-cut (0.17 inch) mowing treatment, anthracnose was least severe in plots that were not vertical mowed, and it was significantly lower than that in plots which were vertical mowed shallow (0.13 inch) or deep (0.20 inch) (Fig.1). A relative reduction of 74% in disease severity was achieved in plots that were not vertical mowed compared to those that were cultivated deeply (0.20 inch).

The effects of vertical mowing on disease severity were observed when the mowing height was reduced to the medium (0.12 inch) or lowest (0.08 inch) height of cut. Effects of vertical mowing and mowing height on disease severity showed a similar pattern in the second disease assessment that was conducted two weeks later. Overall, basal rot anthracnose severity slightly increased over two weeks; in the second assessment, there were significant differences in disease severity among the three vertical mowing depths under all three mowing heights. Similarly, under no vertical mowing, disease severity in plots that were mowed at the highest height of cut (0.17 inch) was significantly lower than that in plots that were maintained at the medium height of cut (0.12 inch), which in turn was significantly lower than that of the plots maintained at the lowest height of cut (0.08 inch) (Fig. 2). Effects of mowing height and vertical mowing on development of anthracnose basal rot disease in the second disease assessment followed a similar pattern.

Fig. 2. Effects of vertical mowing on basal rot anthracnose severity in annual bluegrass maintained under 0.17-inch (A)

Fig. 2. 0.12-inch (B), and 0.08-inch (C) mowing heights of cut.

Fig. 2. 0.08-inch (C) mowing heights of cut.

The experimental turf area in this study originally contained local population of moderately virulent C. cereale isolates. Inoculation of turf with C. cereale originally isolated from this site provided fairly uniform distribution of basal rot anthracnose in the annual bluegrass population. Inoculation of plants soon after employing the treatments appeared to have accelerated the infection process as indicated by relatively high basal rot anthracnose disease severity in the experimental turf area compared to adjacent areas.

It has been established in the literature that C. cereale is a stresspathogen that effectively infects the host plants through mechanical injury (3, 5). Although it has become apparent in recent years that the fungus can also effectively infect plants that are growing under non-stressed conditions, the injury factor appears to remain critical during the infection process. This was evidenced by increased disease severity in plots that were vertical mowed with reduced mowing height. Depth of vertical mowing appeared to play important role in basal rot anthracnose development.

This study indicates that deep vertical mowing may have caused relatively greater injury to crown, roots, and stolon tissue; thus, more severe disease. Extreme reduction of mowing height removes considerable amount of photosynthates, provides greater exposure of crown tissue to the pathogen, and contributes to summer stress, all of which can be conducive to infection and disease development. Effects of close mowing in this study clearly demonstrated such effects on basal rot anthracnose severity in annual bluegrass.

Conclusion

This study revealed the effects of mowing height and vertical mowing depth in basal rot anthracnose development in annual bluegrass putting green. Such cultural practices evidently caused significant mechanical injury; thus increased disease severity. Devising a turfgrass management strategy focusing on proper timing of cultural practices that minimize plant injury will be strongly desirable for turf managers. The most practical approach to addressing these two issues may be careful application of these practices. Vertical mowing may be performed while disease is least active at reduced depths; and the correct mowing height may be formulated with a compromise between disease management and appropriate green speed. Employing cultural practices that minimizes mechanical injury and reduces disease severity will be instrumental in development of an integrated basal rot anthracnose management strategy.

Literature cited

1. Couch, H. B. 1995. Diseases of Turfgrasses. 3rd ed. Krieger Publishing Company, Malabar, Fla.

2. Danneberger, K. 2007. Basal anthracnose: Springtime ritual. Golfdom Apr. 2007.

3. Landschoot, P., and Hoyland, B. 1995. Shedding some light on anthracnose basal rot. Golf Course Manage. 11:52-55.

4. Nikolai, T. A. 2005. The Superintendent’s Guide to Controlling Putting Green Speed. John Wiley & Sons, Inc., Hoboken, NJ.

5. Smiley, R.W., Dernoeden, P.H. and Clarke, B.B. 2005. Compendium of Turfgrass Diseases. 3rd ed. The American Pathological Society, St. Paul, MN.

6. Vargas, J.M., and Turgeon, A.J. 2004. Poa annua: Physiology, Culture, and Control of Annual Bluegrass. John Wiley & Sons, Inc, Hoboken, NJ.

Dr. Wakar Uddin is Professor in the Department of Plant Pathology, The Pennsylvania State University, University Park, PA