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One of the keys to successful sports turf management in the UK and other temperate areas of the world is control of the weed grass Poa annua L. (annual meadow grass or annual bluegrass) (Adams & Gibbs 1994). For example, P. annua infests virtually every golf green in the UK and in many cases it can represent the majority of grass cover in the sward (Mann 2004). Although selective herbicides have been used as control agents (Johnson 1982), application can result in almost total loss of the sward. This is unacceptable on golf putting greens. Poa annua is a particular problem in fine turf such as putting greens because it is susceptible to drought, so large amounts of irrigation water must be applied to maintain the sward in a playable condition. In addition, it is susceptible to disease, notably microdochium patch [causative organism Microdochium nivale (Fr.) Samuels & I.C. Hallett] and anthracnose [basal rot, causative organism Colletotrichum graminicola (Ces.) Wilson]. In order to maintain fine turf on golf putting greens that contain high proportions of P. Annua, fungicide application is necessary in order to protect against disease (Perris & Evans 1996).
Currently, 16 fungicides containing seven different active ingredients are approved for use on turf grass in the UK (Whitehead 2000). In a recent survey, it was found that golf putting greens receive five times the amount of active ingredient of all pesticides per unit area per annum than is applied to a cereal crop (Garthwaite 1996). Such a level of pesticide application, combined with a high frequency of irrigation, means that there is the potential for leaching of chemicals and contamination of groundwater (Odanaka et al. 1994). Despite this theoretical risk, a recent review found little evidence for groundwater contamination because chemicals persist in the soil profile or are degraded to less harmful substances by microbial action (Cohen et al. 1999).
There are several reasons why turf management on golf courses needs to become less dependant on chemical inputs and more ecologically based. First, reliance on any one chemical cannot be sustained because the target organism is likely to develop resistance. Indeed, fungicide resistance in M. nivale, the most prevalent and damaging pathogen of sport turf, has been reported in the USA (Vargas 1994). Secondly, many chemicals have non-target effects on beneficial organisms, for example fungicides applied to turf can reduce bacterial populations in the soil (Yang et al. 2000). Thirdly, the current pesticides review programme in the European Union (EU) may mean that a number of compounds will be lost to UK growers in the future, including products used by turf managers (Wood 2001). Over the years, pesticide strategies on turf grass have changed from the use of inorganic compounds, such as mercuric chloride, cadmium chloride, Malachite/Bordeaux mixture (containing copper sulphate), Paris Green (containing copper acetoarsenite) and lead arsenate (Greenfield 1962), to the array of modern, less persistent organic pesticides. However, the number of approved compounds is likely to diminish in the future. Biological approaches to turf management are needed to replace chemical control, but these need to be compatible with pesticide residues derived from past or present usage.
It has been shown recently that arbuscular mycorrhizal (AM) fungi may have potential for reducing the amount of P. annua in putting green turf (Gange 1998; Gange, Lindsay & Ellis 1999). These fungi generally form mutualistic associations with about 70% of vascular plants (Hodge 2000), but for most plants there exists a continuum of responses to fungal colonization, from positive (i.e. beneficial) to negative (i.e. antagonistic) (Gange & Ayres 1999). In fine turf, it appears that AM fungi are antagonistic to the growth of P. annua, while being beneficial to the growth of desirable grasses such as Agrostis spp. (Gemma et al. 1997; Gange 1998). The mechanism is thought to be one in which carbon outflows to the mycorrhiza exceed nutrient inflows in P. annua, thus resulting in a net reduction in plant growth (Gange, Lindsay & Ellis 1999). AM fungi therefore have the potential to be an important part of an integrated control programme for P. annua. However, for this to be successful management techniques must be developed to encourage and sustain high levels of these fungi in sports turf to sustain sufficient coverage of desirable grass species.
There is a potential conflict in the use of turf fungicides and AM fungi, because many chemicals in common use have been shown to reduce AM abundance in ecological experiments (e.g. chlorothalonil, Venedikian et al. 1999; iprodione, Gange, Brown & Farmer 1990). Furthermore, if lack of pesticide leaching from golf greens is indicative of chemical retention within the soil profile (Cohen et al. 1999; Armbrust 2001), it is also important to determine if this is a cause of the relative scarcity of these fungi in turf soil. Certainly, levels of AM fungi in putting greens are considerably lower than those of less intensively managed areas (Koske, Gemma & Jackson 1997a; Gange, Lindsay & Ellis 1999). This could be due to persistent elements, such as cadmium, lead and arsenic, derived from compounds applied many years ago, or from frequent applications of modern organic fungicides. However, to date no study has examined whether application of fungicides to fine turf affects the colonization of grass roots by AM fungi.
In this study we described recent fungicide application patterns at four golf courses in southern England to determine which chemicals are most commonly applied. In addition, we measured levels of arsenic (As), cadmium (Cd), copper (Cu) and lead (Pb) and three of the most commonly used organic pesticides in putting green soils, and performed regression analysis of these data against the abundance of AM fungi. Clearly, it is not desirable or relevant to conduct experiments involving historically applied compounds such as lead arsenate or cadmium chloride. Therefore, we took an observational approach to identify relations between element levels and AM colonization. We also conducted experiments with modern fungicides where three of the most commonly used chemicals were applied to a working golf green and AM abundance levels were measured over a 6-month period. This experiment was designed to test the hypothesis that fungicide application will reduce AM colonization in sports turf, given that one of the most commonly used chemicals (iprodione) has been shown to do so in a natural plant community (Gange, Brown & Farmer 1990).
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- Materials and methods
AM fungi have great potential in aiding the establishment and growth of perennial grasses in golf putting greens (Gemma et al. 1997). Furthermore, evidence suggests that the abundance of these fungi in fine turf is negatively related to that of P. annua (Gange 1998) and that addition of AM inoculum to an established green may result in a decrease in the abundance of this weed (Gange, Lindsay & Ellis 1999). If this biological management strategy is to be successful, it is important that AM fungi form part of an integrated programme of turf management in which their presence is compatible with the array of chemicals currently in use. Studies have shown that AM colonization levels of grasses in putting greens are considerably lower than in more natural situations (Koske, Gemma & Jackson 1997a; Gange, Lindsay & Ellis 1999), although a surprising number of species have been found in sports turf (Koske, Gemma & Jackson 1997b). The results from the current study indicate that fungicidal chemicals do not appear to have an adverse effect on levels of AM colonization in fine turf.
In the past, lead arsenate and copper acetoarsenite (Paris Green) were widely used as insecticides on turf, while cadmium chloride and various salts of mercury and copper were used to control fungi (Greenfield 1962). Lead arsenate was probably used on turf until the late 1960s, and the immobility of lead in the soil, coupled with the generally undisturbed nature of putting green soils, led us to hypothesize that levels of lead and arsenic would be relatively high in the older putting greens. Although there were positive correlations between lead and arsenic and between copper and arsenic levels in the older courses, indicating the source of these elements, we found that levels of As, Cd, Cu and Pb were only a little above ambient levels reported elsewhere for UK soils (Lepp 1981). The metal extraction procedure used in this work probably leads to a lower metal recovery (especially for non-volatile copper) than from a more established total digestion method such as hydrofluoric/perchloric acid. The latter procedure is more likely to be used for measuring non-volatile element concentrations, although some guidance methods for the determination of elements (including non-volatiles) in environmental samples involve weaker digestions, such as 20% HNO3 or aqua-regia (US Environmental Protection Agency 600/R-94-111 method). However, none of the levels of any of the four elements in any green were likely to result in phytotoxicity (Carbonell et al. 1998; Das, Samantaray & Rout 1997; Aksoy, Hale & Dixon 1999).
There are few reports of the effects of heavy metals on AM fungi, and these either show negative effects on colonization (Gildon & Tinker 1983; del Val, Barea & Azcón-Aguilar 1999) or no effect (Weissenhorn, Mench & Leyval 1995). A consistent feature of these studies is that different AM fungal ecotypes exhibit varying degrees of tolerance to heavy metals (Haselwandter, Leyval & Sanders 1994) and that indigenous fungi from polluted sites show high degrees of tolerance (Weissenhorn, Leyval & Berthelin 1993). An excellent example of this was found with Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe, in which metal-tolerant isolates exist (Joner, Briones & Leyval 2000) as well as isolates that are strongly inhibited by heavy metals (del Val, Barea & Azcón-Aguilar 1999). Glomus mosseae was the dominant species in spore counts from similar golf courses (Gange, Lindsay & Ellis 1999) and has been found in other turf grass situations (Koske, Gemma & Jackson 1997b). It is therefore possible that putting green populations of AM fungi have become more tolerant of historic heavy metal concentrations, although it must be appreciated that the current mean concentrations of Cd, Cu and Pb in our study equate with the ‘non-contaminated’ control levels of del Val, Barea & Azcón-Aguilar (1999). It is quite possible that metal concentrations have become ‘diluted’ over the years, with the cessation of application and regular application of top dressing that contributes to the soil profile (Perris & Evans 1996). If putting green AM fungi are relatively tolerant, this would explain the lack of significant relations found between element levels and AM abundance, as found by Weissenhorn, Mench & Leyval (1995).
Our data show that there is no evidence to suggest that levels of heavy metals in putting green soils from past fungicide applications are likely to account for the low abundance of AM fungi. However, if metal-tolerant fungi are a feature of golf greens, future work involving the inoculation of greens with AM fungi must use isolates that have been obtained from turf grass. Indeed, this is the only ecologically realistic approach to AM manipulation (Read 2002) and may explain why a previous experiment, in which non-turf grass isolates were used, was only a partial success (Gange, Lindsay & Ellis 1999).
The three most commonly applied fungicides in this study were fenarimol, chlorothalonil and iprodione. Among the ‘others’ category, carbendazim (for earthworm control) was the most prevalent but only contributed to 8% of the total fungicide use at maximum. This chemical can reduce AM root colonization (Venedikian et al. 1999), but there are also studies where it has been shown to be ineffective (Schweiger, Spliid & Jakobsen 2001). We consider that in no case was application of carbendazim or other compounds frequent enough to have been a likely confounding factor in any relationship between AM levels and fungicides. Chlorothalonil and iprodione have been used in a number of ecological experiments, where they have been found to be effective in reducing the abundance of AM fungi (Gange, Brown & Farmer 1990; Aziz, Habte & Yuen 1991; Wan, Rahe & Watts 1998). We found very little evidence that AM fungal abundance was related to the levels of any fungicide in the soil and, overall, levels of the three fungicides were very low. This may be because the root zone of a putting green is generally free-draining and thus chemicals are rapidly leached through it. However, previous studies indicate that this is unlikely to be a common occurrence (Cohen et al. 1999). Instead, it is more likely that the chemicals are degraded by microbial action (Mercadier, Vega & Bastide 1997; Armbrust 2001) and the most probable site where this occurs is the thatch layer in a golf green (Sigler et al. 2000). This layer of undecomposed plant material occurs at the soil surface and in some cases can be several centimetres deep. A moderate thatch layer is required, to improve ball bounce and putting quality (Perris & Evans 1996), but too much is regarded as problematic, as it prevents the ingress of fertilizers or even water to the root zone and encourages disease outbreaks.
Generally, there was little indication that frequency of pesticide application was related to the amount of chemicals in the root zone, providing further evidence that chemicals may be entrapped and degraded within the thatch layer before they reach the roots. These data show the difference in pesticide usage by course managers, and it was encouraging to see that all four courses used a variety of chemicals, in an attempt to lessen the chances of resistance to any one chemical occurring. Concentrations of fenarimol were highest in the newest course, in which the root zone was composed predominantly of sand rather than soil. It is known that retention of fenarimol in a soil is positively related to the cation exchange capacity (CEC) (Wehtje, Walker & Shaw 2000) and this may be why levels were higher in this course, as inorganic products designed to increase the CEC had been added to the root zone.
The experiment failed to show any effects of chlorothalonil, iprodione or fenarimol on AM colonization levels. It was conducted over a 6-month period, which should have been sufficient for any detrimental effect of iprodione or chlorothalonil on root colonization levels to become apparent (Gange, Brown & Farmer 1990; Aziz, Habte & Yuen 1991). Given the efficacy of these chemicals in reducing AM levels in other experiments (above), this is perhaps a surprising result. It is possible that AM fungi in putting greens are resistant to fungicides, although to our knowledge such a phenomenon has never been recorded. Indeed, it seems unlikely for fenarimol, which is a relatively recent addition to the pesticide market. However, this is not the first time that fungicides have failed to reduce AM abundance. Sukarno, Smith & Scott (1993) have attributed this to the way in which colonization levels have been assessed. For example, they have shown that some fungicides affect root production, thus biasing results, while in other cases the amount of living fungal structures may be reduced, a fact not revealed by conventional staining methods. In our study we could detect no effect of fungicides on root production of either P. annua or A. stolonifera, and chose to record arbuscules only. As these are relatively transient structures, we believe that if the fungicide had a detrimental effect on the symbiosis it would have been revealed by this method over the course of a 6-month period. Instead, we suggest that the degradation of chemicals within the thatch layer meant that insufficient pesticide reached the roots and thus had no effect on AM colonization (Negre et al. 1997).
The data reported here do not provide any evidence that toxic chemicals are the reason for the relatively low levels of AM fungi in putting green soils, and we suggest two alternative explanations. The first concerns soil P levels, which can be high in some putting greens (Baker, Binns & Cook 1997). Indeed, it is well known that AM fungi tend to become less abundant and less functional at high soil P (Demiranda & Harris 1994). However, if this is so, it still does not explain why such a relatively high diversity of AM species has been recorded in turf grass soils (Koske, Gemma & Jackson 1997b). There has been no study of the occurrence of AM fungi in fine turf in relation to soil P status and we suggest that this is an important area that needs to be addressed.
The second feature of fine turf is the intensity of mowing throughout the year, which in a golf green results in a height of about 4·5 mm for most of the growing season (Perris & Evans 1996). Although occasional mowing may have little effect on AM colonization (Eom et al. 1999; Smilauer 2001), such an intensity of foliage removal is certain to affect the capacity of the plant to direct carbon to the mycorrhiza (Jakobsen, Smith & Smith 2002). Therefore, in any future manipulation of AM populations in turf, factors that affect the carbon economy of the plant will also need to be considered.