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Diversity in vegetation composition and structure within cropping systems can influence pathogen and arthropod pest populations. While the overall effects may differ between systems and species, the published findings are of a general reduction in density and subsequent damage by the causal agent, compared with a system comprising only a single genotype (Chin & Wolfe 1984; Andow 1991). The concept that mixtures can be developed to produce a degree of functional diversity (diversity that limits pathogen and pest expansion), and hence a more sustainable form of cropping, has been discussed for a wide range of agricultural systems (Andow 1991; Finckh & Wolfe 1998). At present, mixtures have been deployed primarily in annual crops (Garrett & Mundt 1999) where, if pests or diseases develop rapidly, pesticides can be applied or the choice of genotypes used as components in the mixture can be changed the following season. In long-term perennial crops, such as willows grown as short-rotation coppice (SRC), it may not be possible to change constituent genotypes or feasible to use pesticides. In addition, the complex of pests and pathogens likely to attack such crops may not show the same effects in response to the use of mixtures compared with pest complexes of annual crops. Boudreau & Mundt (1997) argue that the most important mechanisms that contribute to changes in insect and disease levels in variety mixtures are ones that affect dispersal of these damaging agents. A passively dispersed agent, such as an air-borne fungal pathogen, will be dependent on wind or rain for its dispersal, whereas an actively dispersed agent, such as a highly mobile insect pest, can to some extent choose its direction and rate of movement. It is possible that the mode of dispersal, and subsequent damage, will be different for each agent within a common cropping system, and therefore that different pests may be affected in different ways by genotype mixtures.
Willows grown as SRC have been shown to be a suitable species as a source of renewable energy to provide an alternative to fossil fuels (Stott et al. 1985). However, SRC willows are attacked by a number of potentially debilitating diseases and pests (Hunter, Royle & Arnold 1996). Cropping systems that incorporate plantings of willow genotypes that differ in susceptibility to the most serious disease and pest have been proposed (Parker, Arnold & Royle 1996; Tabbush & Parfitt 1999). An understanding of the potential competition effects between neighbouring trees (Parker, Arnold & Royle 1996) should also be considered when choosing genotypes for use in mixtures.
Willow rust disease, caused by Melampsora spp., is a wind-borne and rain-splashed pathogen that is one of the main causes of yield loss in SRC willows. Melampsora exists as a number of species and pathotypes, each of which is capable of infecting a characteristic range of willow species and hybrid genotypes (Pei, Royle & Hunter 1996). First symptoms are often seen on susceptible genotypes in UK willow plantations from mid-May to early June (McCracken & Dawson 1990). Parker, Arnold & Royle (1996) have shown evidence for the highly aggregated nature of rust spread within a plantation consisting of plantings of a single willow genotype. Dispersal within a plantation is thus initially from a focal source, and subsequent development is dependent on microclimate conditions and suitability of host genotype for epidemic progress. It is known that the majority of rusts affecting willows are unable to complete their life cycle on the willow host but instead need to pass through an alternate host such as European larch Larix decidua (Mill.) (Pei, Royle & Hunter 1993), consequently infections of foliar rust arise each season from external sources. The use of genotype mixtures has been suggested in the control of this rust pathogen (Dawson & McCracken 1995; Parker, Arnold & Royle 1996; McCracken & Dawson 1998).
In coppiced willow, the most damaging invertebrate pests are chrysomelid beetles: the blue willow Phratora vulgatissima (L.) and the brassy willow beetle Phratora vitellinae (L.). Phratora vulgatissima prefers to feed on willows low in phenylglucosides and P. vitellinae prefers willows high in these compounds (Pasteels & Rowell-Rahier 1992). Differences in preference have been noted between favourable willow types under both laboratory and field conditions (Kendall et al. 1996) indicating that beetles actively select their preferred willow food source. A study of the responses of the willow beetle to mixed willow genotype plantations shows a reduction in the overall density, damage and oviposition in the mixtures compared with the monocultures (Peacock & Herrick 2000). Beetles overwinter mainly outside plantations (Kendall & Wiltshire 1998), enter the plantation during April, feed, mate and lay eggs. Phratora vulgatissima are known to form aggregations and are highly mobile (Peacock, Herrick & Brain 1999), the position of highest infestation moving throughout the planting during the season (Sage et al. 1999).
The coppice willow system is therefore subjected to serious diseases and pest infestations that enter from sources outside of the plantation early in the growing season. The success of a specific willow plantation design in reducing the damage caused by beetles and rust will depend to a large extent on how easily these organisms are able to disperse throughout the plantation. Because rust is sessile and passively dispersed, and beetles are highly mobile and actively disperse, requirements for effective plantation design for control of either may be very different. Basic information on the spatial dynamics of these differentially dispersed organisms is necessary for the effective deployment of willow genotypes in a plantation, as dispersal within the crop may have significant effects on the implementation of pest management techniques (Jeger 1999). This study assessed the damage incurred from beetles and rust within willow plantations with random and regular design configurations of three and five genotypes, by intensive, simultaneous, assessments of the same stems in mid-growing season for 2 consecutive years (1998–99). The patterns of damage were then compared and contrasted between the different willow plantation designs. The sampling technique used also permitted the assessment of possible interactions between rust and beetle damage.
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- Materials and methods
The concept that mixtures can be developed to produce a degree of functional diversity that limits pathogen or pest expansion (Andow 1991; Finckh & Wolfe 1998) has been utilized primarily in annual crops, with little work involving long-term perennial crops (Aluja et al. 1997; Docherty & Leather 1997; McCracken & Dawson 1998). Response to uniform or mixed willow plantings of different host genotypes by each member of the complex of pests and pathogens likely to attack this crop will determine the success of mixtures. It is known that the mode of dispersal for willow rust and beetles is different, consequently the functional diversity of a common cropping system may be different for each damaging agent. However, the present study shows the beneficial suppressive effect of mixtures against both organisms, as reflected in lower beetle feeding and rust damage compared with that found in the monocultures. The over-riding factor influencing beetle and rust damage was willow genotype numbers, with no consistent significant effect observed with structural design. However, the trend was for decreased overall damage when the spatial configuration was random rather than regular.
The comparison between plantation designs for the pattern of damage distribution along stems and within plots showed differences for the two organisms, reflecting their passive and active dispersal. There was a strong positive correlation in rust severity between leaves along stems, with a similar disease distribution between designs (but at reduced severity) as the number of host genotypes increased. For beetles, there were weak positive correlations in damage between leaves in alternate sections along stems, with relatively more damage on leaves in the monoculture compared with the mixtures, especially towards the shoot tip.
The strong positive correlation in rust severity between leaves along stems can indicate a spread of inoculum from neighbouring leaves on the plant. Parker et al. (1995) suggest that the spread of rust within an individual plant would be a reflection of what might occur within a monoculture in that susceptible host tissue is always in close proximity. Thus, in a monoculture rust can disperse radially to neighbouring trees, resulting in spatially aggregated distributions of incidence and severity (Parker, Arnold & Royle 1996). At a later stage in the epidemic, there would no longer be discernible aggregations (as our results showed for 1998) as more plants became infected. The occurrence of rust was later in 1999, thus at the time of rust assessment disease development would probably have been at an earlier stage, with the resulting aggregated distribution patterns found in monocultures in that year.
In mixed plantings, rust spores landing on resistant plants would not propagate; thus the spread of disease through a mixture of willow genotypes would be slowed down relative to the monoculture, as was suggested for other crops (Chin & Wolfe 1984). Therefore, in 1999, as rust in the mixtures would most likely be at an even earlier stage of development than in the monoculture, the non-aggregated spatial distribution reflects the initial random spore deposition and germination pattern. With time, the rust would spread and form discernible aggregations, as shown in our 1998 assessments. Thus, the difference in the spatial distribution of rust between the monocultures and the mixtures for both assessment years is probably due to a delay in epidemic development in the mixed plantings.
Differences in beetle damage along stems, especially between plantation designs, can indicate a movement of beetles from trees or, as suggested by Power (1988) for insects in heterogeneous plantings, an emigration from mixtures over time. Because the willow beetle is very mobile and discriminates between host-plants within a mixture (Peacock, Herrick & Brain 1999), time would be lost looking for suitable plants, especially in plots with several different willow genotypes and less regular spatial configurations (Peacock & Herrick 2000). These mobile aggregations of adults would result in a distribution of damaged plants throughout the plots. The spatial patterns of damage incidence in the present study were primarily regular or random, indicating beetles were not consistently choosing adjacent trees. There was also no evidence that beetles were choosing trees on the basis of leaf number, as there were very few associations between beetle damage and leaves per tree. Beetles may be responding to other factors, such as conspecifics or kairomones, as shown previously (Peacock, Lewis & Herrick 2001). This plant choice behaviour makes predictions of insect movement, especially in relation to configuration of genotypes within a plantation, very difficult.
The sampling of insect and rust damage on the same stems allowed for an assessment of interactions between the two causal agents. In the first year of this study, there were significant negative correlations between the extent of rust and beetle damage on stems. It is unclear what agent was responsible for these interactions as there were no consistent correlations between rust and beetle damage by position along stems. Only three of the 15 plots showed spatial dissociation, suggesting that beetles were not choosing or avoiding areas of rust infection. These dissociations occurred when rust damage was greater than 1%. Therefore any negative effects of rust on beetle feeding may be noticeable only at relatively high rust severity. As willow beetles tend to be present on host-plants before rust infections take place, the pattern and degree of leaf damage by beetles may affect subsequent rust infection. This occurs in Rumex spp., as feeding by the chrysomelid beetle Gastrophysa viridula causes a significant reduction in the infection of the rust Uromyces rumicis due to the induction of a resistance response throughout the undamaged portion of the damaged leaf (Hatcher et al. 1994).
In conclusion, the results shown here indicate the usefulness of deploying coppiced willow mixtures to reduce the impact of both rust and beetles within the same willow cropping systems. The spatial distribution of rust throughout the willow plantings differed from the distribution of beetles, reflecting their different modes of dispersal. Nevertheless, surveys of rust and beetle damage must consider the potential differential aggregations for both agents. However, it has been shown that both pest organisms can be assessed simultaneously. Results also suggest that spatial design of willow plantations affects rust distribution, and hence development, to a greater extent than it does beetle distribution. Therefore, this must be taken as a major criterion in selection of mixture configuration. Thus, further work is required to determine the optimum design and deployment for various willow genotype mixtures in renewable energy plantations.