Systemic Diseases on Hosts with Different Growth Patterns

Abstract

To date, several reviews and books have focused on fungal pathogens in natural plant populations (Burdon 1987, Augspurger 1988, Alexander 1990). All stress the importance of fungal pathogens in shaping plant populations, since they have the potential to cause high mortality in adult plants (Paul and Ayres 1986), seedlings (Augspurger 1984) and seeds (Hendrix and Campbell 1973). Despite the awareness of the importance of pathogens, we still know very little about the general nature of host-pathogen interactions in natural populations. At present, we have no good way of predicting the immediate outcome of any interaction between a given fungus and its host plant, i.e. the effects of pathogens on the survival, biomass, and reproduction of individual hosts. Not surprisingly then, we cannot predict the likely long-term consequences of a given interaction at the population level. One reason why predictions about host-pathogen systems are few, may be a consequence of treating pathogens as if they belong to a single homogeneous group (but see Burdon 1991, 1993). However, a large number of fungal genera and families are known to include pathogenic species. Studying them closely reveals that elements of life history strategies, such as longevity, reproduction, host specificity and growth pattern, vary considerably between species of pathogens. One important life history trait is whether the fungus is non-systemic or systemic. The former group consists of fungal pathogens that are not able to grow away from the site of infection, while the latter group consists of those pathogens that are able grow internally from the site of infection to other parts of the plant, and may produce pustules along the way. Many species of rusts and smuts are systemic and important pathogens of herbs, grasses and trees (Wilson and Henderson 1966, Lindeberg 1959). Systemic fungal diseases often form a perennial mycelium within the plant. They often form confluent pustules that strongly deform the diseased part, e.g. stems can be severely twisted (Parker 1987, Wennstrdm and Ericson 1992). Disease symptoms may be seen on the entire plant (Bryzgalova 1928), but more commonly, pustules are only seen on specific parts of the plants such as leaves (e.g. Puccinia pulsatillae on Pulsatilla pratensis, Wennstrbm and Ericson 1991) or flowers (e.g. Microbotryum (Ustilago) violaceum on Silene dioica, Carlsson and Elmqvist 1992). The extent of damage caused to the host plants varies considerably between systemic diseases. For example, the systemic Puccinia pulsatillae enhances growth and increases survival of its host, Pulsatilla pratensis (Wennstrom and Ericson 1991), while the systemic Urocystis trientalis reduces survival and growth of its host, Trientalis europaea (Wennstrom and Ericson 1990). One way to understand and hopefully also develop a predictive theory of how fungal pathogens behave in plant populations and communities is to incorporate studies of the life-history of the pathogen (cf. Andrews and Rouse 1982). Life history studies should, however, also include the life-history strategies of the host, e.g. host longevity, reproduction and growth pattern. The growth pattern, and especially variation in the lateral growth of the host, may prove to be important in host-pathogen interactions, as incomplete transmission of systemic diseases has been reported for host plants with a strong lateral growth (e.g. Pady 1939, van den Ende et al. 1987, Wennstr6m and Ericson 1992). Although about 70% of all plant species in temperate zones of the world are clonal, the exact growth pattern varies considerably from species to species (van Groenendael and de Kroon 1990). For example, ramets can be formed close to the parent (hereafter called plants with weak lateral growth) or on lateral shoots or roots at some distance from the parent (hereafter called plants with a strong lateral growth). In this paper, I concentrate on perennial systemic rusts and smuts on host plants with different growth patterns. I compare characteristics of disease expression (disease levels, infection rate), and host response between hosts with weak lateral growth, and those with strong lateral growth.