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Predicting Invasions of Nonindigenous Plants and Plant Pests
The ability to use light efficiently may enhance a plant’s ability to live in areas with extensive canopies. Consequently, the ability to establish may be related to this trait. The extent to which shade tolerance is an attribute shared widely among naturalized plants is unknown but deserves systematic survey. Baruch et al. (2000) examined 10 physiological and morphological plant traits of four invasive members of the Melastomataceae (two herbs, a shrub, and a tree) in Hawaii and found that the invasive species were better suited to capturing and using light than a large group of natives.
Life-history traits important for the establishment of plant pathogens include reproductive strategies and genetic variability related to fitness, virulence, and host compatibility.
As with plants, pathogens use many reproductive strategies. Some pathogens (such as viruses and some fungi) reproduce only in the presence of their hosts, whereas others (for example, many fungi) are facultative saprophytes and do not require the plant host for reproduction. Some fungi reproduce only sexually, whereas many pathogens, such as viruses and some fungi, reproduce only asexually. Some pathogens can complete several generations in a single year, whereas others require several years to complete a single generation.
Asexually reproducing pathogens are thought to establish most easily (Agrios 1988). Another major characteristic of invasive pathogens is a high rate of survival when the plant host is not present (for example, in the winter for pathogens that infect the leaves of annual plants and deciduous perennials) or when the physical environment is totally unfavorable. Survival can occur in a dormant state (for example, in overwintering spores), in a saprophytic condition, or as infections in alternative hosts (Agrios 1988).
The most successful pathogens display a short time between one infection cycle and the next, have a high rate of production of infectious units (spores, bacterial cells, nematode cysts or eggs, or viruses), and have a long infectious period–the time that infectious units are produced or plants are contagious (Campbell and Madden 1990). Races of Puccinia helianthi, the sunflower rust pathogen, illustrate this point. The rust’s superior colonizers have higher spore germinability, more rapid spore germination, more rapid formation of appressoria (spore-producing structures), and higher spore production than other genotypes or races (Prudhomme and Sackston 1990).