Parasitism

A parasite is an organism that depends on another organism (its 'host') for nutrition, support or shelter; parasitic plants do this by physically infecting and/or climbing on their hosts (Table 9.3). Plants that are entirely dependent on their host (holoparasites) are usually white because they lack chloro-

Table 9.3. Definitions of the various types of parasitism.

Type of parasitism Explanation

Holoparasite Entirely dependent on their host for carbon, water and nutrients

Hemiparasite Rely on their host for some resources, but are self-sufficient in others

Obligate hemiparasite Can survive only when associated with the appropriate host

Facultative hemiparasite Can survive without host but are usually associated with the host

Epiphyte Rely on host for physical support phyll and cannot photosynthesize. Plants that rely on their host for only some resources (hemiparasites) form either obligate or facultative relationships. Some parasitic species are dependent on their host for physical support (epiphytes). Orchids, ferns, bromeliads, lichens, mosses and many mistletoes are epiphytic. Epiphytes live upon other plants and may or may not have a negative effect on their host. One example of a parasitic epiphyte is the strangler fig (Ficus leprieurii) (Fig. 9.2).

Similarly to herbivory, natives may be more chemically or morphologically suitable than exotic weeds to native parasites. There are exceptions. Some weed species are parasitized by the same species as natives or crops, and weeds may be used as a 'trap' crop to reduce the impact of a parasite on desirable species - the same approach can be used in using weeds to attract herbivores (Scholte, 2000; Rambert et al., 2001). Weeds can be parasitized by fungi that are somewhat akin to a 'sexually transmitted disease' (Kaltz and Schmid, 1995); this essentially describes fungi that attack male sexual structures like anthers, e.g. in white cockle (Silene latifolia) (Shykoff and Kaltz, 1998). Many weed species are parasitized by fungi like 'fusarium wilt fungi' -though the weeds do respond with inducible defences as they might to herbivores (Jennings et al., 2000). As with herbivores, biological control of weeds may involve introduction or augmentation of parasites, though using multiple types of biological control can create very complex interactions that are difficult to manage (Rosenheim et al., 1995).

Weeds themselves can be parasitic

Fig. 9.2. Establishment of the parasitic strangler fig (Ficus leprieurii) on the palm (Elaeis quineensis). The fig germinates in the canopy of the palm and sends aerial shoots downward. Once the fig's roots reach the ground, the shoots increase in size. Eventually the fig overgrows the palm and the palm dies but the fig remains. (Longman and Jenik, 1974; with permission of Pearson Education.)

Table 9.4. Examples of important agricultural parasitic weeds.

Common name Genera

Host species

Comments

Dodder Mistletoe Broomrape Witchgrass

Cuscuta

Loranthus

Arceuthobium

Viscum

Orobanche

Aeginatia

Striga Alectra

Non-specific

Coniferous trees

Carrots, broad beans, tomatoes, sunflowers, red clover,

Species specific: sorghum, millet, maize, cowpeas, groundnuts, other crops

Twines around stems the seedling makes contact with, and inserts haustoria into stem Shoots hemiparasite

Root holoparasite

Root hemiparasite, called witchgrass because it harms crop before the parasite has emerged from the soil

(Table 9.4). Parasitic agricultural weeds are more important in developing countries (Zimdahl, 1999), e.g. witchweeds (Striga spp.) infest approximately 44 Mha in Africa and infestations can reduce crop yield by more than 20%. North Americans are more concerned with parasitic forestry weeds like dwarf mistletoes (Viscaceae). Dwarf mistletoes are parasites of many conifer species; their impact on the North American forest industry is on the same scale as witchweeds in African agriculture (Baker and Knowles,

1992). The mistletoes preferentially parasitize young trees because the bark is thinner and easier to penetrate (Parker and Riches,

1993). Forestry practices have increased infestations because cutting and replanting mean there are more younger trees of the same age and vulnerability to infection, and fires are no longer allowed to burn - these once reduced dwarf mistletoe infestation. Managing parasitic weeds is difficult and often requires herbicides. Sometimes trap cropping is used to stimulate germination and growth of parasitic weeds so they can be managed in a crop that is not economically important before the weeds infest more important crops (Chittapur et al., 2001). In these cases, the 'trap crop' is not usually a weed but crop species like flax (Linum spp.) not meant to be harvested for human use.

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