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Higher plants may cause disorders of or damage to other plants, either by acting directly as parasites diverting nutrients and water or as vigorous competitors or antagonists within mixed populations. Parasitic angiosperms are rare enough to be curiosities in many cool temperate countries, but elsewhere they are nuisances or economically important parasites (Table 1.2, Figure 1.6). The dwarf mistletoes, Arceuthobium, can kill or deform pines and other conifers, and even minor attacks reduce the quality of timber by causing the production of numerous large knots and irregularly grained, spongy wood. These parasites spread their sticky seeds by an explosive dispersal mechanism, leading to patches or foci of infestation within a plantation. By contrast, root parasites such as Orobanche and witchweed, Striga, produce numerous tiny seeds which lie dormant in the soil. The seeds are triggered to germinate by a stimulant from host roots. The parasite then attaches itself to the root by means of a specialized organ and diverts water and nutrients, leading to wilting, chlorosis, and stunting of the host. These parasites are difficult to control due to the large number of seeds they produce (in the case of Striga, as many as 200 000 per plant) and the long periods over which they remain viable.

On a world scale the most important angiosperm parasite is Striga hermonthica which attacks cereals such as maize, sorghum, millet, and rice. In many of the agricultural areas where it is most prevalent, for example sub‐Saharan Africa, there are insufficient resources to support expensive control measures and infested land may eventually be abandoned. Recently, cultivation systems have been developed that reduce Striga infestation by intercropping the cereal host with a different crop, usually a legume, that suppresses infection by the parasite (see later in this chapter). Orobanche is a significant problem in sunflower, tobacco, tomato, and especially faba bean, with a substantial proportion of the crop area in the Mediterranean region affected.

Table 1.2 Angiosperms parasitic on other higher plants

Family, common name, genus	Geographic area	Crops attacked
Convolvulaceae Dodder (Cuscuta)	Europe, North America	Alfalfa, clover, potatoes, sugar beet
Lauraceae Dodder (Cassytha)	Tropics and subtropics	Citrus trees
Loranthaceae Dwarf mistletoe (Arceuthobium)	Worldwide	Gymnosperms
American true mistletoe (Phoradendron)	North America	Angiosperm trees
European true mistletoe (Viscum)	Europe	Angiosperm trees, especially apple
Orobanchaceae Broom rape (Orobanche)	Europe	Tobacco, sunflower, beans
Scrophulariaceae Witchweed (Striga)	Africa, Asia, Australia, North America	Maize, sorghum, rice, cowpea

The deleterious effects of other higher plants are due to competition for space, light, water, and nutrients. Species which are vigorous competitors with crop plants are usually described as weeds. As well as affecting crop development, weeds may interfere with harvesting and their seeds can contaminate grain samples. They may also be important as alternative hosts for pests or pathogens which can subsequently spread to crops. In addition to direct competitive effects, some plants produce chemicals which inhibit the growth of neighboring plants. This phenomenon, analogous to microbial antibiosis, is known as allelopathy. Plant roots release a diverse range of chemicals which can act as potential inhibitors or defense compounds, but it is difficult to determine the extent to which these interactions operate in nature. Allelopathy is believed to influence plant succession and distribution in natural communities, and may also have significant effects in agricultural systems. The chemicals involved are of interest both as potential herbicides and as signal molecules affecting the growth and behavior of other organisms. The suppression of Striga by some legumes, described earlier, has been shown to be due to a combination of compounds that stimulate “suicidal germination” in the absence of the host and inhibitors that interfere with infection of roots.

Figure 1.6 Poplar tree infested by European mistletoe Viscum album.

Source: Photo provided by John Lucas.