ABSTRACT
Parasitic plants have evolved three times on the asterid I clade (Figure 17.1). In addition to Cuscuta in Convolvulaceae, the order Lamiales contains Orobanchaceae, one of the most economically important parasitic plant families. Although the vast majority of Orobanchaceae are benign root parasites, two genera, Striga (witchweeds) and Orobanche (broomrapes) together cause more damage to crop plants than all other parasitic plants combined. Striga asiatica (Figure 17.3), a pathogen of maize (Zea mays), is native to Africa, but was found in the eastern United States in the 1950s. Since then, a governmental program has been successful in eradicating witchweed from the affected regions. Several species of Orobanche are agricultural pathogens throughout the Middle East and Europe, and some species have been accidentally introduced to North America. Like Striga, Orobanche seeds are tiny and require specific molecules exuded by host roots to stimulate germination. The final group of economically important parasitic plants that will
Concept Box
TABLE 17.1 The 12 Clades of Haustorial Parasites in Flowering Plants
be considered here are the mistletoes (Mathiasen et al., 2008). Taxonomically, mistletoes can be found in five different families within the sandalwood order (Santalales), but in North America, only Arceuthobium (Figure 17.4) and Phoradendron (Figure 17.5) (both Viscaceae) occur. As parasites of conifers in the pine family in North and
Central America, the dwarf mistletoes (Arceuthobium) cause billions of dollars of damage on commercial timber trees. One example is A. douglasii, which causes significant losses to Douglas fir (Figure 17.4).
