VII.

Parasites of Plants

Similar organisms that parasitize animals also infect plants (see Diseases of Plants). Fungi cause the majority of plant diseases. Although they typically feed on dead organic matter, fungi can also feed directly on living tissues. The fungus that causes Dutch elm disease is often carried from tree to tree by beetles. It attacks and eventually kills by blocking water flow through the plant. Protozoans such as phytoflagellates can parasitize milkweed plants. Bacterial plant diseases include fire blights, certain soft rots, and citrus canker. The bacteria that cause these diseases destroy tissue or block the passage of water through the plant. Numerous viruses, such as the tobacco mosaic virus, also attack plants. Certain insects and worms, particularly nematodes, parasitize the roots, stems, and leaves of plants. They secrete chemicals that induce plant cells around the parasite to rapidly divide and produce large growths known as galls. Galls formed by the root knot nematode can cause serious physical damage to the roots of important crops including tomatoes and potatoes.

Some higher plants feed on other plants and cause them harm. One group known as hemiparasites, or water parasites, absorbs water and nutrients from their plant hosts. Witchweed is a hemiparasitic seed plant that damages sugarcane, corn, and other grasslike crops by attaching itself to the host’s roots and absorbing minerals and water, eventually killing the host. Mistletoe, another hemiparasite, parasitizes broadleaf trees including ash, maple, walnut, birch, and some conifers. Mistletoe roots bore into the host’s branches in order to draw out water and nutrients. Birds eat mistletoe berries, which pass through their digestive tracks, are excreted, and sometimes stick to a tree branch where they produce a new mistletoe bush.

True plant parasites lack chlorophyll and cannot photosynthesize. These plants must obtain carbohydrates as well as minerals and water from their plant hosts. True plant parasites include dwarf mistletoe, which primarily parasitizes conifers; dodder, which parasitizes important agricultural crops such as alfalfa, clover, sugar beets, and woody perennials such as olive trees; and broomrape, which causes extensive damage to tomato crops.