Rust is the common name of a group of diseases caused by fungi that are parasites on plants. Rusts are especially harmful to cereal crops. The rust diseases are named for the spores produced by the fungi. These spores are brownish and resemble iron rust. Rusts have special organs resembling threads that grow among the cells of the host plant and absorb the food of the plant cells. This action robs the plant of nutrients and may cause the leaves and stems to wither. Badly rusted crops produce shriveled and worthless grain. Rust-causing fungi also attack other types of plants. For example, asparagus rust damages asparagus, blister rust attacks white pine trees, and cedar rust harms apples.
Every species of rust-causing fungi goes through a certain life cycle. Each period or stage in this life cycle is marked by a different type of spore formation. Some rust-causing fungi have as many as five different types of spores, while others have only two or three. Some species of rust-causing fungi spend their entire life cycle on one host. These species are called autoecious. Other species must spend their life cycle on two different hosts. This type is called heteroecious. The second host is known as the alternate host.
A common heteroecious type of rust-causing fungus causes black stem rust of wheat plants. This species has five different kinds of spores and must live its life cycle on two hosts, the wheat plant and the American barberry plant (see Barberry ). In the spring, small cups filled with spores appear on the lower side of the leaves of the barberry plant. These spores are carried by the wind and spread to wheat plants. The spores germinate and send out threads. These enter the tissues of the wheat plant and there produce reddish spores that are carried to other healthy wheat plants. This is the first stage. The second stage occurs in the fall, when there is a growth of tiny black spores on the stalks and stubble. These black spores sprout in the spring and produce small colorless spores called sporidia, which mark the third stage of life. The sporidia cannot grow on the wheat plant and will grow only on the barberry plant. These sporidia are carried by the wind to the barberry. The fourth stage is the development of tiny yellow spores on the upper surface of the barberry leaves. Later, yellow-orange cups containing spores appear on the undersurface of the barberry leaves. This marks the fifth stage. These spores are not able to infect the barberry plants. They must be carried on the wind to wheat plants. There the life cycle begins again.
One method of controlling a rust that grows on two hosts is to destroy the alternate host. In the case of black stem rust, wheat crops have been saved by destroying barberry plants. Another method is to breed rust-resistant plants. Rust is sometimes controlled by destroying or uprooting crops affected by the disease. Crop rotation also helps prevent rust.
