John H. Martin, S. C. Salmon.
Wheat, oats, barley, and rye may be attacked by eight distinct species or subspecies of rust fungi.
Wheat is subject to stem rust (Puccinia graminis tritici), leaf rust (P. rubigovera), and stripe rust (P. glumarum).
Oats are attacked by a stem rust (P. graminis avenae) and by crown rust (P. coronata avenae).
Barley may fall prey to the same stem rust (P. graminis tritici) that attacks wheat and a leaf rust (P. hordei).
A stem rust (P. graminis secalis) and a leaf rust (P. rubigo-vera) attack rye.
Each of the eight rusts is made up of several or many -different races, which may attack certain varieties of a particular cereal crop but not others.
Stem rust of wheat causes the most spectacular and perhaps the greatest losses. Leaf rust of wheat and crown rust of oats occur more frequently, usually affect larger acreages, and so may cause greater average losses year in and year out. Before the extensive Use of resistant varieties, stem rust was most destructive in spring wheat in the northern Great Plains. Severe losses sometimes occur in the southern Plains, many Eastern States, California, and occasionally in localities in the Pacific Northwest and Intermountain States.
Leaf rust of wheat and crown rust of oats occur wherever wheat and oats are grown. They cause little damage west of the Rocky Mountains or in the drier parts of the Great Plains. Some damage occurs in the eastern half of the United States nearly every year. Reductions in acre yields are especially great in the Southern and Southeastern States although neither wheat nor oats is grown extensively in much of that area.
Most varieties of barley, some of oats, and all varieties of rye escape considerable damage because of their early maturity as compared with wheat. Stem rust sometimes injures barley seriously. Leaf rust is not usually serious but is sometimes locally destructive on spring barley. The stem and leaf rusts do not cause serious losses in rye. Stripe rust occurs most commonly on wheat but only in special situations in the United States has it caused serious losses.
Upper left: Section of wheat stem rust pustule with spores in the wintering stage. Upper right: Section of a pustule of wheat stem rust with red, summer-stage spores. Lower left: Wheat leaves infected with the summer stage of the leaf rust fungus. Lower right: Wheat stem infected with the red or summer stage of stem rust fungus.
WHEAT STEM RUST is characterized by pustules that develop and break through the surface of the stems, leaves, and sheaths and often the chaff and beards of the wheat plant. Myriads of brick-red spores escape from the pustules and are carried by the wind to other wheat plants.
The crop is damaged by the growth of the rust fungus on the wheat stems and leaves and by the developing spores, both of which use up the water and nutrient materials needed for developing the wheat kernels. The water requirement in rusted wheat is much higher than in healthy wheat.
As a result, the kernels are badly shriveled, many of them being so light and chaffy that they are blown out with the chaff in threshing. The remaining grains may be shrunken to one-half or two-thirds normal size. Losses range up to 85 or 90 percent; at that point the crop is not worth harvesting and is a total loss. The rusted straw turns brown, becomes dry and brittle, and soon breaks over.
Wheat stem rust also attacks barley and occasionally rye. It attacks many wild grasses, including wild barley or squirreltail grasses (Hordeum species); certain wheatgrasses (Agropyron species); wild-rye grasses (Elymus species); bottlebrush grasses (Hystrix species); and some bromegrasses. It does not attack oats.
The rust lives over the summer on volunteer grains and wild grasses in the Southern States and in northern Mexico. These spores and those blown down from the north in late summer and early fall infect fall-sown wheat or barley. The rust lives over winter in the red rust stage in the southern part of the United States and in northern Mexico but not in the Northern States.
If weather conditions are favorable in the spring, the rust multiplies and the spores sweep northward with the advance of the crop season. Thus a heavy rust epidemic in Texas is a threat to the wheatfields of Oklahoma and Kansas, and the latter, in turn, are sources of inoculum for the grain-fields in the North Central and Intermountain States.
The red rust spores that spread the disease are about one-thousandth of an inch long. They fall upon a wheat leaf and may germinate in an hour in warm, humid weather or in several hours at temperatures of 40 to 50 F. Germinating spores send out germ tubes, which grow along the surface of the leaf or stem of the plant until they reach a breathing pore (stoma), where they enter, send out branches that grow within the tissues of the plant, draw nourishment from them for a week or longer, and then produce the red pustules with another crop of spores. At low temperatures, or when there is but little sunlight, it may take 2 or 3 weeks from the time the germ tube first enters the plant until the spores form. In warm, moist weather, however, they may reach full development within a week.
Thus a new generation of rust spores may be produced every 10 to 14 days during the spring and summer, starting in Texas and advancing northward with the progressive development of the wheat crop at different latitudes. Since a single rust pustule may produce 350,000spores, the rust can spread very rapidly. Rising air currents can lift the spores up to altitudes exceeding 10,000 feet and from such heights winds can carry the spores many miles before they fall to the ground.
A heavy rust epidemic on the nearly 4 million acres of wheat in South Dakota could produce about 2 sextillion rust spores. If only one in 10,000 of the spores blew north into North Dakota, four spores would be provided for every wheat plant in the State. Spores carried northward reach young wheat plants that are in a succulent stage, in which they are easily infected with rust. Spores blown southward in early summer fall on ripening or ripe wheat, which is not readily infected with rust. Those blown southward in late summer can attack volunteer wheat, early-sown wheat, and certain grasses. These plants in turn serve as a source of inoculum for fall-sown wheat in the South, where the rust lives over winter.
An additional source of rust menaces the wheat in the northern half of the country rust that develops on the barberry. The production of the brick-red uredospores ceases as wheat approaches maturity and black spores (teliospores) are produced in the same pustules. The latter stage is important only in the Northern States because the spores cannot survive the hot summers in the South.
The teliospores usually will not germinate immediately after they are formed but require a relatively long resting period, somewhat like the hard seeds of alfalfa and clover. They are not blown about by the wind, but remain on straw or stubble throughout the winter and germinate in the spring, especially in moist, cool weather. On germination, they produce small colorless spores (sporidia), which germinate and readily infect certain species of barberry.