Economic Importance
Stem rust, caused by the fungus Puccinia graminis, is a
serious disease of wheat, oats, barley, and rye, as well as of many
cultivated and wild grasses. It is present in Illinois every year
on one or more of the small grains and grasses.
The prevalence and severity of stem rust varies from year to year,
depending chiefly on the weather, the amount of rust inoculum (spores)
blown into Illinois in the spring by southerly winds, and the reaction
of small grain and grass varieties to the races of rust present
in the inoculum. Wheat losses are now quite small in most years
but have run as high as 12 percent. In oats, the estimated annual
loss is 0.1 percent, yet one loss of 16 percent has been recorded
for Illinois. Reductions of up to 3 percent for barley have occurred
in the northern half of the state (see Illinois Natural History
Survey Circular 48). Rust can also reduce both hay and seed production
in many economic grasses.
Losses from stem rust have been reduced because of the extensive
use of resistant, early maturing cultivars and the removal of rust-spreading
barberries. Barberries once served as an early source of infection
and led to new races that were often capable of attacking previously
resistant cultivars. With the development of these controls, losses
are now greatly reduced.
In the United States, stem rust has lowered grain yields by as
much as 200 million bushels in a single year. It also lowers test
weight, reduces grain quality, and causes shriveled kernels and
lodging. The rust fungus uses food and water that would normally
be used in the growth and development of the kernel (Figure 1).
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Figure
1. Ergot of wheat.
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Symptoms and Disease Cycle
Nature of Stem Rust
When the stem rust fungus was described, the name included "forma
specialis" (abbreviated f. sp.), which is used to indicate the host
on which it was identified. For example, Puccinia graminis f. sp.
tritici, was first described on wheat, but also infects barley
and a number of other grass hosts. The following is a list of several
forma specialis of stem rust:
1. Puccinia graminis f. sp. tritici chiefly attacks wheat,
barley, triticale, wheatgrasses, wild barley, and wild ryes.
2. Puccinia graminis f. sp. avenae occurs mostly on oats
and grasses–including oatgrass, orchardgrass, timothy, bromegrasses,
fescues, mannagrass, velvetgrass, and bluegrasses.
3. Puccinia graminis f. sp. specialis principally attacks
rye. Other hosts include barley, wild rye, wild barleys, quackgrass, bromegrasses,
dropseed, and wheat-grasses.
4. Puccinia graminis f. sp. phlei-pratensis is found primarily
on timothy, but fescues, orchardgrass, and wild ryes may also be infected.
5. Puccinia graminis f. sp. poae attacks bluegrasses.
6. Puccinia graminis f. sp. agrostis infects redtop, bentgrasses,
orchardgrass, and bluegrasses.
7. Puccinia graminis f. sp. lollii infects ryegrasses and
orchardgrass.
Physiologic Races
Each forma specialist that infects a specific cereal or grass may be
further subdivided into numerous physiologic races. Races are defined
by their ability to infect or not infect a series of differential cultivars.
Ideally each differential cultivar would have only one resistance gene,
but some or most cultivars have multiple resistance genes. Isolates of
the pathogen can then be described in terms of the resistance genes that
are either effective or not effective. The number of races present during
a specific year is not as important as the proportion of isolates capable
of infecting cultivars with specific resistance genes. Therefore, cultivars
that are resistant to stem rust in one year or one location, may be susceptible
in another year or location depending upon the races of the pathogen present.
More than 350 physiologic races of the wheat-attacking form tritici are
known. Fortunately, only a dozen or so are common and able to cause damage
in the U.S. each year. In Illinois, about three to six races of this type
are identified in any given year.
There are more than 51 known physiologic races of the form avenae, which
attacks oats, and also several races of the form secalis, which attacks
rye.
SYMPTOMS
Stem rust produces elongated, reddish brown pustules on the stem, leaf
sheaths, leaf blades, glumes, beards, and occasionally on the young kernels
(Figure 2). The pustules, which usually begin to appear in late May or
early June in southern Illinois, increase in number until the cereal or
grass has headed and the seed (grain) is ripe.
Soon after the pustules appear, they rupture the epidermis, exposing a
powdery, reddish brown mass of summer spores (urediospores). A single
pustule may produce 350 thousand spores over a period of time. The elongated
shape of the pustules, their larger size, and the fragments of epidermis
that adhere to their sides and ends and give them a ragged appearance
are the characteristics that distinguish stem rust from leaf rust. Individual
stem rust pustules may be 1/4 inch or more long. If rust is abundant,
two or more pustules may merge to form a streak. As the crop matures,
the reddish brown pustules gradually turn black as a result of the formation
of overwintering spores (teliospores).
DISEASE CYCLE AND PATHOGEN VARIABILITY
The disease cycle of the stem-rust fungus involves several different
kinds of spore forms. The fungus also requires an alternate host, the
European or common barberry (Berberis vulgaris), to complete its full
cycle (Figures 3 and 4). The alternate host is important in two ways:
(1) New races of rust may be produced on barberry as the result of the
hybridization or cross-fertilization of existing races. (2) Rust spreads
from barberry to nearby cereals and grasses two or three weeks before
the rust spores are normally carried by the wind from the south into Illinois.
With this early start, rust can severely damage young grain plants.
New races of stem rust are believed to originate largely by mutation,
but also by recombination due to hybridization on barberry. Since this
fungus has the capacity to develop new races that infect previously resistant
cultivars it greatly complicates the work of the plant breeder in developing
and maintaining rust-resistant cultivars of wheat, oats, barley, rye,
and grasses. New races, occurring in the large summer population or from
barberry, may be blown south in the fall to infect winter grains and grasses.
Here they may overwinter, multiply in spring, and spread northward to
attack Illinois grain crops that were previously resistant. Every common
barberry bush is a potential source of new races of stem rust. Many more
races appear in localities where barberries are numerous than in areas
where there are no bushes. With the widespread eradication of barberry,
however, this problem has been greatly alleviated.
Spring. Black spores (teliospores) of the rust fungus survive the winter
in Illinois and other northern states on grain stubble, straw, and various
grasses. The overwintering spores germinate in the early spring, producing
another type of spore (sporidia or basidiospores) that can infect only
the leaves of the common barberry. Within a week to ten days, the spring
or "cluster-cup" stage of the rust appears on the undersides
of the barberry leaves. Spores (aeciospores) produced in the orange cluster-cups
(Figure 5) are carried by air currents and infect nearby grain and grass
plants that are susceptible. Billions of spores may be produced on a single
barberry bush.
Summer. With favorable weather (high humidity, foggy conditions, frequent
heavy dews or light rains, and temperatures of 70 F [21 C] or above),
billions of red rust spores (urediospores) are produced on susceptible
cereals and grasses. New "crops" of red spores may be produced
every 7 to 14 days, causing a local buildup of disease, and may be blown
away by the wind to attack other grain and grass plants. The rust spreads
rapidly from plant to plant, from field to field, and from south to north
until harvest–as long as weather is favorable. Grain fields many
miles from a barberry plant or other source of spores may be affected
in this manner.
Fall and Winter. As the grains and grasses mature, the black overwintering
spores are again produced. These thick-walled spores survive the winter
to start another local cycle of infection in the spring. The black-spore
form of stem rust cannot infect grains and grasses directly. Removing
the alternate host (barberry) breaks the hybridization stage of the life
cycle of the fungus.
Ergotism
Barberry Eradication
The rust-spreading barberry is not native to Illinois. Early settlers
planted the first barberries in the state at Galena in 1844. Common barberries
were used for hedge and ornamental plantings until 1918, when the shrub
was outlawed.
Seed produced on barberry bushes is scattered by birds and other natural
means to nearby fencerows, woodlots, timbered areas, and stream banks.
Today, the problem is principally one of finding and destroying the bushes
that are growing wild in difficult-to-reach areas so as to continue to
maintain the present level of removal. Until 1975, federal, state, and
local agencies in Illinois and eighteen other states cooperated in barberry
eradication. From 1975 through 1980, the federal eradication program was
gradually terminated. The states now are independently responsible for
eradication. The federal program continues to test new cultivars and species
for resistance to rust and to inspect bushes in commercial trade.
Since the start of the eradication program in 1918, nearly three million
common barberry bushes have been destroyed on over 20,000 Illinois properties.
Less than 879 of these locations where bushes have been found still required
rechecking by 1973. Areas consisting of more than 55,815 square miles
in Illinois were cleared of barberry bushes (Figure 6). The annual cost
of this eradication program, including all federal and nonfederal funds,
averaged about a hundredth of a cent per acre for the small grains harvested
in Illinois each year. However, scattered bushes remain, so it is necessary
to constantly watch for common barberry bushes and continue to remove
those in the vicinity of small grain and grass hay fields. Currently,
there are no known areas in the wheat- and oat-producing areas of the
Great Plains of the United States where rust spreads from barberry to
commercial fields. There is, however, always a risk
of barberry becoming reestablished in fencerows and field areas.
Medical Uses
How to Identify Barberries
The common barberry that harbors and spreads stem rust is normally about
six feet tall. Specimens may range from tiny seedlings to erect, woody,
fifteen-foot shrubs. There are both green- and purple-leafed varieties.
The red- and green-leaf Japanese barberries, now widely used for hedge
and ornamental plantings are immune to rust. These harmless, low-growing
shrubs (seldom more than five feet tall) are easily distinguished from
the common barberry, as shown in Figure 7. A few other kinds of harmless
barberries (such as Mentor) are sold by nurseries and may be planted wherever
desired.
CONTROL
1. Sow small grain cultivars that are rust-resistant and that mature
early. Use those recommended for your locality by the Department of Agronomy
at the University of Illinois and by your nearest Extension adviser.
2. Use chemical control wisely. There are several fungicides available
that can be applied as sprays, usually by aircraft, that control stem
rust and other diseases. Recommended fungicides and their timing for maximum
control are given in the Illinois Agricultural Pest Management Handbook
which is updated annually and available at your nearest Extension office.
It is very important to scout fields and to know when the disease can
cause a yield reduction. In Illinois, if rust is present on the flag leaf
or leaf below the flag leaf at heading, yield and grain quality losses
may occur. Some fungicides must be applied at specific growth stages,
so it is important to carefully follow label directions.
3. Sow spring grains early. Plant winter crops on recommended seeding
dates. Winter wheat should be sowed as soon as possible after the hessian
fly-free date that is announced for your area. Waiting until this date
minimizes the "green bridge" between crop seasons required by
stem rust.
4. Be alert to the possible presence of barberry bushes to avoid the
local spread of rust and to prevent possible new, virulent, physiologic
races of rust from becoming established. Remember that the development
and spread of a single new race of stem rust has caused crop losses amounting
to many millions of bushels of grain, caused farmers to discontinue growing
otherwise excellent cultivars of grain, and reduced farm income by millions
of dollars.
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