Agriculture Reference
In-Depth Information
Leaf rust is characterized by the uredinial stage
of small round, brown to orange pustules that
occur on the upper and lower leaf surfaces, and
less frequently on the leaf sheaths (Color Plate 9).
The pustules remain discrete, without coalescing.
The uredinia are capable of producing up to 3,000
urediniospores per day (Roelfs et al., 1992) if the
host plant tissue remains healthy. The uredinio-
spores are 20 μm in diameter, echinulate, dikary-
otic (
n
+
n
), wind-disseminated, and deposited in
rain events on host plants in the immediate vicin-
ity and potentially also on hosts hundreds of kilo-
meters distant. The urediniospores germinate on
wheat plants, producing the specialized infection
structures of appressoria, germ tube, and penetra-
tion peg (Harder 1984) that allow the fungus to
penetrate the host stomata. Further specialized
structures, the substomatal vesicle and haustoria,
are produced which allow the fungus to obtain
nutrients from host mesophyll cells without
killing them. Infectious hyphae of the fungus
spread throughout the mesophyll layer. Leaf rust
infections are initially visible as faint fl ecks on leaf
surfaces 3-4 days after inoculation. Uredinia
erupt and break through the epidermal leaf surface
8-10 days after initiation of the infection process.
The clonally produced urediniospores can cycle
indefi nitely on wheat hosts.
Resistance responses to leaf rust are character-
ized by small uredinia surrounded by necrosis, or
by an abundance of hypersensitive fl ecks pro-
duced in response to infection (Color Plate 9).
Non-hypersensitive resistance is characterized by
fewer and small uredinia compared to a suscepti-
ble response. As the uredinia age, teliospores are
formed in the uredinia. Teliospores are dark,
16 μm wide, and thick walled and have two
dikaryotic cells. The pycniospores and aecio-
spores, which are spore stages associated with
sexual reproduction on alternate hosts, are not
produced on the wheat host.
Leaf rust infections occur in the US during
September to November on fall-planted winter
wheat from Texas to South Dakota, and from the
Gulf Coast states to North Carolina. Leaf rust
infected volunteer winter wheat plants that
survive the summer are the inoculum source for
INTRODUCTION
The rusts of wheat are among the most important
and common diseases of wheat in the US and
worldwide. Rust has affl icted wheat for thousands
of years as references to wheat rust can be found
in the Bible and the classical literature of ancient
Greece and Rome (Chester 1946). In the early
20th century widespread epidemics of wheat
rusts provided the impetus for early advances
in genetics of disease resistance in plants, epide-
miology of plant pathogens, and genetics of host-
parasite interactions. Today the rust diseases
continue to cause regular yield losses worldwide,
threatening the sustainable production of wheat.
The continuing evolution of virulent rust races in
response to the release of rust resistant wheat
cultivars poses a constant challenge to wheat
researchers.
WHEAT LEAF RUST
Distribution and epidemiology
Leaf rust, caused by
Puccinia triticina
Eriks., is
the most common and widely distributed of the
three rust diseases of wheat. In the US, leaf rust
is commonly found on soft red winter wheat
grown in the southeastern states and Ohio Valley
region, on hard red winter wheat from Texas to
South Dakota, and on hard red spring wheat in
South Dakota, North Dakota, and Minnesota.
Leaf rust also occurs on spring wheat that is
grown in California, and to a lesser extent on
winter and spring wheat in Oregon, Washington,
and Idaho. Worldwide, leaf rust is a major conti-
nent-wide disease in the western prairies of
Canada; the South American region of Argentina,
Chile, Brazil, and Uruguay (German et al., 2007);
the Central Asia region of northern Kazakhstan
and Siberia; southern and central Europe; the
Middle East; and parts of the Indian Subconti-
nent (Roelfs et al., 1992). Leaf rust is less wide-
spread and occurs at a local level in eastern and
western Australia, China, eastern Africa, and in
South Africa.
