Puccinia graminis: Life cycle,
symptoms and control measures
By- Dr. Ekta Khare
Department of Microbiology,
Chhatrapati Shahu Ji Maharaj University, Kanpur
• DISEASE: Stem rust (black rust)
• PATHOGEN: Puccinia graminis f. sp. tritici
• HOSTS: Wheat and barley, common barberry (and some
additional Berberis, Mahoberberis, and Mahonia spp.)
• Stem rust was once the most feared disease of cereal crops. It is not as
damaging now due to the development of resistant cultivars, but
outbreaks may occur when new pathogen races arise against which the
existing kinds of resistance are ineffective.
• Stem rust remains an important threat to wheat and barley and, thus, to
the world food supply.
• Anton deBary first demonstrated the heteroecious life cycle of a rust
fungus with Puccinia graminis, the causal agent of stem rust.
Symptoms and Signs
• On wheat and other grass hosts:
• Plants do not usually show obvious disease symptoms until
7 to 15 days after infection when the oval pustules
(uredinia) of powdery, brick-red urediniospores break
through the epidermis (Figures 1, 2).
• Microscopically, these red spores are covered with fine
spines (Figures 3, 4).
• The pustules may be abundant and produced on both leaf
surfaces and stems of grass hosts.
• Later in the season, pustules (telia) of black teliospores
begin to appear in infected grass species (Figure 5).
• Microscopically, teliospores are two celled and thick walled
(Figure 6).
Pathogen Biology
• Rust fungi are obligate parasites. In nature, they require living host tissue for
growth and reproduction; they cannot exist as saprophytes.
• In the absence of living host tissue, they survive as spores.
• In most rust fungi, only the teliospores are adapted to survive apart from a
living host plant for more than a few months under field conditions.
• Puccinia graminis is heteroecious. This word describes rust fungi that require
two unrelated host plants, such as wheat and barberry, to complete their life
cycle.
• Puccinia graminis is macrocyclic, producing all five spore stages:
– basidiospores,
– pycniospores (spermatia),
– aeciospores,
– urediniospores (uredospores),
– teliospores.
• Anton deBary, in 1865, first recognized the nature of the heteroecious life
cycle, but the role of each spore stage was not completely understood until
John Craigie, a Canadian scientist, studied the pathogen in 1927.
... Pathogen Biology
• Although stem rust is caused by a single species of fungus, Puccinia graminis,
there is considerable genetic variation within the species.
• In 1884, Eriksson discovered host-specific subspecies or "special forms" of the
fungus.
• Each special form is designated in Latin as a forma specialis or "f. sp."
• All of the formae speciales have an identical appearance, but vary in host range.
• The pathogen that causes stem rust of wheat (Triticum aestivum) is Puccinia
graminis f. sp. tritici.
• Other formae speciales include P. graminis f.sp. secalis, causal agent of stem rust
of rye (Secale cereale), and P. graminis f.sp. avenae, causal agent of stem rust of
oat (Avena sativa).
• Both Puccinia graminis f. sp. tritici and P. graminis f.sp. secalis cause stem rust in
barley.
• About 1916, E.C. Stakman and others determined that within P. graminis f.
sp. tritici are further genetic subdivisions called races. Later, races were found
within other formae speciales as well.
Disease Cycle and Epidemiology
• The disease cycle of wheat stem rust starts with the exposure of each new
wheat crop to spores of Puccinia graminis f. sp. tritici, which are the primary
inoculum.
• The source of the first spores that infect the new wheat crop differs depending
on the region in which the wheat is grown.
• In warm climates, wheat is planted in late fall and harvested in early summer.
• The first spores to infect the young wheat plants in the fall are urediniospores.
• They generally come from infected volunteer wheat plants.
• Seed spilled in the field or on roadsides at harvest time often sprout and
produce scattered volunteer plants.
• These plants can become infected from spores produced on late-maturing
wheat plants still in the field.
• The infected volunteer wheat plants serve as a bridge that carries P. graminis f.
sp. tritici through the summer to the next fall-sown crop of wheat.
• In regions with temperate climates, wheat may
be planted either in the fall (winter wheat) or the
spring (spring wheat) depending on the severity
of the winters.
• The first rust spores to infect wheat in the spring
in temperate regions may be aeciospores from
barberry, the alternate host, or urediniospores
from infected wheat in distant regions with
milder winters.
• Therefore, we describe two disease cycles for
stem rust - with or without barberry.
• Uredinial Stage
• The uredinial, or red summer, stage is initiated by germination of a urediniospore on its
grassy host, penetration, development of an intracellular mycelium with intracellular
haustoria, and subsequent sporulation of uredinia to form new urediniospores.
• The recycling of the uredinial stage is the major means whereby the fungus initiates
and perpetuates an epidemic.
• The urediniospores of P. graminis are dikaryotic (n+n), dehiscent, thick-walled and
covered with spines. They are elliptical and about 20 x 30 µm.
• Telial Stage
• As infected plants mature, urediniospore formation ceases and teliospore formation
commences, either in the same, or in new (telia), fruiting structures.
• At this stage, the infections become black, hence the name black rust.
• The ontogeny of teliospores is the same as urediniospores, but the teliospores remain
attached.
• The teliospores are two-celled, thick-walled (with up to five wall layers) and are
thickened at the apical end.
• Teliospores are important because they are constitutionally dormant, enabling the
fungus to survive severe cold or drought.
• The mature teliospore represents the only true diploid state of the fungus.
Basidiospore Stage
• The germination of teliospores and subsequent meiosis in the
basidium results in the formation of haploid basidiospores.
• Four basidiospores, two of each opposite mating types, are
produced from each basidium.
• If basidiospores are deposited on the surface of the alternate host
(mainly Berberis vulgaris) they germinate, penetrate directly
through the host epidermis and form a haploid mycelium.
• The fungus is most capable of infecting Berberis only when the
leaves are young and tender.
• The fruiting structure, formed as a result of basidiospore infection,
is called a pycnium.
Spermatial Stage
• The pycnia are normally formed on the adaxial leaf surface, often
in clusters.
• The important features of the pycnia are the formation of flexuous
(receptive) hyphae and haploid spermatia.
• The spermatia, produced successively from the terminal ends of
sporophores, are exuded in a nectar.
• The nectar attracts insects, which in addition to splashing rain
drops, serve to transport the spermatia to flexuous hyphae of the
pycnia of opposite mating types, where fusion occurs.
Aecial Stage
• Following union of the opposite mating types, dikaryotization occurs.
• The spermatial nuclei migrate to the protoaecium, where mitosis occurs,
the nuclei reassort into dikaryons and the aecial structure forms.
• The aecia of P. graminis are elongated, cylindrical structures.
• The ornamented, dikaryotic aeciospores are produced successively in
chains from the aeciosporophores.
• The aeciospores infect the grassy host, completing the fungal life cycle.
• Aeciospores differ from urediniospores, which also infect wheat, in their
appearance - slightly warty rather than spiny - and in the way in which
they are formed - in chains in an aecium rather than on individual stalks in
a uredinium.
Fig 7. Barberry plants.
Fig 8. Pycnia are often in small
clusters and exude pycniospores
in a sticky honeydew.
Fig 9-10. Cup-shaped structures
filled with orange-yellow,
powdery aeciospores break
through the lower leaf surface.
Fig 11. Microscopically,
aeciospores have a slightly
warty surface.
Epidemiology
• Stem rust is favored by hot days (25-30ºC/ 77-86ºF), mild nights (15-20ºC/
59-68ºF), and wet leaves from rain or dew.
• Both aeciospores and urediniospores require free water for germination
as do the other spore stages.
• Infections occur through stomata.
• If disease develops in individual foci within a wheat field, the source of
urediniospores is probably overwintering mycelia and/or uredinia.
• Rusted plants in foci from overwintering sources have heavy infection in
lower leaves and less infection in the younger leaves formed higher on the
wheat plants.
• In the absence of barberry or other alternate hosts, urediniospores are the
only functional spores in the disease cycle of P. graminis.
• In tropical and subtropical climates, mycelium and urediniospores on
volunteer wheat and noncrop grass hosts begin epidemics.
• Urediniospores are generally unable to survive harsh winter conditions.
• In the Northern Hemisphere, inoculum for spring wheat arrives from
southern areas.
... Epidemiology
• In Southern Hemisphere, urediniospores arrive from milder areas in the north.
• Occasionally, P. graminis can overwinter in wheat volunteers, noncrop grass
hosts, and winter wheat, but usually only where snow cover insulates both the
wheat leaves and the fungal mycelium.
• One uredinium can produce at least 100,000 urediniospores.
• Explosive epidemics can occur during favorable environmental conditions,
resulting in losses of 50 to 70% over a region.
• Stem rust causes cereal yield losses in several ways:
– The fungus absorbs nutrients from the plant tissues that would be used for
grain development in a healthy plant.
– As pustules break through the epidermal tissue, it becomes difficult for the
plant to control transpiration, so its metabolism becomes less efficient.
– Desiccation or infection by other fungi and bacteria also can occur.
– Interference with the vascular tissues results in shriveled grains.
– Stem rust also can weaken wheat stems, so plants lodge, or fall over, in
heavy winds and rain.
Disease Management
• Barberry eradication: An expensive and extensive barberry survey and
eradication program was initiated in 1918 in the U.S. and continues to a
limited extent today.
• Cultural practices: It has long been known that moisture on leaves and
excessive foliar nitrogen favor infections by rust fungi.
• Farmers consider these factors in spacing, row orientation, and fertilizer
schedules.
• Mixed cropping with suitable crops.
• Genetic resistance: Genetic resistance is the most commonly used and the
most effective means to control stem rust.
• Grow resistant varieties like PBW 343, PBW 550, PBW 17
• Chemical control: Fungicides that inhibit the synthesis of sterols [i.e.,
sterol biosynthesis inhibitors (SBIs) or demethylation inhibitors (DMIs)] are
particularly effective, but the cost of application is generally prohibitive for
routine use in most wheat-growing areas in the U.S.
• Spray Zineb at 2.5 kg/ha or Propioconazole @ 0.1 %.
