Introduction to Fungi, Third Edition

622 UREDINIOMYCETES: UREDINALES (RUST FUNGI)
Fig 22.9 Puccinia graminis. (a) T.S. spermogonium on a leaf of Berberis vulgaris.The spermogonium has penetrated the upper
epidermis.The wall of the spermogonium is lined by tapering annellides which give rise to spermatia. (b) T.S. leaf of B. vulgaris
showing a spermogonium and a proto-aecium. (c) T.S. leaf of B. vulgaris showing an aecium in section.The aecium has burst through
the lower epidermis of the host leaf. Note the columns consisting of alternating large and small cells.The large cells are the
aeciospores.
spore chains, whose outer walls are thick and
fibrous. This layer forms a clearly defined border
or peridium surrounding the spores. Eventually
the peridium and spore chains burst through the
lower epidermis. The peridium ruptures and
aeciospores are now visible as orange-coloured
cells enclosed by the white cup-like peridium
(Fig. 22.8b). Since the aecia are commonly seen
clustered together beneath a spermogonial
lesion, this stage is popularly known as the
cluster-cup stage. In a section through the centre
of a group of young aecia, it is usually possible to
find the spermogonia penetrating the upper
epidermis and the aecia penetrating the lower.
The aeciospores are violently projected from the
end of the spore chain by rounding off of the
flattened interface between adjacent spores
(Ingold, 1971). Aeciospores are unable to reinfect
barberry but readily infect wheat or
other grasses.
It is relatively easy to establish and maintain
P. graminis in garden situations by planting
B. vulgaris and allowing Agropyron repens to grow
underneath (Webster et al., 1999). The form most
likely to grow is P. graminis f. sp. secalis which
readily alternates between Berberis and grasses
in Britain (Wilson & Henderson, 1966). In other
countries the connection between Berberis and
P. graminis f. sp. tritici is strong, and the first
barberry eradication laws were implemented
long before de Bary had formally proven the
connection. The first recorded cases of such laws
were in Rouen in 1660 and Massachusetts in 1755.
The greatest effort by far was the barberry
eradication campaign undertaken as a consequence
of the 1916 epidemic in the United States.
Widespread eradication of susceptible Berberis
spp. started in 1918 and continued on a massive
scale well into the 1930s, and locally for several
decades afterwards. Overall, this campaign is
considered to have been successful, with wheat
rust epidemics much reduced in severity, especially
on a local scale. Because aeciospores are not
particularly long-lived and are not produced in
vast numbers, the direct impact of barberry
bushes on wheat crops is limited to less than
2 miles. Perhaps more importantly, barberry
eradication also retarded the evolution of new
races of P. graminis due to the reduced ability of
the pathogen to reproduce sexually. Campbell
and Long (2001) have provided a highly readable
account of the eradication campaign in the USA.