Introduction to Fungi, Third Edition

624 UREDINIOMYCETES: UREDINALES (RUST FUNGI)
follows the usual pattern for dikaryotic propagules,
i.e. via an appressorium through a stoma
as shown in Fig. 22.5b. Within about 7 21 days
of infection, a new crop of urediniospores is
formed so that inoculum can build up rapidly
within a crop. A single uredinium may produce
between 50 000 to 400 000 spores, and there may
be 4 5 generations of urediniospores in the
growing period of a wheat crop. Disease development
occurs best at daytime temperatures
around 30°C. This may explain why stem rust
of wheat is particularly prevalent in areas with
a continental climate, whereas it is not a serious
disease, for example, in Britain.
Puccinia graminis f. sp. tritici can survive the
winter in the mild climates, e.g. of Mexico and the
Southern USA (see Fig. 22.11) as uredinial infections
on its principal host, i.e. wheat crops and
volunteer plants. Despite the almost complete
elimination of the barberry, infections with
P. graminis f. sp. tritici still occur every year in
North America from uredinial lesions overwintering
in the South. Urediniospores are longer-lived
than aeciospores, and waves of inoculum can
move northwards with the prevailing southerly
winds. The track taken is called the ‘Puccinia
pathway’ (Agrios, 2005). Using small aeroplanes,
urediniospores have been detected at altitudes
as high as 5000 feet. The total distance travelled
each year from the Southern USA to Canada
can be in excess of 2000 miles (Stakman &
Christensen, 1946), but in most cases migration
occurs in several shorter intervals, with new crops
of uredinia being produced en route (Eversmeyer &
Kramer, 2000). Consequently, infections start
at successively later dates the further north
the epidemic moves (Fig. 22.11). Attempts have
been made to interrupt this migration by planting
cultivars with different resistance genes in
different regions of the Puccinia pathway (Frey
et al., 1973). Towards the end of the season there
may be a reversal of the flow of air, so that wheat
Fig 22.11 The‘Puccinia pathway’ in North America, with dates indicating the average annual arrival of P. graminis f. sp. tritici in
the wheat fields.The fungus regularly overwinters in the uredinial state south of 30° Northern latitude, and occasionally as far north
as 34°. Redrawn from Roelfs (1986) and several other sources.