Exotic Brome-Grasses in Arid and Semiarid Ecosystems of the Western US

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7.2 Principal Pathogens on Bromus tectorum

7.2.1 Ustilago bullata (Head Smut Pathogen)

The head smut disease of B. tectorum , caused by the cosmopolitan basidiomycete
pathogen U. bullata , is the B. tectorum disease most familiar to land managers,
ranchers, and recreationists. It is macroscopically visible and conspicuous late in
the spring as smutted flowering heads on diseased plants that are interspersed
in B. tectorum stands among healthy plants in seed (Fig. 7.1a ). Under epidemic

cd

Fig. 7.1 ( a ) Normal Bromus tectorum infl orescence ( right ) and smutted infl orescence infected
with Ustilago bullata ( left ). ( b ) Dormancy loss during 15 weeks in dry storage at 30 °C for a rep-
resentative U. bullata teliospore collection as measured by germination time courses during incu-
bation at 15 °C on PDA for 48 h (each data point represents proportion of teliospores germinated
out of 100 examined, corrected for viability; the data points are independent. The experiment was
repeated in time for multiple teliospore collections with similar results). ( c ) Field disease incidence
following inoculation with teliospores of eight U. bullata populations and planting either in early
fall or late fall (from Boguena et al. 2007 ). ( d ) Disease incidence on 12 perennial grass species
after inoculation with pathogen strains from B. tectorum. (Group 1: AGCR Agropyron cristatum ,
ELLA Elymus lanceolatus , ELTR Elymus trachycaulus , BRCA Bromus carinatus. Group 2: THPO
Thinopyrum ponticum , THIN Thinopyrum intermedium , PASM Pascopyrum smithii , LECI Leymus
cinereus , LESA Leymus salinus , BRIN Bromus inermis. Group 3: PSSP Pseudoroegneria spicata ,
ELEL Elymus elymoides , ELGL Elymus glaucus. Group 4: ELCA Elymus canadensis )

S.E. Meyer et al.