Agriculture Reference
In-Depth Information
The pathogen causes root necrosis of wheat
and, to a lesser extent, of barley, rye, and some
grasses.
between the second and fourth year. In regions
where soil organic matter is plentiful and the soil
environment is favorable for microbial growth,
continued production of wheat then leads to a
decline of disease severity until a balance is
reached at a severity less than that during years
of maximum disease expression. Several biologi-
cal hypotheses were proposed to explain this phe-
nomenon, named take-all decline (Hornby 1979).
Current agreement focuses on the important role
of fl uorescent pseudomonads as biological antago-
nists of the take-all pathogen (Weller et al.,
2007).
Mathematical models (Brasset and Gilligan
1989) incorporate components for primary and
secondary infection, together with reduction
of inoculum potential over time. A routine
DNA-based assay recently refi ned as a real-time
PCR test is provided as a service to Australian
farmers to quantify take-all pathogens in soil
samples and to serve as a basis for predicting
potential yield loss (Herdina and Roget 2000).
Symptoms and epidemiology
The take-all fungus survives during the intercrop
period on root and shoot debris of a previously
infected crop, or on grasses and volunteer cereals.
It infects seminal roots, causing characteristic
black necrosis, sometimes after root surface colo-
nization by brown runner hyphae of the fungus.
Secondary infections occur mainly from root-to-
root contact, extending disease to the neighboring
plants, but spreading only a short distance result-
ing in a patchy distribution of the disease within
a crop (Cook 2003; Gosme et al., 2007). The root
system can become entirely affected. In moist
conditions a black necrosis can develop on the
lower stem (Color Plate 12d), but this symptom
seldom occurs in low-rainfall regions where wheat
matures with little or no summer rainfall or irri-
gation. Perithecia (sexual stage) may form on the
lower stem and discharge ascospores, but the
importance of ascospores in the existing crop
is likely limited because perithecia are formed
late in the growing season. Due to restricted
capture of nitrogen and water by roots (Schoeny
et al., 2003), infected plants develop poorly
and appear as stunted patches in spring. Prema-
ture ripening in summer is expressed as white-
heads (Color Plates 13a, 14c) bearing shrivelled
grains.
Severe disease that develops early in the season
causes dramatic yield losses by reducing all yield
components (Schoeny et al., 2001). Even when
disease severity is low or moderate, major yield
losses are to be expected when a dry period occurs
during grain formation and fi lling. Take-all is
severe when the autumn and winter are mild and
humid, allowing early infections to greatly increase
the level of primary inoculum (Hornby 1978). As
the take-all fungus is not a good saprophyte, high
levels of inoculum are the result of host plants
being infected. Wheat monocultures increase
inoculum levels through several seasons, increas-
ing disease severity to a maximum generally
Causal organism
Gaeumannomyces graminis (Sacc.) Arx & Olivier
var. tritici Walker ( Ggt ) is responsible for take-all
of wheat, triticale ( Triticosecale rimpaui Wittm.),
barley, and rye, in decreasing order of susceptibil-
ity. These crops can also be attacked by G. grami-
nis var. avenae ( Gga ), which causes take-all of
oat and take-all patch of turfgrass (Smiley et al.,
2005a). Varieties Ggt and Gga can be differenti-
ated by host range and pathogenicity tests or by
measuring the mean length of ascospores: 70-
105 μm for Ggt and 100-130 μm for Gga (Freeman
and Ward 2004). Two other G. graminis varieties
are known to colonize roots of wheat but are con-
sidered as weakly or nonpathogenic, G. graminis
var. graminis causing dieback of Bermudagrass
[ Cynodon dactylon (L.) Pers.], and G. graminis var.
maydis causing take-all on maize (Freeman and
Ward 2004).
Gaeumannomyces species such as Ggt and Gga
are homothallic, meaning individual isolates are
able to produce perithecia. When Gaeumannomy-
ces -like fungi are isolated from plants with take-all
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