Agriculture Reference
In-Depth Information
promote fl ushes of microbial activity that inhibit
Rhizoctonia
.
In moist soil, root rot caused by
R. solani
AG-8
is most severe at 12-15 ºC (Smiley and Uddin
1993; Gill et al., 2001c). However, in dry soil the
pathogen is equally and highly virulent at tem-
peratures from 10-25 ºC (Gill et al., 2001c).
Pathogen virulence in warm, moist soils is some-
what suppressed by a high level of activity by
associated soil microorganisms (Gill et al., 2001b).
Rhizoctonia oryzae
has a higher temperature
optimum for disease (Ogoshi et al., 1990; Smiley
and Uddin 1993), causing maximum disease at
20-27 ºC.
Reduction in root mass can result in nutrient
defi ciency. Increasing nutrient availability with
starter fertilizer banded below or beside the
seed enables plants to tolerate the disease even
where seedling infection rates are amplifi ed by
the starter fertilizer (Smiley et al., 1990; Cook
et al., 2000). Application of nitrogen has
shown variable effects on disease, and application
of zinc reduced bare patch in zinc-defi cient soils
in Australia (MacNish and Neate 1996) but not
in zinc-suffi cient soils in the US (Cook et al.,
2002a).
Infected roots of grassy weeds and crop volun-
teers enable
Rhizoctonia
to survive and expand
inoculum density between crops. When these
plants are killed with preplant or in-crop herbi-
cides, this nectrophic pathogen can extensively
colonize the dying root system, serving as a
bridging reservoir of amplifi ed
Rhizoctonia
inoculum. If a preplant herbicide is applied to
infected plants soon before planting, the newly
planted wheat seed germinates and seedlings
emerge at a time when the
Rhizoctonia
inoculum
level is at a maximum level (Smiley et al., 1992).
This phenomenon is known as the
green
bridge
and is especially evident following applica-
tion of glyphosate, which curtails plant defenses
because of inhibition of a key enzyme in the
shikimate pathway (Lévesque and Rahe 1992).
Residual levels of certain other herbicides in
soil from a previous crop may also increase
Rhizoctonia disease (Smiley and Wilkins
1992).
Causal organisms
A complex of species causes root rot on wheat,
including
Rhizoctonia solani
Kühn (teleomorph
=
Thanatephorus cucumeris
Donk),
R. oryzae
Ryker
and Gooch (teleomorph
Waitea circinata
Warcup and Talbot), and binucleate species with
a sexual stage in the genus
Ceratobasidium
.
Rhi-
zoctonia solani
is divided into a number of sub-
groups called anastomosis groups (AGs), based
on fusion of hyphae between isolates (Sneh et al.,
1991); AG-8 is associated with stunting and bare
patch symptoms on wheat and barley. Other
weakly virulent AGs also have been isolated
from wheat, including AGs 2, 2-1, 4, 5, 9, and 10
(Ogoshi et al., 1990; T.C. Paulitz, unpublished
data).
Rhizoctonia oryzae
also is an important cause of
root rot in the Pacifi c Northwest US (Ogoshi et
al., 1990; Smiley and Uddin 1993; Mazzola et al.,
1996b; Paulitz et al., 2002a,b). The sexual state,
W. circinata
, consists of subgroups that attack rice
(
Oryza sativa
L.;
W. circinata
var.
oryza
e) and
turfgrasses (
W. circinata
var.
circinata
and
W. cir-
cinata
var.
agrostis
). The most pathogenic isolates
on wheat in the Pacifi c Northwest appear to be
var.
circinata
, based on DNA sequencing (P.A.
Okubara, pers. comm.).
Binucleate
Rhizoctonia
AGs CI, E, H, K, and
D have also been isolated from wheat, although
their pathogenicity on wheat roots is not well
known (Mazzola et al., 1996a; Tunalı et al., 2008).
Most of these binucleate isolates appear to be
weakly virulent on cereals and more virulent on
broadleaf rotation crops (T.C. Paulitz, unpub-
lished data).
Rhizoctonia
AG-D is also known
as
Rhizoctonia cerealis
Van der Hoeven, which
causes sharp eyespot, a basal culm disease
similar in appearance to eyespot caused by
Oculimacula
species and discussed later in this
chapter.
Rhizoctonia
species can be diffi cult to identify
because they do not produce spores.
Rhizoctonia
solani
forms rather thick hyphae (4-15
=
m diam.)
with characteristic right-angle branching, doli-
pore septa near each hyphal branch, and a slight
constriction at the branching point (Sneh et al.,
μ
