Agriculture Reference
In-Depth Information
In the soil, the propagules (chlamydospores) of the fungus close to or on onion
roots germinate to produce limited or more extensive mycelial growth on the root
surface (Abawi and Lorbeer, 1971b). From this, hyphal penetration of the root may
be direct through uninjured root tissue or via wounds; appressoria may or may not
be formed (Abawi and Lorbeer, 1971b). Although the pathogen may remain latent in
onion plants and bulbs, crop yield may be reduced and basal rot may develop in the
stored bulbs (Stadnik and Dhringa, 1997). Bulb rot begins as soon as the pathogen
spreads from the stem base plate to the leaf sheath tissues (Abawi and Lorbeer,
1971b; Holz and Knox-Davies, 1985).
Soil temperature is the main limiting factor in the epidemiology of the disease
but inoculum concentration and certain soil factors are also important.
Inoculum was found to be unevenly distributed in organic soils (Abawi and
Lorbeer, 1971a), ranging from 300 to 6,500 propagules of
F. oxysporum
f.sp.
cepae
per gram of oven dried soil in the top 15 cm of soil.
Abawi and Lorbeer (1972) demonstrated that a population of 50,000 propagules
g
-1
of oven dried soil was needed before significant disease development could be
detected in field soil.
Farms with a history of the disease usually had the highest populations of the
fungus, but in some cases there was evidence of biotic and or abiotic effects where
fields with high populations of
F. oxysporum
f.sp.
cepae
did not have a high
incidence of basal rot (Abawi and Lorbeer, 1971a).
There are suppressive and conducive soils that affect the incidence of diseases
caused by
Fusarium
wilt fungi (Mace
et al.,
1981). General suppression is conferred
by overall activity of the microbial community resident in the soil, but specific soil
suppressiveness is active against a specific pathogen (Mazzola, 2004). Thus,
although microbial interactions in the rhizosphere are extremely complex, specificity
in soil-plant-microbe interactions for disease suppression exists (Whipps, 2001).
Wilt-suppressive soils are specific to
Fusarium
wilts and are not effective against
diseases caused by non-vascular
Fusarium
species (Weller
et al.,
2002). Certain
soilborne bacterial and fungal genera contribute to suppression. Suppression of
pathogenic
F. oxysporum
has been attributed to the activity of non-pathogenic
F. oxysporum
and fluorescent bacteria (
Pseudomonas
spp.) (Weller
et al.,
2002).
19.5.4
Pyrenochaeta terrestris
(cause of pink root)
(a) Pathogen and disease
Pyrenochaeta terrestris
is the pycnidial asexual state of an ascomycete fungus; there
is no teliomorph. It is a soil inhabitant and a weak necrotroph.
Pink root is a devastating disease of onions, especially of those grown in the
tropics and sub-tropics (Entwistle, 1990;
Sumner, 1995; Pagès and Notteghem,
1996). Also, leeks may be affected (Biesiada
et al.,
2004).
Unlike the diseases already described, isolates of the fungus are weakly asso-
ciated with the roots of non-allium crops - for example, pepper, eggplant, soyabean,
