Agriculture Reference
In-Depth Information
tomato, peas, barley, wheat, oats, cucumber, cauliflower and many other plant
species (Sumner, 1995).
Hyphae of the fungus penetrate the root tips, which become light pink; as
infection progresses, roots become deeper pink, turning to deep purple. Affected
roots become water-soaked, dry out and disintegrate. Diseased plants exhibit
drought symptoms; stunting results and bulb size is reduced.
Unlike
F. oxysporum
f.sp.
cepae
the fungus does not invade the stem base plate
or the bulbs of onions. Like
F. oxysporum
f.sp.
cepae
this fungus is most active in
the upper 15 cm of soil.
Resistance to
P. terrestris
has been tested for in a yellow short-day onion in New
Mexico, USA (Corgan and Holland, 1993) and in Australia, cultivars with prolific
root systems were more tolerant of the pathogen than those with weak root systems
(Rogers and Henderson, 1989). Yellow Bermuda is the most resistant of commercial
onion cultivars (Horst, 2001).
(b) Epidemiology
The pathogen probably survives in soil as chlamydospores, pycnidia or pycnidiospores
or in colonised roots or plant debris (Sumner, 1995). Populations of fungus in the soil
increase rapidly in the presence of onions (Entwistle, 1990).
Temperature is the main epidemiological constraint that applies to
P. terrestris
as it does to
F. oxysporum
f.sp.
cepae
.
The optimum temperatures for the growth of
the pathogen and for disease development range from 24 to 28
o
C; infection is
reduced at 20
o
C and there is little disease at less than 16
o
C (Sumner, 1995). The
fungus is little affected by soil moisture. Thus, the damaging effects of pink root are
determined by the seasonal patterns of soil temperature and the growth period of the
host (Entwistle, 1990).
Both the above fungi fit the same environmental niche and often have been
present together in a disease complex in onion roots; this makes it difficult to decide
which of the two is the main cause of damage (Lacy and Roberts, 1982). It is now
accepted that the pathogens can act independently to cause disease:
P. terrestris
reduces numbers of plants but not weights of bulbs in organic soils, whereas
F. oxysporum
f.sp.
cepae
reduces plant numbers and weights of bulbs (Lacy and
Roberts, 1982).
19.6 CONCLUDING REMARKS
Completely different fungal species occupy similar disease niches in temperate
compared with hot climates. All of them, regardless of function (biotrophic or
necrotrophic), means of survival (chlamydospores, teliospores, sclerotia, etc.)
infection (conidia, sporangia, etc.) and host parts invaded (seed, root or leaf) are
subject to epidemiological constraints. Provided inoculum (seedborne, soilborne or
airborne) is present at a threshold concentration capable of causing disease, then
temperature and moisture (as rainfall, dew, soil moisture and/or relative humidity)
are the main epidemiological factors that limit or facilitate the outbreak and the
