Agriculture Reference
In-Depth Information
To an extent this has been achieved by Clarkson
et al.,
(2004) who showed in
laboratory assays that isolates of
Trichoderma viride
effectively degraded sclerotia
of
S. cepivorum
(four isolates) in four onion soil types (silty clay, sand, silt and peat)
under a range of temperature and soil moisture conditions.
19.5.2
Urocystis cepulae
(cause of onion smut)
(a) Pathogen and disease
Urocystis cepulae
is a basidiomycete fungus and obligate biotroph that produces
infective thick-walled compound teliospores which remain viable in the soil for as
long as 25 years (Thaxter, 1890). The pathogen is specific to
Allium
spp., principally
affecting onion, leek and shallots (Horst, 2001).
Smut is a most destructive disease of onions (Horst, 2001) and is of considerable
economic importance in temperate onion-growing areas of the world (Utkhede and
Rahe, 1980; Crevier and Stewart, 1988). Monnet and Thibault (2000) described the
symptoms of
Allium
diseases, including smut, on onions and shallots in France. In
the UK, Brooks (1953) described disease outbreaks of smut as serious but local in
green onions grown from seeds.
Dark lead-coloured lesions appear on the cotyledons of infected onions soon after
emergence. Young lesioned leaves become thickened and twist or curl backwards.
Lesions rupture, releasing the black powdery teliospores of the fungus back into the
soil. Also, the spores may be disseminated by running water, tools, the roots of
transplanted vegetables, and by the feet of people and animals (Horst, 2001).
Teliospores comprise a single thick-walled central cell enclosed by a series of
smaller, hyaline cells that serve as nurse cells. Teliospores give rise to uninucleate
thalli of + or - hyphal mating types which, separately, are not pathogenic. Fusion of
hyphae of opposite mating types results in a binucleate mycelium which is
pathogenic (Tachibana and Duran, 1961) and which infects the emerging cotyledon
leaf below ground level.
Both white rot and onion smut are initiated by fungal propagules, which remain
dormant yet infectious in the soil for considerable periods of time. Unlike the white
rot fungus, which can attack onions at all stages of crop growth,
Urocystis cepulae
can only infect seedlings and very young plants. These are killed within 3 to 4 weeks
after emergence; those that survive are stunted, with brittle, distorted, lesioned leaves.
(b) Epidemiology
Host stimulation of sclerotial germination is a feature of the epidemiology of onion
white rot. There appears to be no similar comparison for
U. cepulae
teliospores. Soil
temperature is a major factor in the epidemiology of the disease. There is abundant
infection of onions at soil temperatures of 10 to 12
o
C and up to 25
o
C; there is no
infection at 29
o
C and there is a rapid decline in disease (Walker and Wellman, 1926).
Optimum germination and growth of teliospores and hyphal fragments occurs in
temperate soils between 13 and 22
o
C; above 25
o
C teliospore germination and above
