ONION DISEASES - The Epidemiology of Plant Diseases - page 501

The pathogen usually associated with purple blotch symptoms is Alternaria porri .
However, these symptoms are generally indistinguishable from those of Stemphylium
leaf blight caused by Stemphylium vesicarium and purple leaf blotch disease is now
considered by Suheri and Price (2000a,b) to be a complex caused by both pathogens.
19.5 CASE HISTORIES: SOILBORNE DISEASES
Soilborne pathogens are located and localised in an environment where their
epidemiology is determined by edaphic effects of the soil and its microflora. In the
soil, host roots grow to contact pathogens in their immediate locality; above ground
and by comparison, leaf pathogens are disseminated in the air to infect new crops
over a wide area.
The main temperate soilborne pathogens described here are Sclerotium
cepivorum (cause of onion white rot), a specialised necrotroph, and Urocystis
cepulae (cause of onion smut) a biotroph (Table 19.2).
The warm-climate pathogens described are Pyrenochaeta terrestris (cause of
pink root) and Fusarium oxysporum f.sp. cepae (cause of basal rot); both function as
necrotrophs (Table 19.2).
Soilborne pathogens of onions affect the rooting systems of the plants commonly
causing wilting, loss of turgor, stunting and death.
Most root diseases are disseminated by the movement of infected vegetative
material, or by any action that moves soil containing the pathogen - for example, in
soil clinging to farm machinery, implements and workers' boots (Entwistle, 1990).
19.5.1 Sclerotium cepivorum (cause of onion white rot)
(a) Pathogen and disease
Sclerotium cepivorum is an ascomycete fungus, which produces black, nearly
spherical sclerotia (about 0.2 to 0.5 mm in diameter); there is no sexual, or conidial
state.
The pathogen is specific to Allium spp. and in that respect is a specialised
necrotroph. White rot occurs in most parts of the world where onions are grown
under temperate conditions. It varies in severity from place to place. New outbreaks
commonly occur where the disease is prevalent locally but, occasionally, the disease
can remain completely localised for considerable periods of time i.e. over 20 years
(Entwistle, 1990).
There are no commercial Allium varieties with sufficient resistance to S. cepivorum
(Utkhede et al., 1982; Brix and Zinkernagel, 1992).
Onions may be attacked at all stages of crop growth. Early attacks cause poor
crop establishment; later infections produce yellowing and wilting, which in some
cases results in the complete collapse of the plant. Affected plants have a grey-white
fluffy mycelium on their stem bases, which gives the disease its name: white rot.
The disease is caused by the sclerotia of Sclerotium cepivorum, which perennate in
the soil. Sclerotia germinate producing hyphae that infect the roots and stem plates
of onions. New sclerotia are formed in the stem base tissues of onions and these are