ONION DISEASES - The Epidemiology of Plant Diseases - page 505

28 o C hyphal growth, respectively, are severely restricted. At these higher tempe-
ratures, seedlings grow more quickly through the susceptible state.
Seedlings are susceptible only from the second day after seed germination for a
period of 10 to 15 days (Horst, 2001). During this time, infection hyphae may
penetrate the cuticle of the cotyledon; if they invade the meristematic tissues, then
successive leaves (which are produced basipetally in onion) become infected during
the short period of susceptibility.
Thus the ecological limits of smut are determined by soil temperature.
19.5.3 Fusarium oxysporum f.sp. cepae (cause of basal plate rot)
(a) Pathogen and disease
Fusarium oxysporum is a common soilborne plant pathogen with a worldwide
distribution. As a species, it probably causes more damage to agricultural crops than
any other plant pathogen (Correll, 1991). Within the species there is a high level of
specificity with over 120 described formae speciales and races capable of causing
vascular wilt diseases of many agricultural crops (Armstrong and Armstrong, 1981).
Fusarium oxysporum f.sp. cepae is one of these and it is the imperfect state of an
ascomycete fungus. In this form, the pathogen is a soilborne necrotroph that causes
disease outbreaks in onions at high soil temperatures (25 to 35 o C). Basal plate rot is
prevalent in areas of the world where onions are grown under high temperature
conditions (Sherf and Macnab, 1986). Thus, over the years, the disease has been
responsible for significant losses in field and stored crops of dry bulb onions in
many parts of the world, for example, in Colorado, USA (Swift, et al., 2002), in
Turkey (Özer et al., 2003) and in Italy (Fantino and Schiavi, 1987). Also, the fungus
attacks shallot and garlic (Horst, 2001).
Infection may occur at any time during the growth of the crop; below ground the
roots, and via these the stem base plate (modified stem) from which the leaves are
produced, are attacked, resulting in a progressive yellowing ( Fusarium yellows) or
necrosis of the leaves from the tips downward culminating in wilting ( Fusarium wilt)
in some cases (Havey, 1995). The roots ultimately rot; infected stem plates may show
a brown discolouration and affected bulbs rot progressively in store (Havey, 1995).
Wilt - resistant, intermediate- and long-day onion cultivars have been developed
but resistant short-day cultivars are lacking (Cramer, 2000).
Disease severity during onion tissue colonisation varies depending on variation
in production of pectolytic isoenzymes produced by the fungus (Özer et al., 2003).
(b) Epidemiology
The fungus commonly occurs in the soil where long-term survival is by means of
thick-walled resting spores (chlamydospores); macroconidia and less often
microconidia are also produced. Soil temperatures of 25 to 28 o C are optimal for
disease development; at 15 o C the disease is rarely seen (Havey, 1995). The fungus
may be active at any soil moisture that permits plant growth.