Agriculture Reference
In-Depth Information
neck rot (Maude, 1983). In the UK, a drying temperature of 30
o
C, at which the
fungus ceases to grow in culture (Fig. 19.1), is used to inhibit the growth of the
fungus on the necks of freshly-topped harvested onions.
Table 19.2. Effects of temperature on growth of fungal pathogens
in vitro
Temp. range
and optima
for growth °C
Disease
Pathogen
Reference
Cool climates
Downy mildew
Peronospora destructor
1-28
(13)
Yarwood, 1943
Smut
Urocystis cepulae
12-28 (20)
Dow and Lacy,
1969
White rot
Sclerotium cepivorum
< 5-29 (20-24)
Walker, 1926
Neck rot
Botrytis aclada
5-25 (22-23)
Maude, 1990a
Leaf rot
Botrytis squamosa
9-31 (24)
Shoemaker and
Lorbeer, 1977
Hot climates
Purple blotch
Alternaria porri
10-30 (25)
Fahim, 1966
Pink root
Pyrenochaeta terrestris
13-32 (26)
Hansen, 1929
Basal plate rot
F. oxysporum
f.sp.
cepae
9-36
(24-27)
Abawi and Lorbeer,
1972
Black mould
Aspergillus niger
10-40
(31)
Maude, 1990a
Blue/green mould
Aspergillus fumigatus
20-45
(37.5)
Maude, 1990a
All
in vitro
measurements were made on agar
High temperature fungi, such as
Aspergillus niger
, grow optimally in culture at
32.5
o
C (Fig. 19.1); they rarely, if ever, occur on onion seed samples produced under
temperate conditions but occur increasingly on seed samples produced under sub-
tropical and tropical conditions (Table 19.3) achieving maximum incidence where
seeds are produced under hot, dry tropical conditions with temperatures in seed
production fields in excess of 30
o
C.
Aspergillus niger
is seed-transmitted (Hayden
and Maude, 1992) and causes black mould of bulb onions produced and stored in
hot climates, e.g., Texas (USA), Egypt, Sudan and India (Maude, 1990a). Thus,
black mould is rarely a problem of the temperate onion crop; however, in 1982 there
were serious outbreaks of this disease in onions in bulk stores in the UK, where bulb
drying temperatures in excess of 30
o
C and relative humidities in excess of 80% RH
for long periods elicited infection by
A. niger
(Maude and Burchill, 1988).
