Agriculture Reference
In-Depth Information
With this disease, the duration of conditions favourable for infection and subsequent
RH are important in determining the type of lesion that is produced. Thus, conidia
formed under dew durations of 12 h or less and low RH cause sterile leaf flecks,
while those formed under dew conditions lasting for 16 h or more with high RH
cause typical lesions (Fig. 19.4) (Bock, 1964; Everts and Lacy, 1990a). It was
suggested that flecks may be caused by immature conidia, which are not sufficiently
aggressive to infect (Everts and Lacy, 1990a). However, in onion leaf inoculation
studies, germinated conidia were found near the centre of flecks but these flecks did
not develop into lesions (Everts and Lacy, 1996).
Aveling
et al.
(1994) found that 96% of normal conidia germinated at 25
o
C within
24 h of inoculation and that penetration of leaf tissue is direct or through stomata.
Miller
et al.
(1978) reported that the fungal necrotroph
Stemphylium vesicarium
alone or in complex with
A. porri
caused a severe leaf blight of onions in 1976 in
Texas, USA.
Stemphylium vesicarium
produced white and purple lesions on
commercial garlic crops in Spain (Basallote-Ureba
et al.,
1999). The symptoms were
reproduced when the fungus was inoculated into garlic and onions.
In recent Australian research (Suheri and Price, 2001) either or both pathogens,
Alternaria porri
and
Stemphylium vesicarium
were isolated from purple leaf blotch
lesions on field leeks grown in the state of Victoria. In this field situation
S. vesicarium
was the dominant pathogen. However, the diurnal pattern of
A. porri
and
S. vesicarium
conidia in the leek crops was similar to the periodicity of
A. porri
in
onion crops (Meredith, 1966; Everts and Lacy, 1990b), but the purple blotch lesions
developed under cooler (10 to 13
o
C)
conditions (Suheri and Price, 2001).
Figure 19.4. Effect of duration of dew and post-infection relative humidity on symptom expression
by
Alternaria porri
(redrawn from Bock, 1964).
