Agriculture Reference
In-Depth Information
The first symptoms on leaves are small water-soaked lesions that develop white
centres and enlarge to become zonate and brown to purple in colour. Older leaves
are more susceptible than younger ones to infection (Miller, 1983; Lakra, 1999).
Severely affected foliage may die back and seed stalks may fail to produce seed
heads causing serious losses in onion seed crops (Lakra, 1999).
(b) Epidemiology
The epidemiological factors that control the development of temperate foliar
pathogens such as
Botrytis squamosa
and
Peronospora destructor
also apply to
Alternaria porri
. A major difference is that
A. porri
is active over a higher
temperature range with an optimum of 25
o
C (Fahim, 1966; Miller and Lacy, 1995).
Otherwise, conidia are formed at night on leaf lesions at relative humidities of 90%
or more and are fully mature after 15 h of dew. Sporulation occurs at night at high
relative humidities and conidia are released between 0800 and 1400 h as humidity
decreases on calm days (Fig. 19.3) (Meredith, 1966). The concentration of conidia in
the air increases on windy days and also after rainfall, irrigation or spraying
(Meredith, 1966).
Figure 19.3. Diurnal periodicity of
Alternaria porri
conidia in calm dry weather (redrawn from
Meredith, 1966).