Agriculture Reference
In-Depth Information
alone or mixed with a fungicide, in winter can markedly reduce overwintering
mildew.
Pear leaf blight and fruit spot
This disease is caused by
Fabraea maculata
Atk. It leads to defoliation of both
nursery and orchard trees and to disfigured, cracked and misshapen fruits.
'Clapps Favourite' and 'Bartlett' have been found to be resistant in some
studies but not in others. Highly resistant material of potential use in breeding
programmes has been identified (Bell
et al.
,
).
Apple rust
The cedar-apple rust caused by
Gymnosporangium juniperi-virginianae
Schw. is the
most important rust disease of apple and has the North American red cedar
as its alternate host. A number of good quality cultivars, e.g. 'McIntosh', are
resistant: others, e.g. 'Jonathan', fully susceptible (Brown,
).
Apple scab
Apple scab, caused by
Venturia inaequalis
(Cooke) Winter, is evident as scabbed
areasonleavesandfruitsand,underfavourableconditions,onshoots,budsand
flowers. The spots on leaves are initially light in colour and then become dark
olive-green, showing a visible mat of fungus. The spots expand and run into
each other, extending along the veins. Infected leaves tend to fall prematurely.
The scab spots on fruit are almost black in colour. Scabby fruits cannot be sold
in the high quality grades and the scab lesions are readily colonized by rotting
organisms.
The fungus can infect unripened wood and may persist for up to
years
in the bark, from which it produces asexual spores in spring and early sum-
mer. It also overwinters on fallen infected leaves on which it produces sexual
spores (ascospores) in spring. The ascospores are released within
minutes
of wetting, a maximum rate being reached after
hours. The spores are
then carried by the wind to leaves on which they will germinate if the surface
remains wet for long enough for a 'Mills period' of, for example,
-
◦
C
hat
◦
C. Infection pressure is very limited in areas with dry spring
and summer conditions.
Initial infection pressure can be reduced by pruning, by soil cultivation
which increases earthworm populations and consequently leaf burial, and
by post-harvest, pre-leaf-fall, applications of urea to accelerate decay of
fallen leaves. Control during the cropping season is based on routine sprays
from budburst and repeated at
or
hat
-
-
-week intervals. Knowledge of infection
