New Zealand Journal of Crop and Horticultural Science abstracts
Incidence of Venturia inaequalis on apple fruit
during the second half of the season
under different fungicide and weather regimes
L. J. PENROSE
NSW Agriculture
Agricultural Research and Veterinary Centre
Forest Road
Orange, NSW 2800
Australia
K. A. DODDS
Batlow Fruit Cooperative Ltd
Batlow, NSW 2730
Australia
Abstract The incidence of apple scab (
Venturia inaequalis) on
apple fruit was estimated from December to March in orchards with different
levels of leaf and fruit scab and under different fungicide programs. Twenty
orchards were monitored at Batlow, NSW, Australia from 1990 to 1993. Moderate
levels of primary infection in the 1990/91 season resulted in levels of leaf
scab in December/January, which varied from 0 to 6.2% (mean 0.8%), and fruit
scab in December from 0 to 7.1% (mean 0.9%). Fruit scab incidence measured in
March was 0.4% (after the normal practice of removal of fruit to increase
remaining fruit size). A mean of 1.6 fungicide sprays were applied from January
to March, with a range of 0-7 sprays. It is likely that infection criteria
would have been met on three occasions during December-March. No significant
increase of scab occurred in seven orchards where no fungicides were applied
over this period. In the 1991/92 season, a dry spring resulted in five
infection periods, four of which were low severity. No leaf scab was detected
in December. A mean of 0.02% fruit infection (range 0-0.2%) was detected in
December. The mean level of fruit infection was 0.03% in March. No fungicides
were applied during January-March in 12 orchards (mean 0.6, range 0-4 sprays)
without any increase in scab. Infection criteria were met on only one occasion
over this period. The spring of 1992/93 was wet, resulting in 13 primary
infection periods from mid September to the end of November. A mean fruit scab
incidence (after excluding three orchards with management problems) of 0.1% was
recorded in December although leaf scab was only present in trace amounts. The
mean fruit scab level in March was 0.2%. During January-March a mean of 2.8
fungicide sprays were applied (range 0-7). No significant scab increase
occurred in 17 orchards where fruit scab levels were low in December. In the
two orchards where management problems resulted in failure to apply fungicides
and one orchard where fungicide resistance occurred, fruit infection levels
increased from December through the season, in the worst instance from 12.3 to
23.1%. Infection criteria were met on three occasions during December- March. A
combination of low levels of inoculum in December, arrived at by effective
primary infection control, together with increasing fruit resistance reduces
the risk of disease incidence increase during the second half of the season in
districts where rainfall is low. Our results suggest that in orchards with low
levels of inoculum, careful disease incidence monitoring should enable
protective spraying for apple scab control to cease at the end of the primary
infection season, with the benefit of reducing the amount of fungicide applied
to apples.
Keywords apple scab; disease control; fungicide management; Venturia
inaequalis
New Zealand Journal of Crop and Horticulrural Science, 1994, Vol. 22:
251-261
0114-0671/94/2203-0251 $2.50/0 (c) The Royal Society of New Zealand
1994
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