Necrotic Leaf Blotch
Necrotic leaf blotch is a relatively common but minor disease of Golden Delicious and cultivars with Golden Delicious as a parent (e.g. Aurora Golden Gala, Gala, Mutsu/Crispin, Sunrise, Silken, etc.).
The disease is caused by a physiological disorder that usually starts to appear in apple orchards in late June or early July. Symptoms become most severe later in the season, and include leaf blotching followed by defoliation. In severe cases, up to 50% of leaves fall off over several days.
Identification
Generally symptoms appear on mature leaves on the base of the centre of upright, succulent, rapid growing shoots. Affected leaves initially have necrotic yellow blotches on them. These blotches are usually contained by the larger leaf veins. Affected leaves turn yellow within a few days, then brown prior to defoliating. Defoliation can be severe. This disease usually appears in two to four (or more) waves of symptoms during periods throughout the later half of the growing season. All sports of Golden Delicious on any root stock appear to be equally susceptible.
Biology
This disease does not appear to be caused by any biotic factors, including fungi, bacteria, air pollution or nutrient deficiency. It is considered to be a physiological disorder occurring when a cool rainy period of four to five days precedes several hot, sunny days. It is considered to be linked to a hormonal imbalance in the tree.
Scouting Notes
During weekly monitoring, look for blotching on leaves of Golden Delicious and related cultivars. Also check the ground under the trees for leaf drop.
Management Notes
- Although necrotic leaf blotch is not associated with any fungus or bacteria, researches in the United States recommend the use of ethylenebis (dithiocarbamate) (EBDC) fungicides during cover sprays to reduce symptoms of this disorder.
- In most years, control measures are not necessary.
- Trees with an annual moderate crop of fruit have less necrotic leaf blotch than trees with a biennial bearing habit or a light fruit crop.
- Golden Delicious trees pruned to a central leader also have less leaf blotch than those pruned to an open centre.
