Over the past week I've received a few phone calls regarding concern over leaves on Golden Delicious suddenly developing leaf spots/blotches, quickly yellowing, and then prematurely defoliating. Understandably, there was widespread concern that Glomerella leaf spot had appeared in the affected trees just prior to harvest, despite the intensity of fungicide applications throughout the season. Fortunately I do not suspect the symptoms to be caused by a pathogen in this case. Rather Necrotic Leaf Blotch (NLB), a physiological disorder, is most likely the culprit on rapidly defoliating Goldens.

Cultivar

• GLS: All cultivars with 'Golden Delicious' parentage. Severity of symptoms is generally the same across all cultivars
• NLB: Symptoms are usually observed first and are most severe on the 'Golden Delicious' cultivar. Symptoms may be observed on other cultivars (particularly those with Golden parentage) but do not tend to be as severe.

Causal Agent

• GLS: A fungal pathogen: Colletotrichum spp. It is a disease of apple.
• NLB: Cause is unknown but is believed to be associated with a zinc deficiency. It is a physiological disorder of apple.

Leaf Lesion Appearance

• GLS: Symptoms usually begin in early to mid summer as small purple flecks on leaves and rapidly expand into irregular concentric lesions with purple borders. Lesions can be located anywhere on leaves, but are not usually observed on leaf margins or initiating from the leaf mid-vein.

• NLB: Symptoms (see photo above) are most often observed later in the summer and can appear suddenly. I have not observed early "purple fleck" symptoms with NLB. Mature leaf lesions/blotches are in general larger than GLS (though not always the case) and appear more like chemical burn. Lesions lack the concentric ring appearance.

Leaf Appearance

• GLS: Yellowing (chlorosis) appears on heavily infected leaves. GLS lesions are always present on leaves prior to yellowing and defoliation. A relatively gradual progression of symptoms is often observed (purple flecks to concentric lesions to yellow/"leopard spotted" leaves to defoliation). Leaves defoliate from any location within a branch, but premature defoliation tends to initiate in the top and inner canopy. Defoliation can approach 100% in some years.  
• NLB: Yellowing (chlorosis) tends to be observed more suddenly and later than GLS and yellowing may occur with minimal blotch (e.g. lesion) severity on leaves. Similar to GLS, premature defoliation occurs, however, defoliation is most apparent on the lower third of shoots. Defoliation usually does not exceed 50% in even the most severe cases.

Fruit Appearance

• GLS: Early fruit infections appear as small indented spots/lesions on fruit. As fruit matures (or after harvest) lesions expand and cause a fruit rot (bitter rot). Under the lesion, a V-shaped pattern can be observed in apple flesh.

• NLB: Fruit are only indirectly affected by NLB. If heavy defoliation occurs near harvest, fruit can become sunburned which may predispose the fruit to rapid rot development.

Environmental Conditions

• GLS: Warm/hot + humid weather.
• NLB: Symptoms most often appear during a period of hot, dry weather that follows a period of humid, rainy weather.

How to Manage?

• GLS: Fungicides: Strobilurin, mancozeb, and/or captan on 7 to 10 day interval beginning at petal fall
• NLB: While not labeled specifically for NLB, trees that have been treated during summer covers with fungicides containing zinc (Zn) ions tend to have less NLB than trees in which Zn-containing fungicides are absent. On Golden Delicious, consider applying ziram occasionally in summer cover sprays. Alternatively, zinc oxide (ZnO) may be effective.