Necrotic Leaf Blotch of Golden Delicious and Related Cultivars

Necrotic leaf blotch (NLB) is definitely one of the most intriguing and scary looking conditions that I know of. This year, growers reported lots of defoliation occurring in Goldens by the end of August and into early September. This coincided with a dramatic change in weather from a pretty cool late August to a relatively warm and sunny early September.

Dr. David Rosenberger reports from New York State that Dr. Turner Sutton studied the disorder extensively in the early 1970’s in North Carolina. He concluded that the disorder is not caused by a fungus, bacterium or air pollution and that it is not related to foliar nutrient concentrations.

The sudden death and abscission of shoot leaves after seeing a perfectly normal looking tree can send shivers down the back of the most open-minded of growers. Just after working to establish that perfect crop load and fruit growth rate, beating primary scab and several generations of various insects, you loose up to 50% of the mature foliage in the canopy more or less overnight (see Figure 2).

Some observations this year that I thought would add to our overall knowledge of this disorder included watching the experimental cultivar ‘Chinook’ go through an equally serious defoliation from NLB. In checking the parentage of ‘Chinook’, we find ‘Gala’, which has ‘Golden Delicious’ as one of its parents.

Also, the defoliation occurs almost exclusively on the mature shoot leaves of the current season’s growth. These leaves do appear to finish the job of maturing their associated buds in the axil area between the leaf petiole and the woody branch on which the leaf is borne. The spur leaves and the shoot terminal leaves don’t generally appear to be affected by this disorder.

In general, the disorder does not appear to affect overall tree performance or fruit size and finish unless it is very severe. It does appear to be worse when a relatively cool growing period is followed by a period of fairly intense heat and light. Tree vigour may affect response while rootstock appears to have minimal influence on the severity of the symptoms.

Figure 2. Necrotic Leaf Blotch in various stages of development.

The one factor we had throughout the 2004-growing season, even though we got off to a cold wet start, was the intense UV light from day to day.

Michigan’s Al Jones described the disorder as being related to air temperature, light intensity and soil moisture. There appears to be a relationship between certain plant hormones and the disorder. The leaf symptom is enhanced by gibberellins and inhibited by abscisic acid.

The EBDC protectant fungicides appear to help alleviate the severity of the symptom while fungicides like Captan are not effective.