Apple measles, also called internal bark necrosis, is an abiotic disorder due to manganese toxicity caused by acidic (low pH) soil. The disorder most frequently occurs on the apple variety Red Delicious. Measles also affects Fuji, Jonathan, and Stayman Winesap and occasionally Golden Delicious and Rome Beauty varieties.

Identification

Measles first appears as small, red pustules on the smooth bark of young twigs. The surrounding bark may have a discolored, water-soaked appearance. Tissue in the centers of the pustules is dead, and the lesion extends into the cortex (tissue just under the surface) or to the phloem on the inside of the bark. Surface tissue of affected bark may crack, thicken, and slough (drop) off over a large area, resulting in cankers with roughened bark. Cutting into the roughened bark reveals brown flecking or streaking of internal bark tissue. Cracking and sloughing of the bark surface may continue for several years.

Damage

Apple measles causes cracking and cankering of bark. Affected bark may fall off the tree. Affected trees can have reduced fruit yield. It is usually difficult for severely affected trees to recover.

Solutions

Before planting apples, especially Red Delicious, check the soil pH. If it is too low (less than pH 5.5), raise pH by mixing lime (burnt or calcined limestone) into the soil.

To learn whether apple measles is the cause of symptoms, you will need to have a plant diagnostic laboratory analyze leaf tissue. Manganese levels in leaves from trees affected by measles are usually well over 100 ppm.

Where soil is too acidic, if trees are fertilized with nitrogen use a nitrate formulation that contributes to increased soil pH, such as calcium nitrate or potassium nitrate. Applying lime after planting to make soil more alkaline (basic) may also slowly remedy symptoms of measles.

References

Adapted from Integrated Pest Management for Apples and Pears, University of California Statewide Integrated Pest Management Program (UC IPM).