Apple Skin Disorders: Scarf Skin and Russet

Both are disorders of the peel (epidermis). The descriptions, causes, and corrections of these disorders are similar in many respects.

Scarf Skin

Scarf skin is a physiological disorder of apple that results in a dull gray (sometimes described as milky or cloudy) appearance (Figure 1). It occurs commonly on Stayman, Rome Beauty, and Gala varieties, but has been observed at times on other varieties. Scarf skin has recently caused loads of Pennsylvania-grown Gala apples to be rejected by retailers, who fear that consumers will mistake scarf skin for pesticide residue. A common nickname in the trade is “bridal veil.” The widely planted red strains of Gala, Buckeye, and Gale are particularly susceptible. The dull appearance is caused by disruption of the hypodermal layer of cells of the skin. This layer is rich in red pigments, but when the cells in this layer are disrupted, the light reflected at our eyes is refracted, and appears dull and lacking in red color.

Russet

Russet is a rough tan or brown skin defect that is also only skin deep (Figure 2). Fruit with more than 10% russet is excluded from both U.S. Fancy and Extra Fancy grades, and fruit with more than 25% russet is excluded from U.S. No. 1 grade. Russet is caused by damaged epidermis cells at the fruit surface. Damage to the surface stimulates the cells to produce a kind of wound-sealing tissue—phellogen, which forms a thin, corky surface. A primary difference between the two disorders is that russet breaks up at the surface, resulting in the formation of phellogen, while scarf skin injury stays below the surface, so phellogen doesn’t form.

Causes of Skin Defects

Both scarf skin and russet can be caused by a number of physical or biological stress factors, such as frost or cold temperatures, rain or high humidity, intense sunlight, and by chemical burns. Russet can also be caused by external factors like skin abrasion and pathogens such as powdery mildew. The chemical agents that can cause russet include surfactants, foliar fertilizers (especially metals, e.g., Cu, Zn), certain pesticides (such as sulfur or captan fungicides), and by the carbamates Vydate and carbaryl.

Less is known about chemical causes for scarf skin. Scarf skin was worsened by the use of Benlate fungicide, which is no longer used. Captan, calcium chloride, and solubor did not affect scarf skin.

The various causes of scarf skin or russet can be additive and may interact. For instance, chemical applications that are usually not harmful can make symptoms worse in a season when environmental stresses are high. Unfavorable weather (frost/cold, or wet/humid) can directly cause scarf skin or russet, and these weather conditions often interact with other causal factors, resulting in damage that wouldn’t be expressed under better (warm/dry) weather conditions.

Weather conditions that cause fruit growth to fluctuate also impact the susceptibility of apple fruits. Cool, cloudy weather can make the fruits more susceptible to skin disorders, especially if followed by a period of rapid fruit growth, stressful weather, or other causal agents. Drought and other stressful weather that cause “stop and go” fruit growth tend to make fruit finish problems greater. Chemical burns are usually worse when accompanied by high temperatures, and symptoms are less severe when temperatures are cooler.

The symptoms of scarf skin or russet indicate that the skin of the fruit has been damaged, without necessarily being diagnostic for the exact cause of the damage. Sometimes the pattern of the russet can indicate a sub-lethal frost during bloom, resulting in “frost rings.” This may result in a band of russet that circles all or part of the circumference of the fruit, usually between the fruit equator and the blossom end (Figure 2, Left). However, frost-induced damage can also be expressed without the characteristic ring pattern.

Russet that occurs on the non-red side of the fruit, which faces the interior of the tree canopy, may be associated with slow drying conditions that can arise from high humidity alone. But extended wetting can also result in increased chemical uptake associated with a spray application of a caustic foliar fertilizer or pesticide.

Scarf skin and russet occur during the first 40 days of fruit development, starting at petal fall. Apple fruits develop from the flower receptacle and grow rapidly after fertilization. The outermost layers of the developing fruit, the cuticle, epidermis, and hypodermis, must grow very rapidly to keep up with the growth of the expanding fruit. While some of this growth is the result of cell division, the cells in the peel must also expand to keep pace with the rapid growth. Skin cells that have been arrested or damaged lose the elasticity necessary to keep up with the rapidly expanding fruit tissues beneath. The earlier these tissues are damaged, the more extensive the resulting defects. The older fruit has a thicker skin and becomes less susceptible to scarf skin and russet. It should be noted, however, that russet caused by chemical injury can be expressed even later in the growing season. Scarf skin is often worst on the shoulders of the apple, where the stress and strain of rapid fruit growth is most acute.

While almost any variety can have skin defects, sensitivity to skin disorders is an inherited trait. Certain apple varieties that are genetically more prone to scarf skin include Rome, especially Law Rome, and Gala, especially Buckeye and Gale Gala. Golden Delicious is an example of a variety that is prone to russet. Red Delicious is an example of a variety that is somewhat resistant to russet. No variety is immune to skin disorders.

Management

Several sprays of a proprietary mix of gibberellins (GA₄₊₇) at 10-day intervals starting at petal fall (PF) are known to reduce the severity of scarf skin and russet. GA₄₊₇ is effective for reducing skin defects due to unfavorable environmental conditions, and less so against chemical causes.

Examples of commercially available products used for this purpose include ProVide (Valent BioSciences) and Novagib (Fine Americas). The concentration of active ingredients differs between products, so consult the label for rates. Generally speaking, sprays of the higher range of permitted labeled rates give better results for reducing scarf skin and russet.

Four sprays of GA₄₊₇ at 10-day intervals are made. Enhance absorption by applying sprays at 100 gallons per acre, under slow drying conditions. If possible, apply sprays in slow drying conditions (early morning or late evening) with an expected minimum drying period of six hours. Gibberellins should not be applied with surfactants or other spray additives, and preferably not tank mixed with other crop protectant chemicals.

Gibberellins, particularly GA₇, inhibit flower bud formation and can reduce the potential crop in the following year. While not a particular problem with the very annual Gala variety, the reduction in bloom could be a problem for varieties such as Golden Delicious, which are more prone to biennial bearing. Therefore, when GA₄₊₇ is used for managing skin disorders of biennial varieties, the grower should also use a program of NAA or ethephon sprays for promoting return bloom. See the Penn State Tree Fruit Production Guide for details.