Taking an environmentally sensitive approach to pest management
Russet is a rough, tan skin defect that commonly occurs on some apple cultivars in Missouri, such as Golden Delicious, Golden Russet, Ginger Gold, Rome, Idared, Empire, Stayman, Cortland, Jonagold, and McIntosh (Figure 1). However, late-maturing strains of Golden Delicious sometimes have less russet than early-maturing strains. In contrast, Red Delicious is relatively resistant to russet. While russeted fruit is unattractive, it is safe to eat. Russeted apples can be salvaged by simply peeling the fruit or using them for juice or cider.
Russet symptoms develop when there are cracks in the cuticle of the fruit. Epidermal cells underneath the cuticle become injured, turn brown, and are then pushed upward to become exposed at the peel surface as the fruit develops. Russet injury occurs from the pink stage of floral development to the first 40 days after petal fall on apple. Often the side of the fruit that faces the interior of the tree canopy has more severe russeting, which is often associated with the accumulation of moisture on apples and slow drying conditions.
Unfavorable weather conditions, such as cool temperatures, prolonged cloud cover, rainfall, and heavy dew, promote russet formation. This type of weather may also contribute to fungi that can russet fruit following infection. Aureobasidium pullulans and Rhodotorula glutinis are two types of fungi commonly found on apple fruit surfaces and foliage that cause russeting. Also, the fungus that causes powdery mildew, Podosphaera leucotricha, produces russet on susceptible cultivars, such as Jonathan and McIntosh.
Fungicides can also promote the development of russet when they penetrate the fruit cuticle. Copper, captan, sulfur, and liquid lime-sulfur products can cause phytotoxicity, especially when trees are sprayed in the evening, when high humidity occurs and there is low wind speed. When using copper for fire blight suppression, apply it no later than the green tip bud stage (when leaf tips are just visible). Also, copper applied later in the summer can result in blackened lenticels on fruit. If summer copper sprays are applied, use low-rate copper products such as Phyton or Cueva under rapid-drying conditions when the foliage is dry. Another way to minimize russeting on susceptible cultivars is to omit Captan from sprays and use a different fungicide from bloom through 40 days after petal fall. Consult The Midwest Fruit Pest Management Guide at https://ag.purdue.edu/hla/hort/documents/id-465.pdf for alternative fungicides.
For organic growers, avoid using sulfur or liquid lime sulfur products when the temperature at the time of application or for the next four days will be over 85°F. During hot weather, substitute low-rate copper products (Cueva, Previsto) for those containing sulfur. Although the low-rate copper products may induce some russet when applied during warm temperatures, they will cause less injury than sulfur products sprayed under similar conditions.
Commercial products containing GA4+7 (gibberellic acid), such as ProVide and Novagib, can mitigate russet injury. For this use, the product is applied four times at 10-day intervals. However, the high cost of these products may not justify their use, depending on the apple cultivar and its use.
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REVISED: February 21, 2017