Russeting is a brown, corky netlike condition on the skin of apples. It may appear on only a small portion of each fruit, or may cover its surface. Severe russeting may be accompanied by fruit cracking which usually renders the fruit useless.
Russeting has been associated with specific environmental conditions, damage from harsh chemicals, excess nitrogen, or infection by certain fungi, bacteria and viral organisms. The fruit of younger or vigorously growing trees seem more prone to russeting than older and slower-growing trees. A little russeting is normal on certain varieties of apples, crabapples, and pears, and is not considered a defect in these varieties. 'Chestnut' crab is a good example.
'Haralson,' one of our most popular apple varieties, is one of the most susceptible to severe russeting. 'Regent' and 'Jonathan' also are susceptible, but aren't as widely grown here. When russeting occurs early, the skin becomes thin and tight. It can't stretch with the developing, enlarging fruit, so it cracks. These cracks into the flesh allow fungi and bacteria to enter and rot the fruit. When russeting occurs later in the fruits' development, cracks don't form but the skin becomes brown, dull, and rough.
Weather-induced or physiological russeting, initiated within 30 or 40 days of bloom, is probably the principal form of the disorder in Minnesota. It is thought to be caused by late frost and a humid, rainy spring. High temperatures and excess nitrogen may also be involved. Such physiological russeting may occur uniformly over the entire surface of each fruit, but it might be worse on some fruits than others.
Russeting may also be caused by caustic sprays. Spray-induced russeting often occurs only on the exposed side of apples. The use of pesticide formulations called emulsifiable concentrates (or ECs for short) is more likely to result in russeting than using wettable powders (WPs).
Diseases are not likely to cause russeting on apples in Minnesota.
You probably will not be able to eliminate apple russeting since much of it is due to factors beyond your control. Here are suggestions for reducing it, however.
- Plant varieties such as Sweet Sixteen, Fireside, or Honeycrisp, that are known not to be prone to russeting.
- Avoid spraying emulsifiable concentrates (ECs).
- Do not apply chemicals during slow drying conditions, high humidity, or temperatures above 90°F.
- Prune properly to encourage good air circulation and speed fast drying after rains.
- Thin fruits to only 1 or 2 per cluster within 3 to 4 weeks of peak bloom.
- Fertilize a bearing tree only if the main shoots grow less than 8" a year. Have the soil analyzed at the University Soil Testing Laboratory to determine its fertilizer needs.
Learn more about soil testing from the Soil Testing Laboratory.
Surface russeting may damage the appearance and skin of the apples but not the flavor or nutritive value of their flesh. As long as they remain uncracked, russeted fruits should store acceptably if refrigerated. Even so, it's a good idea to use them first.
Remember, just because you have trouble with russeting one season, it doesn't mean you'll have problems every season. Sometimes even susceptible trees can go for years without much difficulty. If, however, it is a problem every year, you might consider replacing the tree with a different variety.