See:
Apple (Malus spp.) - Fruit Russeting
Cause The d'Anjou pear is susceptible to undesirable fruit russeting while 'Beurre Bosc' gets compete russeting which is normal for this cultivar. Fruit russet results from the damage to epidermal cells that occurs within the first 30 to 40 days after petal fall. Once damaged, a brown layer of suberized cells form in the lower epidermal region. As cork cells develop in this area, they push outward and become exposed to the surface as the fruit matures.
There are many reasons for fruit to russet including cool, wet weather, frost, pesticides, viruses, fungi, and bacteria. Each cultivar is affected differently by the above factors. Cool (not necessarily freezing) weather and wet fruit, especially from pink-blossom stage until 3 weeks after petal fall can cause russeting. This kind of weather may be the cause or may only encourage growth of the fungi and bacteria discussed below. Many spray materials (including surfactants), especially emulsifiable concentrates and materials containing copper, zinc, or calcium, if not evenly distributed by spray equipment can result in a russet.
Several fungi can also russet fruit including Podosphaera leucotricha, which causes powdery mildew. Cultivars susceptible to powdery mildew can develop this type of russeting. Several different kinds of bacteria including Erwinia herbicola and Pseudomonas sp. that produce high levels of the plant hormone indole-3-acetic acid (IAA) cause russeting. These bacteria have been shown to increase russeting when inoculated onto pear fruit.
Symptoms Russeting caused by cool weather and wet fruit often is associated with corky lenticels and tan markings shaped like rain-splashed water droplets. These markings are more abundant at the stem end.
A band or ring that forms either partially or completely around the fruit is usually what characterizes frost russeting. Russeting from spray materials is likely to be found where spray droplets accumulate, such as the lowest portions of the fruit. Russeting from powdery mildew is tan to gray and has a netted appearance.
Cultural control
- Manage trees for powdery mildew.
- Check for uneven spray distribution or worn nozzles.
- Use only virus-indexed scion wood.
Chemical control Using these products to control scab during bloom also aids russet control.
- Luna Sensation at 4 to 5.8 fl oz/A. Do not use within 14 days of harvest. Group 7 + 11 fungicide. 12-hr reentry.
- Mancozeb (such as Dithane, Manzate, or Penncozeb) up to 6 lb/A prebloom or at 3 lb/A after bloom. Do not combine the 6 lb/A prebloom or the 3 lb/A all-season schedules. Do not apply within 77 days of harvest. See label restrictions. Group M3 fungicides. 24-hr reentry.
Note: The biological control products Botector and Blossom Protect contain Aureobasidium pullulans and may increase fruit russet under certain conditions. An acidic buffering agent is also used with Blossom Protect and may increase russeting when used close to applications of copper-based materials.
Reference Sutton, T. B., Aldwinkle, H. S., Agnello, A. M., and Walgenbach, J. F. 2014. Compendium of Apple and Pear Diseases and Pests, 2nd ed. St. Paul, MN: APS Press.