Russeting, microcracks and moisture loss in apples in 2011
A large number of fruit are struggling with skin defects, making them prone to excessive water loss in storage. Monitoring the propensity of fruit samples being pulled from storage may not be a bad idea.
Every year seems to offer new challenges to producing fruit with a good finish. For some apple varieties such as Golden Delicious and the relatively new Minnieska (sold as SweetTango), getting a good finish can be a perennial challenge. This year, in particular, there seems to be a large number of fruit having surface cracks, russeting, and other skin defects. There are a number of environmental factors, including field-applied chemicals, that can cause surface defects like russet. Among the most common causes of russet are cool weather and frost injury, chemical damage such as copper and some surfactants, and fungal infections, such as powdery mildew.
Russeting results from damage to the epidermis of the fruit and the formation of wound periderm. From the post-harvest perspective, it is important to realize that the impact of the russet is to prevent the formation of a complete cuticle and wax barrier over the surface of the fruit. As a result, skin that is russeted will lose water many times faster than non-russeted skin, causing mild to severe shrivel. Importantly, disruptions of the cuticle may be present, but without the obvious symptoms of russeting.
Some varieties like Golden Delicious and Braeburn tend to form microcracks in the cuticle. These very small cracks are not visible to the naked eye, but have the potential to lose water many times faster than the intact cuticle around them. Evidence suggests that microcracks form as part of the natural process of fruit growth and expansion. However, just as the case for russet, fruit are more prone to the formation of microcracks and, therefore, prone to excessive water loss in storage. Monitoring the propensity of fruit samples being pulled from storage is not a bad idea, especially with the varieties like Golden Delicious. This year, with its peculiar weather patterns, warrants extra vigilance.