Apple rough

Apple rough, commonly referred to in pomology as russeting, is a physiological disorder that affects the appearance of apple fruit. It is characterized by the formation of corky, rough patches on the skin, which can range from small, localized markings to severe netting that covers the entire surface of the fruit.

The development of russeting is closely linked to the early stages of fruit cell division. If the expansion of the fruit's epidermis is disrupted by environmental stress, the fruit produces periderm tissue to protect the exposed cells. This corky tissue is the physical manifestation of the plant's healing process, triggered by external damage to the epidermis.

Key symptoms include the loss of the fruit's natural luster and the appearance of a brown, net-like pattern. In some varieties, the texture changes significantly, becoming dry and leathery. While this is primarily a cosmetic issue, it is a significant concern for growers, as fruit quality grades are strictly based on the smoothness and color uniformity of the skin.

The conditions favorable for the development of apple rough include high relative humidity and cold temperatures shortly after petal fall. Furthermore, the use of certain agrochemicals, such as surfactants or highly concentrated copper-based fungicides during the fruitlet stage, can exacerbate the condition by burning the sensitive skin of the developing apple.

The economic impact of this disorder is severe, as it leads to a reduction in the marketability of the crop. Apart from aesthetic degradation, russeted apples have higher rates of transpiration. This leads to increased water loss during post-harvest storage, causing the fruit to shrivel more quickly and increasing susceptibility to secondary infections by pathogens.

Effective management strategies focus on reducing environmental and chemical stress during the most critical periods of fruit development:

• Selection of less susceptible cultivars for new orchard plantings.
• Avoidance of phytotoxic chemical applications during the critical window of fruitlet expansion.
• Implementation of controlled irrigation to maintain steady growth rates of the fruit cells.
• Application of specialized calcium sprays to support the structural integrity and elasticity of the epidermal layer.