Bitter pit

Malus domestica 'Summerred' bitterpit, kurkstip (c)

Malus-Lädericher1

Bitter pit is a physiological disorder that affects the fruits of apple trees, particularly those of the Malus domestica species. It is characterized by small, dark, sunken spots or pits on the skin of the fruit, which may extend into the flesh. The condition is non-pathogenic, meaning it is not caused by disease organisms such as fungi, bacteria, or viruses, but rather by a deficiency of calcium in the fruit tissue. Bitter pit can significantly affect the aesthetic and commercial value of apples, making it a concern for apple growers worldwide.

Causes and Development

Bitter pit is primarily caused by an inadequate supply of calcium to the fruit, which is essential for normal cell growth and development. Several factors can contribute to calcium deficiency in apples, including:

• High levels of potassium and magnesium in the soil, which can compete with calcium for uptake by the roots.
• Excessive nitrogen fertilization, which can promote vigorous vegetative growth at the expense of fruit development.
• Drought stress, which limits the tree's ability to absorb calcium from the soil.
• Large fruit size, which dilutes the available calcium per unit of fruit tissue.
• Genetic factors, as some apple varieties are more susceptible to bitter pit than others.

Symptoms

The symptoms of bitter pit typically appear on the fruit as small, dark, sunken spots. These spots are usually more common near the calyx end of the fruit but can occur anywhere on the surface. As the disorder progresses, the affected areas may become brown and corky, extending into the flesh of the apple. The pits can vary in size and depth, and in severe cases, can render the fruit unmarketable.

Management

Managing bitter pit involves a combination of cultural practices aimed at improving calcium uptake and reducing factors that exacerbate calcium deficiency:

• Soil and leaf analysis to monitor nutrient levels and adjust fertilization practices accordingly.
• Application of calcium sprays to the foliage and fruit during the growing season to increase calcium availability to the fruit.
• Irrigation management to ensure consistent soil moisture, enhancing calcium absorption.
• Thinning of fruit to reduce competition for calcium and improve fruit size and quality.
• Selection of rootstocks and varieties less susceptible to bitter pit.

Prevention

Preventive measures are crucial for minimizing the incidence of bitter pit. These include:

• Choosing appropriate apple varieties known to be less susceptible to the disorder.
• Maintaining a balanced soil nutrient profile, avoiding excessive levels of potassium, magnesium, and nitrogen.
• Implementing regular soil and leaf testing to guide fertilization and irrigation practices.
• Applying calcium sprays to the foliage and fruit, particularly during key stages of fruit development.