Black rot of apple

Black rot, caused by the fungus Diplodia malorum (often synonymized with Botryosphaeria obtusa), is a serious fungal disease that significantly impacts apple and pear production worldwide. This pathogen is known to attack all parts of the tree, including the bark, foliage, and fruit, leading to substantial economic losses.

The fungus primarily targets pome fruit trees. It is particularly dangerous because it can colonize healthy tissue through wounds caused by pruning, insects, or environmental stress factors like hail damage, winter injury, or sunscald, making management challenging in orchards with poor hygiene.

Symptoms are distinct: on branches, it causes sunken, dark cankers that may enlarge over time, eventually girdling and killing the limb. On leaves, it manifests as small, purple-edged spots. On fruit, the characteristic symptom is a firm, brown rot that later turns black and shriveled, often covered in small, pimple-like fruiting bodies (pycnidia).

The development of the disease is favored by warm, humid weather. Spores are released during rainfall and spread via wind and water splashes. The fungus survives the winter primarily in mummified fruits remaining on the tree and in necrotic bark tissues, serving as a continuous inoculum source for the following season.

Management strategies require an integrated approach. Essential practices include pruning out and destroying all infected wood during the dormant season, which is critical for reducing inoculum. Applying copper-based fungicides during the dormant period, protecting tree trunks from sunscald with white paint, and ensuring good air circulation are vital steps to keep the disease under control.