Branch wilt

Branch wilt, caused by the fungus Hendersonula toruloidea (syn. Nattrassia mangiferae), is a devastating disease affecting various fruit and nut trees, particularly the walnut. It is classified as a vascular and cortical disease that interferes with the translocation of nutrients, eventually leading to the death of the infected branches or the entire tree.

The disease is most prevalent in regions with high summer temperatures and low humidity, which are the optimal environmental conditions for the fungus. Besides walnuts, it attacks almonds, citrus trees, and grapes. Because the fungus acts as a secondary opportunistic pathogen, it predominantly targets trees already stressed by drought, poor soil management, or mechanical injuries.

The primary symptom of the disease is the sudden wilting and dieback of branches. Upon close inspection of the bark, one can observe brown necrotic patches and longitudinal cracks. When the bark is stripped away, a characteristic black, soot-like fungal mass consisting of arthroconidia becomes visible, which confirms the presence of the pathogen.

The transmission of the fungus occurs through the dispersal of conidia by wind, splashing rain, or pruning equipment. The pathogen requires a point of entry, which is usually created by sunburns, pruning wounds, or existing bark fissures. Once inside the xylem and phloem, the fungus produces toxins and mycelium that disrupt the tree's vascular system, leading to rapid tissue death.

Management of branch wilt requires a strict phytosanitary protocol. All infected branches must be pruned at least 20-30 centimeters below the visible lesion and immediately burned to prevent further spore distribution. Pruning tools must be sterilized between cuts to avoid cross-contamination. Furthermore, providing adequate irrigation and whitewashing tree trunks to prevent sunburns significantly reduces the risk of infection.