A canker is a dead area on the stem or trunk of a tree appearing when the underlying vascular tissue gets damaged and dies. It is a symptom of an underlying disease (not the cause of it). Botryosphaeria canker (or band canker) is an important fungal disease symptom caused by various genera including:

  • Botryosphaeria dothidea, which has a large circulation in Algeria, thriving in both temperate, humid environments and semiarid environments. It's having an impact on different countries including Italy, France, Portugal and Spain. [1] This pathogen is the dominant species on almonds of Spain. [2]
Dieback, wilting, streaking, necrosis and wood discoloration in almond trees of Mallorca (Image source: Armengol et al, 2012)
  • Diplodia seriata, causes canker and dieback on stem and twig, forms pycnidia to produce spores on infected tissues and causes wood discoloration visible in cross section. [5] The fungus is the dominant on prunus species of South Africa [7]
  • Neofusicoccum mediterraneum
  • Neofusicoccum nonquaesitum
  • Neofusicoccum parvum  (more aggressive)  
  • Diplodia gallae
  • Diplodia mutila
  • Dothiorella plurivora
  • Botryosphaeria sarmentorum
  • Botryosphaeria viticola
  • Lasiodiplodia citricola
  • L. mahajangana
  • Lasiodiplodia theobromae
  • Neoscytalidium novaehollandiae, which was reported to appear in almond trees of Diyarbakır, Turkey. It's observed to cause defoliation of leaves, stem canker, branch dieback, gummosis, vascular discoloration. The tree either dies or its vigor is reduced. [8] Neoscytalidium is an important cause of cankers and dieback reported in Crous et al. (2006).
Branch dieback, vascular discoloration and necrotic lesion caused by Neoscytalidium novaehollandiae (Image source: Bayraktar et al, 2020)

Pathogens of Botryosphaeria are not host-specific and co-occurring infections of grapevines and almonds are reported in the literature, especially in places like Spain where grapevines and almonds are grown in close proximity or in the same soil after one another. [6] Apple trees, cherry trees and other species in Prunus such as peaches, plums are frequently affected as well. These pathogens are opportunistic and plants are open to attack when stressed and the tree vigor is low.

These factors could predispose plants to fungal infection:

  • Excessive pruning
  • Over-fertilization of young trees
  • Repeated defoliation caused by insects
  • Drought
  • Injury caused by a harsh winter
  • Physical wounds caused by hail, mower, insects etc.
  • Diseases of the root

These fungi often survive in the host tree as endophytes, living between plant cells, long before they cause any visible symptoms. Once they are established on the leaves, branches and the trunk, the canopy, of the tree; they produce and disseminate spores through pycnidia that erupt through the bark. These fruiting, spore disseminating bodies of fungus appear as black specks on the bark.

Pycnidia on honeylocust and pistachio (Image source: honeylocust above from UMass Center for Agriculture, Food and the Environment and pistachio below from Wunderlich et al, 2012)

These factors could help spore dispersal and amplify disease impact:

  • Splashing rainwater in heavily wet periods
  • Pruning during wet periods in the spring
  • Insects feeding on the tree
  • Contaminated tools used for pruning

Different areas of the host tree or nearby trees are then infected with these spores through any wound or natural opening in their barks. Spores can survive the winter in either infected or dead branches.

Cankers of Botryosphaeria often look cracked and show signs of dryness and discoloration. While the canker surrounds the stem, leaves start to wilt, gradually fade into yellow and brown. Young twigs may show signs of weakness and curl downward. When the disease injures the stem, a twig dies starting from that point to the tip. If the stem is not completely surrounded, the tree may stay partially green and tree death can be limited to one side. It's important to identify these stressed, disease-stricken trees early on in order to protect nearby trees.

Management

Pathogens belonging to Botryosphaeria are stubborn disease-causing agents. The following ideas could serve to limit their impact:

  • A careful distribution of the trees allowing ample space for growth and maintenance could help. When planting, consider the size the trees will reach when they mature.
  • Plant at a proper depth. First, this could help to limit future injury by providing a better balance to the tree. Second, this could delay and decrease the amount of potential pathogens from the organic mulch reaching the root.
  • Avoid planting too soon after disease struck other species on the same soil.
  • Prune nearby, intruding plants to allow the tree a better-growing space and better airflow.
  • Identify the mineral level of soil and fertilize properly
  • Retain soil moisture by keeping an organic mulch over the root zone that is well-maintained with good compost.
  • Avoid mechanical injury in the orchard through proper use of sharp, damaging tools.
  • Prepare against long dry periods exposing trees to drought stress and irrigating on regular intervals
  • Maintain a well-drained soil. Divert the drainage that the tree is exposed to if needed.
  • Avoid excessive pruning
  • Use tree species that have already adapted to local conditions in your area

[1] Occurrence of Botryosphaeriaceae species associated with grapevine dieback in Algeria

[2] Fungal trunk pathogens associated with wood decay of almond trees on Mallorca (Spain)

[3] Collophora hispanica, a new pathogen and potential threat to the almond industry in Iran.

[4] Novel Phaeoacremonium species associated with necrotic wood of Prunus trees

[5] Morphological and molecular characterization of Diplodia seriata, the causal agent of canker and twig dieback disease on mulberry in Iran

[6] Characterization and Pathogenicity of Botryosphaeriaceae Species Isolated from Almond Trees on the Island of Mallorca (Spain)

[7] Botryosphaeriaceae as potential pathogens of Prunus species in South Africa, with descriptions of Diplodia africana and Lasiodiplodia plurivora sp. nov.

[8] First report of Neoscytalidium novaehollandiae associated with stem canker and branch dieback of almond trees

[9] Phylogenetic lineages in the Botryosphaeriaceae