Infection Biology

Entry to host trees occurs through wounds into bark and sapwood. Infected superficial wounds lead to rapid colonization of the inner bark (secondary phloem), vascular cambium and sapwood, with longitudinal spread of up to 2 – 2.5 m per year (in P. x hispanica) in the sapwood xylem; Further growth of C. platani at a rate of 50 – 100 cm per year occurs in the inner bark, causing dark staining in those tissues and the underlying sapwood. The fungus also grows through medullary rays, sometimes reaching the heartwood.

Growth of C. platani can occur between 10 and 45°C, but the fungus survives at temperatures as low as -17°C. Optimum temperature for growth is 25°C. In the soil, asexual spores may survive for at least 105 days during winter, but are killed when soil temperatures exceed 35-40°C (Accordi, 1989). 

Another common method of transmission from infected trees to adjacent healthy trees is via root-to-root contact, which is common in many Platanus species, especially in the (largely) clonal P. x acerifolia (Accordi, 1986). Insect transmission is common in the Ceratocystidaceae (de Beer et al., 2014) and other ophiostomatoid fungi (Wingfield et al., 1993), and there is some evidence that bark boring and other insects may carry C. platani between trees, at least in in riparian ecosystems (Crone, 1962; Soulioti et al., 2015); this possibility requires further verification. In urban trees, C. platani is frequently transmitted via pruning wounds and damage caused by other human activities on and around the trees (Tsopelas et al., 2017). Sawdust from diseased trees is highly infective, and is probably the main method of transmission in urban Platanus, particularly those that are pruned regularly.

None of the spore types produced by C. platani appears to be airborne under natural conditions (Panconesi, 1999), but the pathogen certainly spreads locally on infected sawdust generated during pruning and sanitation felling operations. If these operations occur near a watercourse, infected sawdust may be carried downstream and lead to infections of riparian Platanus trees. Human activities are, therefore, the main mechanisms of spread.

Natural spread from tree to tree via root contacts, although slow, is of importance in infection foci. Terracing machinery used in construction activities has been demonstrated to carry infested soil and contaminate previously healthy areas. Spread of the fungus may also occur in colonized Platanus timber, in countries where this is used: this pathway is probably how the pathogen was spread from North America to Italy (Panconesi, 1999; Tsopelas et al., 2017). Trade in symptom-free Platanus may inadvertently transfer infected host plants internationally. The most significant pathways for transfer of C. platani into new areas are likely to be host plants for planting, wood (e.g., timber, wood packaging material, wood chips, dunnage, firewood) and machinery (EFSA, 2016).