Ceratocystis platani
(canker stain of plane)

C. platani is an aggressive fungal pathogen that attacks plane trees (Platanus spp.) by entering through wounds and causing canker stain disease and tree death. It is thought to be native to south-eastern USA and has spread throughout urban P. acerifolia populations planted in the large cities of the East coast. It seems likely that it was introduced to Europe in wood packing materials during World War II and, although spread was initially slow, it has accelerated recently in southern France and was reported for the first time in Greece in 2003. The disease threatens natural and planted populations of economically, ecologically and aesthetically important plane trees.

C. platani is an EPPO A2 quarantine organism (OEPP/EPPO, 1986). The localization of the fungus is well known but the spread of the disease from infected areas of a country to unaffected neighbouring, temperate areas is a serious threat. Indeed, considering the widespread occurrence of plane trees in southern mediterranean countries, and the sensitivity of Platanus acerifolia to the fungus, the phytosanitary risk is constantly present. The risk of introducing the fungus to unaffected countries by trade in unknown infected plants or Platanus wood used in carpentry is high. The recent introduction of the pathogen to Greece is thought to have been via another European country (Ocasio-Morales et al., 2007). 

The main hosts of C. platani are Platanus acerifolia, P. occidentalis and P. orientalis, either isolated trees or groups of trees growing in cities, in the countryside and along roadsides.

Genetics

The genetic variability of C. platani was studied in Italian populations using RAPD and minisatellite markers and compared with a number of C. fimbriata isolates from various hosts growing in different parts of the world. A high level of homogeneity was revealed in the Italian population, whereas a certain variability was recognized in isolates from others hosts. The Italian population appears to be clonal, with the same genotype occurring throughout the country (Santini and Capretti, 1998, 2000).

Physiology and Phenology

C. platani can survive for several years in vitro (at -17°C) and in soil and dead trees. The fungus can survive for 7-15 days on the surface of a wound and 30 months in decaying parts of plane trees (Grosclaude et al., 1990, 1995). Survival of the pathogen is facilitated by the production of numerous chlamydospores.

Reproductive Biology

It is extremely difficult to observe reproductive structures in infected trees; however, it is easy to obtain them on the surface of bark pieces or on wounds taken from the edge of a canker and incubated at around 25°C. The conidia germinate when they come into contact with a plant wound and the resulting mycelium colonizes the exposed tissues; reproductive structures develop at the same time. The asexual form appears first (within 2-3 days). It consists of two endoconidial stages characterized by endoconidiophore hyphae, which produce cylindrical hyaline conidia and barrel-shaped sub-hyaline conidia. After 6-8 days the fungus produces numerous thick-walled, olive-brown conidia (chlamydospores). Dark perithecia (sexual stage) containing asci and ascospores are produced after 10-20 days. Sometimes asexual and sexual reproductive structures are produced inside vessels or wounds and in the medulla (Clérivet and El Modafar, 1994; Clérivet et al., 2003).

Isolates of C. platani can be either homothallic or heterothallic (Webster and Butler, 1967) and several different strains exist. An Italian study showed that the self-fertile strain is the most common. It has two endoconidial forms, chamydospores and perithecia. The self-sterile protoperithecial strain is also frequent and possesses the same asexual forms and primordia of immature, more or less well-developed perithecia called protoperithecia. Other strains have been identified, both in nature and in culture (sectorial mutation). The most common strains are: self-sterile, with only the three asexual forms; perithecium sterile, with the three asexual forms plus a perithecium which, though morphologically normal, is sterile because it lacks ascospores; and the protoperithecium self-fertile strain, which possesses the three asexual forms plus a fertile perithecium. The self-sterile strain can be induced to produce perithecia by hybridization with cylindrical conidia from the self-fertile strain (Panconesi, 1999).

Environmental Requirements

The optimum temperature for the growth and development of C. platani is 25°C; the fungus does not grow below 10°C or above 45°C. A high level of hygrometric pressure is also necessary for growth and development. The most favourable period for fungus development is May to September. A wound in the bark or wood of the roots, trunk or branches is necessary for fungal penetration. The urban environment (pollution, water deprivation, wounds, etc.) increases the sensitivity of plane trees to attack.

The fungus can easily be isolated within 2 days from pieces of wood removed from cankers and plated on malt agar or PDA at 25°C. It can also be detected from soil and sawdust samples under the same culture conditions.

The original technique used to trap the fungus, described by Grosclaude et al. (1988), is more complex but provides good results. Healthy branches of Platanus acerifolia are stripped of their bark and pieces of the wood are placed in contact with the collected sample (wood, soil, sawdust, etc.) in a jar full of aerated water; wood blackening within a few days at room temperature is a positive result.

An ELISA test was perfected by Diop-Bruckler (1991). A diagnostic protocol has been published by EPPO (EPPO, 2003). Pillotti et al. (2012) developed a sensitive and effective Real-Time PCR method based on a Taqman probe for the diagnosis of C. platani on members of the genus Platanus.

The ability to inspect a tree to detect an attack by C. platani is dependent on the size of the tree and the accessibility of the roots. Attention must be paid to chlorotic foliage and sudden widespread withering of the foliage as these are usually the first symptoms of canker stain disease. Attack by C. platani must be considered a high possibility if all other symptoms (described under Symptoms) are present.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Cultural Control and Sanitary Measures

As the disease is mainly spread by human activity, clean propagation and production methods can limit its dispersal (Smith, 1985). Planting material should be obtained from regions where the disease does not occur and plants should be grown in a place found to be free from the fungus during the last growing season (OEPP/EPPO, 1990). All pruning tools should be disinfected with alcohol before any pruning operations, even in uninfested regions. In infested regions, disinfection should be repeated between each tree.

In principle, all terracing machinery used in the vicinity of infested trees should be treated by water-jet and sprayed with 8-hydroxyquinoline sulfate, benomyl or sodium hypochlorite solution before moving to another site. When infected trees are felled, all debris and sawdust should be sprayed abundantly with fungicides before sweeping up and disposal. All wood should be burned (Vigouroux, 1979; Blankart and Vigouroux, 1982; Panconesi, 1999).

The wood used in industry for the manufacture of woodchip board should be treated by high temperatures (90-100°C) during the manufacturing process to kill the fungus (Panconesi, 1999).

Chemical Control

Chemical substances do not provide very satisfactory preventive or curative control of the disease. External treatments with substances that are effective against the fungus are not efficient on a diseased tree because the fungus grows in the conducting vessels of the wood. The injection of fungicide under pressure into the vascular system temporarily halts the infective process but the pathogen is not eliminated (Panconesi, 1999).

Biological Control

No biological control methods exist against C. platani. Several experiments with Bacillus subtilis or Trichoderma harzianum, a fungus known to have a strong antagonistic activity against the closely related C. fimbriata in vitro, failed to give the desired results (Turchetti and Panconesi, 1982; Mutto Accordi, 1989).

The necrotrophic action of some fungi against C. fimbriata is known but there are no reports of the use of these mycoparasites to control C. fimbriata (Panconesi, 1999).

Host-Plant Resistance

Host-plant resistance is unknown in the species Platanus acerifolia. However, a natural source of resistance was highlighted in the American species Platanus occidentalis in the south-eastern USA (FI McCracken, US Department of Agriculture, Southern Hardwoods Laboratory, Stoneville, Mississippi, USA, unpublished data). Unfortunately this source cannot be directly exploited in Europe as the species is not acclimatized to the region. A genetic improvement programme (Vigouroux, 1992; Clérivet et al., 2003) aimed at restoring the original hybrid (Platanus acerifolia) using Platanus orientalis and the resistant clones selectively obtained by McCracken as parents, yielded some clones with a good level of resistance against the fungus (Vigouroux and Olivier, 2001, 2004). However, the durability of resistance is not yet clearly established under the natural conditions of plane tree development.