Cankers on Broadleaves - Nectria spp.
A group of fungi in the genus Nectria (Hypocreales) are responsible for perennial cankers in different broadleaves. In this kind of canker, typically target-shaped, a series of callus ridges are formed by the host for stopping the fungus, but, during each dormant season, the last ridge is newly invaded. The canker can also serve as an entry point for wood decay fungi.
In Europe the main hosts are Acer, Fagus, Fraxinus, Platanus, Robinia and Tilia. The fungal species involved are N. ditissima Tul., N. coccinea (Pers.) Fr., N. galligena Bres.,
N. cinnabarina Tode (Fr.). When the main stem is attacked, trees are subject to wind breakage. Infection occurs through wounds or alterations that reach the cambium (branch stubs, frost cracks, pruning wounds, etc.). Infection is also possible through leaf scars.
Dutch Elm Disease (DED) - Ophiostoma ulmi (Buism.) Nannf. and O. novo-ulmi Brasier
This infamous disease has caused great losses of adult Ulmus in Europe. The Ophio-stomatal fungus, formerly named Ceratocystis ulmi (Buism.) Moreau, was first detected in Europe (France and Belgium), at the beginning of the 20th century (during World War I) and successive epidemics of aggressive strains of the fungus (e.g. the famous one of the 1970s) have devastated the susceptible elms during the past century.
Infection starts when fungal mycelia or conidia reach a susceptible host, via insect vectors or root anastomoses. Once the fungus is in the vessels, the host reacts by producing gums and tyloses that block them and stops water flow. As a result, the vessels turn brownish and the tips of the top branches wilt and eventually die, producing vascular wilt. The elm bark beetles (Scolytus spp.) play a very important role. Some authors have suggested that the vector probably has a greater influence on disease incidence in the Mediterranean area where the climate is warmer, and less in northern European countries. Two groups of fungal strains can be distinguished: the less aggressive strains, responsible for the first pandemic of DED, and the highly aggressive strains, responsible for the current one.
Verticillium wilt is an important problem in northern Europe, even if it is present also in warmer conditions. It is caused by two main Verticillium species, distinguishable only in culture: V. dahliae, a thermophilous species forming sclerotia, and V. albo-atrum, which stops growing at 30 °C and in culture only forms a dark resting mycelium. These anamorphic fungi are mainly soil-borne and infect the plant through rootlets in contact with the remains of previously infected crops. They can be transmitted also by tools. Frequently, plants are infected in the nursery.
The main host is Acer, but also Aesculus, Fraxinus, Olea, Platanus, Robinia, Tilia and Ulmus are mentioned. Affected trees can be killed in the first year of infection, or they can show chronic symptoms, with progressive dieback and sparse yellow foliage.
Cypress Canker - Seiridium (Coryneum) cardinale (Wagner) Sutton and Gibson
This anamorphic fungus lives in the bark of stems and twigs of different Cupressaceae (Cupressus, Thuja, Chamaecyparis species), where it forms girdling cankers that finally kill the distal part. The infection often begins in small cracks of the bark and becomes evident with the appearance of some resin flow. The fungus produces its asexual fruit bodies, recognisable as very small black pustules in the dead part of the bark, near or inside the canker. No sexual stage is known. Conidia are dispersed by the wind that carries droplets during rains. Some cypress beetles can also be vectors.
Canker Stain on Plane Tree - Ceratocystis fimbriata Ellis and Halsted f. spp. platani Walter
This ascomycete (Microascales) causes a destructive tracheomycosis in Platanus spp. The disease is present in the USA and in Italy, France and Switzerland. The name of the disease leads astray, as no canker is detectable on the stem, and the staining is an internal one, in the wood.
A single branch with sparse, more or less chlorotic foliage is usually seen first. On the side bearing this branch, the bark becomes necrotic, pale-brown, cracked and adheres to the tree. The edges of the lesions show no wood callus formation, and often extend in bluish-black filaments or veins, more evident when bark is peeled off. Infected trees die in 3-7 years. When a tree is felled, on the surface of timber the very evident symptom is a dark color, often in a radial pattern and along the younger woody rings.
Contaminated pruning tools and terracing machinery, which causes damage to the roots, transmit the pathogen. It may be transmitted also by root contact (anastomoses). Infected tissues of a dead plant can transmit the disease. In fact, the pathogen can survive for more than 5 years in woody fragments in soil. Sawdust from diseased trees is highly infective.
Parasitic nematodes are among the most important pathogens of cultivated plants. They are also frequently associated with trees. Most species cause direct damage to plant tissue, but some of them (e.g., Xiphinema spp. and Longidorus spp.) can also serve as important vectors of viruses. Economic losses of seedlings and young trees are occasionally reported from ornamental and forest bare-root nurseries and plantations. Several species of Meloidogyne, Trichodorus, Xiphinema and Pratylenchus were recorded on roots of broadleaf trees. Conifers are frequently infested with various species of Haplolaimus, Tylenchorhynchus, and Pratylenchus (Sutherland and Webster 1993). Information on nematode-related damage caused to mature trees is, however, very limited in temperate climates. So far no data is available on such cases in urban forests of Europe.
Recent establishment of the pine wood nematode, Bursaphelenchus xylophilus in Portugal (Mota et al. 1999) may, however, change this situation soon. The nematode is known to cause a devastating pine wilt disease, which presently spreads over the eastern regions of Asia, and North America. The disease can develop on various species of conifers, mainly pines (Pinus spp.), and in suitable environmental conditions it may kill even mature trees within only a few weeks. Infective juveniles of the nematode are transmitted by wood dwelling long horn beetles from the genus Monochamus. After reaching the host nema-todes start to feed on epithelial cells of resin canals and cause fast wilting and subsequent dying of the infested tree. The nematode is presently subjected to strict quarantine regulations in Europe; however its control after establishment in the forest has shown to be extremely difficult. We can only hope that B. xylophilus and pine wilt disease will not manage to establish in other regions of Europe. Otherwise, this species could become a real threat to pine trees grown not only in open forests, but also in the urban environment.
Interestingly, a recent study of Braasch et al. (1998) has also revealed severe pathogenicity of two other Bursaphelenchus species, naturally present in Europe (B. mucro-natus, B. sexdentati), to pine. Extent of this phenomenon in the field conditions is still under study.
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