Some serious fungal pathogens

Many serious problems facing trees are due to fungi, as a number of diseases are sweeping through various areas of the globe. Phytophthora cinnamoni, introduced into Western Australia in 1921, is killing very large numbers of trees across southern Australia. The closely related sudden oak death Phytophthora ramorum causes serious problems in southern Oregon and California, where it has already killed more than 100 000 tanoak Lithocarpus densiflorus, Californian black oak Quercus kelloggii and other related species. The fungus thrives in warm, moist conditions and its spores can be carried in water or in mud on shoes or vehicle tyres; so far there is no concrete evidence that it is spread by birds or wild mammals. In Europe it is a threat not only to alien oaks and the native oaks Quercus robur and Q. petraea, but also to beech, sweet chestnut, maples and a variety of shrubs. It was found in England on established Viburnum, Magnolia, Camellia, Rhododendron and Pieris in the summer of 2003 by inspectors from the Department for Environment, Food and Rural Affairs (Defra). These were destroyed and the soil disinfected. In the autumn of that year came the first record of it infesting an oak in Britain; a bleeding canker was discovered in Sussex on the trunk of a 100-year-old exotic southern red oak Quercus falcata from central and southeastern USA. The two English Quercus species are members of the white oak group and not thought to be so vulnerable to the fungus. Moreover, despite urgent and detailed searches, the disease has not so far been discovered in the wild. Many of the trees infected have been in nurseries, growing next to infected viburnums and sharing the same watering system. The problem is made more difficult because Europe has been suffering from oak decline for decades, linked to a variety of causes including root-infecting fungi, including other Phytophthora species, drought and various insect attacks; it will be difficult to discern what sudden oak death is likely to contribute. Oak is a valuable timber tree and also important to many historic sites, for example the Boscobel oak in Shropshire in which Charles II is said to have found refuge after his defeat by Cromwell at the Battle of Worcester. The UK Government is treating this fungal threat with the highest priority; it may well be contained by vigorous inspection and the immediate elimination of infected trees. The danger remains that it will be spread on ornamental plants imported from continental Europe that have bypassed the statutory health checks required under legislation introduced in 2002.

Some fungal diseases, for example Dothistroma pini needle blight which afflicts radiata pine in New Zealand plantation forests, are relatively easy and cheap to control in large monocultures. In this instance the fungicide copper oxychloride applied at the rate of 2.5 kg in 20 litres of water per hectare is very effective; not more than three applications are needed in a 30-year even-aged rotation. This pine develops some natural resistance after the age of 15 years.

Root diseases are a common problem since the soil provides excellent conditions for fungal growth. The susceptibility of the highly adaptable species Port Orford cedar or Lawson's cypress Chamaecyparis lawsoniana to the root disease caused by Phytophthora lateralis was first noted in a horticultural nursery in Seattle, Washington in the 1920s. Its long-lived spores have since been carried far to the south by motor vehicles, on the feet of grazing animals and in floods of water. The disease also moves from tree to tree via root grafts. Infection at the root tip rapidly spreads to the whole root system; even large trees often die within a year of infection. Quarantine remains the best defence, though research into genetic resistance continues (Greenup, 1998). More recently this disease has also been shown to infect the yew Taxus brevifolia which commonly grows with Port Orford cedar in California and Oregon.

The basidiomycete Ganoderma causes root- and butt-rot (i.e. at the base of the tree) in a wide range of trees in many parts of the world. Ganoderma applanatum, which has a bracket-type fruiting body and is widespread in the northern hemisphere, attacks mainly deciduous trees including maples, beeches, limes, poplars, planes, oaks, horse chestnuts, birches, alders, ashes and willows, but is also found on conifers including firs and spruce. It is primarily saprotrophic rather than parasitic, being largely associated with trees whose roots already have large wounds. Trees infected by fungi often respond by producing chemically modified barriers, termed R-zones which help to arrest the spread of fungal hyphae and the consequent xylem dysfunction. Tests of this important ability against Ganoderma were made by Schwarze and Ferner (2003) who used sterilized wood blocks taken from a single London plane Platanus x acerifolia. The sapwood used as host material already contained naturally occurring R-zones. Five sides of each test block were sealed with paraffin wax while the sixth was exposed to one of the fungal species concerned. This novel approach demonstrated considerable differences in invasive ability and speed of decay between the species involved.

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