Phytophthora diseases

Annual Summary of Research
1 April 2003–31 March 2004

General

Much of the work on the pathogenicity and host range studies with Phytophthoras from trees planned for 2003/04 was deferred because of research demands created by the quarantine pathogen Phytophthora ramorum (see Programme Report: Phytophthora ramorum Cause of Sudden Oak Death in the USA) and also because there has been an overall reduction in funding by the FC compared with 2002/03. However, some work was undertaken on Phytophthora related problems, particularly on Phytophthora disease of alder which also has additional funding from the Environment Agency.

Phytophthora Disease of Horse Chestnut

Bleeding cankers caused by Phytophthora have been recorded in this country on horse chestnut (Aesculus) and lime (Tilia). Elsewhere, other species (such as sycamore, birch, oak and willow) have also been found to suffer from this disease. Over the previous 4-5 years there has been a marked increase in the reports of bleeding canker of horse chestnut. Strouts and Winter (Diangosis of Ill-health in Trees, 2nd edition, published: Stationary Office 2000) reported the disease as uncommon and only found in southern England. However, by 2002–03 more than 20 enquiries about diseased horse chestnuts were received – many with visible signs of cankers and crown dieback. This number increased further in 2003–04 with more than 60 enquiries related to horse chestnut have come into DDAS, most related to bleeding cankers. The similarity of the symptoms with those caused by Phytophthora ramorum has also greatly increased public awareness of the disease, and many of these cases have been referred to the Branch by PHSI, arboriculturists and members of the public. Based on the number of reports and the widespread planting of horse chestnut, we estimate that more than 20,000 trees have been severely affected or killed by the disease.

Both P. cactorum and P. citricola have been found to cause the aerial stem cankers on the stem of affected tree. The outward signs of diseases are scattered drops of gummy liquid oozing from patches of dying bark on the stem of affected trees. In some cases the problem can be so severe that the stem is girdled; mature trees can be killed as a result of these stem cankers. Although both P. cactorum and P. citricola also cause root disease and can be isolated from soil, bleeding canker disease is distinct from Phytophthora root disease where the aerial lesions are the result of fungal growth upwards from the roots. It has not yet been elucidated exactly how the pathogens reach the aerial parts of trees, germinate and infect the bark.

Several features mark a change in this disease:

• It is now widespread and not limited to southern Britain. Cases as far north as Lancashire and Glasgow have been identified.
• In the past, the disease was apparently mainly confined to mature trees, but now is also commonly seen on young trees. Individuals trees planted as recently as 8 years ago have been found to be infected, many in the 20-40 year range have been reported.
• The disease no longer affects one individual tree out of several, but often entire plantings are affected, sometimes 10-50 trees at one time.
• Circumstantial evidence suggests that trees may be infected with the causal agents in the nursery, but the disease on become visible several years later and at that time may also spread to more mature trees planted nearby.
• The etiology and dissemination pathways are likely to be very similar to P. ramorum.

In response to the many enquiries about the disease, an information note has been drafted and, as part of a small project on the subject, the disease is being examined in greater detail at four sites. The intention is to confirm which species of Phytophthora is most commonly responsible for the aerial cankers and to define the best method for isolating and making a firm diagnosis (to avoid any confusion with P. ramorum). In addition, there is a need to have better data on the distribution of this disease and determine if there are site associated factors that predispose trees to infection.

Phytophthora Disease of Alder

Alder Disease Survey: Several plots have had to re-established because of tree losses, in some cases as a result of trees being uprooted during winter flooding and in others due to riverside felling and coppicing. This year the all the plots were assessed and the survey was conducted between July and early October 2002. The data for 2003 are currently being analysed, but the annual incidence of the disease appears to be less than in 2002. Observations again indicated that the disease is very severe in the southern part of the country, particularly the West Country, and diseased alders can be seen on most river systems in this part of the country. However, there are also several reports that the disease is now becoming much more visible in northern parts of England, where previously the disease was considered to be relatively uncommon.

Coppicing for regeneration: The experiment set up in 1996 at Hadley Brook in Worcestershire to examine the impact of coppicing on regeneration of alder was assessed for the fourth time. This area was selected for coppice because it was one of the first river systems where the disease was recorded. It is also interesting because not only the standard type of the alder Phytophthora has been found there but at least three segregant genotypes which are less aggressive than the standard type. This may indicate more than one introduction of alder Phytophthoras and it may also explain why the progression of the disease has been relatively slow despite the length of time the pathogen has been infecting trees along that particular river system.

When coppicing was undertaken all the trees, healthy and diseased, were coppiced along both sides of the river. Since then, shoot growth from healthy and many diseased stools has been very vigorous (up to 11 m in height). Not surprisingly, stools with the most vigorous growth (more than 50 shoots) had been healthy trees prior to coppicing. However, in some cases shoot growth from diseased stools was very similar to growth from healthy stools (see Table 1 below). In addition, it was clear from the assessment of individual stools that some trees considered to be healthy prior to coppice were already infected, as disease symptoms were clearly visible in some of the regenerated sprouts. Symptoms included slower growth, chlorotic and smaller leaves.

The Alder Phytophthoras: The alder Phytophthoras comprise a standard hybrid type and a range of variant hybrid types, each virtually equivalent to a distinct species in properties. A formal taxonomic description of the various taxa has now been drafted and submitted for journal peer review. The standard type is designated as Phytophthora alni subspecies alni, the main variant as P. alni subspecies uniformis, and P. alni subspecies muliformis comprises at least three other variants.

Table 1 Vigour of regeneration after coppicing of Alnus glutinosa

Biology of Oak Phytophthoras

Oak decline is a condition of mature trees characterised by yellowing of leaves, gradual thinning of the canopy, defoliation and die-back of shoots causing crowns to acquire a 'stag-headed' appearance and below the soil affected trees often have a lack of fine feeder roots. The life of affected trees is likely to be shortened significantly and in acute form trees die over a relatively short time (5-10 years).

The condition is apparently the result of several interacting factors (drought stress, insect pests and fungal pathogens) but in relation to pathogens there is good evidence which demonstrates that Phytophthoras are associated with the syndrome in Europe. In several European countries including UK a number of soil-borne Phytophthoras have been identified from oak. Some are new species, others are well characterised. However, many of the ecological and epidemiological aspects of their involvement in oak decline are unresolved.

The concentration of research effort has meant that this project has not been progressed to any great extent but in relation to oak decline and the involvement of Phytophthoras some areas have been addressed. They are:

• A literature search to identify the most recent research on factors associated with oak decline including the involvement of Phytophthora species.
• Contacts with land owners and foresters to identify oak decline sites have been made.
• Visits to oak decline sites and more detailed examination and sampling at two of the sites to establish the methodologies required for further work.
• Field and lab visit to Dr Thomas Jung, University of Munich, who is one of the leading experts on Phytophthoras associated with diseases of oak, beech and alder. Reference cultures and papers were obtained during the visit.
• Infrastructure and equipment necessary to undertake the work have been obtained or are being developed.