2008 Phytophthora Workshop in Italy

1. Persistence of Phytophthora
kernoviae and P. ramorum on
infested sites: impact on disease
management
Joan Webber, Forest Research
3rd International Phytophthora Workshop

2. P. kernoviae / P. ramorum
Both apparently introduced invasives in Britain
Both aerial Phytophthoras which cause foliar/shoot
symptoms on rhododendron
Rhododendron is the UK ‘bay laurel’
Both cause bleeding lesions on trees, mainly beech
(Fagus sylvatica)
Both thrive under similar climatic regimes typical of
Cornwall in south west England, so the majority of
outbreaks are there

3. Disease outbreaks: 2002/03 - 08
England, Scotland and Wales
Pathogen Nurseries/ retail Managed/
plant sales unmanaged
P. ramorum 611 (498*) 224 (68*)
P. kernoviae 4 (3*) 55 (2*)
Total 615 (501*) 279 (70*)
* eradicated outbreaks
Data derived from Defra Consultation document 2008

4. XXXXXX
Distribution of Pr Distribution of Pk
XX
XX
XXXXXX

5. Current measures in the UK
• EU emergency measures apply to Pr
– eradication of infection in nurseries
– containment/eradication of infection in
natural and semi-natural environments
• Similar measures apply to Pk
– containment or eradication via the
removal of infected understorey plants
as the majority of outbreaks are in
woodlands with a dense understorey of
R. ponticum

6. Impact of rhododendron eradication
over 1-3 years
• Time frame for persistence in naturally
infected leaves of rhododendron?
• Persistence in litter and soil?
• Regrowth and re-infection of the
rhododendron?

7. How long does
inoculum persist?

8. Persistence of Pk
inoculum
Naturally infected leaves put into bags
and air suspended or put in litter layer

9. Survival of Pk in naturally infected leaves:
2005 - 06
Air exposed Litter embedded
100
90
80
% leaves with Pk
70
60
50
40
30
20
10
0
0 1 2 3 4 5 6 8 12
Month

10. Deterioration of leaves over in the litter layer
3 months 9 months
6 months 12 months

11. Survival of Pk in naturally infected leaves:
2006 - 07
Air exposed Litter embedded
100
90
80
% leaves with Pk
70
60
50
40
30
20
10
0
0 1 2 3 4 6 8 10 12
Months

12. Survival of Phytophthora in naturally
infected leaves: 2006 - 07
45
P. kernoviae P. citricola
40
35
30
% recovery
25
20
15
10
5
0
0 1 2 3 4 6 8 10
Months

13. Survival of Pr in naturally infected leaves:
2006 - 07
100
% leaves with P. ramorum
90 Air exposed Litter embedded
80
70
60
50
40
30
20
10
0
0 1 2 3 4 6 8 10 12
Months

16. Pre and post R. ponticum removal in a Pk
infested woodland
80
Litter Soil
Percent Pk positive samples
70
60
50
40
30
20
10
0
Pre-treatment Post-treatment Post-treatment
(Sept '04) (Sept '05) (Sept ’07)

17. What happens to inoculum………
◄ Re-sprouting from
rhododendron stumps with
infection
Recruitment of new ►
rhododendron seedlings

18. Seedling Foliage Soil Roots
1
2
Impact of persistent Pk inoculum
3
4
on rhododendron recruitment
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
Fichtner et al., 2008: APS

19. Conclusions
• Both Pr and Pk are proving to be difficult to
eradicate from infected natural or semi- natural
environments
– eradication process must involve litter removal
– but persistence is extended and signs of disease by
Pk return after more than 3 yr following eradication
– additional issue of asymptomatic infection of
rhododendron roots by Pk
• Is it worthwhile?
– removing the infected rhododendron does safeguard
trees in woodlands from Pk infection
– reduces inoculum and therefore likely to reduce the
opportunities for Pk to get into the nursery trade
– consultation on ‘is it worthwhile’?
www.defra.gov.uk/corporate/consult/phytophthora-ram-
kern/index.htm