Cavity spot of carrots
Dez Barbara
Cavity spot of carrots
•
Rated #1 disease by UK carrot growers.
•
Soil-borne, causing disfiguring lesions on roots
In UK mainly Pythium violae (an oomycete
fungus)
(P. sulcatum found only occasionally)
(Overseas can be either of these or other
Pythium species entirely)
Some “cavity spot” lesions caused by one of four other
species – all possibly particularly in wet conditions.
- may be source of some conflicting
reports in past.
Cavity spot on late harvested
carrots
Lesions initiated by P. violae
- but much of damage done by
secondary invaders.
- by this stage P. violae gone
It is a very “patchy” disease
Distribution of cavity spot on individuals within a row of
carrots
(intensity of colour is proportional to numbers of lesions on root)
4
2
1
1
1
4
5
2
1
21
1 5 3 2
27 4
5
5
3
20 22 7 19 1
5
1
1 1
4
1 34 3 25 25132
11
6
5 14 12
2
4 1
1
31
3
2
1
1 4
2 3
1
3 5 12 1 1 2
8
1
2
10 3 12 14
1 2 4
2 1
3
3
3
4
3
3 5 6 5
4 23
5
14 2
2 3 1
2
10
4 2
3 4 1 2 21
5
1
Aggregation of disease and other evidence suggests:
• that primary infections scattered
• that secondary spread is very important
Growth of P . violae and levels of disease are
primarily driven by soil moisture
Averages of PCR for soil off carrots surfaces
3.50
3.00
(irrigated = solid lines/
non-irrigated = pecked)
PCR
2.50
2.00
1.50
1.00
0.50
0.00
13.6.06
17.7.06
23.8.06
20.9.06
Date
17.10.06
27.11.06
Cavity spot does not build up with repeated
cropping of the same site over 4 years.
Lesions
Oct Nov
F/F/F/C
F/F/C/C
F/C/C/C
C/C/C/C
13
14
3
12
12
6
18
4
% roots
affected
Oct Nov
50
45
15
40
F/F/F/C = 3 years fallow followed by carrots……..to….
C/C/C/C = continuous carrots for 4 years
40
10
50
20
Cottage field sequential harvest : Lesions v P. violae in SOCS
Numbers of lesions on indivual roots
1.00E+00
0
2
4
6
8
10
12
14
16
DNA (ng/ul extract)
1.00E-01
1.00E-02
1.00E-03
1.00E-04
1.00E-05
1.00E-06
1.00E-07
There seems not to be no fixed relationship between amount of
fungus and the level of disease
So its “efficiency” as a pathogen varies
How does P. violae persist and why does cavity spot
sometimes occur after long periods without carrots
1
DNA in pg/µl
0.1
0.01
0.001
0.0001
species
root surface soil
Pythium violae in wild plants (weeds)
root tissue
Even for susceptible crop species not all roots
will be infected in the field
So how do I think about P. violae now?
Best thought of as a rhizosphere (root surface)
organism
Doesn’t benefit from causing disease on carrots
• active fungus eliminated from soil and roots
• doesn’t build up in soil
• can grow on other hosts with no apparent disease
Seems to be an accidental pathogen
(which is of no comfort to growers!!).
Options for control
•
fungicides
•
long-rotations (dubious value)
•
calcium treatments
Fungicide
Syngenta/Agrovista 2006 - Notts
1.8
% Surface Area Visually Infected 26/10
1.53
1.49
U/T
1.39
1.17
1.2
0.83
0.6
15 Jun 1.3 l/ha
13 Jul 1.3 l/ha
04 Sep 1.3 l/ha
15 Jun fb 04 Sep
0.65 fb 0.65 l/ha
0
cv. Nairobi - drilled 12/05
T1 = 2 lvs (15/06), T2 = 6 lvs (13/07), T3 = 35cm tall (04/09)
LSD = 0.47
Thanks to Jon Ogborn &
Syngenta (and apologies that
I couldn’t copy the logos).
Resistant varieties are becoming available
2008 Cottage Field in replicated beds.
Volcano
3 / 60 (lesions / roots)
Nairobi
38 / 80 (lesions / roots)
Fairly typical 80%+ reduction for this variety
relative to Nairobi
……these are resistant not immune and will
still get some disease, especially where
conditions favourable
But we need much better assays for assessing
resistance!
One possible target is thickness of skin on
roots
(Resistance in field is inversely correlated
with other susceptibility to other root damage)
“Soil health” seems crucial in
determining level of disease
Intercropping seemed a possibility
Can be very effective at
reducing disease
but leads to major
crop loss.
No lesions
40
Treatment
Dose 1
Dose 2
Total lesions on 20 roots
35
30
25
20
15
10
5
0
Control
Untreated
29/9/2008
sampling
A mild soil pasteurisation treatment can greatly increase
disease (but with little effect on levels of fungus)
One promising approach is using pre-plant green manures
Total lesions from 20 roots
12
10
8
Number of lesions
0 0 8
0 0 10
6
4
2
0
10000
approx number genomes
Light crop of mustard
but appears to greatly
reduce disease in
Autumn (upper)
Variety 1
Variety 2
1000
100
10
1
P. violae in SOCS
but relatively little effect
on P. violae (lower)
So much of the effect is
of reducing the
efficiency of fungus in
causing disease.
0.1
29/9/2008 sampling (similar in 10/2008)
Even judicious choice of previous crop can affect
disease:
Most have little effect but clover-grass and
potato seem to reduce disease by 50% or more
Preceding crop
forage rape
carrot
sugar beet
clover
wheat
barley
broccoli
clover-grass
potato
first
second
three-year
two-year rotation two-year rotation rotation
19
50
47
16
35
42
13
51
38
15
30
37
6
44
35
11
54
34
11
30
26
10
30
19
4
28
15
Notes:
1) 3-year rotation is same preceding crop twice then carrots
2) Control is repeated carrots
3) first 2-year rotation not irrigated
Summary:
1. Essentially rhizoplane organism and only accidental
pathogen
2. Growth of fungus driven by environmental factors
3. Doesn’t build up in soil and past history of field not
good indicator of whether disease will occur
4. Conventional (fungicide) treatments can give control
but need to ensure at right time and in right state.
5. Disease and the fungus affected by interaction with
other soil microbiota.
6. Green manures offer prospect of effective control
but need further investigation.
Possible future work:
Green manures (both “hot” and “cold”)
(HDC application in)
Better assays for screening for resistance