CONTROLLING
ANTHRACNOSE IN
AVOCADO BY ENHANCING
NATURAL FRUIT
RESISTANCE: THE ROLE OF
ROOTSTOCKS AND
NUTRITION
Sonia Willingham, Ken Pegg, Lindy
Coates, Tony Cooke, Peter Langdon and
Jan Dean (QHI, Indooroopilly DPI)
Natural Resistance
• Plants have natural defence mechanisms in
place to combat disease
• Plants can defend themselves
– physically (eg., strengthen cell walls by crosslinking, depositing lignin, callose and suberin)
– biochemically (eg., phytoalexins, specific
antifungal compounds, PR-proteins such as
chintinases and beta-1, 3-glucanases)
*Defences may be preformed or inducible*
Diene Concentration
Disease Susceptibility
Appearance of
disease
symptoms
Time after Harvest
Systemic Acquired Resistance
(SAR)
• induced by a local necrotizing pathogen
infection
• long lasting response
• broad spectrum, acts against viruses,
bacteria, fungi and nematodes
• resistance is not 100%
• multiple inductions can enhance
resistance
SAR Triggered:
Biologically
Chemically
avirulent strains of the same species
different non-pathogenic species
plant and microbe extracts
salicylic acid (SA)
Bion ® /INA/BTH
phosphonates
Messenger®
Physically
heat shock (eg., 40 sec at 50oC)
UV-C light
High CO2
rubbing
Benefits of SAR
residue free
non-toxic to the environment
very low risk of pathogen resistance
developing
long-term sustainable control
Plant Resistance Depends on:
• Plant part
• Plant age
• Environmental factors eg.,
temperature, light, moisture
• Cultural factors eg., nutrition,
rootstock
Nutrients Can Affect Disease
Susceptibility by:
1. Influencing the production of defence
compounds via the Shikimic Acid
pathway (N, Mn)
a. directly by altering rate of metabolism
b. indirectly by altering Mn availability
Defence Products via Shikimic
Acid Pathway
Carbon Dioxide
Photosynthesis
a. Rate of
metabolism
b. Mn availability
Carbohydrates
Glycolysis
Shikimic Acid
Simple Phenolics
eg., coumarins, flavenoids
Complex Phenolics
eg., lignins, tannins
Nutrients Can Affect Disease
Susceptibility by:
2. Restricting access to cell walls and
middle lamella by fungal pectolytic
enzymes (Ca)
3. Preventing or delaying ‘attack’ signal to
fungus (Ca)
4. Inhibiting fungal enzymes (Mn)
‘Duke 6’
‘Velvick’
Rootstock Effects - Young
‘Hass’ Trees
Shelf % Anthracnose % Mark.
Rootstock life(d)
a
sev.
inc.
b
fruit
b
a
Velvick
7.0
7.7
61.9
66.1
Duke 6
6.7b
41.8a
93.2a
13.6b
Rootstock Effects - Older
‘Hass’ Trees
Shelf % Anthracnose % Mark.
Rootstock life(d)
sev.
inc.
fruit
Velvick
9.1a
15.6b
50.0b
64.5a
Duke 6
a
a
a
b
8.9
39.5
77.0
33.6
Rootstock Effects - Young
‘Hass’ Trees
Rootstock
Velvick
Duke 6
Diene
(mg/g FW leaf)
2.45a
1.74b
Rootstock Effects - Older
‘Hass’ Trees
Rootstock
Velvick
Duke 6
Diene
(mg/g FW leaf)
3.30a
2.57b
Rootstock Effects - Nursery
Stock Trees
Rootstock
Velvick
Duke 6
Diene
(mg/g FW leaf)
1.01a
0.08b
Rootstock Effects - Young
‘Hass’ Trees
N
Rootstock (% DW)
Velvick
Duke 6
b
2.3
a
2.5
N/Ca
ratio
b
0.9
a
1.1
Rootstock x Nitrogen Study
‘Hass’ trees on ‘Duke 6’ and ‘Velvick’
rootstocks were treated with 3 different
nitrogen fertiliser levels:
1. Control - standard rate (133 g NH4+N/tree/month)
2. Low N - no nitrogen fertiliser applied
3. High N - double rate (266 g NH4+N/tree/month)
Applied from flowering until harvest
Anthracnose severity (%)
Rootstock Effects - Young
‘Hass’ Trees
70
r = 0.82**
(67%)
60
50
40
30
20
10
0
0
0.001
0.002
0.003
0.004
N/Ca
0.005
0.006
0.007
Anthracnose Severity (%)
a
70
a
a
60
50
40
a
a
Control
Low N
High N
a
30
20
10
0
'Velvick'
'Duke 6'
Rootstock Effects
Shelf % Anthracnose % Mark.
Rootstock
Velvick
Duke 6
life
(d)
a
9.3
8.7
b
sev.
inc.
b
32.4
a
63.9
fruit
b
64.0
a
90.0
a
46.9
b
16.4
Leaf N Concentration (%DW)
a
3.10
3.05
a
a
3.00
2.95
a
a
a
2.90
2.85
2.80
'Velvick'
'Duke 6'
Control
Low N
High N
Leaf Ca Concentration (%DW)
1.8
ab
a
1.6
1.4
1.2
bc
cd
cd
d
Control
Low N
High N
1.0
0.8
0.6
0.4
0.2
0.0
'Velvick'
'Duke 6'
Rootstock Effects on Leaf
Minerals (%DW)
Rootstock
Velvick
Duke 6
N
Ca
b
2.9
a
3.0
Mg
a
1.6
b
1.3
a
0.4
b
0.3
K
b
0.5
a
0.7
Anthracnose severity (%)
Rootstock Effects
100
r = 0.49**
(24%)
80
60
40
20
0
0
10
20
30
N/Ca
40
50
Anthracnose severity (%)
Rootstock Effects
100
r = 0.57**
(33%)
80
60
40
20
0
0
0.05
0.1
Ca+Mg/K
0.15
0.2
Conclusions
Rootstock influences postharvest
anthracnose susceptibility by
influencing the accumulation of
mineral nutrients and antifungal diene
compounds in the scion tissue.
Rootstock discovery will provide a
new long-term sustainable disease
control strategy that is less reliant on
chemical control.
Future Research
Assess nitrogen effect after two
consecutive seasons of fertiliser
applications.
Evaluate the effect of N form (ie.
ammonium vs nitrate) on
anthracnose susceptibility, mineral
nutrient and diene accumulation.
Evaluate foliar applications of plant
activators (eg., Bion®, Messenger®)
to boost antifungals.
Acknowledgments
We wish to thank the Australian Avocado
Growers Federation, Horticulture
Australia Limited and the Queensland
Horticulture Institute for funding this
project and a special thanks to
Graham Anderson for his collaboration
with field experiments.