Halo Blight of Mungbean in Australia - Opinion - MDPI
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Opinion
Halo Blight of Mungbean in Australia
Araz Sedqi Abdullah *,† and Col Douglas
Department of Agriculture and Fisheries, Hermitage Research Facility, Warwick 4370, Australia;
col.douglas@daf.qld.gov.au
* Correspondence: araz.solman@daf.qld.gov.au
† Author also known as Araz Solman.
Abstract: Halo blight, one of the major diseases of mungbean, is caused by the bacterium Pseudomonas
savastanoi pv. phaseolicola. The pathogen infects the foliar parts of the plant, causing water-soaked
spots that eventually develop surrounding yellow margins. The disease is particularly destructive
under moderate temperature and high humidity, especially when it occurs during late vegetative
through to early reproductive stage. In such conditions, severely infected crops could experience a
yield loss up to 70%. Halo blight can be widespread on mungbeans grown in Southern Queensland
and Northern New South Wales. However, due to its seedborne and cryptic nature of transmis-
sion, the disease is likely to be under-reported. This report addresses major aspects of halo blight
symptomology, pathology and epidemiology.
Keywords: Vigna radiata; P. phaseolicola; bacterial disease; disease management
1. The Pathogen and Disease Symptoms
Halo blight is a major mungbean disease caused by the foliar bacterium Pseudomonas
savastanoi pv. phaseolicola. The disease was first reported in Queensland in the early
1980s and has since become a widespread constraint affecting the reliability and yield
Citation: Abdullah, A.S.; Douglas, C. of mungbeans. The bacterium’s seedborne nature and broad host range contribute to
Halo Blight of Mungbean in Australia. the widespread distribution of the halo blight disease. However, due to unfamiliarity
Crops 2021, 1, 3–7. https://doi.org/ with disease symptoms and the possibility of mixed infections, halo blight is likely to be
10.3390/crops1010002 under-reported. Recent genomic analysis suggests the existence of several pathotypes of
the causal agent of halo blight [1]. This highlights the significance of conducting annual
Academic Editor: Il-Ryong Choi surveillance at a level sufficient to monitor and characterise pathogen virulences as they
Received: 9 April 2021 respond to changes in host genotype and growing conditions. Virulences identified can
Accepted: 13 May 2021 then be evaluated against current breeding lines and commercial varieties to inform and
Published: 17 May 2021 direct future breeding activities.
While halo blight can occur on seedlings early in the season, the main damage often
Publisher’s Note: MDPI stays neutral occurs when the infection reaches the epidemic stage during and after flowering. Typical
with regard to jurisdictional claims in
symptoms of the disease initially appear as small, dark, circular spots. These eventually
published maps and institutional affil-
develop necrotic lesions surrounded by yellow margins (Figures 1 and 2). Advanced
iations.
lesions can join, giving rise to large necrotic areas scattered on both sides of the infected leaf
(Figure 2B,C). Stress plants, including those exposed to waterlogging, high wind rainfall
events or water stress, are more likely to show disease symptoms and suffer significant
yield loss. Even stress associated with flowering may be sufficient to induce susceptibility
Copyright: © 2021 by the authors. for significant halo blight disease. The disease is wind- and rain-dispersed, spreading the
Licensee MDPI, Basel, Switzerland. infection to uninfected leaves and neighbouring plants throughout the crop.
This article is an open access article
Studies have shown that halo blight bacterium can rapidly reach high densities with-
distributed under the terms and
out causing visible disease symptoms [2]. This suggests that apparent lack of symptoms
conditions of the Creative Commons
is not indicative of a pathogen-free crop. In fact, dispersal of bacteria is likely to precede
Attribution (CC BY) license (https://
symptom development, making physical movement of the pathogen within and between
creativecommons.org/licenses/by/
crops less detectable. Symptoms can subsequently express following physical injury from
4.0/).
Crops 2021, 1, 3–7. https://doi.org/10.3390/crops1010002 https://www.mdpi.com/journal/crops
Crops 2021, 1 4
Crops 2021, 1, FOR PEER REVIEW 2
rain and hail; these are thought to trigger a widespread pathogenic shift [2]. Therefore,
rain and hail;
movement of these are thought
equipment throughto trigger a widespread
and between pathogenic
paddocks shouldshift
be [2]. Therefore,
carefully considered,
movement of equipment through and between paddocks should be carefully
as symptomless mungbeans can harbour the pathogen. Mechanical damage and physical considered,
as symptomless
movement maymungbeans
spread thecan harbour
disease the pathogen.
across Mechanical
the paddocks [3]. damage
Growers and physical
and agronomists
movement may spread the disease across the paddocks [3]. Growers and agronomists
should implement a ‘come clean-go clean’ approach, which has proven successful in many
should implement a ‘come clean-go clean’ approach, which has proven successful in many
other crops such as cotton [4]. Vehicles and machineries should be properly cleaned before
other crops such as cotton [4]. Vehicles and machineries should be properly cleaned before
leaving one paddock so that they arrive clean at the next paddock.
leaving one paddock so that they arrive clean at the next paddock.
Figure 1. Examples of mungbean leaves, cultivar Berken, showing halo blight symptoms. On the
Figure 1. Examples of mungbean leaves, cultivar Berken, showing halo blight symptoms. On the
cotyledons, symptoms initially appear as irregular shaped spots scattered throughout the tissue
cotyledons, symptoms
((A) picture collected initially
6 days appear as irregular
after inoculation). These can shaped spots
join, causing scattered
large, throughout the tissue
irregular-shaped
((A) picture
chlorotic collected
lesions 6 days
((B) picture after 12
collected inoculation). These can
days after inoculation). Onjoin, causingleaves,
the trifoliate large,initial
irregular-shaped
symptomslesions
chlorotic appear ((B)
as regular-shaped, water-soaked
picture collected 12 days circular spots surrounded
after inoculation). On bythelarge chlorotic
trifoliate leaves, initial
margins ((C) picture collected 6 days after inoculation). These develop large necrotic lesions by
symptoms appear as regular-shaped, water-soaked circular spots surrounded by large chlorotic
margins ((C) picture collected 6 days after inoculation). These develop large necrotic lesions by about
12 days after inoculation (D). White arrows in A and B indicate the presence of chlorotic tissues. Black
and red arrows in C and D indicate the presence of necrotic and water-soaked lesions, respectively.
Crops 2021, 1, FOR PEER REVIEW 3
about 12 days after inoculation (D). White arrows in A and B indicate the presence of chlorotic
Crops 2021, 1 5
tissues. Black and red arrows in C and D indicate the presence of necrotic and water-soaked le-
sions, respectively.
Figure
Figure 2.
2. The
The ‘5Rs’
‘5Rs’ diagnostic
diagnostic scheme
scheme showing
showing symptom
symptomprogression
progressionon on aa halo
halo blight
blight infected
infected leaf
leaf (A).
(A). (1)
(1) Ringed
Ringed with
with
yellow
yellow margin,
margin, (2)
(2) rectangular
rectangular shape
shape lesions,
lesions, (3)
(3) restricted
restricted by
by the
the leaf
leaf veins,
veins, (4)
(4) reddish-brown
reddish-brown colouration,
colouration, (5) (5) rampant
rampant
throughout
throughout the
the leaf.
leaf. Halo
Halo blight
blight symptoms
symptoms on on the
the upper
upper (B)
(B) and
and lower
lower (C)
(C) sides
sides of
of the
the leaf.
leaf.
2.2.Pathogen
PathogenSurvival
Survivaland andDisease
DiseaseSpread
Spread
The
The causal agent of halo blight survives
causal agent of halo blight survives thethe intercropping
intercropping periods
periods onon alternative
alternative
hosts,
hosts, in infected seeds and plant debris from previous seasons. Infected seed represents
in infected seeds and plant debris from previous seasons. Infected seed represents
the
themajor
major mode
mode of of survival
survival andand transport
transport ofof the
the pathogen
pathogen [3].[3]. The
The pathogen
pathogen can
can invade
invade
the
theplant
plantthrough
throughstomatal
stomatal openings
openings but, more
but, morenotably, through
notably, throughwounds and and
wounds injuries cre-
injuries
ated by heavy rainfall and wind. The disease can be spread by
created by heavy rainfall and wind. The disease can be spread by rain-splash, contactrain-splash, contact be-
tween
between wetwetleaves
leaves andand irrigation
irrigation water,
water,asaswell
wellasasbybypeople
people andand animal
animal movements
movements
through infested crops. Rain splash permits disease transmission,
through infested crops. Rain splash permits disease transmission, especially when especially whentherethereis
isa aprevailing
prevailing wind that facilitates movement of the pathogen over long distances
wind that facilitates movement of the pathogen over long distances (Figure 3). (Figure
3).
Halo Halo blight
blight flourishes
flourishes ininmoderately
moderatelylow lowtemperatures
temperatures(18–26(18–26◦°C) with periods
C) with periods ofof high
high
relative
relative humidity (i.e., during and following heavily rainfall). The disease causes yieldyield
humidity (i.e., during and following heavily rainfall). The disease causes loss
loss by reducing
by reducing leaf leaf
areaarea available
available for photosynthesis
for photosynthesis [5]. Yield
[5]. Yield losseslosses
up toup to have
70% 70% have
been
been documented
documented in heavily
in heavily infected
infected cropscrops
[6]. [6].
Crops 2021, 1 6
Crops 2021, 1, FOR PEER REVIEW 4
Figure
Figure 3. Schematic
3. Schematic representation
representation of theof
lifethe lifeof
cycle cycle of Pseudomonas
Pseudomonas savastanoi
savastanoi pv. Phaseolicola,
pv. Phaseolicola, causing
causing halo halo
blight blight of mung-
of mungbeans.
beans.
3. Management of Halo Blight
3. Management of Halo Blight
Management of halo blight is difficult as there are no registered chemicals for effective
Management
in-crop control of halo Tolerance
of the disease. blight is difficult
level in as
thethere
seed are no such
is low registered chemicals
that, when for effec-
conditions
are tive in-crop only
conducive, control
oneofinfected
the disease.
seed Tolerance
in 10,000 is level in the seed
sufficient to startis low such that,[7].
an outbreak whenThiscon-
ditions are conducive, only one infected seed in 10,000 is sufficient
prompted many countries to undertake highly sensitive and specific testing protocols to to start an outbreak
[7].the
detect Thispresence
prompted of many countries
halo blight to undertake
bacterium in seedhighly
crops sensitive
[8]. Crops and arespecific
rejectedtesting
whenpro-
the bacterium is detected or observed in or near seed production sites. Currently,rejected
tocols to detect the presence of halo blight bacterium in seed crops [8]. Crops are the
when the
Australian bacteriumAssociation
Mungbean is detected requires
or observed in orcrops
all seed near seed
to beproduction
tested, based sites.
onCurrently,
DNA
the Australian
detection method, for Mungbean
the presenceAssociation
of the halorequires
blightall seed crops
bacterium to be
in the tested,
seed. Thisbased on DNA
represents
detectionstep
a significant method,
towardsfor the presence
producing of the halo
certified, haloblight
blightbacterium
free seed. in the seed. This testing
Comprehensive represents
a significant
of seed crops forstep towards
all the majorproducing certified,
bacterial diseases halo
will blight potential
identify free seed.disease
Comprehensive
problems,test-
ing of seed crops for all the major bacterial diseases will identify
reducing the risk of pathogen spread. Diagnostic methods, such as those that employ potential disease
DNA prob-
markers, appear to be more robust in detecting bacterial pathogens such as halo blight,em-
lems, reducing the risk of pathogen spread. Diagnostic methods, such as those that
evenploy
whenDNA markers,
there appearsymptoms
are no visible to be more onrobust in detecting
the infected bacterial pathogens such as
seeds [8,9].
halo blight, resistance
Improved even whentothere halo are no has
blight visible symptoms
become availableon in
thetheinfected
last five seeds
years[8,9].
through
Improved
variety releases resistance
from National to halo blight has
Mungbean become available
Improvement. Opal-AU in the(released
last five years
2020)through
is a
variety
large greenreleases from National
shiny-seeded mungbean Mungbean
in the same Improvement.
market classOpal-AUas Crystal (released 2020) is a
and Jade-AU.
large green
Opal-AU shiny-seeded
is adapted to Southern mungbean
Queensland in theandsameNew market
SouthclassWales as and
Crystal and Jade-AU.
represents the
Opal-AU
biggest singleisstep
adapted
forward to Southern
in diseaseQueensland
resistance for and New South
mungbean. In Wales
the niche andsmall
represents
green the
biggest single
shiny-seeded stepclass,
market forward in disease
Celera resistance2015)
II-AU (released for mungbean. In the niche
has good protection small
from green
halo
blight. More details
shiny-seeded on yield
market andCelera
class, agronomic
II-AUperformance,
(released 2015) disease profiles
has good and marketing
protection from halo
of these areMore
blight. provided
detailsin on
theyield
Variety
andManagement Packages at http://www.mungbean.org.
agronomic performance, disease profiles and marketing
au/agronomy.html
of these are provided(accessedinonthe 17 May 2021).
Variety Management Packages at http://www.mung-
bean.org.au/agronomy.html (accessed on 17 May 2021).
Author Contributions: A.S.A. conceived the idea of the paper, revised the literature and drafted the
manuscript. C.D. drafted theA.S.A.
Author Contributions: halo blight management
conceived the idea section and provided
of the paper, revised comments on and
the literature the article.
drafted
All authors have read and agreed to the published version of the manuscript.
the manuscript. C.D. drafted the halo blight management section and provided comments on the
article.This
Funding: Allresearch
authorsreceived
read andnoapproved the final
external funding version
other of theisarticle.
than what specified in the acknowledgments.
Crops 2021, 1 7
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments: This paper has been developed as part of the National Mungbean Improvement
Program (NMIP). The NMIP is a joint investment between the Department of Agriculture and
Fisheries and Grains Research and Development Corporation aiming to improve productivity and
reliability of the Australian mungbean industry.
Conflicts of Interest: The authors declare no conflict of interest.
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