Mycological Progress (2020) 19:1537–1543
https://doi.org/10.1007/s11557-020-01644-x
ORIGINAL ARTICLE
Downy mildew of lavender caused by Peronospora belbahrii
in Israel
Marco Thines 1,2
&
Anthony Buaya 1 & Sebastian Ploch 1 & Yariv Ben Naim 3 & Yigal Cohen 3
Received: 6 July 2020 / Revised: 20 October 2020 / Accepted: 22 October 2020
# The Author(s) 2020
Abstract
Peronospora belbahrii is one of the most destructive downy mildew diseases that has emerged throughout the past two decades. Due to
the lack of quarantine regulations and its possible seed-borne nature, it has spread globally and is now present in most areas in which
basil is produced. While most obligate biotrophic, plant parasitic oomycetes are highly host-specific, there are a few that have a wider
host range, e.g. Albugo candida, Bremia tulasnei, and Pseudoperonospora cubensis. Recently, it was shown that Peronospora
belbahrii is able to infect Rosmarinus, Nepetia, and Micromeria in Israel in cross-infection trials, hinting an extended host range for
also this pathogen. In this study, a newly occurring downy mildew pathogen on lavender was investigated with respect to its
morphology and phylogeny, and it is shown that it belongs to Peronospora belbahrii as well. Thus, it seems that Peronospora
belbahrii is currently extending its host range to additional members of the tribe Mentheae and Ocimeae. Therefore, it seems advisable
to scrutinise all commonly used members of these tribes in order to avoid further spread of virulent genotypes.
Keywords Downy mildew . Evolution . Host jump . New host . Peronosporaceae
Introduction
Currently, there are more than 700 species of oomycetes known
to cause downy mildew disease on a great variety of
Angiosperm hosts (Constantinescu 1991; Thines and Choi
2016). Even though not members of the kingdom Mycota,
but of the largely unrelated Straminipila (Beakes and Thines
2017), the downy mildew pathogens have evolved into filamentous, osmotrophic, and obligate biotrophic pathogens in
parallel to their fungal counterparts. Several downy mildew
pathogens, such as Plasmopara destructor (Görg et al. 2017),
Pl. halstedii (Sharma et al. 2015; Trojanová et al. 2017),
Section Editor: Meike Piepenbring
* Marco Thines
m.thines@thines-lab.eu
1
Senckenberg Biodiversity and Climate Research Centre,
Senckenberganlage 25, 60325 Frankfurt am Main, Germany
2
Department of Biological Sciences, Institute of Ecology, Evolution,
and Diversity, Goethe University, Max-von-Laue-Str. 13,
60486 Frankfurt am Main, Germany
3
Faculty of Life Sciences, Bar Ilan University, 529000 Ramat
Gan, Israel
Peronospora belbahrii (Thines et al. 2009, 2020b), Pe. effusa
(Choi et al. 2015a, Klein et al. 2020), and Pe. tabacina
(Derevnina et al. 2015), cause economically important downy
mildew diseases, e.g. of balsamines, sunflowers, basil, spinach,
and tobacco, respectively.
Most downy mildew species have only very limited host
ranges (Gäumann 1923, Gustavsson 1959), as evidenced by
both cross-infection trials (e.g. Gäumann 1923) and molecular
phylogenetic investigations (e.g. Voglmayr 2003; Göker et al.
2004; García-Blázquez et al. 2008). However, there are a few
downy mildew species that have broader host ranges, e.g.
Pseudoperonospora cubensis (Runge and Thines 2009,
2012; Runge et al. 2012, Cohen et al. 2015), Bremia tulasnei
(Choi and Thines 2015), Pe. somniferi (Voglmayr et al. 2014),
and Pe. belbahrii (Ben Naim et al. 2019). While in the first
two cases it can be assumed that the broad host range is due to
different stages of radiation after a host jump event (Thines
2019), the latter two cases are more likely a reflection of an
alien pathogen extending its host range to related plants, previously naïve to downy mildew infestation (Thines 2019). The
latter case can pose a substantial threat to plant production, if
there are related commercially produced hosts that can be
infected by the invasive pathogen.
Within just 5 years after its emergence in basil production
in Europe, Pe. belbahrii became a major pathogen throughout
1538
the world, likely due to the trade with infested seed lots and
the delayed recognition of it as a new, previously
unrecognised species (Belbahri et al. 2005; Falach-Block
et al. 2019; Thines et al. 2009). Apart from basil, other species
have been reported as potential hosts for Pe. belbahrii, such as
coleus, sage, nepeta, and rosemary (Ben Naim et al. 2019;
Thines et al. 2009). While the pathogen of coleus was recently
described as a species of its own (Hoffmeister et al. 2020),
rosemary, nepeta, and some sage species where identified as
potential hosts of Pe. belbahrii by cross-inoculation experiments (Ben Naim et al. 2019). However, cross-infection trials
usually involve pathogen loads that are much higher than expected under natural conditions and thus are not conclusive
with respect to realised host ranges under field conditions.
This is exemplified by Bryonia dioica, which is a common
perennial member of the Cucurbitaceae in Central Europe.
While the species can be readily infected with Ps. cubensis
under laboratory conditions (Runge and Thines 2009, Runge
et al. 2012), it has, to our knowledge, not been reported as a
host under natural conditions.
During a scrutiny of culinary herbs in nurseries in Israel,
lavender plants (Lavandula angustifolia) exhibiting symptoms of downy mildew disease were observed. So far, lavender has not been reported as a host to any downy mildew
species. Lavender belongs to the same tribe as basil
(Ocimeae) and the same subfamily (Nepetoideae) as other
species that could be infected with Pe. belbahrii in a laboratory setting. Thus, it was the aim of the current study to investigate, if the observed infection was caused by this species,
which would call for a close monitoring of basil downy mildew in areas in which lavender is grown, in order to minimise
the risk of a host jump to this important perennial herb and
ornamental.
Mycol Progress (2020) 19:1537–1543
(minimum-)mean minus standard deviation-mean-mean plus
standard deviation(-maximum). A dried specimen was deposited in the Herbarium Senckenbergianum (FR) under the accession number FR-0246023.
Infectivity tests
Conidia were collected from downy mildew–infected lavender plants into cold distilled water; their number was adjusted
to 5000 spores/ml, and were spray-inoculated onto the upper
leaf surfaces of basil plants (cv. Peri at 2–4 leaf stage) using a
fine glass atomiser. Inoculated plants were incubated in a dew
chamber at 18 °C in the dark for the first 15 h after inoculation
to ensure infection and, thereafter, for 6 days at 25 °C under
continuous illumination (120 μmole m2 s−1) to allow for
symptom production. Plants were returned to the dew chamber on the seventh day post inoculation to enable sporulation
of the pathogen on leaves of the inoculated plants.
Uninoculated plants served as control.
DNA preparation, PCR, and sequencing
DNA was extracted from small leaf parts containing conidiophores and hyphae as described earlier (Mishra et al. 2018).
PCR amplifications of the barcode loci ITS and cox2 were
done as outlined by Choi et al. (2015b). The PCR products
were sequenced bidirectionally using the primers employed in
PCR at the laboratory centre of the Senckenberg Biodiversity
and Climate Research Centre (SBiK-F). Sequences of the ITS
and the cox2 barcode were deposited in GenBank (https://
www.ncbi.nlm.nih.gov/Genbank/) under the accession
numbers MT588812 and MT602519, respectively.
Alignment and phylogenetic analysis
Material and methods
Plant material and microscopic investigation
Lavender plants showing symptoms of downy mildew disease
were collected in summer 2019 from a nursery at Nehalim,
Israel. Specimens were air-dried between paper towels and
stored in closed envelopes for 2 months to ensure that no live
structures could accidentally be released to the environment.
Small parts of the specimens were then collected in 2-ml tubes
for DNA extraction or alternatively distributed onto microscopic slides in drops of 70% aqueous lactic acid solution
and covered with coverslips. Afterwards, observations of the
morphological characteristics as seen in DIC were done at ×
400 magnification using a Zeiss Imager 2 microscope with an
Axiocam colour camera (Zeiss, Oberkochen, Germany).
Measurements were done in Axiovision (Zeiss, Oberkochen,
Germany) on the pictures taken. Measurements are reported as
Sequences similar to those obtained in the present study were
downloaded from GenBank (https://www.ncbi.nlm.nih.gov/
Genbank/) by searching the nucleotide database using blastn
(Altschul et al. 1990) and selecting representative reference
sequences. Aligned sequences were downloaded and from the
results, and subsequently, over-represented sequence types
were removed to keep only two (in unclear cases up to five)
sequences per sequence type. Subsequently, alignments were
done using the TrEase webserver (https://thines-lab.
senckenberg.de/trease) with muscle (Edgar 2004) applying
standard settings. Leading and trailing gaps were removed,
and afterwards, alignments were uploaded to the TrEase
webserver (https://thines-lab.senckenberg.de/trease) for
phylogenetic inference. Minimum evolution inference was
done using FastTree (Price et al. 2010) with the GTR substitution model and 1000 bootstrap replicates. Maximum likelihood inference was done using RAxML v8 (Stamatakis 2014)
with the GTRGAMMA substitution model. Bayesian
Mycol Progress (2020) 19:1537–1543
Inference with six gamma categories was calculated with
MrBayes v3.2 (Ronquist et al. 2012) running for 5 million
generations, with sampling every 10,000th tree, and to ensure
sampling from the stationary phase, the first 30% of the trees
were discarded before calculating posterior probabilities.
Results
Infectivity tests
Typical downy mildew lesions with profuse sporulation were
observed in the inoculated basil plants at 7 days postinoculation (dpi), supporting that the lavender downy
mildew pathogen can cause downy mildew in sweet basil.
Morphology
Leaf areas of lavender plants infected with downy mildew were
slightly discoloured as seen from above (Fig. 1a). Lesions were
not clearly vein-delimited and gradually transitioning into the
healthy leaf colour at their outer limits. On the lower leaf surface, a sparse to slightly crowded outgrowth of hyaline conidiophores from the stomata was observed. Conidia were very
dark purplish-brown, rendering the lesions easily visible when
turning the leaves. Conidiophores were erect and monopodially
branched to sometimes sub-dichotomously branched with up to
7 (rarely 8) orders, and were (369–)451–606–761(−936) μm
long, (11–)11.2–12.3–13.4(−15) μm wide, sometimes slightly
swollen at the base; trunk (240–)280–375–470(−595) μm, ratio
total length to trunk length (1.5–)1.5–1.6–1.7(−1.8), n = 10.
Ultimate branchlets were usually curved, with an obtuse to
slightly pointed tip, mostly paired (70%) and differed in length,
with the longer ones measuring (8–)12.9–17.3–21.7(−30) μm,
the shorter ones measured (4–)6.4–9–11.6(−17) μm, and the
ratio of the longer to the shorter ultimate branchlet was
(1.08–)1.53–1.98–2.43(−3.95) μm, n = 100; the branching of
the ultimate ramification was mostly rectangular. Conidia were
broadly ellipsoidal, (21.5–)25.7–28–30.3(−33) μm long,
(17–)21–22.9–24.8(−27.5) wide, with a length to breadth ratio
of (1.06–)1.14–1.23–1.32(−1.52), n = 100, directly germinating
with a germ tube. Thus, the main characteristics (ratio of the
longer to the shorter ultimate branchlets and conidial dimensions) agree very well with the measurements reported in
Thines et al. (2009), leaving no doubt that the species affecting
lavender in Israel is Peronospora belbahrii. An overview of the
morphology is given in Fig. 1.
Phylogeny
Both in the cox2-based (Fig. 2a) and in the ITS-based (Fig. 2b)
phylogenetic reconstruction, the downy mildew pathogen
from lavender in Israel grouped together with samples of Pe.
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belbahrii from Ocimum basilicum from several countries in all
phylogenetic analyses and mostly with strong support. Within
Pe. belbahrii the phylogenetic resolution was low with some
sequence variation, but no clear-cut geographical pattern was
seen. The recently described species Pe. choii was inferred as
the sister group to Pe. belbahrii in all analyses, with moderate
support. These two species were embedded in a clade containing several other species on culinary and ornamental herbs,
including the economically important sage pathogen Pe.
salviae-officinalis. The whole group, all of which are pathogens of the subfamily Nepetoideae of the family Lamiaceae,
was supported with strong to maximum support in all analyses
on both loci. Apart from this group, two species groups with a
limited host range, one including pathogens from
Caryophyllales (including the economically important species
Pe. variabilis) and the other containing pathogens from
Ranunculaceae (including the widespread species Pe.
ficariae), were resolved as monophyletic with moderate support in the cox2-based phylogenetic reconstruction (Fig. 2a).
In the ITS-based phylogenetic reconstructions, no additional
groupings apart from the Nepetoideae-infecting species received significant support. Based on both cox2 and ITS sequences, the downy mildew pathogen from lavender in Israel
could unambiguously be assigned to Pe. belbahrii.
Discussion
Many oomycete species are restricted in their host ranges and
by this originally tied to the natural distribution of their hosts.
This originally applied not only to wild plants but also to cultivated ones. However, with the increasing mobility of people
and the global spread of popular crops, new cultivated and
domesticated species were widely distributed, such as potato
(Solanum tuberosum) and tomato (So. lycopersicum). The
resulting global trade resulted in the risk to distribute pathogens
from the natural range of the host as well, and consequently,
pathogens soon followed their hosts, such as Phytophthora
infestans, causing potato late blight (de Bary 1876; Yoshida
et al. 2013); Plasmopara viticola, causing grape downy mildew
(Viennot-Bourgin 1949, Fontaine et al. 2013; Yin et al. 2017;
Brilli et al. 2018); Pl. halstedii, causing downy mildew of sunflower (Novotel’nova 1962, 1966; Trojanová et al. 2017); and,
more recently, Peronospora aquilegiicola, causing Aquilegia
downy mildew (Thines et al. 2019, 2020a). Some of these
introductions had devastating effects: Ph. infestans, which triggered the Great Potato Famine, caused the death of hundreds of
thousands of people in Ireland and continental Europe (Gráda
2000; Zadoks 2008). Apart from these introductions, for which
the source is known, there are cases of emerging diseases with
unclear origin of the pathogen, such as in the tobacco blue
mould, Pe. tabacina (Lucas 1980); balsamine downy mildew,
Pl. destructor (Görg et al. 2017); and basil downy mildew, Pe.
1540
Fig. 1 Symptoms and
morphology of Peronospora
belbahrii on lavender. a Infected
plants with faint chlorotic lesions
and sporulation on the lower leaf
surfaces. b Close-up of
sporulating lesions. c
Sporangiophore. d Terminal
branches and ultimate branchlets.
e Conidia, the arrow pointing to
the secession scar at the former
attachment site of an ultimate
branchlet to the conidium. Scale
bars equal 100 μm in c and 50 μm
in d and e
Mycol Progress (2020) 19:1537–1543
a
b
c
d
e
belbahrii (Thines et al. 2009). For these species, it seems possible that they have originated by a host jump to the cultivated
crop, similar to the situation of Peronoscleropsora maydis,
which became a maize pathogen in Southeast Asia (Suharjo
et al., 2020) and Australia (Telle et al. 2011, Shivas et al.
2012) after a host jump from locally occurring Sorghum species. These host jumps are facilitated if the introduced crop is
naïve to downy mildew infection because no pathogen causing
the disease was present in its original distribution range (Thines
2019).
In the case of basil downy mildew, the pathogen was possibly contracted by basil in Africa (Hansford 1933, 1938),
from where it has rapidly spread, most likely due to infested
seed lots (Belbahri et al. 2005; Falach-Block et al. 2019;
Thines et al. 2009). Peronospora belbahrii is astonishingly
diverse for a newly occurring pathogen, suggesting either high
mutation rates or multiple introductions. As the pathogen has
become a threat to basil production only recently, despite the
crop being cultivated for decades before, the latter scenario
seems unlikely. Should the high degree of diversification be
the result of increased mutation rates, it would enable Pe.
Fig. 2 Minimum evolution phylogenetic reconstruction based on cox2
(a) and ITS (b) sequences. Numbers on branches indicate bootstrap
support in minimum evolution and maximum likelihood analyses as
well as Bayesian posterior probabilities, in the respective order. Support
values are only given, if at least two independent phylogenetic tools
provided support. Values below 60% bootstrap support or 0.85
posterior probability are not displayed. A minus sign indicates support
below the reporting threshold for the presented or an alternate topology
Mycol Progress (2020) 19:1537–1543
a
1541
cox2
Peronospora belbahrii ex Lavandula angustifolia (Israel)
MF687312 Peronospora belbahrii ex Ocimum basilicum (USA)
60/62/0.91 MN546959 Peronospora belbahrii ex Ocimum basilicum (Germany)
62/-/0.95
Peronospora belbahrii
MN546960 Peronospora belbahrii ex Ocimum basilicum (Germany)
99/71/0.98
MN546976 Peronospora belbahrii ex Ocimum basilicum (Germany)
70/60/0.96
KJ654229 Peronospora belbahrii ex Ocimum basilicum (Germany)
KT828760 Peronospora choii
99/100/1.0 FJ394339 Peronospora choii
MN151371 Peronospora choii
KJ654299 Peronospora salviae-plebeiae
100/98/1.0
MN546955 Peronospora salviae-officinalis
100/99/1.0 MN546956 Peronospora salviae-officinalis
76/68/0.94
MN546965 Peronospora salviae-officinalis
KJ654217 Peronospora glechomae
91/-/0.99
FJ527435 Peronospora elsholtziae
99/98/1.0
KJ654235 Peronospora polygoni
69/-/0.89
KX777249 Peronospora cf. ducometii
MF595895 Peronospora variabilis
100/100/1.0 KJ654246 Peronospora scleranthi
95/62/0.99
KJ654277 Peronospora cerastii-brachypetali
89/60/0.97
KJ654197 Peronospora kochiae-scopariae
100/100/1.0
87/83/1.0
KJ654286 Peronospora alpicola
KM058113 Peronospora pulveracea
100/100/1.0 KJ654239 Peronospora ficariae
KM058109 Peronospora ficariae
100/98/1.0
KJ651397 Peronospora sordida
JX982637 Peronospora sordida
KJ654209 Peronospora erodii
KJ654219 Peronospora violacea
82/-/0.99
KJ654245 Peronospora knautiae
KJ654208 Peronospora dipsaci
0.005 substitutions per site
ITS
AY884605 Peronospora belbahrii ex Ocimum basilicum (Switzerland)
100/91/0.99
KC756923 Peronospora belbahrii ex Ocimum basilicum (Canada)
EF153666 Peronospora belbahrii ex Ocimum basilicum (Iran)
93/84/0.99
EF153667 Peronospora belbahrii ex Ocimum basilicum (Iran)
Peronospora belbahrii ex Lavandulaangustifolia (Israel)
KF419290 Peronospora belbahrii ex Ocimum basilicum (Cyprus)
89/66/0.82
DQ479408 Peronospora belbahrii ex Ocimum basilicum (South Africa)
FJ394335 Peronospora belbahrii ex Ocimum basilicum (Germany) ex-TYPE
100/99/0.99
LC011940 Peronospora choii
DQ980194 Peronospora choii
KT828758 Peronospora choii
FJ527442 Peronospora elsholtziae
100/100/1.0
FJ394345 Peronospora salviae-officinalisex-TYPE
GQ390794 Peronospora salviae-officinalis ex Agastache mexicana (UK)
MN308036 Peronospora salviae-officinalis
88/62/0.99
FJ527445 Peronospora salviae-plebeiae
100/100/1.0 MT510002 Peronospora saturejae-hortensis
JN882274 Peronospora saturejae-hortensis
70/63/0.96
MH730801 Peronospora argemones
KJ651427 Peronospora somniferi
100/100/1.0
DQ885373 Peronospora arborescens
EU295529 Peronospora arborescens
94/96/1.0
AY198297 Peronospora violacea
100/100/1.0 KT795476 Peronospora potentillae-reptantis
AY608610 Peronospora sparsa
JX982638 Peronospora sordida
EU513600 Peronospora perillae
0.005 substitutions per site
Peronospora belbahrii
FJ346561 Peronospora belbahrii ex Ocimum basilicum (USA)
b
1542
belbahrii to radiate to new hosts in his new distribution range,
especially in a setting with frequent contacts and large population sizes of potential new hosts that are related to its current
host (Thines 2019). Even though infection trials in a laboratory setting are exaggerating the susceptibility of hosts because of optimised infection parameters and high inoculum
load, the results of Ben Naim et al. (2019) seem to support
the potential risk of the spread of Pe. belbahrii to other members of the Nepetoideae.
The confirmation in this study of Pe. belbahrii infection in
lavender under field conditions suggests that the aggressiveness observed in the laboratory setting might also be present in
the field, which might result in jumps to new widely cultivated
hosts. Thus, it seems advisable to closely monitor the pathogen in related crops and wild species and to counter infections
in basil fields with appropriate phytosanitary measures.
Acknowledgements The first author gratefully acknowledges Christian
Printzen, herbarium FR, section for cryptogams, for his continued help in
accommodating unusual specimens in the Herbarium Senckenbergianum.
Authors’ contributions MT and YC conceived the study; YC and YBN
provided material; YBN produced Fig. 1a, b, and MT produced all other
figures; ATB and SP processed the samples; MT performed the morphometric analysis, did the phylogenetic reconstructions, and analysed the
data; MT wrote the manuscript with contributions from the other authors.
Funding Open Access funding enabled and organized by Projekt DEAL.
MT is supported by LOEWE in the framework of the Centre for
Translational Biodiversity Genomics. YC is supported by The Plant
Council of Israel.
Data availability Sequence data have been deposited in GenBank.
Compliance with ethical standards
Ethics approval and consent to participate Not applicable.
Consent for publication Not applicable.
Competing interests The authors declare that they have no competing
interests.
Open Access This article is licensed under a Creative Commons
Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as
you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were
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statutory regulation or exceeds the permitted use, you will need to obtain
permission directly from the copyright holder. To view a copy of this
licence, visit http://creativecommons.org/licenses/by/4.0/.
Mycol Progress (2020) 19:1537–1543
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