Dynamic simulation of grape downy mildew primary infections - Assam

Fig. 1. Relational diagram of the model simulating primary P.
viticola infection on grape. See Tab. 1 for acronym explanation.
RH
T
LW
LW
S
U
R
OLL
MMO
PMO
GEO
ZLL
ZGL
ZCI
ISS
MMR
DOR
GER
INF
INC
R
LW
T
R
t
T
T
LW
T
RH
T
LLM
VPD
RH
which are oospores that have completed the germination
process and have produced a macrosporangium on the
leaf litter surface. Oospores change from MMO to PMO
at the end of dormancy, and from PMO to GEO when the
germination process is completed; the corresponding
rates (DOR and GER, respectively) both depend on air
temperature (T) when leaf litter moisture (LLM) is not a
limiting factor, but rainfall (R) is necessary to moisten
the leaf litter and trigger germination. LLM depends on
the balance between water absorption from and
desorption to the atmosphere, measured by means of the
vapour pressure deficit (VPD).
In the presence of a film of water (LW)
macrosporangia release zoospores (ZLL); otherwise they
can survive for a few days and then die: the survival rate
(SUR) depends on T and relative humidity (RH).
These zoospores, swimming in the film of water
covering the leaf litter, reach the grape leaves (ZGL) by
splashes and aerosols triggered by rainfall. If the litter
surface dries up before rainfall they do not survive;
therefore the SUR of these zoospores depends on LW.
Zoospores in the ZGL stage go to the next stage of
zoospores causing infection (ZCI) according to an
infection rate (INF) which depends on LW and T during
the wet period. During this period zoospores swim in the
direction of stomata, form a cyst and produce germ tubes
that penetrate the stomatal rimae. If the leaf surface dries
before penetration, the zoospores dry out; therefore their
survival depends on a combination of LW and T.
110
At the end of incubation, the infection sites become
visible as disease symptoms (ISS); incubation progress
(INC) is influenced by T and RH.
Model running. Considering that oospores usually
reach the MMO stage in autumn, it is assumed that the
population of oospores formed at the end of a downy
mildew epidemic is all in the MMO stage on the 1 st of
January of the following year.
Since it is well known that oospore germination is a
gradual process over the grape-growing season, the
model considers that the oospore population of a
vineyard overcomes dormancy in many subsequent
groups (cohorts), and that the density of each cohort
follows a normal distribution. Therefore, the model
calculates the time when the first cohort of oospores
enter the PMO stage at the end of its dormancy; further
cohorts enter this stage progressively (Fig. 2). A
measurable rainfall moistening the leaf litter is the event
triggering germination of the oospore cohorts in the
PMO stage at that time. Times required for completing
both dormancy and germination are calculated by the
variables DOR and GER, respectively, using two
counters that increase according to T when LLM is not a
limiting factor. When these counters reach a fixed
threshold the model assumes that both dormancy and
germination of an oospore cohort are finished.
Afterwards, the model calculates the time when the
macrosporangia produced by the germinated cohort of
oospores survive, the possibility that they release
zoospores, that these oospores survive, reach the grape
leaves and successfully infect the leaf tissue (Fig. 3).
Finally the model calculates the incubation period and
defines a time period when the downy mildew symptoms
should appear on the affected grape organs.
Model outputs. The model provides tables showing
the hourly progress of the main infection stages (Tab. 2),
and graphs showing the state of the infection cycle on
each day during the primary inoculum season (Fig. 3).
Tab. 2. Example of the tabular output of the model in a day with
a successful infection.
Hours of
the day
Oospore
germination
Sporangium
survival
Zoospore
release
Zoospore
dispersal
Infection
progress
Incubation
progress
2 0.99
3 0.99
4 0.99
5 1.00 0.01
6 0.01
7 0.02
8 0.03 yes
9 yes 0.30
10 0.77
11 1.00
12 0.01
13 0.02