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Abstract
Tomato powdery mildew is caused by the obligate pathogenic fungus Oidium
neolycopersici
Kiss (Erysiphaceae). The host range of the pathogen is broad and it is reported to attack
over
60 species in 13 plant families, particularly members of the Solanaceae and
Cucurbitaceae.
The disease was first discovered in 1986 in England, later, this disease was also described
in
North America and in other countries around the world. O. neolycopersici is an
ectoparasite.
Symptoms of the disease include powdery white lesions on leaf blades which results in a
reduction in photosynthesis and, in severe epidemics, similar lesions may also be
observed on
other organs, i.e., petioles, stems, and sepals, but not on fruits. In severe outbreaks, the
lesions
coalesce and the disease is debilitating. This disease is common in glasshouse tomatoes
worldwide, and its importance on field-grown tomato crops is increasing.
In Israel, O. neolycopersici epidemics occur in unheated greenhouses and net houses in
the spring, summer, and early autumn. The common methods for tomato powdery
mildew
control are repeated applications of chemical fungicides. The public attitude and
environmental concerns towards the use of pesticides, as well as the ongoing
development of
powdery mildew strains resistant to different fungicides have reduced the appeal of
chemicals.
All these constraints have led to the search for alternative methods to disease suppression,
such
as natural products, cultural practices, and biological control agents.
The objectives of this study were to examine the effects of different biotic and a-biotic
factors on different components of the disease cycle (germination, appressoria formation,
conidiation and survival) in tomato plants and to determine which environmental
conditions
may limit the progress of an epidemic and to examine natural products together with
common
means that are used against the disease.
Under controlled conditions, the highest rates of conidial germination were observed at
25±1ºC, 99±1% RH, and minimal light, and was the lowest on leaves adjacent to fruits as
compared with leaves located at other locations. Conidia survived and remained capable
of
germination for over four months when initially incubated at lower temperatures and
higher
RH, as compared with their fast decline under more extreme summer shade conditions.
Optimal conditions for appressoria formation were 25±1ºC, RH ranging from 33 to 99%,
and
1,750 lux light intensity. More conidia were formed at 20°C, 70 to 85% RH, and 5150
lux
light intensity than at 16 and 26±1°C, 99±1% RH, and 480-1750 lux, respectively. In
growth
chamber experiments, disease did not develop at 28±1ºC. Within the range of 70 to 99%
RH,
disease was less severe under the higher RH than the drier conditions. Disease was also
less
severe at lower light intensities.
Data collected in three commercial-like greenhouse experiments carried out during the
fall of 2005 and the spring of 2006 and involving various climate regimes were used to
draw
correlations regarding the effects of temperature and RH on the development of
epidemics. A
positive correlation was observed between disease severity and the length of time that
environmental conditions ranged between 15-25ºC and 60-90% RH. A negative
correlation
was observed between higher temperatures and lower RH levels.
Control of the pathogen was tested for chemical fungicides, products based on plant
extracts, biological control agents or the combination between these methods. Most All of
the
commercial control products, except Serenade (i.e, Heliogofrit, Amistar, Signum etc.)
and
those which are based on plant extracts (Neemgard, Timorex etc.) effectively suppressed
the
disease, both when applied at weekly intervals or as single spray before the inoculation.
In summary, precise microclimatic conditions that affect the pathogen were identified.
Combining high temperatures and low relative humidity in the greenhouse with
alternated
sprays and friendly compounds is expected to effectively control tomato powdery
mildew.
This treatment is expected to be associated with reduction in chemical fungicides use.
This research was supported by SafeCrop Center and funded by Fondo per la Ricerca,
Autonomous Province of Trento, the Chief Scientist of the Israeli Ministry of Agriculture
and
the Israeli Board of Vegetable Growers.