Cucurbit downy mildew caused by
Pseudoperonospora cubensis
is one of the most important
foliar diseases of cucurbits causing significant yield losses in the world. Chemical control is
not always feasible because of the high costs associated with fungicides, their application,
and side effect on human and nature. Systemic acquired resistance (SAR) is known to play an
important role in plant diseases and pest resistance. Benzothiadiazole (BTH) is an effective
in vitro and acts via inducing natural defense mechanisms in plants. It was used in this study
for inducing resistance against downy mildew of cucumber. Seven-day-old seedling
(Superdaminos cultivar) were transferred to potting soil, kept at 25 °C 16 hours photoperiod
and at a relative humidity (RH) of 70%. Seedling at the tree-to four- leaves stage inoculated
with BTH at concentration of 0.01, 0.02, 0.04 mg/ml. for inoculation four holes were
punched in to the soil around the base of seedling in each pot. Different concentrations of
BTH solution were poured in to the holes. Plant were kept in greenhouse condition for two
days. Then three Leaves of each plant were detached and inoculated with pathogen. The
inoculated leaves were kept in plastic box with 22°C and 90 percent relative humidity. For
preparing of
P. cubensis
inoculum leaves with symptoms of disease were collected from
disease nursery and kept in the dark place at 100% RH overnight for producing of sporangia.
Sporangia were collected from the lower side of leaves using a brush and suspended in
sterilized water. The concentration of sporangia adjusted to 5×10
4
sporangia per milliliter
using a hemacytometer. Seven days after inoculation of leaves the disease were evaluated by
scoring the infected leaves using six scores (0, 1, 3, 5, 7, 9) according the number and
extension of lesions on leaves. Results showed that the disease index were 55, 40, 27 in
concentration of 0.01, 0.02, 0.04 mg/ml respectively. Statistically analyses of data showed the
). The b