Table 1.
Origin and characteristics of 26 isolates of Pseudoperonospora cubensis used in this study.
Fig 1.
The inhibitory effect of oxathiapiprolin on cystospore germination of P. cubensis.
A-D In vitro; E-H in vivo. A, C—Fluorescent micrographs (with or without bright light) showing cystospore germination in water. B, D—No cystospores germination occurred in oxathiapiprolin of 0.0001 mg/l. Bars in A, B = 50 μm; in C, D = 10 μm. E—Scanning electron micrographs showing a germinating cystospore a control leaf. F—inhibitory effect of oxathiapiprolin on cystospore germination on a treated leaf. G—Fluorescent micrographs showing extensive mycelial growth in a control leaf at 1 dpi. Bar = 100 μm. H—Oxathiapiprolin inhibits zoospore discharge on a cucumber leaf surface at 1 dpi. No mycelial growth is seen. Bar = 50 μm.
Fig 2.
Preventive application of oxathiapiprolin inhibits infection with P. cubensis.
A—Detached leaves of cucumber treated with 0.0001–1 mg/l. B—Detached leaves of melon treated with 0.0001–1 mg/l. C—Leaf discs of melon treated 0.0001–1 mg/l. D- Detached leaves of cucumber treated with 0.0001 mg/l and inoculated with three A1 isolates: 19, 42R and BH. E- Detached leaves of cucumber treated with 0.0001 mg/l and inoculated with three A2 isolates: 13, 229P and 98P. Leaf tissues were sprayed and inoculated on lower leaf surface. Photos in A, B and C were taken at 13 dpi with lower surface uppermost; photos in D and E were taken with upper surface uppermost at 10 dpi.
Fig 3.
The effect of a preventive spray with oxathiapiprolin on infection of 2-leaf cucumber plants with P. cubensis.
A—Mean number of lesions per plant at 7dpi. Different letters on bars indicate on significant differences at α = 0.05 (Tukey’s-Kramer HSD-test). B—Disease development at 7 dpi on control and oxathiapirolin treated plants.
Fig 4.
The inhibitory effect of oxathiapiprolin on development of downy mildew in cucumber depends on time of application.
Detached leaves were inoculated on lower leaf surface with sporangial suspension (25 droplets/leaf) and oxathiapiprolin was applied as a spray at 0, 1, 2 or 3dpi. A—No lesions developed when oxathiapiprolin was applied at 0 dpi. Lesions appeared when oxathiapiprolin was applied at 1 or 2dpi, but became smaller as its dose increased. B—Oxathiapiprolin was applied at 3dpi. Note no effect on lesion size. Photos were taken at 6 dpi.
Fig 5.
Preventive and curative effects of oxathiapiprolin on lesion development, lesion expansion and sporagiphore formation of P. cubensis in detached leaves of cucumber.
See legend to Fig 4 for experimental design. A—Lesion size, as measured at 10 dpi. B—Number of sporangiophores produced at 10 dpi per 1 mm2 leaf tissue as counted with the aid of a UV epifluorescent microscope. Different letters on bars indicate on significant differences at α = 0.05 (Tukey’s-Kramer HSD-test).
Fig 6.
Fluorescence micrographs showing the suppression of sporulation of P. cubensis in cucumber leaves treated with oxathiapiprolin at 2 or 3dpi (calcofluor and basic aniline blue staining).
A—Leaves treated with oxathiapiprolin of 0.0001–0.01 mg/l at 2 dpi. B—Leaves treated with oxathiapiprolin of 0.1–10 mg/l at 3 dpi. Photos were taken at 10 dpi. Bar = 50 μm. Note numerous sporangiophores and sporangia in the controls, few or deformed sporangiophores in treated leaves, and heavy callose encasement of haustoria at 10 mg/l.
Fig 7.
The effect of oxathiapiprolin on sporulation of P. cubensis in cucumber.
A—Detached leaves were treated with oxathiapiprolin at 2dpi and examined for sporulation at 7 dpi. B—Detached leaves were treated with oxathiapiprolin at 5dpi and examined for sporulation at 7 dpi. Different letters on bars indicate on a significant difference at α = 0.05 (Tukey’s-Kramer HDS test). C- Sporulation of P. cubensis on an untreated control leaf of cucumber at 7 dpi. D—Encasement with callose of the houstoria of P. cubensis in a cucumber leaf treated with 1 mg/l oxathiapiprolin at 5dpi. Leaf discs were removed at 7 dpi and stained with basic aniline blue for callose (yellow) and calcofluor for cellulose (blue). Images were taken with an epifluorescent microscope under mixed UV-incandescent light illuminations. Bar = 30 μm. Note in C—the dark sporangia and blue sporangiophores, and in D—the callose-encased haustoria (yellow). Bar = 30 μm.
Fig 8.
The effect of oxathiapiprolin on sporulation of P. cubensis in cucumber leaves naturally-infected with downy mildew.
A—In leaves collected from Net-house 1 on April 27, 2015 and May 1, 2015. B—In leaves collected from Net-house 3 on June 9, 2015 and June 14, 2015. Different letters on bars indicate on a significant difference at α = 0.05 (Tukey’s-Kramer HDS test). C—Close up photo showing heavy sporulation in a control leaf. D—Close up photo showing no sporulation in a leaf treated with oxathiapirolin of 0.0001 mg/l on June 9. Photos were taken on June 10, 2015.
Fig 9.
The effect of oxathiapiprolin on sporulation of P. cubensis in nature.
Downy mildew-infected plants in Net-house 3 were sprayed with oxathiapiroplin of 3 mg/l ai at 8 am 11.6.2015, while control plants were left untreated. Leaves were collected (after a dewy night) at 8 am 12.6.2015 and photographed. Note the heavy sporulation in control leaves but no sporulation in treated leaves.
Fig 10.
Translaminar control efficacy of oxathiapirolin against P. cubensis in detached cucumber leaves.
The compound was sprayed at a concentration of 0.0001 mg/l on the lower or the upper leaf surface and leaves were inoculated with P. cubensis on the opposite leaf surface. Disease records were taken at 14 dpi. A—Lesions per leaf (out of 15 droplets inoculated). B—Lesion size, mm. Different letters on bars indicate on significant differences at α = 0.05 (Tukey’s-Kramer HSD-test).
Fig 11.
Translocation of oxathiapirolin from a treated organ to leaves.
A—Leaf 1 (in 1-leaf plants) was treated by spray. B—Leaf 1 (in 1-leaf plants) was treated with 10 droplets of 25 μl each. C—hypocotyl of 2-leaf plants was treated by spray. D—Soil drench in 2 leaf plants. In A and B plants were inoculated after 4 days when leaf 2 has expanded. In C and D plants were inoculate one day after treatment. Photographs were taken at 7dpi.