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Kaori Itagaki, Toshio Shibuya, Motoaki Tojo, Ryosuke Endo, and Yoshiaki Kitaya

The development of powdery mildew fungus (Podosphaera xanthii) is suppressed on cucumber (Cucumis sativus L.) seedlings acclimatized to higher red-to-far-red ratio (R:FR) than natural R:FR (≈1.2), but its early development and any limiting factors are still unclear. The present study evaluated conidial germination, initial invasion, and subsequent development of P. xanthii on cucumber seedlings raised under light-emitting diode (LED) lights with R:FRs of 1.2, 5.0, or 10. There were no differences in conidial germination or initial invasion between the treatments, so there was no effect of acclimatization to R:FR on either. But, the development of hyphae, hyphal cells, and haustoria after inoculation were suppressed on seedlings acclimatized to higher R:FR. Because differences occurred only after the initial invasion, nonstructural properties of the host leaves may have affected conidial development. Higher R:FR also suppressed conidial development under natural light filtered through a photo-selective film, which absorbs near-infrared (NIR)-light. However, this effect was reduced when the plants were moved to natural R:FR after inoculation, possibly because of reacclimatization of the seedlings.

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Kaori Itagaki, Toshio Shibuya, Motoaki Tojo, Ryosuke Endo, and Yoshiaki Kitaya

The present study evaluated the development of powdery mildew fungus (Podosphaera xanthii) on leaves of cucumber (Cucumis sativus L.) acclimatized to different CO2 concentrations ([CO2]) to examine plant–pathogen interactions under the wide range of [CO2] that can occur in greenhouse cultivation. Seedlings of resistant and nonresistant cultivars were acclimatized to reduced (200 µmol·mol−1), ambient (400 µmol·mol−1), or elevated (1000 µmol·mol−1) [CO2]. Powdery mildew spores were inoculated onto the adaxial surface of cotyledons or first true leaves, and colonization was measured after 7 days. Colony density decreased as acclimatization [CO2] increased at the cotyledon stage but increased at the first-true-leaf stage in both cultivars. This result implies that when the effects of [CO2] on plant–pathogen interactions are described, growing stage must be specified. The acclimatization [CO2] was correlated positively with leaf mass per area, dry matter content, and carbon (C) content and negatively with nitrogen (N) content at both stages. Therefore, these leaf properties could not explain the changes in host-plant susceptibility between stages. The effect of acclimatization [CO2] was greater on the resistant cultivar than on the nonresistant cultivar, indicating that the resistant cultivar was more responsive.

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Toshio Shibuya, Kaori Itagaki, Motoaki Tojo, Ryosuke Endo, and Yoshiaki Kitaya

We investigated the effects of fluorescent illumination with a high red-to-far-red ratio (R:FR) on the resistance of cucumber (Cucumis sativus) seedlings to powdery mildew fungus (Sphaerotheca cucurbitae; PM). Seedlings were grown at a photosynthetic photon flux (PPF) of 300 μmol·m−2·s−1 provided by fluorescent lamps with high R:FR light (R:FR = 7.0; FLH) or low R:FR light (R:FR = 1.1; FLL) until cotyledons or the first foliage leaf were fully expanded. Spores of PM were then inoculated onto the leaves, and the seedlings were grown for 7 days (from cotyledon stage) or 9 days (from foliage–leaf stage) under FLH. The number of PM colonies on FLH seedlings was 0.80× (cotyledons) and 0.62× (foliage leaves) the number on FLL seedlings. The reduction on the FLH seedlings was probably the result of changes in leaf morphological characteristics such as a thicker epidermal tissue as a result of the higher R:FR illumination. The number of PM colonies on cotyledons of the FLH seedlings was also smaller than that on seedlings grown under metal-halide lamps providing a spectrum similar to that of natural light (R:FR = 1.2).