キュウリうどんこ病菌に及ぼすレシチンの影響 Effect of Soybean Lecithin on Cucumber Powdery Mildew, Sphaerotheca fuliginea, at Various Growth Stages

キュウリうどんこ病菌Sphaerotheca fuligineaを供試し, 本菌の生育過程に及ぼす大豆レシチンの影響を検討した.キュウリ子葉裏面に大豆レシチン2000ppmを処理し, うどんこ病菌分生胞子を接種し, 分生胞子の発芽, 菌糸の伸長, 分生子梗, 分生胞子の形成を経時的に走査電顕により観察した.分生胞子の発芽には著しい抑制はみられなかったが, 無処理区に比し, 発芽管がやや太く, 短くなり, 菌糸の伸長が遅くなる傾向がみられた.もっとも顕著な影響は, 処理区の接種3日目以降の菌糸先端部に現われた.すなわち, 伸長した菌糸の先端部周縁に薄膜が見られ, うなぎ尾状を呈し, 日数の経過とともに膜の部分が拡大することであった.また無処理区に比し, 分生子梗, 分生胞子の形成が遅れ, 数も少なく, とくに分生胞子が分生子鎖から離脱せずに, そのままたれ下がり, 全体的にゼンマイ状を呈するものがみられた.

Lecithin obtained from soybean plants at the time of producing soybean oil controls effectively several powdery mildews and rice blast disease. Since lecithin is a ubiquitous component in living organisms and its safety as a food additive is established, it is expected that this compound may be a safe agricultural fungicide and is applicable to vegetables even during harvest. Therefore, we examined the effect of lecithin on the growth of the cucumber powdery mildew fungus, Sphaerotheca fuliginea with a scanning electron microscope. After spraying 2000 ppm of the lecithin on the lower surfaces of cucumber cotyledons, conidiospores of the pathogen were inoculated on the same place. Spore germination, hyphal elongation, conidiophore formation and sporulation were observed at certain intervals. The lecithin did not inhibit remarkably spore germination, but gave some effects on spore germination, hyphal elongation and sporulation. That is, germ tubes on treated lower surfaces were stunted and became shorter than that of control. Hyphal elongation on treated surfaces was delayed, and conidiophore formation and sporulation were poorer than those on control lower surfaces. It was a striking feature that hyphal tips on the lower surfaces of cucumber cotyledon treated with lecithin became deformed, namely thin membrane-like substance appeared around hyphal tips and expanded with time. More delayed and fewer conidiophores and conidiospores were observed on the lower surface treated with the lecithin than those on the control lower surfaces. In addition, many conidiospores which were unable to release from conidiospore chain were observed on conidiophores, and such conidiospores dangled like osmund from the conidiophores on lower surfaces of cucumber cotyledon treated with the lecithin.