Change in Panicle Susceptibility Associated with Flowering Rate of Spikelets in Bacterial Grain Rot of Rice Caused by Pseudomonas glumae.

Abstract

Change in susceptibility of rice spikelets (cvs. Koshihikari, Koganebare and Asominori) to bacterial grain rot, caused by Pseudomonas glumae, with time was examined by spraying bacterial suspension to panicles at different stages of flowering. Percentage of diseased spikelets of these varieties was highest when they were inoculated on flowering day and remained high when they were inoculated within 3 days after flowering. The disease was very low when spikelets were inoculated 2 days before or 4 days after flowering, even though spikelets were incubated under high humidity (RH > 95%) condition for 24 hr after inoculation. These results indicate that susceptibility of rice spikelets changes with time during a short, critical period before and after flowering and that high humidity at this stage is crucial to the infection of spikelets. Analysis on the relationship between susceptibility of a whole panicle and the flowering rate of spikelets within a panicle showed that panicle susceptibility depends on the rate of flowering per day after heading. This correlation indicates that susceptibility of rice plants to the disease in the field may be estimated from the heading rate of panicle in the field and from the extent of susceptibility of each panicle per day. The relative value for cumulative susceptibility of rice plants (T_t) in the field on the day t after first heading was represented by the value for total susceptibility of all panicles on the day t which was calculated by daily heading rate of panicles on days 0 to t after first heading in the field and by the panicle susceptibility on each day after heading. The T_t value of rice plants increased similarly from heading time, which is defined as the day when more than 40% of panicles have headed, and then decreased 5 to 6 days after heading time. T_t was very low about 12 days after heading time, suggesting that rice plants in the field are susceptible to the disease during a short length of time, i.e., from the heading time to about 11 days after that. イネもみ枯細菌病に対するイネの感受性の経時的な変化を3品種 (コシヒカリ, 黄金晴, あそみのり) を供試して検討した. 開花前後の籾に細菌懸濁液を噴霧接種し, 24時間湿室に保持後温室に移して発病を調査した. その結果, 籾の感受性は開花日に最も高く, ついで開花後3日間は高く経過したが, 開花後4日以降と開花前の籾では顕著に低かった. 開花前に接種した籾では, 開花日に再度相対湿度95%以上の条件下に置かれた時に激しい発病が見られ, 感受性の高い期間の高湿度条件が発病に重要であることが示唆された. 一穂での開花頻度 (観察値) と籾の日別発病度 (実験値) から, 穂の日別感受性値 (相対的累積感受性値, TP)を求め, 試験で得られた穂の日別発病度 (実験値) と比較したところ, 両者における感受性の経時的変化はほぼ一致した. このことから, 穂の感受性は穂を構成する籾の開花頻度に大きく依存していることが示唆された. そこで, 穂の日別発病度 (実験値) と本田での出穂頻度 (観察値) から本田のイネ群落での感受性を推定した結果, 得られた値 (相対的累積感受性値, T_t) は3品種ともに出穂期 (出穂率40%以上の日) 後急速に増加し, 同4〜5日後に最も高く, 同12日後には著しく低下した. 以上から, 本病に対するイネ群落の感受性の高い期間が出穂期から11日間程度ときわめて短いことが示された.