Masataka Aino

Suppressive effect of potassium silicate on powdery mildew of strawberry in hydroponics

From 1996 to 1997, potassium silicate (SiO2) was tested at 0, 25, 50, and 100 mg l−1 in hydroponics to control powdery mildew. Other elements were added in the usual amounts, and the strawberries were cultivated hydroponically in a greenhouse for 4 months (from October to January). The powdery mildew spread in the control plot, but little mildew developed in the plot with 25 mg l−1 silicate, and none in plots with more than 50 mg l−1 silicate. The suppressive effect lasted for about 4 months on fruits and even longer on leaves. On analysis of mineral content in the leaves, only the silicate content differed markedly between the control and treated plants. Nitrogen, phosphate, potassium, and calcium contents did not differ greatly. The maximum silicate content was about 24 times that of the control, and disease severity decreased significantly when the content was more than 1.5% in the leaves. The hardness of the strawberry leaves, measured by a creep meter, was increased by the silicate treatment.

Suppression of conidial germination and appressorial formation by silicate treatment in powdery mildew of strawberry

The mode of action of soluble silicon against strawberry powdery mildew (Sphaerotheca aphanis var. aphanis) was investigated in four experiments. First, silicon-treated leaves from plants grown with silicate (Si+) and control leaves were excised, inoculated with conidia, and subsequent germination and formation of appressoria in a petri dish was assessed after 24 h. The germination rate was 49.7% on Si+ leaves, and was 67.2% on control leaves (t-test, P < 0.01). Second, we soaked cellulose membranes in various solvents and then placed the membranes on 4% water agar, dusted the membranes with conidia, and examined after 12 h. No difference was apparent between any treatment and the control (distilled water). Third, strawberries growing hydroponically with additional silicon in the medium were inoculated with conidia, and leaves were observed with a scanning electron microscope 1–2 days after inoculation. Germ tubes and secondary hyphae were shorter and had fewer branches on Si+ leaves than on the control. Moreover, penetration appeared to be inhibited. Fourth, the cuticle was separated from leaves from plants grown as in the third experiment, placed on water agar, and dusted with conidia. Germination of conidia, observed with a light microscope, on Si+ leaves was suppressed markedly to 40%–60% of that of the control. These results suggested that soluble silicon induced physiological changes in the cuticle layer after absorption by the plant. In addition, soluble silicate reduced germination of conidia, formation of appressoria, and possibly the penetration of powdery mildew.

Race distribution of Phytophthora sojae on soybean in Hyogo, Japan

Since 1987, Phytophthora root and stem rot of soybean [Glycine max (L.) Merr. cv. Tanbakuro], caused by Phytophthora sojae Kaufman and Gerdemann, has been increasing in the Sasayama, Nishiwaki, and Kasai regions in Hyogo, the most famous soybean (cv. Tanbakuro)-producing areas in Japan. In 2002 to 2004, 51 isolates (one from each field) of P. sojae were recovered from 51 fields in Hyogo. These isolates were tested for virulence on six Japanese differential soybean cultivars used for race determination in Japan, and three additional ones containing four Rps genes used in Indiana, USA. Race E was the most prevalent from 2002 to 2004, followed by races A, C, D, and four new races (proposed as races K, L, M, and N). Interestingly, none of the new races had high virulence on the Japanese differential cultivars, compared with other races in each area. One (race N) was avirulent on all six soybean differentials. There was a difference in race distribution on each of three individual areas; race E seemed to be a major component of the P. sojae population in Sasayama, whereas race A and the new race M were the most prevalent in Nishiwaki and Kasai, respectively. Rps6 (cv. Altona) and Rps1a + Rps7 (cv. Harosoy 63) were infected by 90.2% and 33.3% of all isolates, respectively. However, Rps1d (cv. PI103091) was not susceptible to any of the 51 isolates, nor was cv. Gedenshirazu-1. These two soybean cultivars were considered to be potential sources of resistance to breed new resistant cultivars with the desirable characteristics of cv. Tanbakuro for this region.

Suppressive Potential of Paenibacillus Strains Isolated from the Tomato Phyllosphere against Fusarium Crown and Root Rot of Tomato

The suppressive potentials of Bacillus and Paenibacillus strains isolated from the tomato phyllosphere were investigated to obtain new biocontrol candidates against Fusarium crown and root rot of tomato. The suppressive activities of 20 bacterial strains belonging to these genera were examined using seedlings and potted tomato plants, and two Paenibacillus strains (12HD2 and 42NP7) were selected as biocontrol candidates against the disease. These two strains suppressed the disease in the field experiment. Scanning electron microscopy revealed that the treated bacterial cells colonized the root surface, and when the roots of the seedlings were treated with strain 42NP7 cells, the cell population was maintained on the roots for at least for 4 weeks. Although the bacterial strains had no direct antifungal activity against the causal pathogen in vitro, an increase was observed in the antifungal activities of acetone extracts from tomato roots treated with the cells of both bacterial strains. Furthermore, RT-PCR analysis verified that the expression of defense-related genes was induced in both the roots and leaves of seedlings treated with the bacterial cells. Thus, the root-colonized cells of the two Paenibacillus strains were considered to induce resistance in tomato plants, which resulted in the suppression of the disease.

Phylogenetic analyses of plant-growth-promoting rhizobacteria isolated from tomato, lettuce, and Japanese pepper plants in Hyogo Prefecture, Japan

Approximately 30,000 fluorescent bacterial strains isolated from tomato, lettuce, eggplant, Chinese cabbage, and Japanese pepper plants at seven different locations in Hyogo Prefecture, were screened for plant-growth-promoting (PGP) activity to induce disease resistance against bacterial wilt in tomato. The 37 strains that had higher PGP activity were subjected to molecular phylogenetic analyses using the sequences of the 16S rRNA, gyrB and rpoD genes. Most of the strains were identified as Pseudomonas fluorescens or its close relative, P. putida, while a few strains were grouped with more distantly related bacterial species such as Enterobacter and Stenotrophomonas. The phylogenetic relationships among tomato and lettuce isolates mostly coincided with the source locality and host plants, with a few exceptions. In contrast, isolates from Japanese pepper plants did not form their own cluster but represented several different bacterial species.

Acidic soil conditions suppress zoospore release from zoosporangia in Olpidium virulentus

Lettuce big-vein disease, caused by Mirafiori lettuce big-vein virus and Lettuce big-vein associated virus, is suppressed when the pH of field soil becomes acidic. Therefore, we evaluated the effect of soil pH on the activities of Olpidium virulentus, the vector of the viruses. We found that acidic soil, pH less than 6.0, significantly reduced O. virulentus infection of the root and influenced the detection rate of zoospores released in the surrounding water. We concluded that acidic soil suppresses zoospore release from zoosporangia.

Studies on biological control of bacterial wilt caused by Ralstonia solanacearum using endophytic bacteria

病害虫等の作物に対する生産障害 要因を排除するためには,これまで, 農薬などの化学物質が用いられ,多 大の効果を上げてきた.その反面, これらの化学物質の長期にわたる連 用によって,耕地生態系の単純化や 環境への影響が懸念されるに至って いる.このような背景の中で,化学物質に代わる新たな素 材の開発が世界的に志向され,生物的防除の研究に注目が 注がれるようになった.また,内生細菌は種々の生理活性 物質を産生し,これらの働きによって内生細菌に感染した 植物が病害虫に対して抵抗性になったり,環境ストレスに 対して耐性になったりする.このため,植物への有用形質 付与に内生細菌の利用研究が行われ,また,植物病害に対 して抵抗性を誘導するためのエージェントとして使用する 研究が行われている(Aino et al., 1999; 東山ら,2012). 一方,ナス科青枯病は,細菌の一種である Ralstonia solanacearum によって引き起こされる土壌伝染性病害,とくに 導管病と呼ばれる病害の 1 つである.青枯病菌は寄主植物 の根の傷口から侵入し,導管の中で繁殖して導管閉塞をお こし,トマトを枯死に至らす.地上部の症状は,緑色の状 態で萎凋し枯死する.畑全体に蔓延し,全滅することも希 ではない.トマト青枯病はトマト生産,とくに,栽培が夏 期高温時である抑制栽培で発生が多い(Hayward, 1991). 様々な防除対策が講じられているが効果は不十分である. 近年,生物的防除の成功例が報告されている(Aino et al., 1997; Ariwiyanto et al., 1994; Bustamante et al., 1989; Furuya et al., 1991; Handlman and Stabb, 1996; 原・小野,1991; Phae et al., 1992; Takenaka et al., 2008). 1 内生細菌の分離と有効なエージェントの選抜 兵庫県下で栽培されているトマト,ハクサイ,レタス, ナス,ピーマンの根圏から,約 30,000 菌株の蛍光性シュー ドモナスを分離した.採取した根を流水で 5 分間洗浄後, 根面からは,洗浄した根を殺菌水に浸漬し 30 分間振とう後, 懸濁液を希釈した.根内からは,70%のエチルアルコール に 2 ~ 10 秒間浸清し根の表面を殺菌し,滅菌水でエチルア ルコールを洗浄,乳鉢で磨砕後,滅菌水に懸濁した.それ ぞれの懸濁液を,蛍光性シュードモナス選択培地 P1(Katoh and Itoh, 1983)を用いて分離した.分離株すべてを対象に, トマト青枯病の発病抑制効果について,シードリングバイ オアッセイチャンバー法(Aino et al., 1997)を用いて検定し た.その結果,104 菌株の一次スクリーニング株を得た.そ の中でも青枯病発病抑制効果の高い 2 菌株を選抜すること ができた. 2 種類の蛍光性シュードモナス,Pseudomonas fluorescens FPH9601(P. f. FPH)と FPT9601(P. f. FPT)は,青枯病の 生物的防除エージェントとして利用可能と判断された.両 菌株の分離源は,青枯病の発生している圃場から採取した トマト根内からである.