Introduction
The Cucurbitaceae family, which includes cucumber (Cucumis sativus L.), melon (Cucumis melo L.), watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai), pumpkin, and squash (both genus Cucurbita) are taxonomically divided into 96 genera, including about 1000 species (Renner and Schaefer, 2016). Cucurbits are used as food worldwide. However, cucurbit plants are subject to damage that can further lead to several diseases (Agrios, 2005).
Powdery mildew caused mainly by Podosphaera xanthii is a major, worldwide disease of greenhouse vegetable crops, particularly cucumber (Eskandari et al. 2018; Pérez-García et al., 2009; Rur et al., 2018). Common control strategies applied for cucumber powdery mildew disease mainly involve mainly the use of chemical fungicides. However, over time, these strategies have led to the development of resistance towards some chemicals (Fungicide resistance
action committee, 2018; Ishii et al., 2001; Lebeda et al., 2010; McGrath and Wyenandt, 2017; Miyamoto et al., 2010). In fact, field resistance of cucumber powdery mildew to demethylation (DMI), succinate dehydrogenase (SDHI), and Quinone (QoI) inhibitor fungicides was reported in Japan (Ishii et al., 2001; Miyamoto et al., 2010).
Conversely, development of resistance to microbial fungicides has not been reported. Therefore, it is necessary to examine a microbial collection with wide biological diversity to find new microorganisms with potential for control of plant diseases. A particularly wide variety of biologically active secondary metabolites is produced by actinomycetes of the genus Streptomyces. Some members of genus Streptomyces are well known to produce antimicrobial compounds that play an important role in plant protection against pathogens (Kim et al., 2011; Lee et al., 2005; Park et al., 2006; Zhang and Yuen, 2000).
In a previous study, we suggested that Streptomyces
J. JSATM 26（2）：61－68, 2019
Original Paper
Biological Control of Powdery Mildew Caused by Podosphaera xanthii in Cucumber by
Streptomyces blastmyceticus Strain STS1 Isolated in Shimane Prefecture
Rattrikorn GANPHUNG1,2, Junichi KIHARA1 and Makoto UENO1*
1Faculty of Life and Environment Science, Shimane University, Matsue 1060, Shimane 690-8504, Japan
2The United Graduate School of Agriculture Science, Tottori University, Koyama-Minami 4-101, Tottori 680-8553, Japan
Abstract
Powdery mildew, one of the most important diseases of worldwide vegetable crops under field and greenhouse conditions, is caused mainly by Podosphaera xanthii. Common control strategies applied for powdery mildew disease mainly involve mainly the use of chemical fungicides. However, over time, this has led to the development of resistance towards some chemicals. We isolated strain STS1 of Streptomyces blastmyceticus from a contaminated potato-sucrose-agar culture exposed in the open field in Shimane Prefecture and identified the antifungal activity of strain STS1 against anthracnose. In laboratory experiments, when cucumber leaves were sprayed suspension culture of strain STS1 by using spray, and after 24 h inoculated with P. xanthii, powdery mildew caused by P. xanthii was effectively suppressed by strain STS1 in pretreated cucumber plants. Furthermore,
P. xanthii conidia germination on cucumber leaves was inhibited by pretreatment with a suspension culture of strain STS1. In
diseased leaves treated with the same strain STS1 suspension-culture, mycelia of strain STS1 enveloped conidia of P. xanthii causing them to burst. Successful growth of strain STS1 on LB agar medium was observed at 20°C, 28°C and 37°C, but not at 4°C. In greenhouse experiment, there was a significant difference in disease development between control and strain STS1-treated cucumber plants. Lesion formation of powdery mildew caused by P. xanthii appeared on control cucumber leaves 15 days after transplanting. Furthermore, lesion formation in control cucumber plants increased at 60 days after transplanting. On the other hand, lesion formation in cucumber plants treated with strain STS1 was significantly suppressed. Taken together, our results suggest that strain STS1 might contribute to the development of a new effective agent for biocontrol of powdery mildew in greenhouse-cultivated cucumber.
Key Word : antifungal activity, biological control, cucumber, Podosphaera xanthii, Streptomyces blastmyceticus,
Journal of Japanese Society of Agricultural Technology Management Vol.26 No.2, 2019 blastmyceticus strain STS1 isolated from the field of
Shimane Prefecture can control cucumber anthracnose caused by Colletotrichum orbiculare (Ueno et al., 2012). Further, we found that strain STS1 in the growth media inhibited the mycelial growth of plant pathogenic fungi (Corynespora cassiicola, Monosporascus cannonballus and Stagonosporopsis cucurbitacearum) infecting cucurbits (Ganphung et al. 2018). However, the inhibitory effects of strain STS1 against powdery mildew caused by P. xanthii in cucurbits have not yet become clear elucidated.
In this study, we examined the potential of strain STS1 for the biological control of P. xanthii, the causal agent of powdery mildew, in cucumber plants cultivated in a greenhouse.
Materials and Method Microorganism for biocontrol
S. blastmyceticus strain STS1 was isolated from an open field in Matsue, Shimane Prefecture, Japan by the method described previously (Ueno et al. 2012). Strain STS1 was suspended in 15-20% glycerol solution and stored at -80°C until use. A single colony of strain STS1 was grown on Luria-Bertani (LB) liquid medium (10 g Bactotryptone, 10 g NaCl, 5 g Yeast extract and 1 L distilled water), and used to individually inoculate test tubes containing 20 mL LB liquid medium. The suspension cultures were incubated at 25-27°C for 7 days with constant shaking on a rotary shaker (MMS-3010, Tokyo rikakikai, Tokyo, Japan) at 130 rpm.
Plants and pathogen
Cucumber plants of cultivar ‘Hokushin’ (Takii & Co., Ltd., Kyoto, Japan) were grown under standard cultural practices in greenhouse. Cucumber seeds were sown in plastic pots (9 cm in diameter), containing commercial garden soil (Kumiai Nippi Engeibaido No. 1. Nihon Hiryo Co., Ltd. Tokyo, Japan). Seedlings at the 1-leaf stage were used in inoculation and greenhouse experiments. P. xanthii is obligate biotrophic parasites and unable to grow on culture media. Therefor P.
xanthii was collected from naturally infected cucumber
leaves grown in the greenhouse and conidia were inoculated on healthy cucumber intact leaves every 10-20 days, and maintained at 20-25°C under a 12/12 h light/dark regime in small glass greenhouse with temperature controller (FHB-1508S, PiCa, Osaka, Japan). The synchronously formed conidia were used as inoculum to infect cucumber leaves (Chen et al., 2014; Parada et al., 2012).
Strain STS１ pretreatment and inoculation with P. xanthii The whole cucumber seedlings at the 1-leaf stage were pretreated once with strain STS1 suspension-culture by using spray (1mL / cucumber seedling). LB liquid medium
was used as control treatment. Pretreated cucumber plants were maintained at natural conditions in the laboratory for a designated time, and then strain STS1 suspension-culture-pretreated leaves were inoculated with the conidia suspension
of P. xanthii (1 × 104 conidia/ml). Number of lesion formation
or disease index was observed at the designated time. Disease index was calculated as [Disease index (%) = {(4A
+ 3B + 2C + D) / (total plant leaves investigated × 4)} ×
100], A = number of leaves with and infected area > 50%, B = number of leaves with an infected area > 20%, C = number of leaves with an infected area > 10%, D = number of leaves with an infected area > 0.5%. After observation, leaves were cut into small disks and immersed in lactophenol-alcohol (1:2 v/v) until the chlorophyll was completely removed for 3 days. Discolored leaves were stained with 1% cotton blue in lactophenol for 1 h and then infection behaviors were monitored under a light microscope (BX50, Olympus, Tokyo, Japan). The experiments were repeated three times independently.
Strain STS１ treatment of cucumber leaves
The whole cucumber seedlings at the 1-leaf stage were
inoculated with a conidia suspension of P. xanthii (1 × 104
conidia/ml) using the method described above. The inoculated cucumber plants were maintained at natural conditions in the laboratory for 10 days, and then the suspension culture of the strain STS1 was treated on P. xanthii inoculated cucumber leaves. LB liquid medium was used as a control. Inoculated cucumber plants were maintained at natural conditions in the laboratory. The growth of the mycelium of strain STS1 on cucumber leaves was observed by scanning electron microscopy (Microscope TM3000, Hitachi, Tokyo, Japan) 2 days after inoculation. The experiments were repeated three times independently.
Temperature stability of strain STS1
To investigate its temperature stability, strain STS1 was incubated at different temperatures. The suspension culture of strain STS1 (30 µL) was spread on the surface of the LB agar medium in the petri dishes (9 cm) using the method described above, and then incubated at 4°C, 20°C, 28°C and 37°C. After 2 days, the number of colonies of strain STS1 in the petri dish was estimated.
Greenhouse experiment
The greenhouse experiment was carried out in Shimane University between September 2016 and November 2016.
Cucumber plants were grown in a similar manner using all normal cultural practices. Cucumber seeds were sown in plastic pots (9 cm in diameter), and seedlings at 1-leaf stage were transplanted onto 10-L pots (30 cm diameter) filled with commercial garden soil (Super Mix A; 1 L contain: N
Strain STS1 suppress P. xanthii
= 150-260 mg, P = 70-160 mg and K = 190-320 mg; Sakata Seed Co., Yokohama, Japan), and transferred to a greenhouse without temperature controller, where they were arranged in two rows with five pots in each row. Whole leaves of cucumber plants were treated with suspension culture of strain STS1 by using spray (1mL per each cucumber leaf) at every five days. Control cucumber plants were treated with LB liquid medium. Lesion formation on main stems and lateral shoots was investigated at 15, 20, 30 and 60 days after transplanting.
Data analysis
Data are reported as means ± standard deviation (SD). Significant differences of data were determined by t-test
(P < 0.05) or Tukey-Kramer test (P < 0.05) using statistical package for the social sciences (IBM SPSS version 22.0).
Results
Inhibition of powdery mildew in cucumber plants by pretreatment with a suspension culture of strain STS1
To determine the inhibitory activity of strain STS1-pretreatment against P. xanthii, cucmber leaves were pretreated with strain STS1 and subsequently inoculated with
P. xanthii conidia 24 h after pretreatment. Lesion formation
was significantly inhibited in leaves pretreated with strain STS1 compared with control leaves pretreated with LB liquid medium (Fig. 1A). Disease index of pretreated and control plants was 0 % and 100%, respectively (Fig 1B). In addition, we investigated the germination of conidia and conidiation of P. xanthii in cucumber plants pretreated with strain STS1. Conidia germination and conidiation by
P. xanthii was significantly inhibited in cucumber leaves
subjected to a 24 h pretreatment with strain STS1, but not in control leaves (Fig. 2).
Time course of the inhibitory activity of strain STS1 on powdery mildew-lesion formation in cucumber plants
Next, we investigated the effect of treatment duration on the inhibitory activity of the strain STS1. In the control treatment, the number of lesions formed per leaf was 98.9 ± 19.6 (Fig. 3). The number of lesions formed per leaf after treatment with strain STS1 for 1, 3, 5, 7 and 9 days, were 14.9 ± 3.6, 15.5 ± 2.9, 2.8 ± 2.9, 34.6 ± 6.4, and 48.0 ± 7.9, 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 STS1 Control
Fig. 1 Effects of pretreatment with strain STS1 on powdery mildew lesion-development in cucumber plants. The whole cucumber seedlings at the 1-leaf stage were pretreated once with strain STS1 suspension-culture or LB liquid medium (control) by using spray (1mL / cucumber seedling) and then kept at natural conditions in the laboratory for 24 h. Plants were then inoculated with suspension of Podosphaera xanthii (1 × 104 conidia/ml). Disease
incidence was investigated at 7 days after inoculation (A). [Disease index (%) = {(4A + 3B + 2C + D) / (total plant leaves investigated × 4)} × 100], A = number of leaves with and infected area > 50%, B = number of leaves with an infected area > 20%, C = number of leaves with an infected area > 10%, D = number of leaves with an infected area > 0.5%. (B). Three independent experiments were conducted, each with 10 plants. Bars represent means ± SD. Asterisk indicates significant difference (t-test, P < 0.05).
Figure 1 Rattrikorn Ganphung 縮尺率2/3 D ise as e inde x (% ) 100 90 80 70 60 50 40 30 20 10 0 Control STS1 A B *
Fig. 1 Effects of pretreatment with strain STS1 on powdery mildew lesion-development in cucumber plants. The whole cucumber seedlings at the 1-leaf stage were pretreated once with strain STS1 suspension-culture or LB liquid medium (control) by using spray (1mL / cucumber seedling) and then kept at natural condi-tions in the laboratory for 24 h. Plants were then in-oculated with suspension of Podosphaera xanthii (1
× 104 conidia/ml). Disease incidence was investigated
at 7 days after inoculation (A). [Disease index (%) = {(4A + 3B + 2C + D) / (total plant leaves investigated × 4)} × 100], A = number of leaves with and infected area > 50%, B = number of leaves with an infected area > 20%, C = number of leaves with an infected area > 10%, D = number of leaves with an infected area > 0.5%. (B). Three independent experiments were conducted, each with 10 plants. Bars represent means ± SD. Asterisk indicates significant difference (t-test, P < 0.05). 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 C C DD E E FF
Fig. 2 Effects of pretreatment with strain STS1 on conidia germination and conidiation by
P. xanthii in the cucumber leaves. The whole cucumber seedlings at the 1-leaf stage
were pretreated once with strain STS1 suspension-culture or LB liquid medium (control) by using spray (1mL / cucumber seedling) and then kept at natural conditions in the laboratory for 24 h. Plants were then inoculated with a suspension of P. xanthii (1 × 104 conidia /ml). After 7 days, leaves were cut into small disks and
immersed in lactophenol-alcohol (1:2 v/v) until the chlorophyll was completely removed for 3 days. Discolored leaves were stained with 1% cotton blue in lactophenol for 1 h. Infection behaviors were monitored under a light microscope. C: Conidia
Figure 2 Rattrikorn Ganphung. 縮尺率2/3
60µm 60µm
Control STS1
C
C
Fig. 2 Effects of pretreatment with strain STS1 on conidia germination and conidiation by P. xanthii in the cucumber leaves. The whole cucumber seedlings at the 1-leaf stage were pretreated once with strain STS1 suspension-culture or LB liquid medium (control) by using spray (1mL / cucumber seedling) and then kept at natural conditions in the laboratory for 24 h. Plants were then inoculated with a suspension of P. xanthii
(1 × 104 conidia /ml). After 7 days, leaves were cut
into small disks and immersed in lactophenol-alco-hol (1:2 v/v) until the chlorophyll was completely removed for 3 days. Discolored leaves were stained with 1% cotton blue in lactophenol for 1 h. Infection behaviors were monitored under a light microscope. C: Conidia
Journal of Japanese Society of Agricultural Technology Management Vol.26 No.2, 2019
respectively (Fig. 3).
Inhibition of powdery mildew in cucumber plants by treatment with suspension culture of strain STS1
To investigate the healing effect of treatment with a suspension culture of strain STS1 on powdery
mildew-diseased cucumber plants, cucumber leaves were inoculated with P. xanthii and subsequently treated these leaves with the suspension culture of strain STS1 for 2 days. The results showed that mycelia of strain STS1 enveloped conidia of P.
xanthii and caused to burst (Fig. 4).
Effect of strain STS1 on natural infection of powdery mildew in greenhouse cucumber plants
The effect of strain STS1 on development of powdery mildew caused by P. xanthii in greenhouse cucumber plants was investigated. As shown in Fig. 5, there was a significant difference in disease development between control and strain STS1-treated cucumber plants. Lesion formation of powdery mildew caused by P. xanthii appeared on control cucumber leaves 15 days after transplanting. However, no change in the increase of lesion formation was observed at 20 and 30 days after transplanting. And then lesion formation in control cucumber plants increased at 60 days after transplanting. On the other hand, lesion formation in cucumber plants treated with strain STS1 was significantly suppressed against control.
Effect of temperature on the growth of strain STS1. To investigate its temperature stability, strain STS1 cultured on LB agar medium in petri dishes was incubated at 4°C, 20°C, 28°C and 37°C. Successful growth was observed at 20°C, 28°C, and 37°C, but not at 4°C (Table 1). The number of colonies was largest at 28°C, 199.4 ± 45.3, followed by 20°C and 37°C, 142.1 ± 43.0 and 91.0 ± 27.9, respectively. But none of colonies was formed at 4°C. 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Figure 3 Rattrikorn Ganphung 縮尺率2/3 140 120 100 80 60 40 20 0 N um be r o f le sion fo rm atio n on le af Control 1 3 5 7 9 Fig.3 Time course of the inhibitory activity of strain STS1 on lesion formation of powdery
mildew in cucumber plants. The whole cucumber seedlings at the 1-leaf stage were inoculated with a P. xanthii conidia suspension (1 × 104 conidia/ml) after pretreatment
with strain STS1 suspension-culture at 1, 3, 5, 7 and 9 days. Control plants were pretreated with LB (Control) liquid medium. Ten days after inoculation with P. xanthii, the number of powdery lesions formed was recorded. Experiments were repeated three times and five cucumber leaves were examined each experiment. Bars represent means ± SD. Means followed by different letters indicate significant difference (Tukey-Kramer test, P < 0.05).
a
d d
e
c b
Days after strain STS1 pretreatment
Fig. 3 Time course of the inhibitory activity of strain STS1 on lesion formation of powdery mildew in cucumber plants. The whole cucumber seedlings at the 1-leaf stage were inoculated with a P. xanthii conidia
sus-pension (1 × 104 conidia/ml) after pretreatment with
strain STS1 suspension-culture at 1, 3, 5, 7 and 9 days. Control plants were pretreated with LB (Control) liquid medium. Ten days after inoculation with P.
xanthii, the number of powdery lesions formed was
recorded. Experiments were repeated three times and five cucumber leaves were examined each experi-ment. Bars represent means ± SD. Means followed by different letters indicate significant difference (Tuk-ey-Kramer test, P < 0.05). 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Control STS1
Figure 4 Rattrikorn Ganphung 縮尺率2/3
Fig. 4 Effects of strain STS1 treatment on the inhibition to powdery mildew in cucumber plants. The whole cucumber seedlings at the 1-leaf stage were pre-inoculated with a suspension of P. xanthii (1 × 104 conidia/ml). Ten days
after inoculation, the leaves were treated with strain STS1 or LB liquid medium (control) and then kept at natural conditions in the laboratory. Two days after treatment, mycelia growth of strain STS1 on the cucumber leaves was observed by scanning electron microscopy. C: Conidia of P. xanthii, M: Mycelia of strain STS1 C C M 20µm 20µm
Fig. 4 Effects of strain STS1 treatment on the inhibition to powdery mildew in cucumber plants. The whole cucumber seedlings at the 1-leaf stage were
pre-in-oculated with a suspension of P. xanthii (1 × 104
conidia/ml). Ten days after inoculation, the leaves were treated with strain STS1 or LB liquid medium (control) and then kept at natural conditions in the laboratory. Two days after treatment, mycelia growth of strain STS1 on the cucumber leaves was observed by scanning electron microscopy. C: Conidia of P.
xanthii, M: Mycelia of strain STS1
18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Figure 5 Rattrikorn Ganphung 縮尺率2/3
Fig. 5 Effects of strain STS1 on natural powdery mildew infection in the greenhouse cucumber. Cucumber plants were treated with a suspension culture of strain STS1 at every five days. As a control, cucumber plants were treated with LB liquid medium. Lesion formation was observed at 15 and 60 days after transplanting. 15days
60 days
Control STS1
Fig. 5 Effects of strain STS1 on natural powdery mildew infection in the greenhouse cucumber. Cucumber plants were treated with a suspension culture of strain STS1 at every five days. As a control, cucumber plants were treated with LB liquid medium. Lesion formation was observed at 15 and 60 days after trans-planting.
Strain STS1 suppress P. xanthii
general, inhibition of conidia germination could be due to proteolytic functions, such as chitinase and β-1,3-glucanase, that mediate cell-wall and cell-membrane damage in plant pathogens (Fridlender et al., 1993; Zhang and Yuen, 2000), or to low-molecular-weight organic compounds (Kim et al., 2011; Lee et al., 2005; Park et al., 2006). Recently, we found that strain STS1 can produce heat-unstable compounds in culture filtrate (Nguyen et al., 2016). In addition, strain STS1 exhibited chitinase production on the media (data not shown). It was reported that expression of chitinase gene in grapevines and wheat exhibited resistance against Powdery mildew (Nirala et al., 2010; Yamamoto et al., 2000). These results suggest that chitinase production by strain STS1 is one of the important factors for control of powdery mildew in cucumber.
On the other hand, members of the genus Streptomyces are known as potential biocontrol agents due to their inhibitory ability against various plant pathogens (Shimizu et al., 2000; Shimizu et al., 2009; Kim et al., 2011). However, inhibitory compounds that reportedly provide this protection have not yet been identified in strain STS1. Furthermore, we investigated that infection behaviors of some cucumber diseases (C. cassiicola, C. orbiculare, and P. xanthii) were inhibited by culture filtrate of strain STS1 (data not shown). Thus, further studies are required to identify the active inhibitory compounds present in the culture filtrate of strain STS1.
The maximum and minimum average temperatures in Shimane Prefecture are 35-36°C and 4-5°C, respectively. In our experiment, the growth of strain STS1 was observed at between 20°C to 37°C on the growth medium (Table 1). Furthermore, the effect of strain STS1 on development of powdery mildew caused by P. xanthii in cucumber plants was investigated in greenhouse plants. Thus, growth of strain STS1 was observed in the presence of fungicides against powdery mildew (data not shown). In the experiments reported herein, lesion formation in cucumber plants treated with strain STS1 was suppressed. Generally, the optimum temperature for growth of cucumber is 25-30°C. Also, the conidial germination of P. xanthii is observed in temperature of 20–30°C. Additionally, strain STS1 could be grown at 37°C. These results suggested that the strain STS1 might be able to control powdery mildew caused by
P. xanthii in greenhouse-grown cucumber plants. However,
suppression by strain STS1 of other pathogenic fungi and bacteria infecting cucurbits in greenhouse-grown cucumber plants has not yet been clearly elucidated. Therefore, further studies are required to investigate the control of multiplex cucumber diseases by strain STS1 in greenhouse-grown
Temperature (°C) Number of colony per Petri dish
4 0Z ay
20 142.1 ± 43.0 b
28 199.4 ± 45.3 c
37 91.0 ± 27.9 d
ZValues represent were mean ± SD
yDifferent letters within a column indicate significant
differences by Tukey-Kramer test at P<0.05.
Table 1 Effect of temperature on growth of strain STS1 on LB agar medium.
Discussion
Control strategies against plant pathogenic fungi mainly involve the use of chemical fungicides whose extensive use has led to development of resistance to these chemicals. Therefore, it is necessary to research for the development of new microbial fungicides, in which case, development of resistance has not been reported.
Previously, we reported that strain STS1 can control cucumber anthracnose caused by C. orbiculare (Ueno et al., 2012). And strain STS1 can inhibit the mycelial growth of pathogenic fungi infecting cucurbits on the growing medium (Ganphung et al. 2018). In this study, we found that powdery mildew lesion formation was significantly suppressed on cucumber leaves pretreated with strain STS1 (Fig. 1). Similarly, conidia germination of powdery mildew on cucumber leaves pretreated with strain STS1 was also inhibited (Fig. 2). Further, powdery mildew caused by
P. xanthii was suppressed in cucumber leaves pretreated
by spraying with suspension culture of strain STS1 for 1 to 9 days (Fig. 3). It is well known that Bacillus-based biological control agent such as Impression Clear (Bacillus
amyloliquefaciens, SDS Biotech K.K.) and Botokilar®
(Bacillus subtilis, Idemitsu Kosan Co., LTD.) suppress powdery mildew. However, Streptomyces-based biological control agent against powdery mildew has not been development. Importantly, the suspension of strain STS1 did not show a phytotoxic effect on cucumber leaves. However, the phytotoxic effects on cucumber flower and fruit have not yet been investigated in strain STS1. Thus, further studies are required to investigate the phytotoxic effect on cucumber flower and fruit used in the culture suspension of strain STS1. These results suggest that powdery mildew in cucumber plants can be suppressed by strain STS1 without exerting toxic effects on cucumber leaf.
We observed that mycelia of strain STS1 adhered to conidia of P. xanthii causing them to burst (Fig. 4). In
Journal of Japanese Society of Agricultural Technology Management Vol.26 No.2, 2019
cucumber plants.
In conclusion, Streptomyces blastmyceticus strain STS1 may become of great use as a biological control agent and a promising source for the development of a new microbial fungicide to prevent cucumber diseases, such as powdery mildew caused by P. xanthii in cucurbitaceous crops.
Acknowledgments
The authors gratefully acknowledge the faculty of Life and Environmental Science in Shimane University for financial support to publish this report.
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Journal of Japanese Society of Agricultural Technology Management Vol.26 No.2, 2019
