Chemical and Biological Controls of Balloon Flower Stem Rots Caused by Rhizoctonia solani and Sclerotinia sclerotiorum

Abstract

Division of Agronomy and Herbal Medicine Resources, Gyeongnam National University of Science and Technology, Jinju 660-758,Korea(Received on October 22, 2011; Revised on February 20, 2012; Accepted on February 21, 2012)Stem rots caused by Rhizoctonia solani and Sclerotiniasclerotiorum have been known as devastating diseases inballoon flower plants. Antifungal activities of four fungi-cides, azoxystrobin, polyoxin B, trifloxystrobin andvalidamycin A were evaluated in vitro, showing effectivesuppression with mycelial growth of the fungal isolateson PDA media. Efficacies of the four fungicides werealso demonstrated in stem tissues of balloon flower plantsagainst R. solani and S. sclerotiorum. A commerciallyavailable Bacillus subtilis strain Y1336 was tested interms of antagonistic biological control of stem rotdisease of balloon flower plants. The bacterial strainrevealed its antifungal activities against R. solani and S.sclerotiorum demonstrated by dual culture tests usingpaper discs and two plant pathogenic fungi on PDAmedia, as well as by plant inoculation assay, indicatingthat this antagonistic bacterial strain can be incorporatedinto disease management program for balloon flowerstem rot diseases together with the four chemical fungi-cides. Keywords: antagonistic bacteria, fungicide, Platycodongrandiflorum, stem rot diseasesBalloon flower ( Platycodon grandiflorum A. DC.) has beencultivated as an edible rootcrop and traditional medicinalherb to treat diverse disease symptoms such as inflam-mation (Kim et al., 2006). Continuous cropping of balloonflower more than 3 years provoked drastic increase of soil-borne disease occurrence such as stem rot (personalcommunication). A variety of fungal species includingColleotrichum dematium, Rhizoctonia solani, Sclerotiniasclerotiorum and Septoria platycodonis were identified ascausal agents for balloon flower diseases in South Korea(Korean Society of Plant Pathology, 2009). Species ofCladosporium sp., Fusarium sp., Phyllosticta sp. andStemphylium sp. were not identified so far requiring to beconfirmed. R. solani infection led to disease occurrence in awide range of plant species including monocots and dicots(Jeon et al., 2010). Different R. solani isolates originatedfrom balloon flower plants grown in six locations of SouthKorea, showed differential pathogenicity in relation to theiranastomosis and tissue specificity (Kim et al., 1992).However, stem rot disease of balloon flower plants by S.sclerotiorum was rarely demonstrated.Cultural, chemical and biological controls of soil-bornedisease such as R. solani and S. sclerotiorum have beenapplied for many different crop productions in combinationwith each other (Csinos and Stephenson, 1999; Howell etal., 2000; Vinale et al., 2009; Van Beneden et al., 2010).Large number of fungicides was evaluated and suggestedfor disease control of R. solani and S. sclerotiorum in manycrops. Among those, fungicides like flutolanil, iprodioneand pencycuron were effective to limit mycelial growth ofR. solani (Campion et al., 2003; Csinos and Stephenson,1999). Use of boscalid and vinclozolin were suggested forcontrol of Sclerotinia stem rot in canola and soybean,respectively (Bradley et al., 2006; Mueller et al., 2002).Only azoxystrobin and polyoxin B are allowed to beapplied to balloon flower plants to control leaf spot diseasescaused by Phyllosticta sp. and Septoria sp. in South Korea.In addition, no fungicidal activities including these twofungicides have been determined to control balloon flowerstem rot diseases yet.Pseudomonas aeruginosa strain JS2 and endophyticBacillus sp. strain CY22 isolated from plant growing soiland root interior of balloon flower, respectively, showedin vitro antibiotic activity against several phytopathogenicfungi, Fusarium oxysporum, Phytophthora capsici, Pythiumultimum and R. solani (Cho et al., 2002; Ryu et al., 2000).Treatment of balloon flower seedlings with the strain CY22protected the seedlings against stem rot disease by R. solani(Cho et al., 2003). 2,4-Diacetylphloroglucinol and iturinwere proven as the antifungal compounds produced by theP. aeruginosa and Bacillus sp., respectively (Cho and Yun,