Enkhjargal Baljii, Won-Seok Na, Jin-Won Kim* Control of Root Rot of Sweet Pepper Caused by Phytophthora capsici with Nonionic Surfactants in a Recirculating Hydroponic System Enkhjargal Baljii, Won-Seok Na, Jin-Won Kim* Department of Environmental Horticulture, University of Seoul, Seoul 130-743, Korea Abstract Phytophthora capsici is a common and destructive pathogen of pepper crops, especially zoospores of P. capsici spread from inoculated source plants to healthy pepper plant on the recirculating hydroponic cultural system. In vitro experiments, amending the recirculating nutrient solution with a fungicide (Metalaxyl-M), potassium phosphonate (KH2PO3) and nonionic surfactants (Tween 20 and Triton X-100) which selectively kill zoospores, and were tested and demonstrated for their control capacity against P. capsici root rot of sweet pepper (Capsicum annuum cv. New Wave). Sweet peppers were grown in a mini hydroponic system in a greenhouse. Four weeks after the plants were inoculated with a zoospore suspension of P. capsici, Metalaxyl-M at 0.025 and 0.05 µl/ml treatments showed 65.8% and 77.2% of control value, respectively. KH2PO3 at 100 and 200 ppm treatments showed 14.3% and 28.6% of control value, respectively. Tween 20 at 500, 1000, 1500, 2000 and 3000 µl/ml treatments showed 0%, 20%, 46%, 88.6% and 94.3% of control value, respectively. Triton X-100 at 250, 500, 1000, 1500 and 2000 µl/ml treatments all showed 100% of control value. The nonionic surfactants had a direct lytic effect on the zoospores in vitro. The results of this research indicate that nonionic surfactant was potential significance to control sweet pepper root rot caused by P. capsici in recirculating hydroponic culture. Introduction Root rot caused by Phytophthora capsici, is economically important in almost all hydroponic crops including sweet pepper (Capsicum annuum L.). Symptoms of the disease include root browning and necrosis, wilting, reduced growth, and decreased yield. Recently, the efficacy of synthetic surfactant for control of major zoosporic root-infecting pathogens was demonstrated by Stanghellini and Tomlinson (1987). The objective of this investigation to evaluate the control efficacy of nonionic surfactants in managing zoosporic plant pathogens in recirculating system. Fig. 4. Effect of Metalaxyl-M, Potassium Phosphonate and non ionic surfactants on sporangium and zoospore production A.Control (water) B. Fragments of a Phytophthora capsici colony grown on PDA and placed in sterile water, C-D. Tween-20 500, 3000 µl/ml, E-F. Triton X-100 250, 2000 µl/ml, G-H. Metalaxyl-M 0.025, 0.05 µl/ml, I-J. potassium phosphonate 100, 200ppm Fig. 5. Lytic effect of Triton X-100 on zoospores of Phytophthora capsici A. Biflagellate motile zoospore, B. Granules, and C. Lysis; Material and Methods Sweet pepper (Capsicum annuum cv. New Wave) were grown from seed in 240-cell-rockwool-plugs in a growth room temperature. Four week-old seedlings were transferred to mini hydroponic NFT system. Phytophthora capsici KACC-40158 isolates were obtained from the Korea Agricultural Culture Collection (Suwon). The stock culture was maintained on potato dextrose agar (PDA) Petri dish. Different concentration Metalaxyl-M, potassium phosphonate, nonionic surfactants (Tween 20 and Triton X-100) In vitro experiment Mycelium growth inhibition (Stanghellini and Tomlinson, 1987) . Sporangia formation, zoospore release and zoospore lysis [Sporangia and zoospores were prepared as described by Ristaino (1990)]. Pathogencity test (seedling symptom test on water agar) In vivo experiment Control effect of caused by P. capsici with Metalaxyl-M, potassium phosphonate, nonionic surfactants in a recirculating hydroponic NFT system Disease severity was evaluated after 28 days using a 0-5 scale (Sunwoo et al., 1996). Control value (%)= [untreated plants (dc) − treated plants (dt) / dc ] x 100 Fig. 6. Pathogencity test of Phytophthora capsici. A. Inoculated seedlings, B. Zoosporangium of P. capsici within the root cells and C. Root intercellular space mycelium In vivo experiment Fig. 7. Disease severity and control value four weeks after the plants were inoculated with a zoospore suspension of Phytophthora capsici. Disease severity index (0-5); 0 = no visible disease symptom, 1 = leaves slightly wilted with brownish lesions beginning to appear on stems, 2 = 30 - 50% of entire plant diseased, 3 = 50 - 70% of entire plant diseased, 4 = 70 - 90% of entire plant diseased, and 5 = dead plant Results In vitro experiment Fig. 1. Phytophthora capsici KACC - 40158  A. Colony morphology of PDA culture medium at 250C, 7 days after, B. Offset sporangiophore, C. Sporangia papillate and two papilla with long pedicels and lemon type, D. Medium exit pore, sporangium proliferating from inside an old sporangial wall, E. Zoospores, F. Encyst, G. Germinating zoospores, and H. Oospore Fig. 2. Mycelial growth inhibition of Phytophthora capsici A. Control, B-C. Metalaxyl-M, (0.025 and 0.05 µl/ml), D-E. Potassium phosphonate, (100 and 200ppm), F-J. Tween-20 (500, 1000, 1500, 2000 and 3000 µl/ml), and K-O. Triton X-100 (250, 500, 1000, 1500 and 2000 µl/ml) * ** Fig. 8. Fresh weight (FW) and dry weight(DW) four weeks after the plants were inoculated with a zoospore suspension of Phytophthora capsici. Leaf, stem and roots were separated and oven-dried at 700C for 3 days and dry weights were measured. * He – health control, ** Di – disease control Fig. 9. Four weeks after the plants were inoculated with a zoospore suspension of Phytophthora capsici, A. Health (control) B. Disease (control), C-D. Metalaxyl-M (0.025 and 0.05 µl/ml), E-F. Potassium phosphonate (100 and 200 ppm), G-K. Tween 20 (500, 1000, 1500, 2000 and 3000 µl/ml), and L-P. Triton X-100 (250, 500, 1000, 1500 and 2000 µl/ml) Fig. 10. Four weeks after inoculation root discoloration A. Health (control), B. Disease (control), C-D. Metalaxyl-M, (0.025 and 0.05 µl/ml), E-F. Potassium phosphonate (100 and 200 ppm), G-K. Tween-20 (500, 1000,1500, 2000 and 3000 µl/ml), and L-P, Triton X-100 (250, 500, 1000,1500 and 2000 µl/ml), * ** Fig. 3. Effect of different concentration Metalaxyl-M, potassium phosphonate and nonionic surfactants on mycelia growth inhibition of Phytophthora capsici. Colony radius was measured 3 days after at 25°C. *M.M - Metalaxyl-M, **P.P - potassium phosphonate Table 1. Effect of different concentration of Metalaxyl-M, potassium phosphonate and nonionic surfactants on sporangium and zoospore production of Phytophthora capsici z. Conclusion Treatment* Sporangia (no/mm²) Zoospore release (no/ml) Zoospore lysis (%) Control 40.9 a 9.6 x 10⁴ M.M µl/ml 0.025 1.3 bc 0.2 d 13.6 0.05 0.7 c 0.1 de 23.5 PP ppm 100 0.0 31.2 200 28 Tween 20 (µl/ml) 500 2.6 b 1.0 81.3 1000 1.9 85 1500 1 84 2000 90.7 3000 0.3 e 96.3 Triton X-100 250 100** The results of this research indicate that nonionic surfactant was potential significance to control sweet pepper root rot caused by P. capsici in recirculating hydroponic culture. References Stanghellini, M. E. and Tomlinson, J. A. 1987. Inhibitory and lytic effects of a nonionic surfactant on various asexual stages in the life cycle of Pythium and Phytophthora species. Phytopathology 77:112-114. Ristaino, J. B, 1990. Intraspecific variation among isolates of Phytophthora capsici from pepper and cucurbit fields in North Carolina. Phytopathology 80:1253-1259. Stanghellini, M. E., Kim, D. H., Rasmussen, S. L. and Rorabaugh, P. A. 1996. Control of root rot of peppers caused by Phytophthora capsici with a nonionic surfactant. Plant Dis. 80:1113-1116. Sunwoo, J. Y., Lee, Y. K. and Hwang, B. K. 1996. Induced resistance against Phytophthora capsici in pepper plants in response to DL-B-amino-n-butyric acid. European Journal of Plant Pathology 102: 663-670. ZEach number is an average of 10 replicates. Figure with same letter in each column are not significantly different (P=0.05, Duncan’s range test). *M.M - Metalaxyl-M and P.P - potassium phosphonate; ** All Triton X-100 treatments zoospore lysis in 5 second ☏ 02) 2210-2664 E-mail: jwkim@uos.ac.kr