1. ☏ 02) 6490-2690 E-mail: jwkim@uos.ac.kr Optimal Fungicide Application Time for the Control of Fusarium Wilt on Carnation Suji Park, Ji Won Lee, Jihye Choi and Jin-Won Kim* Department of Environmental Horticulture, The University of Seoul, Seoul 130-743, Korea Carnations are cultivated in greenhouses for a year starting from May and overcome winter for cut-flower production of next year. Fusarium wilt caused by Fusarium oxysporum f. sp. dianthi (Fod) is the most damaging disease in carnation production and substantial efforts are made to control the disease. One of the main factors that influence the development of Fusarium wilt is temperature; for example, carnations remain symptomless under relatively low temperature below 18°C. Therefore, optimal fungicide application time from winter to spring is needed for effective control of the disease. In our experiment from January to May, carnations were inoculated with Fod isolates and a temperature data logger was installed to measure hourly air temperature in the glasshouse. No visible symptom was observed when the mean air temperature was maintained around 15°C even though the daytime air temperature exceeded above 18°C for most days. After the mean air temperature above 18°C sustained for five weeks, the first wilt symptom was detected and the number of withering plants increased. From the results, we suggest that the mean air temperature should be calculated and used to predict the optimal fungicide application time from winter to spring. At the time after the average air temperature above 18°C continues more than 1 week in the greenhouse, the fungicide should be regularly applied to reduce the initial inoculum for effective control of Fusarium wilt. Carnation (Dianthus caryophyllus L.) is the fourth high ranked crop in aspect of the sales and cultivation area in cut-flower industry of Korea. Carnation demand increases particularly in May which has Parents’ day and Teacher’s day; thus, high crop yield with good quality in spring season is especially important. Fusarium wilt caused by Fod causes severe yield loss of carnation production. The development of Fusarium wilt on carnation is known to be influenced by temperature and carnations show no visible symptom at relatively low temperature below 18°C (Ben-Yephet and Shtienberg, 1994). Predicting optimal fungicide application time from winter to spring will enable efficient control of the disease.Introduction Relationship between disease development and average air temperature The first wilt symptom was observed about nine weeks after inoculation and the number of diseased plants gradually increased (Fig. 2). According to Ben-Yephet and Shtienberg (1994), there were temperature limit (max. 34°C and min. 18°C) for Fusarium wilt expression on carnation. However, no visible symptom was detected for nine weeks even though the daytime temperature exceeded 18°C everyday except one day (Fig. 3). The onset of disease was when after the weekly average air temperature around 18°C continued about five weeks. Thus, average air temperature inside the greenhouse seems to be an important factor of the development of Fusarium wilt on carnation. Incubation period In other experiments conducted in 2013 and 2014, under the average air temperature of 22- 26°C, the incubation periods were 10-21 days (data not shown) and 3-3.5 weeks of incubation periods were reported (Baayen and De Maat, 1987; Baayen et al., 1989; Ben-Yephet and Shtienberg, 1994; Harling et al., 1987). The prolonged incubation period obtained in this experiment is thought to result from duration of the relatively low temperature period for the first three weeks. Prediction of optimal fungicide application time According to the results about incubation period, the estimated infection time in this experiment seemed to be around 27 th February. Sarroco et al. (2007) reported that the pathogen infected the roots and xylem vessels at 7-10 days after inoculation. Thus, the optimal fungicide application time appeared to be 20 th February, when the average air temperature inside the glasshouse kept around 18°C for one week. Fusarium wilt did not develop on carnations during relatively low average air temperature period below 18°C regardless of high daytime temperature above 18°C. From the results, we suggest that average air temperature inside the greenhouse should be monitored and used to predict disease onset and optimal fungicide application time from winter to spring for effective control of Fusarium wilt on carnation. Results and Discussion Conclusion - Baayen, R. P. and De Maat, A. L. 1987. Passive transport of microconidia of Fusarium oxysporum f. sp. dianthi in carnation after root inoculation. Neth. J. Pl. Path. 93:3-13. - Ben-Yephet, Y. and Shtienberg, D. 1994. Effects of solar radiation and temperature on Fusarium wilt in carnation. Phytopathology 84:1416-1421. - Baayen, R. P., Van Eijk, C. and Elgersma, D. M. 1989. Histology of roots of resistant and susceptible carnation cultivars from soil infested with Fusarium oxysporum f. sp. dianthi. Neth. J. Pl. Path. 95:3-13. - Harling, R., Taylor, G. S., Matthews, P. and Arthur, A. E. 1988. The effect of temperature on symptom expression and colonization in resistant and susceptible carnation cultivars infected with Fusarium oxysporum f. sp. dianthi. J. Phytopathology 121:103-117. - Sarrocco, S., Falaschi, N., Vergara, M., Nicoletti, F. and Vannacci, G. 2007. Use of Fusarium oxysporum f. sp. dianthi transformed with marker genes to follow colonization of carnation roots. J. Pl. Path. 89(1):47-54.References Abstract Materials and Methods Ten Fod isolates were incubated on potato dextrose agar at 25°C with 12 hours of light period for two weeks and mycelial plugs were made with cork borer (8mm diameter). Rooted carnation cuttings of 17 cultivars (3 replicates) were transplanted to 9cm pots and four mycelial plugs per plant were inoculated on the carnation roots from 13 th to 20 th in January at the glasshouse of the University of Seoul. To induce disease, additional inoculation was made by pouring spore suspension (concentration more than 2×10 ⁵ spores/ml) at five weeks after the mycelial plug inoculation. A temperature data logger, ‘HOBO’ (Onset Computer Corporation, USA), was installed in the middle of glasshouse about 1m above the grow bed. Air temperature inside the glasshouse was recorded at every hour and weekly average air temperature was calculated. Disease investigation was made every other day until the first symptom was observed and the number of diseased plants was recorded at 3-7 days intervals as of the date of disease onset. Disease severity index (Fig. 1) was used to evaluate disease and plants showing index level 1.5 or higher were counted as diseased plants. Fig. 1. Disease severity index; 0= no symptom, 1=one or two leaves wilted at the basal stem, 2=(unilateral) wilt reached half of the stem, 3=severe wilt and crook-neck appeared, and 4=completely wilted and died. 01234 Fig. 2. Weekly average air temperature inside the glasshouse during the entire experiment period (up) and the number of diseased plants per week (bottom). Lowest temperature limit Disease onset Estimated infection time Optimal fungicide application time Fig. 3. Daily air temperature (blue line) and daily average air temperature (orange line) inside the glasshouse until the disease onset.