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Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 4/8/2008 Publication Date: 8/1/2008 Citation: Copes, W.E., Thomson, J.L. 2008. Survival Analysis to Determine the Length of the Incubation Period of Camellia Twig Blight Caused by Colletorichum gloeosporioides. Plant Disease 92:1177-1182. 2008. Interpretive Summary: Camellia twig blight, caused by Colletotrichum gloeosporioides, is a disease problem that commonly occurs on several Camellia species in the southern United States. To determine the potential seasonal differences in time from infection until the appearance of symptoms, stems of Camellia sasanqua ‘Rosa’ plants grown in containers were artificially infected with the fungus in a greenhouse then moved outside under overhead sprinkler irrigation, every month over two years. Continuous temperature data was recorded. Survival tended to be shorter (symptoms developed sooner) in periods with a higher number of hours between 15 – 30 ° C. The lowest and highest hazard ratios (risks of developing blight) occurred in winter and summer months, respectively. The largest overlap when symptoms first appeared occurred in May and June from stems infected from February to May. Knowing when infection occurred in relationship to the appearance of symptoms will help extension and research scientists when testing efficacy of control measures. Technical Abstract: Camellia twig blight, caused by Colletotrichum gloeosporioides, is a prevalent disease problem on several Camellia species in the southern United States. To determine the potential seasonal differences in incubation periods, twelve to sixteen stems of Camellia sasanqua ‘Rosa’ plants grown in pine bark in a 3.8 liter container were wounded and inoculated monthly with the leading edge of C. gloeosporioides mycelium on a 3.6 mm2 block of agar that was sealed around the stem with parafilm. Infection occurred over three days at 25ºC in a greenhouse then plants were grown at ambient conditions on plastic fabric under overhead sprinkler irrigation. The data best fit a proportional hazards model that included and was significant for the optimal temperature range (OTR) for development of stem blight (a continuous variable reflecting cumulative hours within 15 – 30ºC), the period variable (22 periods, identified by the adjusted month or full month following wound-inoculation of stems and the year), and an interaction term between period and time (2 years from July 2003 to May 2005). The coefficient for the temperature term was negative, thus median survival tended to be shorter (symptoms developed sooner) in periods with higher mean hours/day within the OTR. The interaction term was included to account for the non-proportionality between periods. Survival curves revealed similarities between many of the periods and reflected the same general pattern of shorter survival times for periods with higher mean hours/day within the OTR. February 2004 had the fewest mean hours/day in the OTR and the longest median survival, therefore was used as the reference value for calculating hazard ratios for each period. The lowest and highest hazard ratios occurred in winter and summer months, respectively, reflective of mean hours/day in the OTR, which varied year-to-year within a month. Based on the incubation times of individual stems. the largest confluence of dates for first symptom appearance occurred in May and June from stems infected from February to May. |
