Survival Analysis to Determine the Length of the Incubation Period of Camellia Twig Blight Caused by Colletotrichum gloeosporioides.

Camellia twig blight, caused by Colletotrichum gloeosporioides, is a disease common to several Camellia species in the southern United States. To determine the potential seasonal differences in incubation periods, stems of Camellia sasanqua 'Rosea' plants grown in pine bark under ambient conditions were wounded and inoculated monthly with C. gloeosporioides mycelium. The time until appearance of the first symptom of disease (incubation period length) was recorded for all stems. Stems that did not display a disease symptom by the last day of the observation period were recorded as censored observations. Survival analysis using Kaplan-Meier estimates, Cox proportional hazards, and extended Cox models was used to analyze the data. Incubation period length was regressed against stem diameter, monthly mean hours per day in a specified temperature range (15 to 30°C), and a categorical season variable approximating the four host growth stages (winter dormancy, spring leaf and stem growth, summer stem hardening and bud set, fall cessation of leaf and stem growth and opening of flowers) at the time stems were inoculated. Stems of thicker diameter tended to have greater incubation period length, while higher monthly mean hours per day in the specified temperature range decreased incubation period length. In comparison to winter months, spring, summer, and fall months were all associated with significantly higher risks for disease symptom appearance. The median incubation period lengths for the spring, summer, fall, and winter months were 18, 23, 28, and 57 days, respectively.