Statistics for Kiwifruit bacterial canker in 'Hayward' kiwifruit : the application of observational study design and epidemiological techniques to the study of disease outbreaks affecting plant health : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Veterinary Epidemiology, Institute of Veterinary, Animal and Biomedical Sciences at Massey University, Manawatu, New Zealand