Abstract
To generate a causal hypothesis for an association between the disease severity of Alternaria blotch of apple before early July and subsequent severity in late July, a nested case-control study was carried out using 13 years of severity data (1993-2005) from Iwate Prefecture, Japan. The data consisted of 625 cases in 40-56 orchards, and each case included data of leaf disease severity from early June to late July. The cases were stratified into four geographic groups to eliminate confounding factors from climate and epidemic characteristics. Preliminary analysis indicated that one geographic group was considered at low risk for severe epidemics, and the rest were rated as high risk. The results showed that initial disease development in early July was closely related to the occurrence of severe epidemics in late July in both risk groups, where the odds ratio (OR) of the low-risk group was 10.4 and that of the high-risk group was 3.1. The relationships between the initial disease development in late June and the severe epidemics of the disease in late July were not statistically significant in the high-risk group but in the low-risk group, the OR was 3.8. The results reveal a highly probable causality between disease severity at the early and at the late phases of the epidemics under conventional orchard management.
