A long-term study on the effect of agroclimatic variables on olive scab in Spain

Olive scab (OS), caused by Venturia oleaginea, is the most widespread disease in all olive growing regions. However, there is not much information about critical agroclimatic conditions for OS development under natural conditions. To cover this lack of knowledge, the OS severity (OSS) of 92 epidemics, from 14 locations, 16 years and 11 olive cultivars, was annually evaluated. Epidemics were divided in three periods of years: from 1994 to 1998 (period I), from 2002 to 2008 (period II) and from 2010 to 2013 (period III). OSS was compared among periods. Also, Pearson's (r) or Spearman's rank (rho) correlation coefficients were calculated between OSS and monthly weather variables, cultivar resistance categories and OSS value in the previous epidemic year. The OSS in period III was higher than the OSS in the other two periods. Resistance categories and OSS in the previous year were highly correlated with OSS. Relative humidity (RH) and rainfall (R) were the weather variables positively correlated with OSS in all periods. OSS in the previous year, daily average RH (RHm) in July, average of daily maximum temperature (T-mx) in November, average of daily maximum RH (RHmx) in December and average of daily minimum temperature (T-m) in March were selected as agroclimatic variables to develop a linear regression model that relates them with OSS. The present study provides relevant information about the long-term factors that affect the OS development.