ABSTRACT

Since 2002 a disease associated to branch cankers in citrus trees has been detected in the major 
citrus-producing areas in Spain, affecting mainly Clementine mandarins and their hybrids. The 
syndrome was described and Phytophthora citrophthora was identified as the causal agent 
according to isolate frequencies from a 132 citrus orchards survey and pathogenicity tests under 
greenhouse and field conditions. Using RAMS markers to study the population structure of P. 
citrophthora, 14 population groups were found. Around 90 % of the isolates comprising a 
unique population group and the remaining 10 % were divided in 13 dissimilar groups. No 
evidence demonstrated that this population diversity was a consequence of sexual crosses 
because more than 99 % of the studied isolates were sexually sterile. Some isolates also 
displayed high levels of insensitivity to the fungicides metalaxyl and mefenoxam. Assays of 
branch inoculations under field conditions carried out during a period of  27-months showed 
that P. citrophthora was more active in the spring months, followed by autumn, summer and 
winter months. Conversely, it was concluded that inoculations on detached branches it was not a 
valuable methodology to determine the seasonality of the different citrus scions and the 
aggressiveness of P. citrophthora y P. parasitica. Regression analyses between environmental 
and host-linked variables showed that the monthly mean maximum temperature, the relative 
humidity and the relative water content in the bark were the most important factors associated 
with the lesion development. Evidences were found about a vertical dissemination of the 
pathogen from the soil toward the upper canopy by mean of snails of the species Helix aspersa 
that was corroborated in experiments under controlled conditions. H aspersa was able to 
transport propagules of P. citrophthora in its body and also in its digestive tract disseminating 
pathogen inoculum through the feces. In reference to the disease control, it was found that in 
young and mature citrus trees under field conditions, post - infection applications were not 
effectives. However, preventive treatments based on branch or trunk paints were more effective 
than applications as foliar spray, chemigation or injection to the soil. In young trees, the foliar 
sprays showed a good behaviour in greenhouse and no significant differences were found with 
treatments based on paint; however, drench applications showed a limited activity. Treatments 
that consisted of two consecutive applications (20 to 30 days between them) did not improve 
significantly the efficacy of the products compared with one application.