Sources of Phytophthora nicotianae in South African citrus nurseries

Occasional outbreaks of Phytophthora-related diseases, including fibrous root rot of Citrus nursery trees, are indicative of the presence of Phytophthora spp. in citrus nurseries. Diseases caused by Phytophthora spp. are known to occur despite management strategies such as water and rootstock management, testing, and oomycete-fungicide application. The objective of this study was to investigate infection pathways and the sources of Phytophthora inoculum in citrus nurseries in South Africa. Twelve commercial citrus nurseries were sampled for Phytophthora inoculum throughout the major citrus-producing regions of South Africa. Phytophthora nicotianae inoculum was retrieved from soil surrounding the tunnels at all the nurseries sampled, the growing media of pots and unused growing media. Other notable inoculum loci included the gloves and boots of nursery personnel, the cement structures used to raise the pots, moss and leaf litter surrounding pots, filled and used seedling trays, wheelbarrows, tractor wheels and shade netting were found to be contaminated in nurseries. Phytophthora citrophthora inoculum was not recovered from any of the sites sampled. Irrigation water tested at the nurseries showed an absence of Phytophthora inoculum and was thus clean at the time of study. Phytophthora nicotianae inoculum was present in the nursery environment and may be spread to plants through mechanical vectors such as gloves, boots, wheelbarrows and tractors as Phytophthora inoculum was detected on this equipment.