The Interaction Between Phytophthora spp. and Candidatus Liberibacter spp. Damage to Citrus Fibrous Roots

Phytophthora nicotianae (P.n.) causes root rot of citrus, damaging the fibrous roots, which depletes carbohydrate reserves and reduces water and nutrient uptake capacity. Huanglongbing (HLB) is a systemic citrus disease that disrupts photosynthate transport, and is caused by phloem-limited Candidatus Liberibacter asiaticus (Las). High P.n. populations reported as HLB spreads though Florida indicate an interaction between these two pathogens on citrus root. To understand the possible interaction between these two pathogens and host, Cleopatra mandarin (Citrus reticulata) were inoculated with Las, P.n. or both in 2013 and 2014. The P.n. infection, Las movement within plant, root loss, and root exudation-sucrose were assayed. The P.n. infection increased on Las-infected seedlings, indicating that Las reduced their tolerance to P.n. The Las induced higher root exudation suggests that P.n. zoospore was more attracted to root surface by Las. Lower Las detection in Leaves when P.n. presented suggests Las movement was slightly influenced by P.n. The same pattern of P.n. infection and fibrous root biomass on Las positive seedlings through 5 to 11weeks after inoculation suggests that the interaction was regulated by available food supply. Both pathogens caused significant root loss alone, but in combination P.n. did not cause additional loss compared to the total root loss caused by Las alone at all harvest time. Based on these results, we hypothesize that: 1) early in disease development, Las increases susceptibility to P.n. infection by increasing zoospore attraction or facilitating penetration; 2) Las induces root loss resulting in a temporary drop in P.n. population by reducing available food supply; and 3) as new root flushes occur, Las moves to root with phloem sap, and P.n. starts the infection cycle again and repeats until there is a complete loss of the fibrous root system.