Inoculum Sources of Phytophthora nicotianae

One of the most important diseases affecting citrus production in South Texas is Phytophthora root and foot rot caused primarily by Phytophthora nicotianae. P. nicotianae is a fungus-like organism that infects feeder roots and the trunk of citrus trees which leads to poor water and nutrient uptake causing tree decline and eventually tree death.  It is well established that the first step in an effective management of disease caused by biotic agents is the elimination of sources of primary inoculum of the causative agent. In order to identify potential inoculum sources of P. nicotianae within the citrus production system in South Texas, we conducted a survey of citrus nursery facilities and different components of citrus grove operations in the Lower Rio Grande Valley (LRGV). P. nicotianae propagules were isolated, quantified and compared between the different potential inoculum sources including protective sponge wrappings around mature trees, leaf litter found throughout groves as well as soil and irrigation water sources from nurseries and groves using standard practices. Observations were also taken of other fungal pathogens recovered from the same set of samples. Available data revealed the presence of Fusarium, Pythium and P. nicotianae in protective sponge, soil and leaves from mature groves. However, only Fusarium and Pythium were recovered from nursery materials suggesting that inoculum of P. nicotianae in LRGV groves perpetuate within citrus groves and were unlikely to have originated from the nursery system. By identifying, quantifying, and distinguishing sources of these fungal pathogens, growers can more effectively manage the diseases they cause through prophylactic measures such as proper field sanitation as a complement to the use of chemical treatment.