Comparison of Seasonal Infection of Citrus Roots by Phytophthora citrophthora and P. nicotianae var. parasitica.

Excised feeder roots from mature citrus trees located in climatically different regions were infected with zoospores of Phytophthora citrophthora and P. nicotianae var. parasitica at different times of the year under identical laboratory conditions. Zoospores encysted on and caused infection in roots from all locations year round. Both pathogens had the most encysted zoospores on roots from November to January and the least from March to May. Infection by P. nicotianae var. parasitica was consistently higher than P. citrophthora in the excised summer roots (May to September) and lower in January 1998 and November 1997. Infection by both species of Phytophthora dropped to a minimum in March when carbohydrate levels in roots were lowest. Temperature was not solely responsible for determining seasonal fluctuations of P. nicotianae var. parasitica and P. citrophthora. Root carbohydrate content also could not be correlated with seasonal infection. Other physiological or microbial factors associated with citrus roots may cause seasonal Phytophthora infection fluctuations. Infection of citrus seedling roots by P. citrophthora at 16°C reduced root starch content but not glucose compared to P. nicotianae var. parasitica, whereas infection at 30°C by P. nicotianae var. parasitica lowered root glucose but not starch content compared to P. citrophthora.