SOIL, ROOTSTOCK, AND CLIMATIC FACTORS AFFECT POPULATIONS OF PHYTOPHTHORA NICOTIANAE IN SOUTH FLORIDA CITRUS PLANTINGS

Phytophthora nicotianae Breda de Haan is endemic in Florida citrus groves and causes fibrous root rot when rhizo- sphere populations develop to damaging levels. Annual sur- veys of populations of P. nicotianae, soil characteristics, and plant nutrient status were conducted in two citrus plantings on major rootstocks and soil series typical of the southern flat- woods: Grove 1 from 1998-2000, and Grove 2 in 1999 and 2001. In Grove 1, P. nicotianae populations on rootstocks over 3 years ranked: Cleopatra mandarin > Carrizo citrange > sour orange > Swingle citrumelo = Palestine sweet lime > Volkamer lemon. P. nicotianae populations were highest in Myakka fine sand; inter- mediate in Immokalee, Margate, Holopaw, Basinger, and Riviera fine sands; and lowest in Holopaw (Lime Substratum), Boca, and Oldsmar fine sands. Populations were higher in spodosols than entisols and alfisols with the exception of Oldsmar series. P. nicotianae populations were positively correlated with % clay and negatively correlated with % sand. Saturated hydraulic con- ductivity of soil series was predictive of P. nicotianae popula- tions when rainfall was high in 1999, but not predictive in relatively dry years. P. nicotianae populations were weakly cor- related with soil and plant nutrient status. In Grove 2, ranking of P. nicotianae populations for rootstocks was similar to that in Grove 1. Relationships with rootstock susceptibility, soil drain- age, and rainfall patterns define where and when P. nicotianae populations reach damaging levels, as well as the management tactics that are most effective to maintain root health.