Abstract
Phytophthora nicotianae Breda de Haan is endemic in Florida citrus groves and causes fibrous root rot when rhizosphere populations develop to damaging levels. Annual surveys 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 flatwoods: 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; intermediate 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 conductivity of soil series was predictive of P. nicotianae populations when rainfall was high in 1999, but not predictive in relatively dry years. P. nicotianae populations were weakly correlated 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 drainage, 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.
