Phytophthora species cause many major plant disease epidemics worldwide, impacting horticulture, plant trade and production planting industries through the loss of plants and the costs arising from disease management. Phytophthora species have specialised structures which enhance their dispersal, survival and infection. These include motile swimming spores, caducous sporangia and resting spores. Nurseries and orchards use integrated management to control phytophthora diseases, including cultural practises, biological control, resistant host plants and chemical control. The use of fungicides is a very effective control method; however, Phytophthora species can develop resistance to fungicides after prolonged exposure. For example, phosphite tolerant isolates of Phytophthora cinnamomi have been found in avocado orchards in Australia and South Africa. Phosphite is widely used in nurseries and orchards in New Zealand as a preventative and control treatment. The avocado industry largely relies on it for the control of avocado root rot caused by P. cinnamomi. Phosphite is most commonly applied to avocado trees using passive trunk injections but can also be applied as foliar sprays, soil drenches, high-pressure injections and capsule implants. To assess phosphite tolerance of P. cinnamomi isolates from New Zealand avocado orchards, a high-throughput optical density (OD) assay was developed to measure mycelial growth inhibition in the presence of phosphite. The OD assay was used to screen 24 P. cinnamomi isolates from four orchards never treated with phosphite and 32 isolates from eight orchards treated with phosphite for 15 – 37 years. Four isolates had increased tolerance to phosphite and two were intermediately tolerant. These six isolates were from phosphite managed orchards and five were isolated from unhealthy trees. To test if in vitro tolerance would be expressed in planta, three tolerant and three sensitive isolates were tested for their ability to colonise phosphite treated lupin (Lupinus angustifolius) roots. The tolerant isolates were more extensive colonisers of lupins treated with 5 and 10 g/L phosphite. In the presence of phosphite the tolerant isolates were able to produce more sporangia and release viable swimming zoospores more often. Internationally important Phytophthora species from culture collections in New Zealand and the United States were assessed for sensitivity to phosphite and a subset of 32 isolates from the Berkeley collection was tested for mefenoxam sensitivity. The causal agent of kauri (Agathis australis) dieback, P. agathidicida, was found to be highly sensitive to phosphite while the possibly New Zealand native P. kernoviae, was found to be relatively tolerant. Intraspecific variation was found in lineages of P. ramorum, the causal agent of sudden oak death in the United States and the United Kingdom, NA2 was more tolerant than NA1. One P. megasperma isolate was more tolerant to mefenoxam compared to the other isolates tested. Of nine P. cinnamomi isolates from avocado orchards in southern California, one was relatively more tolerant to phosphite. This study has shown phosphite tolerant isolates of P. cinnamomi have developed in phosphite managed avocado orchards in New Zealand and interspecific variation in sensitivity to fungicides is present across Phytophthora species in New Zealand and the United states.