Why many phytophthora root rots and crown rots of tree and horticultural crops remain undetected1

Of the approximately 50 recognized Phytophthora spp., more than half have not been reported to cause root rot or crown rot. Some well‐known phytophthora root and crown rots have still not been diagnosed in many regions or countries in the world. It seems likely that this arises mainly from use of inappropriate isolation media and diagnostic procedures. Thus, various root disorders of tree and horticultural crops have been attributed to other pathogens or to abiotic causes, and found to be due to Phytophthora spp. only when proper isolation techniques involving baiting and selective media were used. These special techniques, however, are still not widely used by many diagnosticians and pathologists today. It seems that most pathologists lack training and specific knowledge on Phytophtohora diseases and tools for their study. They are often unaware of the importance of these fungi in causing below‐ground infections, or of the unique nature of their biology, soil ecology and pathology in relation to root and crown rot development. It is hoped that wider use of special isolation techniques in routine diagnoses in the future will result in an increase in the frequency of reports of occurrence of phytophthora root and crown rots of woody perennials around the world.