A word from the experts: focus on Phytophthora

In this issue, advice comes from horticultural and plant disease expert John Scrace on the serious diseases caused by Phytophthora.

Few plant pathogens have had such a dramatic impact on human history as Phytophthora. The Great Irish Famine of the 1840s was caused by devastating outbreaks of potato blight (Phytophthora infestans). Even today, blight causes annual global yield losses of about £3 billion, whilst new species such as Phytophthora ramorum appear regularly, creating problems for growers.

Nearly one hundred different Phytophthora species have now been documented, and growers of most crops will have at least one species to contend with. Many species rot the roots and stem base of plants, leading to wilting, dieback and often death. These species include P. cinnamomi, P. citricola, P. cactorum and P. nicotianae. Microscopic spores (zoospores) of the Phytophthora swim in water films and are attracted by chemical exudates produced by the roots of the host plant. Long-lived resting spores (oospores) develop in the rotting roots and can contaminate soils, growing media, standing areas and water storage tanks.

Other Phytophthora species affect the aerial parts of plants. These include P. infestans (although this can also attack tubers below ground) and P. ramorum. This latter species is the cause of ‘Sudden Oak Death’, which has devastated forests in parts of the USA, and is a quarantine listed organism in Europe. Any suspected cases must be reported to the relevant plant health authority.

Control of Phytophthora depends on scrupulous hygiene (including the use of disinfectants or soil sterilants) to prevent contamination of the growing system. Be careful about the source of plants you bring onto the nursery. Fungicides can be used on many crops, but these usually only slow the growth of the pathogen rather than killing it outright. For these reasons, early diagnosis of a Phytophthora problem is critical. The Phytophthora Pocket Diagnostic test can be used on-site on roots, stems and foliage to quickly check for a potential infection. If positive results are found, Forsite's new PDplus™ can be used to identify up to four important species of Phytophthora, including P. ramorum and P. kernoviae.

John Scrace is a freelance plant pathologist with many years' experience.