Phytophthora alni

Phytophthora alni is an oomycete plant pathogen that causes lethal root and collar rot in alders. It is widespread across Europe and has recently been found in North America. This species is believed to have originated relatively recently. Phytophythora alni was discovered in 1993 in Southern Britain. It has subsequently been reported in many European countries including the Netherlands, Germany, France, Sweden, Belgium, Austria and Hungary and has recently been found in Alaska. P. alni consists of three subspecies, the most commonly isolated and most virulent subspecies P. alni alni (also referred to as the 'standard form') and two less commonly isolated subspecies P. alni uniformis (also called the Swedish variant) and P. alni multiformis (which contains the Dutch, German and UK variants). A recent genetic analysis has suggested that P. alni alni. was generated on many separate occasions by the hybridization of either P. alni uniformis with P. alni multiformis or their ancestors. The same analysis suggests that P. alni uniformis may have P. cambivora as an ancestor. P. alni multiformis may have been generated by autopolyploidization (the spontaneous duplication of chromosome number within a species) or by a hybridisation of two unknown species a long time ago in evolutionary terms (ancient reticulation). Members of the genus Phytophthora may reproduce by both sexual and asexual methods. P. alni is homothallic meaning that both structures for sexual reproduction (antheridia and oogonia) appear in a single culture. The antheridia are amphigynous, except in some cultures of P. alni. multiformis where paragynous antheridia may occur. Unlike most species of Phytophthora, which are diploid, P. alni alni is near tetraploid and unable to complete meiosis beyond metaphase I. In culture, many oogonia prematurely abort or appear abnormal and only one third of the oospores that appear normal are reported to be viable. As a result, it is believed to spread predominantly via asexual means, namely zoospores which are produced in a specialised structure known as the sporangium. Water temperature has been shown to affect sporulation, with warmer water increasing sporangia production. Temperatures of 8 °C and below prevent production of sporangia.