Resolving confusions about jarrah dieback - don’t forget the plants

The name jarrah dieback has been used for two different disorders, leading to considerable confusion. It was coined in the 1940s to describe the sudden death of groups of jarrah (Eucalyptus marginata) trees in south western Western Australia, which occurred on poorly drained sites, following exceptionally heavy rainfall. In the 1960s these sites were shown to be infested by Phytophthora cinnamomi and jarrah deaths were attributed to it, even though it was only isolated from 5 % of sampled trees. Also the definition of jarrah dieback was expanded to include deaths of many other plants on infested sites, from which P. cinnamomi was more readily isolated. Jarrah trees die from severe water deficiency, indicating problems with water conduction through roots. Xylem vessel diameters vary along roots, being narrow at the root collar, while distally they are larger, providing water storage. Jarrah transpires vigorously during summer, accessing water at depth on sites with deep soil, but being more dependent on internally stored water when root systems are shallower. Following waterlogging, sapwood vessels become blocked with tyloses, reducing both conductivity and potential water storage; such trees may have insufficient water reserves for summer survival. In jarrah P. cinnamomi is unlikely to cause water deficiency because sapwood invasion is rapidly contained in healthy roots. Recent investigations into P. cinnamomi invasion and host responses in other plants show that it can potentially cause a vascular wilt in Banksia spp. and chronic, symptomless infections in herbaceous plants. Susceptibility to waterlogging damage, and/or mortality resulting from infection by P. cinnamomi can only be clarified by detailed knowledge of the hosts and their vulnerabilities. This is essential for making diagnoses, devising management strategies, and avoiding the confusions of the past

Field assessment of avocado rootstock selections for resistance to Phytophthora root rot

Phytophthora root rot (PRR), caused by P. cinnamomi, is a primary constraint on avocado productivity in Australia. Numerous field trials at sites in northern NSW and southern QLD have demonstrated significant variation in tree health amongst commercial rootstocks and recently selected material, grown under high PRR disease pressure. Selections 'SHSR-02', 'SHSR-04', ungrafted 'Hass' (rooted cuttings from clonal propagation) and the commercial rootstock 'Dusa"' were significantly healthier over time than other rootstocks, many of which died during the course of the trials. 'Reed' was consistently highly susceptible. In many cases superior tree health was associated with increased tree height and trunk girth. The trials also clearly demonstrate the negative impact of Phytophthora root rot on establishment of new avocado production blocks, and the importance of identifying and selecting avocado rootstock material that can withstand high P. cinnamomi disease pressure

Soil‐borne pathogens as determinants of regeneration patterns at community level in Mediterranean forests

13 páginas.- 4 figuras.- 4 tablas.- 93 referenciasEmergent diseases are an increasing problem in forests worldwide. Exotic pathogens are now threatening forests where pathogens have not traditionally been considered to be major ecological drivers of tree demography, such as water‐limited Mediterranean forests. However, how pathogens might limit regeneration in invaded forests is largely unknown. Here we used fungicide to analyse the impact of soil‐borne oomycete pathogens on seedling establishment at community level in Mediterranean forests invaded by the exotic oomycete Phytophthora cinnamomi . Fungicide effects were modelled as a function of the tree neighbourhood composition, the seed mass of the target species, and the abiotic environment. Fungicide application had positive effects on seedling performance that varied in magnitude and spatial structure among coexisting species. Seed mass predicted fungicide effects on seedling emergence, but not on survival or growth. Positive fungicide effects were modulated by levels of abiotic resources, mainly water, increasing with soil moisture. Our results support a novel role for soil‐borne oomycete pathogens as one more axis of the regeneration niche of woody species in water‐limited forests. Given the increasing numbers of exotic oomycete pathogens worldwide, more research is needed to understand the role of this relevant microbial group as a factor shaping seedling establishment.This research was supported by the Ministerio de Economía y Competitividad (MICINN) projects CGL2011-26877 and CGL2014-56739-R. JD-B was supportedby a FPI-MEC grant (BES- 2012-055113) and JMA by a FPU-MEC grant (AP2010-0229Peer reviewe