Austrocedrus forests of South America are pivotal ecosystems at risk due to the emergence of an exotic tree disease: can a joint effort of research and policy save them?

GRESLEBIN A.G.; VÉLEZ M.L.; GARBELOTTO, M.

NATURAL RESOURCES FORUM

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2015

0165-0203

A. chilensis threatened by an emergent pathogenStarting in 1948, significant mortality of A. chilensiswas reported in several areas. Disease progressionappeared to be fast leading often to 50% mortality, ormore, of trees of all ages and size classes. Thesymptoms included withering, defoliation, and rootrot. Patterns of spread of mortality suggested aninfectious disease, and the term ?Mal del Ciprés?(Cypress disease) was coined to describe thephenomenon. Its causal agent though remainedunresolved for over 57 years, despite the severalattempts by groups of scientists, including one FAOsponsoredexpedition whose participants suggested tocontinue the studies and to investigate the possibleparticipation of a root pathogen. The lack ofunderstanding of the causes behind the Mal del ciprés,greatly hindered any significant advancement in theformulation of preventive and disease controlmeasures. In 2005, a collaboration between Argentineand US scientists led to the discovery of the causalagent of the disease, since then named ?Austrocedrusroot disease (ARD)?. The causal agent was found to bean undescribed soilborne pathogen belonging to thegenus Phytophthora, soon to be named P. austrocedri(Greslebin et al. 2007). Although research on novelorganisms is extremely complex and demanding, thediscovery has led to the understanding of the diseasecycle, including some key epidemiological aspects.Currently it is understood that higher infection ratesare associated with abiotic factors that favorPhytophthora proliferation (La Manna & Rajchenberg2004, La Manna et al. 2008) and with the proximity toroads and streams, and intensity of grazing (La Mannaet al. 2014). Nine years after the initial discovery, thereis little evidence of natural resistance in populations ofthe tree host, while it has become apparent thathumans are responsible, through cattle grazing,vehicular and foot traffic, and construction, for thespread of the disease both in areas adjacent to olderinfestations and in new areas. Natural preserves andnational parks with limited access show no or limiteddisease. In infested areas, though, mortality can reach90%, and it has been estimated that P. austrocedri isnow present in many sites in at least 3 Argentinestates. In 2011, an apparently identical pathogensurfaced in the United Kingdom, where it appears tobe causing widespread mortality of a key species ofheath and moorlands, Juniperus communis. Although itis still unclear whether the causal agents of the twooutbreaks are really the same, the extremely simplifiedgenetic structure of pathogen populations in the twoareas suggest both outbreaks are caused by anintroduced exotic pathogen, and not by an emergentnative one (Vélez et al. 2013).According to it exotic origin, mitigation measures forARD should be directed to both the protection ofhealthy areas, avoiding the introduction of thepathogen, and to the mitigation of disease impact inaffected areas.