Phytophthora austrocedrae

Overview

Phytophthora austrocedrae Gresl. & E.M. Hansen (2007) was isolated from necrotic lesions of stem and roots of Austrocedrus chilensis (Cupressaceae). It is homothallic with semi-papillate sporangia, oogonia with amphigynous antheridia, and very slow growth. It is the cause of “mal del ciprés”, a lethal disease of Austrocedrus in Argentina. Austrocedrus chilensis (ciprés de la cordillera) is endemic to southern Argentina and Chile. It is the most widely distributed tree species of the few conifers inhabiting the slopes of the Andes Mountains in Patagonia.

Etymology: ‘austrocedrae’ refers to Austrocedrus, the tree that is attacked by this pathogen.

Morphology

Sporangia are borne terminally on mostly unbranched sporangiophores. Sporangiophores frequently have hyphal swellings. Sporangia are ovoid, limoniform or ellipsoid and semi-papillate. They average 50 x 36 μm (range 22-83 μm x 15-58 μm) with the length/breadth ratio about 1.4, and frequently have distorted shapes. Sporangia with hyphal projections and lateral attachment to the sporangiophore are frequently observed in all isolates. Sporangia are not observed in solid media.

Oogonia form in single-strain culture after about 20 days. Oogonia are globose or nearly so, averaging 39 μm diameter (range 22-56 μm), with hyaline to light brown, smooth walls. Oospores are globose, 31 μm diameter, hyaline, with smooth walls. Antheridia are amphigynous and one-celled.  Hyphal swellings usually form in liquid and solid media but are more abundant in the former. Swellings are globose to subglobose and catenulated, sometimes with distorted shapes.

Genetics

The ITS sequence was identical to sequences of Phytophthora DNA extracted from bark of diseased trees. P. austrocedrae is in clade 8 of the Cooke et al. (2000) molecular phylogeny of the genus, with P. lateralis, another pathogen of Cupressaceae, and other aggressive pathogenic species. P. syringae is the closest described relative.

Growth

In V8 and tomato juice (TA) agar the colony is uniform, without growth pattern, cottony, dome-shaped in the center and appressed or mostly submerged at the margins. In corn meal agar (CMA) the colony is appressed, with little or no aerial mycelium and the submerged mycelia shows an arachnoid pattern. In potato dextrose agar (PDA) the colony is uniform, without growth pattern, densely felty to woolly, with abundant and dense aerial mycelium. Growth is very slow and favored by cool temperatures. Optimum temperature is 17.5°C, with no growth at 25°C.

Maximum radial growth rate at optimum temperature on V8 agar ranged from 1.0 – 1.8 mm/day.

Distinguishing characteristics for identification

Phytophthora austrocedrae is isolated from necrotic lesions of stem and roots of Austrocedrus chilensis. It is homothallic, characterized by semi-papillate sporangia, oogonia with amphigynous antheridia, and very slow growth with optimum temperature lower than most Phytophthora species. Phylogenetic analysis indicates that its closest relative is Phytophthora syringae, another species frequently isolated from soil and streams in A. chilensis forests.

The searchable web-based database Phytophthora-ID is useful for rapid identification of Phytophthora species based on sequencing of the ITS or Cox spacer regions, followed by BLAST searching the database. Phytophthora-ID maintains a database of sequences that is selective for sequence accessions that come from trusted sources including published, peer-reviewed studies whenever possible.

Disease History

High levels of mortality of A. chilensis trees were reported in 1948 in Isla Victoria in Nahuel Huapi National Park, in Patagonia, Argentina, near plantings of exotic trees collected from around the world. In 1953, similar mortality was reported in an Austrocedrus stand located near a forest nursery in Epuyen, about 150 km distant from Isla Victoria. Since then, mortality has been reported in many places throughout the range of A. chilensis on the eastern slopes of the Andes in Argentina. In all cases, it has been assumed that the cause was the disease named mal del ciprés (MDC, cypress sickness) (Havrylenko et al. 1989). Although Phytophthora was suspected as a causal agent, no successful isolations were reported until 2007.

Impacts in the Forest

In Argentina Austrocedrus chilensis grows across a broad moisture gradient along the Andean foothills.  A. chilensis can be found either in mixed stands with Nothofagus spp. or in pure Austrocedrus stands on drier sites. It also grows in open, xeric forests or in isolated clumps at the limit of the Andean forest and the Patagonian steppe, acting as a barrier against desert advance. Austrocedrus chilensis is valued not only because of its ecological function but because of the quality of its wood and its scenic importance (Greslebin et al. 2010). It is a protected species, with harvest strictly regulated by the state.

MDC is associated with specific site conditions at both microsite and landscape scales (La Manna et al. 2008), particularly high soil moisture and poor drainage (Baccala´ et al. 1998, Filip & Rosso 1999, La Manna & Rajchenberg 2004). Tree death tends to occur in clusters in forest stands, at least when disease incidence is low (Rosso et al. 1994).

Concern about the disease has increased due to the constant expansion of the affected area. The disease affects tourism, recreation and commercial forestry. Appropriate silvicultural management of affected stands is difficult because the epidemiology of the disease is poorly understood.  Public institutions in charge of forest management have authorized land owners with affected forests to fell the dead trees, which has had the serious consequence of the replacement of native forest with exotic introduced species. (Greslebin et al. 2010).

Forest and Wildland Hosts and Symptoms

In Argentina, Austrocedrus chilensis is the only known host of P. austrocedrae. The susceptibility of other species is largely untested. The North American incense cedar (Libocedrus decurrans) is very similar to Austrocedrus, both in appearance and ecologically. Its susceptibility has not been tested.

The main symptom of P. austrocedrae in naturally infected trees is a necrotic lesion extending from killed roots up to 1 m high on the tree bole. The necrosis affects the entire thickness of the phloem and the sapwood is superficially stained. Both active and inactive lesions are encountered. When active, lesions are bright chestnut brown, moist and flexible. When inactive, they are dark brown, dry and hard, and difficult to distinguish from the outer bark. Defoliation is associated with the amount of root affected, but it is not totally reliable as an indicator of the percentage of necrotic tissues of main roots and root collar (Floria & Greslebin 2009). Sometimes, especially in stands where the disease is very active, older foliage inside the crown turns bright yellow and then red by the end of the summer. This symptom is usually associated with the presence of active lesions at the root collar. Resin exudation is often associated with Phytophthora lesions. Resin flow usually emerges from a resin pocket in the phloem near the active margin of a lesion.