Phytophthora pinifolia

Overview

In 2004 and increasingly through 2006, a new disease of Pinus radiata, referred to as ‘Daño Foliar del Pino’ (DFP) appeared in the Arauco province of Chile and subsequently spread to other areas in central Chile planted with radiata pine. The disease is typified by needle infections, exudation of resin at the bases of the needle bundles and, in younger trees, necrotic lesions in the cambium which eventually girdle the branches. In 2007 a previously unknown Phytophthora sp. was isolated from needle tissue. In 2008 it was formally described as Phytophthora pinifolia by Alvaro Duran and colleagues from Bioforest S.A., Concepción, Chile and the Forestry and Agricultural Biotechnology Institute (FABI), University of Pretoria, South Africa (Duran et al. 2008). This new species is characterized by unbranched sporangiophores and non-papillate, sub-globose to ovoid sporangia that are occasionally caducous. Sexual structures were not observed. Phytophthora pinifolia is the first Phytophthora known to be associated with needles and shoots of a Pinus sp. Etymology : ‘pinifolia’ refers to the occurrence of the organism on the needles of Pinus radiata.

Morphology

Cultures on cornmeal agar selective medium were submerged with coralloid hyphae, while carrot agar (CA) and V8 cultures were fluffy with a regular to rosaceous or petallate margin. Sporangia formed abundantly in soil water and were generally absent in agar culture. Sporangia were borne on predominantly unbranched sporangiophores and were non-papillate, sub-globose to ovoid. Free sporangia with pedicels were occasionally observed after agitation of liquid cultures. Oogonia did not form in single culture or when paired with A1 or A2 tester isolates of heterothallic species.

Genetics

Phytophthora pinifolia resides in Clade 6 of the ITS-based phylogeny of Cooke et al. (2000). Other species in this group include P. gonapodyides, P. megasperma and P. inundata. These species are all ecologically and morphologically different from P. pinifolia despite their phylogenetic affinity. AFLP analysis of isolates collected from throughout the area of infestation in Chile suggests that P. pinifolia exists as a clonal population, with very little genetic variation. This is consistent with a single, recent introduction of the pathogen, although its origins remain unknown.

The phylogenetic tree below is from Duran et al. 2008, Fig. 3. It is one of 10 most parsimonious trees having the same overall topology obtained through analysis of ITS rDNA sequence data, reflecting hypotheses of phylogenetic relationships for Phytophthora pinifolia and other Phytophthora species. The tree is rooted to Pythium aphanidermatum. Bootstrap support values (1000 replicates) are given at the branch nodes.

Growth

Optimal temperature for growth on agar media is 25°C (min 10°C; max 30°C). Growth is slow, about 1 mm/day, and plates are never completely covered by mycelium. Colonies reach maximum diameter (45–55 mm) on carrot agar (CA) in 4 weeks.

Distinguishing characteristics for identification

Morphologically P. pinifolia is similar to other clade 6 species, with unbranched sporangiophores and non-papillate, ovoid sporangia. Deciduous sporangia are infrequent. They are not typical of other species in the clade. P. pinifolia appears to be sexually sterile, like many clade 6 species. Phytophthora pinifolia is the first Phytophthora known to be associated with needles and shoots of a Pinus sp. and its apparent aerial habit is unique in clade 6.

The searchable web-based database phytophthora-id.org is useful for rapid identification of Phytophthora species based on sequencing of the ITS or Cox spacer regions, followed by BLAST searching the database. The database includes only sequences that are associated with published Phytophthora species descriptions or classic Phytophthora phylogenetics references.

Disease History

The pine needle and shoot blight in Chile known as ‘Daño Foliar del Pino’ (DFP) emerged in 2004 and spread in only three years to cover an area of approximately 60 000 ha of radiata pine plantations to become the most important problem affecting plantation pine forestry in Chile. Despite intensive efforts to discover the cause of DFP, etiology was a mystery until July 2007 when a previously unknown Phytophthora species was isolated from diseased needles.

DFP was first recognized in February 2004 as unusual tree mortality in a 6-year-old P. radiata stand of about 70 ha on the Arauco coast of Chile. In the same area in October 2004, a serious needle blight disease was observed and associated with the mortality seen earlier. The area of diseased plantations peaked in 2006 and symptoms were less frequent in 2007. Disease levels remained very low through 2012.

Impacts in the Forest

At the height of the epidemic, the pathogen was isolated across a 600 km range in central Chile from Cardenal Caro Province in the north, south to Valdivia Province. The disease was most intense on east facing slopes near the Pacific Ocean. Only radiata pine has been affected. The long term impact of the disease will depend on the frequency at which epidemics recur and their duration, as well as the added impact resulting from secondary pathogens such as Diplodia. Despite the spectacular needle reddening and defoliation over large areas, few trees were killed in the 2004-2006 epidemic and plantations returned to near normal growth within 3 years with symptoms infrequent and usually confined to young trees adjacent to roads and close to the coast.

Forest and Wildland Hosts and Symptoms

The symptoms of DFP on mature trees begin with a reddening of the current year’s needles in early winter. The first needles to display symptoms are those on the lower sides of the branches. Needles die and become distinctly grey and begin to fall. Initially, dead and dying needles are retained on the branches giving the trees a scorched appearance. Trees can be almost entirely defoliated. New needle growth is not affected and the trees appear to recover unless infection re-occurs in the following season. After two or three years of defoliation, trees occasionally die and this appears to be hastened due to infection by Diplodia pinea, a well-known opportunistic pathogen.

An early symptom on affected needles is a dark resinous band on the green needle, which appears transparent when viewed with backlighting. Drops of resin are often found at the base of the needles and the tissue within the papery brachyblasts is commonly collapsed and has a light grey color. Needle tissue often collapses at the base and needles hang from the branches. When the adjacent twig bark is removed, a distinct brown or reddish brown discoloration may be seen in the phloem.

Young trees are most commonly and most severely damaged, especially when growing alongside larger trees affected by DFP. One of the first and most obvious symptoms to appear on young trees is wilting of growing terminal shoots with necrotic lesions at the base. Necrosis of the cambium is more pronounced on young trees than on mature trees Lesions may coalesce to form cankers in the phloem and outer xylem. Naturally regenerated plants and newly planted seedlings are equally affected by DFP.

DFP has a distinctly seasonal pattern of occurrence. Symptoms appear in July in early winter as temperatures drop to between 6 and 12°C. This is also the start of the rainy season and there appears to be a very close association of the disease with rainfall. Southern slopes (higher humidity, most free water and low sun radiation) are most severely affected.

Pinus radiata is the only known host for P. pinifolia. Native vegetation growing adjacent to diseased radiata pine plantations is unaffected. Similarly, plantations of other exotic conifers including Pseudotsuga menziezii and Pinus pinaster are unaffected. In artificial inoculation trials, distinct lesions developed on P. radiata shoots after 15 days. Inoculated shoots wilted and the needles turned brown, in a manner very similar to symptoms on infected shoots observed in nature.

Educational and Management Materials

• A New Species of Phytophthora Associated with Dying Pine Needles in Chile