New insights into the survival strategy of the invasive soilborne pathogen Phytophthora cinnamomi in different natural ecosystems in Western Australia

Abstract

Despite its importance as one of the most notorious, globally distributed, multihost plant pathogens, knowledge on the survival strategy of Phytophthora cinnamomi in seasonally dry climates is limited. Soil and fine roots were collected from the rhizosphere of severely declining or recently dead specimens of 13 woody species at 11 dieback sites and two dieback spots and from healthy specimens of five woody species at four dieback-free sites in native forests, woodlands and heathlands of the south-west of Western Australia (WA). Phytophthora cinnamomi was recovered from 80.4, 78.1 and 100% of tested soil, fine root and soil-debris slurry samples at the 11 dieback sites, in some cases even after 18-month storage under air-dry conditions, but not from the small dieback spots and the healthy sites. Direct isolations from soil-debris slurry showed that P. cinnamomi colonies exclusively originated from fine roots and root fragments not from free propagules in the soil. Microscopic investigation of P. cinnamomi-infected fine and small woody roots and root fragments demonstrated in 68.8, 81.3 and 93.8% of samples from nine woody species the presence of thick-walled oospores, stromata-like hyphal aggregations and intracellular hyphae encased by lignitubers, respectively, while thin-walled putative chlamydospores were found in only 21.2% of samples from five woody species. These findings were confirmed by microscopic examination of fine roots from artificially inoculated young trees of 10 woody species. It is suggested that (i) the main function of chlamydospores is the survival in moderately dry conditions between consecutive rain events and (ii) selfed oospores, hyphal aggregations, and encased hyphae and vesicles in infected root tissue of both host and non-host species are the major long-term survival propagules of P. cinnamomi during the extremely dry summer conditions in WA.

Genotypic Diversity of Phytophthora cinnamomi and P. plurivora in Maryland’s Nurseries and Mid-Atlantic Forests

“…Third, sexual reproduction can lead to the breakup of coadapted genes in clonal lineages, which would be unfavorable in low selection pressure environments (Dobrowolski et al 2003). For these reasons, sexual reproduction is not always advantageous, which could explain why, even when both mating types of P. cinnamomi are present, outcrossing is limited (Dobrowolski et al 2003;Jung et al 2013). Our study suggests that, although the difference was small, the forest population of P. cinnamomi is more diverse than the Maryland nursery population.…”

“…can also stimulate selfing within heterothallic oomycetes (Brasier 1971(Brasier , 1978Jayasekera et al 2007;Jung et al 2013;Lamour and Kamoun 2009). Differences between forest and nursery systems in terms of these environmental factors could cause different rates of selfing which, in turn, could affect genetic differences between the two systems.…”

“…Selfing in heterothallic oomycetes, known as "secondary homothallism", may confer an evolutionary advantage by enabling heterothallic species to form oospores in adverse environmental conditions (Lamour and Kamoun 2009). In fact, the formation of oospores in A2 populations has been shown to occur regularly in natural forests as a strategy to survive adverse conditions (Crone et al 2013;Jung et al 2013). Another factor to consider is the stimulation of oospores in many water molds when salinity levels of the surrounding environment are consistent with those of their natural habitats (Lamour and Kamoun 2009).…”

Evaluation of PCR markers for Phytophthora infestans mating type determination

“…Although in populations of P. capsici from Peru and in most provinces of China asexual clonal lineages dominate (Hu et al 2013;Hurtado-Gonzales et al 2008). According to Jung et al (2013) isolates of A2 mating type of P. cinnamomi are generally more aggressive and invasive than the A1 ones. When they were introduced to Australia, southern Europe and the USA they caused symptoms of disease on many native species.…”

Pathways to false positive diagnoses using molecular genetic detection methods; Phytophthora cinnamomi a case study

“…Phytophthora species occur worldwide and can be highly invasive plant pathogens (Erwin and Ribeiro 1996;Hansen et al 2012;Jung et al 2013;Jung et al 2015;Scott et al 2013). Phytophthora cinnamomi is widely distributed globally outside its presumed natural range and causes economic losses in both horticulture and in natural ecosystems (Hayden et al 2013).…”