Characterising the growth response and pathogenicity of Phytophthora agathidicida in soils from contrasting land-uses

Abstract

The genus Phytophthora (Oomycetes, Peronosporales, Pythiaceae) is responsible for several forest declines worldwide (i.e. jarrah dieback in Australia (P. cinnamomi) and sudden oak death in California and Europe (P. ramorum)). The recently described pathogen, P. agathidicida, is the causal agent of dieback in remnant stands of New Zealand kauri (Agathis australis), and poses a significant threat to the long-term survival of this iconic species. However, what is least understood are how key physicochemical parameters (e.g. soil pH and soil organic matter) influence growth and pathogenicity of P. agathidicida. This study examined the effects of three contrasting land-uses (kauri forest, grazed pasture, short-rotation pine plantation (Pinus radiata) on the growth and pathogenicity of P. agathidicida in soils sampled from Waipoua Forest, Northland, New Zealand. This was investigated usin: 1) Growth response assay, 2) Pathogenicity trait study with blue lupin (Lupinus angustifolius), and 3) Pasture and pine alternative host infection study. Experiment 1 found that significantly greater sporangia (p < 0.001) and oospore (p <0.01) counts occurred within pasture and pine soils compared to kauri soils, suggesting that they favour P. agathidicida in the early stages of establishment. Additionally, significant increases in oospores (p <0.01) over time in the pine soils potentially suggest their enhanced capacity to act as pathogen reservoirs. Furthermore, two new Phytophthora spp. to New Zealand (P. pini and P. gregata) were identified in this study. Experiment 2 identified non-significant land-use effects on pathogenicity traits (e.g. lesion presence, lesion length etc.) of P. agathidicida inoculation of blue lupin. Experiment 3 also confirmed the potential for pasture and pine to act as reservoirs for P. agathidicida. Overall, the findings of this study revealed that contrasting land-use affects the growth of P. agathidicida in soil, and further detailed study of the activity, distribution and pathogenicity of P. agathidicida in fragmented landscapes is warranted.