Influence of severe drought on the resistance of Pinus yunnanensis to a bark beetle‐associated fungus

Abstract

Summary Research on tree response to pest attacks under drought and heat stress is crucial for predicting the effect of climate change on forest pests and tree health. In this work, we studied the influence of severe water stress on the resistance of Pinus yunnanensis to inoculations of Grosmannia yunnanensis, a pathogenic fungus associated with the aggressive bark beetle, Tomicus yunnanensis. Experiments were performed in two plots, one located at the top of a hill and the other at the foot of a hill, in Xiaojiu, Yunnan, China, from May to September 2010 and March to April 2012. Measurements of soil and needle water content and predawn needle water potentials confirmed that severe droughts occurred during these periods. Maximum water stress intensity even occurred in 2010 during the typical rainy season. Following low‐density inoculations, the reaction zone length and fungal growth were measured in the phloem. Sapwood blue staining and total sapwood occlusion were quantified following fungal mass inoculations. Our results suggest that severe water stress has a negative impact on a tree's resistance to G. yunnanensis. By comparing our results to previous work performed from 1997 to 1999 on the same tree species, and in the same locality under mild water stress, we concluded that drought had opposite effects on tree resistance above and below a certain threshold of predawn needle water potential. The threshold, corresponded to maximum efficiency for tree defence mechanisms, was visible in our results, and was estimated to be approximately ‐1.1 MPa to ‐1.2 MPa for P. yunnanensis under local conditions. We suggest that the existence of such a threshold of water stress intensity may be viable for any conifer undergoing biotic aggressions and, specifically, bark beetle attacks.

Drought impacts on tree phloem: from cell-level responses to ecological significance

“…Even if the mechanisms underlying phloem defence response to drought are not well understood, published studies suggest that often, inducible defences (which comprise structural defences e.g., early lignification of fibers, formation of callus tissues, resin ducts, lesions, or wound periderms, which can directly influence phloem function (Keeling and Bohlmann 2006) and chemical defences e.g., expression of enzymes, phenolics, terpenoids, alkaloids and volatile organic compounds (Franceschi et al 2005;Keeling et al 2008)) are found to be impaired by drought while constitutive defences (production of a suite of defense compounds e.g., terpenoids, phenolics, alkaloids, resins, and enzymes such as chitinases) and mechanical adaptations of the bark (Franceschi et al 2005) appear to be enhanced (Lorio et al 1995;Lombardero et al 2000;Lusebrink et al 2011;Gao et al 2017;Klutsch et al 2017) (Table 2). For example, genes of both the terpene-synthase and chitinase families were upregulated in individuals of Pinus banksiana x Pinus contorta under water limitation (Arango-Velez et al 2014).…”

Drought stress modifies early effective resistance and induced chemical defences of Aleppo pine against a chewing insect herbivore

“…Most studies about damage caused by different pest and pathogens in conifer species agree that severe drought stress would affect negatively plant defensive allocation, because as drought stress increases less photosynthates are available to synthesize chemical defenses, and more damage is expected (Anderegg et al, 2015;Devkota et al, 2018;Gao et al, 2017;Klutsch et al, 2017). Thus, terpene concentration may be reduced when water availability is too low (Bertin and Staudt, 1996;Llusià and Peñuelas, 1998;Klutsch et al, 2017).…”

Differential patterns of ophiostomatoid fungal communities associated with three sympatric Tomicus species infesting pines in south-western China, with a description of four new species

“…Up to now, no data have been provided proving the pathogenicity of these ophiostomatoid species to both indigenous pines, except for L.yunnanense (Liao and Ye 2004, Gao et al 2017). Pathogenicity tests have been done by artificial inoculation of the dominant species into seedlings of the two pines.…”

“…Two species ( Graphilbumfragrans and O.tingens ) were recorded as being associated with T.minor (Zhou et al 2013, Pan et al 2017), whereas only a single species ( O.brevipilosi ) was recorded as being associated with T.brevipilosus (Chang et al 2017). Amongst them, L.yunnanense was the first species newly described from the area (Zhou et al 2000) and is likely the most virulent one (Liao and Ye 2004, Gao et al 2017). Until now, the relative abundance with which these fungi occur, their host (pine and beetle) relationships, and their pathogenicity remain unknown.…”