Developing tools to unravel the biological secrets of Rosellinia necatrix, an emergent threat to woody crops

Abstract

White root rot caused by Rosellinia necatrix is one of the most destructive diseases of many woody plants in the temperate regions of the world, particularly in Europe and Asia. Recent outbreaks of R. necatrix around the globe have increased the interest in this pathogen. Although the ecology of the disease has been poorly studied, recent genetic and molecular advances have opened the way for future detailed studies of this fungus.

Taxonomy: Rosellinia necatrix Prilleux. Kingdom Fungi; subdivision Ascomycotina; class Euascomycetes; subclass Pyrenomycetes; order Sphaeriales, syn. Xylariales; family Xylariaceae; genus Rosellinia.

Identification: Fungal mycelium is present on root surfaces and under the bark, forming mycelium fans, strands or cords. A typical presence of pear-shaped or pyriform swellings can be found above the hyphal septum (with diameters of up to 13 µm). Sclerotia are black, hard and spherical nodules, several millimetres in diameter. Black sclerotia crusts may also form on roots. On synthetic media, it forms microsclerotia: irregular rough bodies composed of a compact mass of melanized, interwoven hyphae with no differentiated cells. Chlamydospores are almost spherical (15 µm in diameter). Synnemata, also named coremia (0.5-1.5 mm in length), can be formed from sclerotia or from mycelial masses. Conidia (3-5 µm in length and 2.5-3 µm in width) are very difficult to germinate in vitro. Ascospores are monostichous, situated inside a cylindrical, long-stalked ascus. They are ellipsoidal and cymbiform (36-46 µm in length and 5.5-6.3 µm in width).

Host range: This fungus can attack above 170 different plant hosts from 63 genera and 30 different families, including vascular plants and algae. Some are of significant economic importance, such as Coffea spp., Malus spp., Olea europaea L., Persea americana Mill., Prunus spp. and Vitis vinifera L.

Disease symptoms: Rosellinia necatrix causes white (or Dematophora) root rot, which, by aerial symptoms, shows a progressive weakening of the plant, accompanied by a decline in vigour. The leaves wilt and dry, and the tree can eventually die. White cottony mycelium and mycelial strands can be observed in the crown and on the root surface. On woody plant roots, the fungus can be located between the bark and the wood, developing typical mycelium fans, invading the whole root and causing general rotting.

Disease control: Some approaches have been attempted involving the use of tolerant plants and physical control (solarization). Chemical control in the field and biological control methods are still under development.

Figure 1

Life cycle of Rosellinia necatrix, covering the sexual and asexual aspects of the cycle. The asexual life cycle occurs via two different spore types, chlamydospores (not considered because they are rarely found under natural conditions) and conidiospores. Conidia originate at the ends of synnemata of conidiogenous cells, which are produced from either sclerotia or brown mycelial masses. The sexual life cycle is mediated by ascospores, which can be easily found in infected tissues. The sexual reproductive structures of R. necatrix are formed inside the perithecium and, when they reach maturity, are expelled. As they age, they contract and acquire a brown–black colour and a dry aspect as a result of hyphal and cell melanization. The asci are projected towards the interior of the perithecium. Perithecia formation takes a long time under natural conditions and has never been achieved previously in a Petri dish.

Figure 2

Characteristic macroscopic symptoms caused by Rosellinia necatrix on avocado. (A) Healthy 15‐year‐old avocado tree. (B) Advanced aerial symptoms of R. necatrix root rot on a 15‐year‐old avocado tree: dry leaves attached to the wilted tree, sparse foliage and dry branches are observed. (C) Hyphal strands and cords of R. necatrix on the soil surface. (D) Hyphal strands and cords of R. necatrix spreading from colonized to healthy avocado roots in an avocado pot plant. (E) Rosellinia necatrix spreading on the wood in a symptomatic 15‐year‐old avocado tree.

Figure 3

Root infection course of Rosellinia necatrix in avocado. This study was conducted used the green fluorescent protein (GFP)‐tagged R. necatrix CH53 strain and visualized by confocal laser spectroscopy and scanning electron microscopy. The whole process takes around 24 days in 1‐year‐old avocado plantlets. The root infection by R. necatrix initiates with fungal development on the root surface without penetration. Then, root penetration can occur in different ways, including natural openings, surface wounds and by direct penetration. Once inside the root, R. necatrix spreads to the inner parts, resulting in xylem invasion, as well as underneath the bark, leading to full symptom expression, as shown in Fig. 2.