Susceptibility of Garden Trees and Shrubs to Armillaria Root Rot

Investigation of Asymptomatic Infection of Phellinus noxius in Herbaceous Plants

The white-rot fungus Phellinus noxius is known to cause brown root rot disease (BRRD) in woody trees and shrubs. To understand the pathogenicity of P. noxius in herbaceous plants, we investigated 23 herbaceous weed and turfgrass species in 32 BRRD-infested sites in Taiwan and/or tested them by artificial inoculation. In the field survey, P. noxius was isolated from seven symptomless herbaceous species (i.e., Typhonium blumei, Paspalum conjugatum, Paspalum distichum, Oplismenus compositus, Bidens pilosa, Digitaria ciliaris, and Zoysia matrella). Potted plant inoculation assays suggested that P. noxius is able to infect Artemisia princeps, O. compositus, and Z. matrella but not Axonopus compressus, Eremochloa ophiuroides, Ophiopogon japonicus, or Cynodon dactylon. A. princeps plants wilted within 2 weeks postinoculation, but inoculated O. compositus and Z. matrella were asymptomatic, and P. noxius could be isolated from only inoculated sites. The colonization of P. noxius in the cortex and vascular cylinder of roots was visualized by paraffin sectioning and trypan blue staining of juvenile seedlings grown on water agar. To evaluate the effect of replantation for the remediation of BRRD-infested sites, P. noxius-inoculated wood strips were buried in soil with or without vegetation. After 4 weeks, P. noxius could be detected only in the bare soil group. For the control of BRRD, the herbaceous hosts should be removed around the diseased trees/stumps and non-host turfgrasses (e.g., A. compressus, E. ophiuroides, O. japonicus, or C. dactylon) planted to accelerate the degradation of P. noxius.

Endophytic Trichoderma spp. can protect strawberry and privet plants from infection by the fungus Armillaria mellea

Armillaria mellea is an important fungal pathogen worldwide, affecting a large number of hosts in the horticulture and forestry industries. Controlling A. mellea infection is expensive, labour intensive and time-consuming, so a new, environmentally friendly management solution is required. To this effect, endophytic Trichoderma species were studied as a potential protective agent for Armillaria root rot (ARR) in strawberry and privet plants. A collection of forty endophytic Trichoderma isolates were inoculated into strawberry (Fragaria × ananassa) plants and plant growth was monitored for two months, during which time Trichoderma treatment had no apparent effect. Trichoderma-colonised strawberry plants were then inoculated with A. mellea and after three months plants were assessed for A. mellea infection. There was considerable variation in ARR disease levels between plants inoculated with different Trichoderma spp. isolates, but seven isolates reduced ARR below the level of positive controls. These isolates were further tested for protective potential in Trichoderma-colonized privet (Ligustrum vulgare) plants where five Trichoderma spp. isolates, including two highly effective Trichoderma atrobrunneum isolates, were able to significantly reduce levels of disease. This study highlights the potential of plants pre-colonised with T. atrobrunneum for effective protection against A. mellea in two hosts from different plant families.

The Threat of Pests and Pathogens and the Potential for Biological Control in Forest Ecosystems

Forests are an essential component of the natural environment, as they support biodiversity, sequester carbon, and play a crucial role in biogeochemical cycles—in addition to producing organic matter that is necessary for the function of terrestrial organisms. Forests today are subject to threats ranging from natural occurrences, such as lightning-ignited fires, storms, and some forms of pollution, to those caused by human beings, such as land-use conversion (deforestation or intensive agriculture). In recent years, threats from pests and pathogens, particularly non-native species, have intensified in forests. The damage, decline, and mortality caused by insects, fungi, pathogens, and combinations of pests can lead to sizable ecological, economic, and social losses. To combat forest pests and pathogens, biocontrol may be an effective alternative to chemical pesticides and fertilizers. This review of forest pests and potential adversaries in the natural world highlights microbial inoculants, as well as research efforts to further develop biological control agents against forest pests and pathogens. Recent studies have shown promising results for the application of microbial inoculants as preventive measures. Other studies suggest that these species have potential as fertilizers.

Early Identification of Root Rot Disease by Using Hyperspectral Reflectance: The Case of Pathosystem Grapevine/Armillaria

Armillaria genus represents one of the most common causes of chronic root rot disease in woody plants. Prompt recognition of diseased plants is crucial to control the pathogen. However, the current disease detection methods are limited at a field scale. Therefore, an alternative approach is needed. In this study, we investigated the potential of hyperspectral techniques to identify fungi-infected vs. healthy plants of Vitis vinifera. We used the hyperspectral imaging sensor Specim-IQ to acquire leaves’ reflectance data of the Teroldego Rotaliano grapevine cultivar. We analyzed three different groups of plants: healthy, asymptomatic, and diseased. Highly significant differences were found in the near-infrared (NIR) spectral region with a decreasing pattern from healthy to diseased plants attributable to the leaf mesophyll changes. Asymptomatic plants emerged from the other groups due to a lower reflectance in the red edge spectrum (around 705 nm), ascribable to an accumulation of secondary metabolites involved in plant defense strategies. Further significant differences were observed in the wavelengths close to 550 nm in diseased vs. asymptomatic plants. We evaluated several machine learning paradigms to differentiate the plant groups. The Naïve Bayes (NB) algorithm, combined with the most discriminant variables among vegetation indices and spectral narrow bands, provided the best results with an overall accuracy of 90% and 75% in healthy vs. diseased and healthy vs. asymptomatic plants, respectively. To our knowledge, this study represents the first report on the possibility of using hyperspectral data for root rot disease diagnosis in woody plants. Although further validation studies are required, it appears that the spectral reflectance technique, possibly implemented on unmanned aerial vehicles (UAVs), could be a promising tool for a cost-effective, non-invasive method of Armillaria disease diagnosis and mapping in-field, contributing to a significant step forward in precision viticulture.

Susceptibility of Garden Plants to Phytophthora Root Rot

Phytophthora root rot (PRR) is a serious disease of horticultural, forest, and ornamental plant species caused by species of the oomycete genus Phytophthora. Their wide host range makes the choice of resistant plants in the management of the disease difficult. We used the Royal Horticultural Society diagnostic dataset of PRR records from U.K. gardens to compare the susceptibility of different host genera to the disease. The dataset was compared with existing reports of plants recorded as notably resistant or notably susceptible to PRR. An index-based approach was used to separate 177 genera of woody plants into three categories: 85 were low index (<0.10: rarely affected), 34 were medium index (0.10 to 0.20: sometimes affected) and 58 were high index (>0.20: frequently affected). Similarly, genera of nonwoody plants were separated into: 45 low index (<0.22), 16 medium index (0.22 to 0.44), and 18 high index (>0.44). Taxus was the genus with the highest index, while most genera in the Malvales and Ericales were in the high-index group. Most genera in the Myrtales, Fabales, and Monocotyledons were low index. While 30 Phytophthora species were recorded in our study, the wide host range spp., P. plurivora, P. cryptogea, and P. cinnamomi, represented 63% of identifications. P. plurivora was the most common species on woody plants and P. cryptogea on nonwoody plants. These results provide confidence in the use of host resistance as part of the integrated management of PRR.

Epidemiology, Biotic Interactions and Biological Control of Armillarioids in the Northern Hemisphere

Armillarioids, including the genera Armillaria, Desarmillaria and Guyanagaster, represent white-rot specific fungal saprotrophs with soilborne pathogenic potentials on woody hosts. They propagate in the soil by root-like rhizomorphs, connecting between susceptible root sections of their hosts, and often forming extended colonies in native forests. Pathogenic abilities of Armillaria and Desarmillaria genets can readily manifest in compromised hosts, or hosts with full vigour can be invaded by virulent mycelia when exposed to a larger number of newly formed genets. Armillaria root rot-related symptoms are indicators of ecological imbalances in native forests and plantations at the rhizosphere levels, often related to abiotic environmental threats, and most likely unfavourable changes in the microbiome compositions in the interactive zone of the roots. The less-studied biotic impacts that contribute to armillarioid host infection include fungi and insects, as well as forest conditions. On the other hand, negative biotic impactors, like bacterial communities, antagonistic fungi, nematodes and plant-derived substances may find applications in the environment-friendly, biological control of armillarioid root diseases, which can be used instead of, or in combination with the classical, but frequently problematic silvicultural and chemical control measures.

Identification of native endophytic Trichoderma spp. for investigation of in vitro antagonism towards Armillaria mellea using synthetic- and plant-based substrates

AIMS

To isolate endophytic Trichoderma species and investigate the potential for biological control of the root rot pathogen Armillaria mellea.

METHODS AND RESULTS

In all, 40 Trichoderma isolates were obtained from a range of host plants and identities were confirmed by ITS, rpb2 and tef1 sequence. When tested in dual culture assays for antagonism against A. mellea, Trichoderma isolates overgrew the A. mellea colonies within four days and by eight days 38 Trichoderma isolates significantly reduced A. mellea colony size. Armillaria mellea was unable to be recovered from five of eight co-cultivations tested, suggesting Trichoderma had killed the A. mellea in these cases. Pre-colonized hazel disks were used to determine what happens in a more heterogeneous situation with A. mellea and a refined set of eight Trichoderma isolates. Similar to plate-based assays, Trichoderma quickly covered A. mellea stopping any further growth and two Trichoderma isolates were able to eradicate A. mellea.

CONCLUSIONS

Of the Trichoderma spp. tested, endophytic isolates of Trichoderma virens and T. hamatum offered the greatest antagonism towards A. mellea. Using pre-colonized hazel disks was of great importance for this work to demonstrate the fungal interactions in plant material.

SIGNIFICANCE AND IMPACT OF THE STUDY

Controlling Armillaria root rot is difficult with chemical treatments, thus an environmentally benign and cost-effective alternative is required. This study highlights the prospect of biological control as an effective, environmentally friendly alternative to chemicals.

De novo sequencing, assembly and functional annotation of Armillaria borealis genome

BACKGROUND

Massive forest decline has been observed almost everywhere as a result of negative anthropogenic and climatic effects, which can interact with pests, fungi and other phytopathogens and aggravate their effects. Climatic changes can weaken trees and make fungi, such as Armillaria more destructive. Armillaria borealis (Marxm. & Korhonen) is a fungus from the Physalacriaceae family (Basidiomycota) widely distributed in Eurasia, including Siberia and the Far East. Species from this genus cause the root white rot disease that weakens and often kills woody plants. However, little is known about ecological behavior and genetics of A. borealis. According to field research data, A. borealis is less pathogenic than A. ostoyae, and its aggressive behavior is quite rare. Mainly A. borealis behaves as a secondary pathogen killing trees already weakened by other factors. However, changing environment might cause unpredictable effects in fungus behavior.

RESULTS

The de novo genome assembly and annotation were performed for the A. borealis species for the first time and presented in this study. The A. borealis genome assembly contained ~ 68 Mbp and was comparable with ~ 60 and ~ 79.5 Mbp for the A. ostoyae and A. mellea genomes, respectively. The N50 for contigs equaled 50,544 bp. Functional annotation analysis revealed 21,969 protein coding genes and provided data for further comparative analysis. Repetitive sequences were also identified. The main focus for further study and comparative analysis will be on the enzymes and regulatory factors associated with pathogenicity.

CONCLUSIONS

Pathogenic fungi such as Armillaria are currently one of the main problems in forest conservation. A comprehensive study of these species and their pathogenicity is of great importance and needs good genomic resources. The assembled genome of A. borealis presented in this study is of sufficiently good quality for further detailed comparative study on the composition of enzymes in other Armillaria species. There is also a fundamental problem with the identification and classification of species of the Armillaria genus, where the study of repetitive sequences in the genomes of basidiomycetes and their comparative analysis will help us identify more accurately taxonomy of these species and reveal their evolutionary relationships.