Biocontrol of Fusarium circinatum Infection of Young Pinus radiata Trees

Characterization of the Fusarium circinatum biofilm environmental response role

The capacity to form biofilms is a common trait among many microorganisms present on Earth. In this study, we demonstrate for the first time that the fatal pine pitch canker agent, Fusarium circinatum, can lead a biofilm-like lifestyle with aggregated hyphal bundles wrapped in extracellular matrix (ECM). Our research shows F. circinatum's ability to adapt to environmental changes by assuming a biofilm-like lifestyle. This was demonstrated by varying metabolic activities exhibited by the biofilms in response to factors like temperature and pH. Further analysis revealed that while planktonic cells produced small amounts of ECM per unit of the biomass, heat- and azole-exposed biofilms produced significantly more ECM than nonexposed biofilms, further demonstrating the adaptability of F. circinatum to changing environments. The increased synthesis of ECM triggered by these abiotic factors highlights the link between ECM production in biofilm and resistance to abiotic stress. This suggests that ECM-mediated response may be one of the key survival strategies of F. circinatum biofilms in response to changing environments. Interestingly, azole exposure also led to biofilms that were resistant to DNase, which typically uncouples biofilms by penetrating the biofilm and degrading its extracellular DNA; we propose that DNases were likely hindered from reaching target cells by the ECM barricade. The interplay between antifungal treatment and DNase enzyme suggests a complex relationship between eDNA, ECM, and antifungal agents in F. circinatum biofilms. Therefore, our results show how a phytopathogen's sessile (biofilm) lifestyle could influence its response to the surrounding environment.

Simply Versatile: The Use of Peribacillus simplex in Sustainable Agriculture

Peribacillus simplex is a Gram-positive, spore-forming bacterium derived from a vast range of different origins. Notably, it is part of the plant-growth-promoting rhizobacterial community of many crops. Although members of the Bacillaceae family have been widely used in agriculture, P. simplex has, so far, remained in the shadow of its more famous relatives, e.g., Bacillus subtilis or Bacillus thuringiensis. Recent studies have, however, started to uncover the bacterium's highly promising and versatile properties, in particular in agricultural and environmental applications. Hence, here, we review the plant-growth-promoting features of P. simplex, as well as its biocontrol activity against a variety of detrimental plant pests in different crops. We further highlight the bacterium's potential as a bioremediation agent for environmental contaminants, such as metals, pesticide residues, or (crude) oil. Finally, we examine the recent developments in the European regulatory landscape to facilitate the use of microorganisms in plant protection products. Undoubtedly, further studies on P. simplex will reveal additional benefits for agricultural and environmentally friendly applications.

The Potential of Wild Yeasts as Promising Biocontrol Agents against Pine Canker Diseases

Native wild yeasts from forest ecosystems represent an interesting potential source of biocontrol organisms in synergy with disease-tolerant forest materials. Yeasts have a combination of competitive mechanisms and low requirements for their biotechnological application as biocontrol agents. The current study aimed to increase the number of biocontrol candidates against Fusarium circinatum and Diplodia sapinea. The enzymatic and antagonistic activities of the biocontrol candidates were evaluated using different screening methods, in which the direct impact on the growth of the pathogen was measured as well as some properties such as cellulose and lignin degradation, tolerance to biocides, volatile compound production, or iron effect, which may be of interest in biotechnological processes related to the management of forest diseases. A total of 58 yeast strains belonging to 21 different species were obtained from oak forest and vineyard ecosystems and evaluated. The application of yeast treatment behaved differently depending on the pathogen and the plant clone. The 2g isolate (Torulaspora delbrueckii) showed the highest inhibitory activity for D. sapinea and 25q and 90q (Saccharomyces paradoxus) for F. circinatum. Clones IN416 and IN216 were the most susceptible and the most tolerant to D. sapinea, respectively, while the opposite was observed for F. circinatum.

Monitoring Forest Health Using Hyperspectral Imagery: Does Feature Selection Improve the Performance of Machine-Learning Techniques?

This study analyzed highly correlated, feature-rich datasets from hyperspectral remote sensing data using multiple statistical and machine-learning methods. The effect of filter-based feature selection methods on predictive performance was compared. In addition, the effect of multiple expert-based and data-driven feature sets, derived from the reflectance data, was investigated. Defoliation of trees (%), derived from in situ measurements from fall 2016, was modeled as a function of reflectance. Variable importance was assessed using permutation-based feature importance. Overall, the support vector machine (SVM) outperformed other algorithms, such as random forest (RF), extreme gradient boosting (XGBoost), and lasso (L1) and ridge (L2) regressions by at least three percentage points. The combination of certain feature sets showed small increases in predictive performance, while no substantial differences between individual feature sets were observed. For some combinations of learners and feature sets, filter methods achieved better predictive performances than using no feature selection. Ensemble filters did not have a substantial impact on performance. The most important features were located around the red edge. Additional features in the near-infrared region (800–1000 nm) were also essential to achieve the overall best performances. Filter methods have the potential to be helpful in high-dimensional situations and are able to improve the interpretation of feature effects in fitted models, which is an essential constraint in environmental modeling studies. Nevertheless, more training data and replication in similar benchmarking studies are needed to be able to generalize the results.

Wild Apple-Associated Fungi and Bacteria Compete to Colonize the Larval Gut of an Invasive Wood-Borer Agrilus mali in Tianshan Forests

The gut microflora of insects plays important roles throughout their lives. Different foods and geographic locations change gut bacterial communities. The invasive wood-borer Agrilus mali causes extensive mortality of wild apple, Malus sieversii, which is considered a progenitor of all cultivated apples, in Tianshan forests. Recent analysis showed that the gut microbiota of larvae collected from Tianshan forests showed rich bacterial diversity but the absence of fungal species. In this study, we explored the antagonistic ability of the gut bacteria to address this absence of fungi in the larval gut. The results demonstrated that the gut bacteria were able to selectively inhibit wild apple tree-associated fungi. Among them, Pseudomonas synxantha showed strong antagonistic ability, producing antifungal compounds. Using different analytical methods, such as column chromatography, mass spectrometry, HPLC, and NMR, an antifungal compound, phenazine-1-carboxylic acid (PCA), was identified. Activity of the compound was determined by the minimum inhibitory concentration method and electron microscopy. Moreover, our study showed that the gut bacteria could originate from noninfested apple microflora during infestation. Overall, the results showed that in newly invaded locations, A. mali larvae changed their gut microbiota and adopted new gut bacteria that prevented fungal colonization in the gut.

Pine Pitch Canker (PPC): Pathways of Pathogen Spread and Preventive Measures

Fusarium circinatum (Nirenberg and O’ Donnell) is the causal agent of pine pitch canker (PPC) disease, one of the most devastating forest diseases worldwide. Long-distance spread occurs mainly through the movement of infected seeds whereas at regional level, the movement of seedlings, substrates, or containers may play an important role in fungal dispersal. Invasion of nurseries takes place via infected seeds and further spread can occur by planting contaminated seedlings, especially due to the possibility of infected plants remaining symptomless. Once established, F. circinatum spreads by rain, wind, and insects. The natural spread of the pathogen is limited due to the short dispersal distances of the spores and the fairly short flight distances of disseminating insects. In this review, we summarize the currently known dispersal pathways of the pathogen, discussing both natural and human-assisted processes. With the purpose of understanding how to best intervene in the disease’s development in nurseries and forests, we outline the epidemiology of the pathogen describing the key factors influencing its spread. Preventive measures to control the spread of F. circinatum locally and globally are described with special emphasis on the challenges in implementing them.

Antifungal potential of Lauraceae rhizobacteria from a tropical montane cloud forest against Fusarium spp

The occurrence of pests and diseases can affect plant health and productivity in ecosystems that are already at risk, such as tropical montane cloud forests. The use of naturally occurring microorganisms is a promising alternative to mitigate forest tree fungal pathogens. The objectives of this study were to isolate rhizobacteria associated with five Lauraceae species from a Mexican tropical montane cloud forest and to evaluate their antifungal activity against Fusarium solani and F. oxysporum. Fifty-six rhizobacterial isolates were assessed for mycelial growth inhibition of Fusarium spp. through dual culture assays. Thirty-three isolates significantly reduced the growth of F. solani, while 21 isolates inhibited that of F. oxysporum. The nine bacterial isolates that inhibited fungal growth by more than 20% were identified through 16S rDNA gene sequence analysis; they belonged to the genera Streptomyces, Arthrobacter, Pseudomonas, and Staphylococcus. The volatile organic compounds (VOC) produced by these nine isolates were evaluated for antifungal activity. Six isolates (Streptomyces sp., Arthrobacter sp., Pseudomonas sp., and Staphylococcus spp.) successfully inhibited F. solani mycelial growth by up to 37% through VOC emission, while only the isolate INECOL-21 (Pseudomonas sp.) inhibited F. oxysporum. This work provides information on the microbiota of Mexican Lauraceae and is one of the few studies identifying forest tree-associated microbes with inhibitory activity against tree pathogens.

Application of Bioactive Coatings Based on Chitosan and Propolis for Pinus spp. Protection against Fusarium circinatum

Pine pitch canker (PPC) is a major threat to pine forests worldwide because of the extensive tree deaths, reduced growth, and degradation of timber quality caused by it. Furthermore, the aggressive fungus responsible for this disease (Fusarium circinatum) can also infect pine seeds, causing damping-off in young seedlings. This study proposes an approach based on coating treatments consisting of natural products to ensure seed protection. Seeds from two pine species (the most sensitive to this disease, Pinus radiata D. Don, and a more resistant one, Pinus sylvestris L.) were coated with single and binary mixtures of low and medium molecular weight chitosan and/or ethanolic-propolis extract. The germination rate, pre- and post-emergence mortality, total phenolic content, and radical scavenging activity were assessed. All treatments, and especially the one based on chitosan oligomers, had a beneficial impact on P. sylvestris seedlings, significantly enhancing survival rates and displaying a positive influence on the total phenolic content and on the seedlings’ radical scavenging activity. Conversely, non-significant negative effects on germination percentages were observed in the case of P. radiata seeds. The proposed treatments show promise for the protection of P. sylvestris seedlings against PPC.