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A special issue of Pathogens (ISSN 2076-0817). This special issue belongs to the section "Emerging Pathogens".

Deadline for manuscript submissions: closed (30 September 2023) | Viewed by 18339

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Prof. Dr. Paolo Gonthier

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Department of Agricultural, Forest and Food Sciences, University of Torino, Grugliasco, Italy
Interests: forest pathology; molecular ecology; invasive plant pathogens; fungal epidemiology; forest decline
Special Issues, Collections and Topics in MDPI journals

Prof. Dr. Matteo Garbelotto

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Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA 94720, USA
Interests: mycology; plant pathology; forest pathology; biodiversity
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Emergent infectious tree diseases are caused by non-native pathogens on native hosts, by native pathogens on planted exotic hosts, or by native pathogens on native hosts as a result of novel host–pathogen disequilibrium caused by climate change or by other types of anthropogenic pressures. Independent of their cause, emergent tree diseases are of significant concern due to their negative effects on the productivity, biodiversity, and ecosystem services that healthy trees can provide in forests, commercial plantations, or urban settings.

Costs to control emergent tree diseases are often prohibitive due to the scale and location of affected systems; hence, there is a pressing need to optimize tree disease management strategies by adopting novel, integrated, and sustainable disease control measures. This Special Issue focuses on the management of infectious emergent diseases of trees caused by a wide range of pathogens, including viruses, phytoplasmas, bacteria, fungi, fungal-like organisms (e.g., oomycetes), parasitic plants, and nematodes. We encourage the submission of studies regarding science-based tree disease management approaches such as pathogen avoidance/exclusion or eradication, and pathogen control approaches employing host resistance, silvicultural strategies, and biological/chemical protection or therapies. Particularly welcome are studies in which the efficacy of any of the approaches mentioned above have been not only designed but also tested in the field. Papers assessing the efficacy of biological control agents or chemical compounds only in vitro will generally not be considered, unless they bring new ideas or develop new analytical methods. Papers reporting disease control or the enhancement of health status of trees in association with the plant microbiome are particularly sought. Likewise, the identification of host resistance using modern approaches is of particular interest. Finally, conceptual and theoretical papers dealing with integrated disease management approaches are also welcome, as long as they are realistic and cost-effective.

Prof. Dr. Paolo Gonthier
Prof. Dr. Matteo Garbelotto
Guest Editors

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Pathogens is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

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Research

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14 pages, 2287 KiB

Open AccessArticle

Thermal Tolerance Data and Molecular Identification Are Useful for the Diagnosis, Control and Modeling of Diseases Caused by Thielaviopsis paradoxa

by Abiodun Abeeb Azeez, Daniel Ofeoritse Esiegbuya, Emad Jaber, Wenzi Ren, Adebola Azeez Lateef, Amarachi Ojieabu and Fred O. Asiegbu

Pathogens 2023, 12(5), 727; https://doi.org/10.3390/pathogens12050727 - 17 May 2023

Viewed by 1436

Abstract

Several economically important diseases of forest trees and agricultural crops in many parts of the world have been linked to the ascomycete fungal pathogen Thielaviopsis paradoxa. This study compared the growth rate of 41 isolates of T. paradoxa sourced from different hosts and two countries (Nigeria and Papua New Guinea (PNG)) under six temperature levels (22 °C, 25 °C, 30 °C, 32 °C, 34 °C and 35 °C). Phylogenetic relationships were obtained from the analysis of their nuclear ribosomal DNA internal transcribed sequence (ITS) data. While all the isolates from PNG and few from Nigeria grew optimally between 22 °C and 32 °C, the majority had their highest growth rate (2.9 cm/day) between 25 °C and 32 °C. Growth performances were generally low between 34 °C and 35 °C; no isolate from the sugar cane grew at these high temperatures. The oil palm isolate DA029 was the most resilient, with the highest growth rate (0.97 cm/day) at 35 °C. Phylogenetic analysis delineated five clusters: a very large clade which accommodates the majority (30 Nigerian and 3 PNG oil palm isolates) and four small clades containing two members each. To a large extent, the clustering pattern failed to address the temperature–isolate relationship observed. However, only the four small clades represent isolates with similar temperature tolerances. It is most likely that wider and robust analyses with more diverse isolates and genetic markers will provide better insight on thermal resilience of T. paradoxa. Additionally, future research to establish relationships between vegetative growth at different temperatures and of different pathogenicity and disease epidemiology merits being explored. The results might provide useful information for the formulation of effective management and control strategies against the pathogen, especially in this era of climate change. Full article

(This article belongs to the Special Issue Prevention and Management of Tree Diseases)

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15 pages, 3814 KiB

Open AccessArticle

Potassium Phosphonate Induces Resistance in Sweet Chestnut against Ink Disease Caused by Phytophthora Species

by Andrea Brandano, Salvatorica Serra, Giles E. St. J. Hardy and Bruno Scanu

Pathogens 2023, 12(3), 365; https://doi.org/10.3390/pathogens12030365 - 22 Feb 2023

Cited by 2 | Viewed by 1584

Abstract

Ink disease, caused by Phytophthora spp., represents a serious threat to sweet chestnuts throughout their distribution area. Among the control strategies, new perspectives have been offered by using potassium phosphonate, which indirectly controls Phytophthora diseases by acting on both host physiology and host-pathogen interactions. In this study, we tested in planta the effectiveness of trunk injection with K-phosphonate against seven different Phytophthora species associated with ink disease. For the two most aggressive species, P. cinnamomi and P. ×cambivora, the treatments were repeated at two different environmental conditions (a mean temperature of 14.5 °C vs. 25 °C) and tree phenology stages. The results obtained in this study demonstrated that K-phosphonate could contain the development of Phytophthora infection in phloem tissues. However, its effectiveness varied based on the concentration applied and the Phytophthora species tested. A concentration of 280 g/L of K-phosphonate was the most effective, and in some cases, callus formation around the necrotic lesion was detected. Overall, this study broadens the knowledge of endotherapic treatments with K-phosphonate as an effective measure for managing chestnut ink disease. Interestingly, the increase in mean temperature had a positive impact on the development of P. cinnamomi lesions on chestnut phloem tissues. Full article

(This article belongs to the Special Issue Prevention and Management of Tree Diseases)

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16 pages, 2581 KiB

Open AccessFeature PaperArticle

An Experiment in Denmark to Infect Wounded Sitka Spruce with the Rotstop Isolate of Phlebiopsis gigantea, and Its Implications for the Control of Heterobasidion annosum in Britain

by Jim Pratt and Iben M. Thomsen

Pathogens 2022, 11(8), 937; https://doi.org/10.3390/pathogens11080937 - 18 Aug 2022

Cited by 1 | Viewed by 1405

Abstract

The formulation of a Finnish isolate of the saprotrophic wood-rotting fungus Phlebiopsis gigantea into the biocontrol agent (BCA) Rotstop, which is used to prevent infection of Norway spruce stumps by aerial basidiospores of H. annosum, has led to its application to more than 200,000 ha of forest in Scandinavia and North Europe. The success of this treatment opens the possibility of introducing the Rotstop strain into Britain for use on Sitka spruce stumps, which at present (2022) are lacking adequate prophylactic treatment. However, Rotstop is probably non-native to Britain and to North America (the ancestral home of this spruce), and we do not know if this xylem-decaying BCA can invade standing trees. In this paper, we describe a trial into this issue conducted for the U.K. Forestry Commission in Denmark, in a country where both Rotstop and Sitka spruce have been naturalised. It was preliminary to further stump treatment trials, and is relevant to long-term issues surrounding stump treatment in Britain. Inoculations into 44-year-old standing Sitka spruce with 20 mm wooden Scots pine plugs pre-colonised with Rotstop resulted in decay of the S1, S2, S3 and middle lamellae of sapwood above and below the wounds after 11–18 months. In contrast, infection of sapwood occurred in only one of 39 wounds on the same trees challenged with oidial spore inoculants adpressed to undamaged xylem sapwood during the same period. While the results suggest that release of Rotstop into the productive stands of Sitka spruce in Britain would be unlikely to cause long-term commercial losses to wounded trees, the work highlights issues relating to the assessment of risk associated with the introduction of non-native BCAs within the forest environment. Full article

(This article belongs to the Special Issue Prevention and Management of Tree Diseases)

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12 pages, 1715 KiB

Open AccessArticle

Potential Role of Biocontrol Agents for Sustainable Management of Fungal Pathogens Causing Canker and Fruit Rot of Pistachio in Italy

by Giorgio Gusella, Alessandro Vitale and Giancarlo Polizzi

Pathogens 2022, 11(8), 829; https://doi.org/10.3390/pathogens11080829 - 25 Jul 2022

Cited by 4 | Viewed by 1800

Abstract

Pistachio (Pistacia vera) is an important Mediterranean crop. In Italy, pistachio is cultivated in the southern regions, of which Sicily is the main production area. Recently, the phytopathological situation of this crop has started to be updated, and new diseases have been discovered, studied, and reported. Botryosphaeriaceae spp. and Leptosillia pistaciae are major canker/rot pathogens, and Cytospora pistaciae and Eutypa lata have been reported as minor canker pathogens. In this paper, we evaluated different biological control agents, belonging to Trichoderma asperellum, T. atroviride and T. harzianum, as well as some Bacillus amyloliquefaciens strains, against above-mentioned pathogens. Results of dual culture assays showed that all the biological products, both fungi and bacteria, were able to inhibit the mycelial growth of the pathogens in vitro. Experiments using detached twigs showed no effect of biocontrol agents in reducing infections, except for Neofusicoccum hellenicum treated with T. harzianum T22 and Leptosillia pistaciae treated with B. amyloliquefaciens D747. Results of detached fruit experiments showed an efficacy ranging from 32.5 to 66.9% of all the biological products in reducing the lesions caused by N. mediterraneum. This study provides basic information for future research on biological control of pistachio diseases and future prospects for search of more effective biological control agents for canker diseases than those studied here. Full article

(This article belongs to the Special Issue Prevention and Management of Tree Diseases)

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15 pages, 4089 KiB

Open AccessArticle

Comparative Efficacy of Chondrosterum purpureum and Chemical Herbicides for Control of Resprouts in Tanoak and Bay Laurel

by Simon Francis Shamoun and Marianne Elliott

Pathogens 2022, 11(5), 485; https://doi.org/10.3390/pathogens11050485 - 19 Apr 2022

Cited by 1 | Viewed by 1955

Abstract

The invasive Oomycete pathogen Phytophthora ramorum has killed millions of susceptible oak and tanoak trees in California and southern Oregon forests and is responsible for losses in revenue to the nursery industry through mitigation activities. In addition, infestation of forests in the United Kingdom by this organism has resulted in the destruction of many hectares of larch plantations. Resprouting stumps can be a reservoir for the inoculum of P. ramorum persisting on a site. In areas where the application of herbicides is not permitted, a biocontrol treatment would be an indispensable alternative. Treatment of stumps with the sap-rotting fungus Chondrostereum purpureum (Pers.) Pouzar has been shown to be an effective tool for the suppression of resprouting on several species, most notably red alder. In this project, the ability of C. purpureum to suppress resprouting was evaluated on stumps of two host species, tanoak (Notholithocarpus densiflorus) and California bay laurel (Umbellularia californica). Laboratory testing of three California isolates of C. purpureum indicated that the fungus can colonize bay laurel stems. Field trials were established near Brookings, Oregon, on tanoak and on bay laurel near Soquel, California. Early results of field testing showed that C. purpureum was able to colonize the stumps of tanoak following treatment and was found to occur naturally on tanoak logs and stumps. Formulations of C. purpureum appear to have some effect on reducing sprout survival in tanoak, but the most effective and rapid treatment for this host is the hack and squirt method of applying the herbicide imazapyr. Sprayed herbicide prevents sprouting on bay laurel, and there was evidence that resprouting was inhibited on stumps treated with C. purpureum. Over time, applications of C. purpureum may be a more permanent solution as the stumps begin to decay. Full article

(This article belongs to the Special Issue Prevention and Management of Tree Diseases)

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19 pages, 1789 KiB

Open AccessArticle

The Epidemiology of Sudden Oak Death Disease Caused by Phytophthora ramorum in a Mixed Bay Laurel-Oak Woodland Provides Important Clues for Disease Management

by Melina Kozanitas, Margaret R. Metz, Todd W. Osmundson, Maria Socorro Serrano and Matteo Garbelotto

Pathogens 2022, 11(2), 250; https://doi.org/10.3390/pathogens11020250 - 15 Feb 2022

Cited by 4 | Viewed by 2129

Abstract

Epidemiological models are important for the understanding of disease progression in plants and for the design of control strategies. Phytophthora ramorum, the pathogen responsible for the disease known as Sudden Oak Death, causes lethal infection on several oaks but relies on California bay laurels for transmission. Here, repeated surveys of bay laurels and oaks indicated that bay laurel disease incidence was positively correlated with rainfall, bay laurel density, and an eastern aspect, and negatively correlated with bay laurel basal area. Oak infection only occurred in years when rainfall was higher than the 30-year average, and although infection rates were greater among larger trees, mortality was greater among smaller trees. Additionally, larger oaks closer to infected bay laurels exhibited greater infection rates. Disease incidence differed among sites, and only a fraction of bay laurels were disease superspreaders, while even fewer individuals were refugial trees harboring active infections during dry periods. Based on this study, reducing bay laurel density in denser stands and the number of superspreaders or refugial trees in less dense stands may reduce disease incidence. However, the selective removal of bay laurel trees 0–10 m from oaks is likely to be more effective in preventing infection of specific oaks. Full article

(This article belongs to the Special Issue Prevention and Management of Tree Diseases)

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15 pages, 940 KiB

Open AccessArticle

Efficacy of Chemical and Biological Stump Treatments for the Control of Heterobasidion occidentale Infection of California Abies concolor

by Adrian L. Poloni, Matteo Garbelotto, Christopher A. Lee and Richard C. Cobb

Pathogens 2021, 10(11), 1390; https://doi.org/10.3390/pathogens10111390 - 27 Oct 2021

Cited by 3 | Viewed by 1537

Abstract

We conducted an experimental evaluation of treatments to limit Heterobasidion occidentale infection of white fir (Abies concolor) stumps and wounds in California mixed conifer forests. We tested the efficacy of urea, borate, and a mixture of two locally collected Phlebiopsis gigantea strains in preventing pathogen colonization of fir stumps and separately, urea and borate as infection controls on experimental stem wounds. These were paired with a laboratory test on ~100 g wood blocks with and without a one-week delay between inoculation and treatment. Urea, borates, and Phlebiopsis treatments all significantly reduced the stump surface area that was colonized by H. occidentale at 84%, 91%, and 68%, respectively, relative to the controls. However, only the borate treatments significantly lowered the number of stumps that were infected by the pathogen. The laboratory study matched the patterns that were found in the stump experiment with a reduced area of colonization for urea, borates, or P. gigantea treatments relative to the controls; delaying the treatment did not affect efficacy. The field wound experiment did not result in any Heterobasidion colonization, even in positive control treatments, rendering the experiment uninformative. Our study suggests treatments that are known to limit Heterobasidion establishment on pine or spruce stumps elsewhere in the world may also be effective on true firs in California. Full article

(This article belongs to the Special Issue Prevention and Management of Tree Diseases)

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16 pages, 619 KiB

Open AccessArticle

Comparative Efficacy of State-of-the-Art and New Biological Stump Treatments in Forests Infested by the Native and the Alien Invasive Heterobasidion Species Present in Europe

by Martina Pellicciaro, Guglielmo Lione, Silvia Ongaro and Paolo Gonthier

Pathogens 2021, 10(10), 1272; https://doi.org/10.3390/pathogens10101272 - 02 Oct 2021

Cited by 9 | Viewed by 1648

Abstract

The Heterobasidion annosum species complex includes major fungal pathogens of conifers worldwide. State-of-the-art preventative stump treatments with urea or with commercial formulations of the fungal biological control agent Phlebiopsis gigantea (i.e., Rotstop^®) may become no longer available or are not approved for use in many areas of Europe infested by the three native Heterobasidion species and by the North American invasive H. irregulare, making the development of new treatments timely. The efficacy of Proradix^® (based on Pseudomonas protegens strain DSMZ 13134), the cell-free filtrate (CFF) of the same bacterium, a strain of P. gigantea (MUT 6212) collected in the invasion area of H. irregulare in Italy, Rotstop^®, and urea was comparatively investigated on a total of 542 stumps of Abies alba, Picea abies, Pinus pinea, and P. sylvestris in forest stands infested by the host-associated Heterobasidion species. Additionally, 139 logs of P. pinea were also treated. Results support the good performances of Rotstop^®, and especially of urea against the native Heterobasidion species on stumps of their preferential hosts and, for the first time, towards the invasive North American H. irregulare on stumps of P. pinea. In some experiments, the effectiveness of Proradix^® and of the strain of P. gigantea was weak, whereas the CFF of P. protegens strain DSMZ 13134 performed as a valid alternative to urea and Rotstop^®. The mechanism of action of this treatment hinges on antibiosis; therefore, further improvements could be possible by identifying the active molecules and/or by optimizing their production. Generally, the performance of the tested treatments is not correlated with the stump size. Full article

(This article belongs to the Special Issue Prevention and Management of Tree Diseases)

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19 pages, 23163 KiB

Open AccessReview

Major Pathogens Affecting Carob in the Mediterranean Basin: Current Knowledge and Outlook in Italy

by Ivana Castello, Giancarlo Polizzi and Alessandro Vitale

Pathogens 2023, 12(11), 1357; https://doi.org/10.3390/pathogens12111357 - 15 Nov 2023

Viewed by 1324

Abstract

The main pathogens affecting the carob (Ceratonia siliqua) tree in the Mediterranean basin are described in this overview. The most widespread diseases periodically occurring in carob orchards are powdery mildew (Pseudoidium ceratoniae) and cercospora leaf spot (Pseudocercospora ceratoniae). The causal agents of “black leaf spots” (e.g., Pestalotiopsis, Phyllosticta and Septoria spp.) are responsible for symptoms similar to those previously mentioned for foliar diseases, but are reported in carob orchards at a negligible frequency. Likewise, canker and branch diebacks caused by fungal species belonging to Botryosphaeriaceae are almost never recorded. Among the rots of wood tissues that may compromise old carob specimens, “brown cubical rot” caused by Laetiporus sulphureus is the most widespread and recurrent issue; this pathogen is also well-known for producing edible fruit bodies that are appreciated for pharmaceutical and industrial purposes. On the other hand, “white rots” caused by Fomes and Ganoderma species are less common and reported for the first time in this review. Gall-like protuberances on twigs of uncertain aetiology or tumors on branches associated with Rhizobium radiobacter are described, although these symptoms are seldom detected, as they are also observed for necrotic leaf spots caused by Pseudomonas syringae pv. ciccaronei. A worldwide list of pathogens not yet recorded but at high risk of potential introduction in Italian carob-producing areas is also provided. Finally, concerns related to new phytopathogenic fungi vectored by the invasive Xylosandrus compactus ambrosia beetle are addressed. All the described pathogens could become limiting factors for carob production in the near future, because they could be favored by high-density orchards, the increasing global network of trade exchanges, and the high frequency at which extreme events related to climate change occur globally. Thus, symptoms and signs, causal agents, epidemiology, and, whenever applicable, recommendations for disease prevention and management are provided in this review. Full article

(This article belongs to the Special Issue Prevention and Management of Tree Diseases)

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16 pages, 2668 KiB

Open AccessReview

Thousand Cankers Disease in Walnut Trees in Europe: Current Status and Management

by Matteo Bracalini, Alessandra Benigno, Chiara Aglietti, Tiziana Panzavolta and Salvatore Moricca

Pathogens 2023, 12(2), 164; https://doi.org/10.3390/pathogens12020164 - 19 Jan 2023

Cited by 2 | Viewed by 1958

Abstract

Thousand cankers disease (TCD) is a new deadly disease in walnut trees (Juglans spp.), which is plaguing commercial plantations, natural groves, and ornamental black walnut trees (Juglans nigra) in their native and invasion areas in the US and, more recently, in artificial plantations and amenity trees in the newly-invaded areas in Europe (Italy). This insect/fungus complex arises from the intense trophic activity of the bark beetle vector Pityophthorus juglandis in the phloem of Juglans spp. and the subsequent development of multiple Geosmithia morbida cankers around beetles’ entry/exit holes. After an analysis of the main biological and ecological traits of both members of this insect/fungus complex, this review explores the options available for TCD prevention and management. Special focus is given to those diagnostic tools developed for disease detection, surveillance, and monitoring, as well as to existing phytosanitary regulations, protocols, and measures that comply with TCD eradication and containment. Only integrated disease management can effectively curtail the pervasive spread of TCD, thus limiting the damage to natural ecosystems, plantations, and ornamental walnuts. Full article

(This article belongs to the Special Issue Prevention and Management of Tree Diseases)

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