The session was delivered by Italy's University of Torino (UNITO) - one of EMPHASIS partners. The talk gave an overview on symptoms and impact of diseases caused by the fungal pathogens included in the Heterobasidion annosum species complex. The biology and epidemiology of these pathogens were presented as well as the main strategies of disease control, with emphasis on integrated forest disease management. Speaker was UNITO researcher Dr Luana Giordano.

1. UNIVERSITY OF TORINO
DEPARTMENT OF AGRICULTURAL, FOREST AND FOOD SCIENCES
AGROINNOVA - Centre of Competence
Overview on Heterobasidion root and butt rots of conifers
Fourth webinar on plant health
November 17th, 2017
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant
agreement No 634179

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General impact of Heterobasidion annosum sensu lato (s.l.)
- Financial losses estimated at about 800 millions Euros/yearin Europe
- Over 200 host species, mostly conifers
- 5 species distributed in Eurasia and North America
Korhonen and Stenlid, 1998
Worldwide distribution

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Heterobasidion annosum s.l.: the pathogen of records
- 1700 scientific papers: one of the most intensively studied forest fungi
- The complete genome sequence is available: the first sequenced plant pathogenic
homobasidiomycete (Olson et al., 2012)
- It is one of the few examples of a forest pathogen that can be and has been controlled in managed
forests (Garbelotto and Gonthier, 2013)
Freely downloadable from:
http://arjournals.annualreviews.org/eprint/Sgi6HxDIHURPYBckxFhw/full
/10.1146/annurev-phyto-082712-102225

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Heterobasidion taxonomy
Heterobasidion annosum (Fr.) Bref. complex
Kingdom: Fungi
Division: Basidiomycota
Class: Agaricomycetes
Order: Russulales
Family: Bondarzewiaceae

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Heterobasidion annosum species complex
3 Eurasian Heterobasidion species
• H. annosum sensu stricto (s.s.) (Fr.) Bref./H. annosum: wide host range, mostly
associated with Pinus spp.
• H. abietinum Niemelä & Korhonen: commonly associated with Abies spp.
• H. parviporum Niemelä & Korhonen: strictly associated with Picea abies (L.) Karst.
2 North American Heterobasidion species
• H. irregulare Garbel. & Otrosina: on Pinus spp., Juniperus spp. and Libocedrus spp.
• H. occidentale Otrosina & Garbel.: wide host range, mostly associated with Picea spp.,
Abies spp., Pseudotsuga spp., Tsuga spp. and Sequoiadendron spp.
Five species regarded as some of the most destructive pathogens of conifers worldwide

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Heterobasidion annosum species distribution
Garbelotto and Gonthier, 2013

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Symptoms and diagnosis: root and heart rot
Root rot (e.g. Pinus spp., Juniperus spp.)
Heart rot (e.g. Picea spp., Abies spp.)

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Symptoms and diagnosis: root and heart rot
Root rot (e.g. Pinus spp., Juniperus spp.)
Heart rot (e.g. Picea spp., Abies spp.)

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Symptoms and diagnosis: root and heart rot
Root rot (e.g. Pinus spp., Juniperus spp.)
Heart rot (e.g. Picea spp., Abies spp.)

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Symptoms and diagnosis: root and heart rot

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Consequences of root and heart rot: reduction of mechanical stability of trees
Forest ecosystems
Negative role: reduction of protection functionPositive role: primary biotic decomposers
Forest ecosystems

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Uprooting resistance of infected trees is 1/3 and 1/5 lower than
that of uninfected trees
Consequences of root and heart rot: reduction of mechanical stability of trees

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Consequences of root and heart rot: canopy gaps in pine stands

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Consequences of root and heart rot: canopy gaps in pine stands

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Signs and diagnosis: fruiting bodies localization

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Signs and diagnosis: fruiting bodies morphology
Fruiting bodies pileate

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Signs and diagnosis: fruiting bodies morphology
Fruiting bodies resupinate

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Signs and diagnosis: fruiting bodies morphology
Incipient fruiting bodies – «popcorn»

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Signs and diagnosis: fruiting bodies morphology
Incipient fruiting bodies – «popcorn»

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Signs and diagnosis: mycelium under the bark
Paper-thin mycelium under the bark of infected roots or at the tree collar

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Signs and diagnosis: mycelium under the bark
Armillaria sp.

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Diagnosis in laboratory: wood samples
One week incubation

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Diagnosis in laboratory: wood samples
Conidiophores of Spiniger meineckellus

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Diagnosis in laboratory: morphological analyses of fruiting bodies
The five Heterobasidion species are morphologically similar to one another
Otrosina and Garbelotto, 2009

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Diagnosis in laboratory: molecular analyses
A number of molecular diagnostic assays based on both nuclear and mitochondrial DNA
have been developed
Nicolotti et al., 2010

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Diagnosis in laboratory: airborne inoculum
Wood disk exposure method (James and Cobb, 1984; modified by Gonthier et al., 2001)
•Wood disks 11-13 cm diameter
•Treatment with 65% ethanol
•24 h exposure in the forest
•Treatment with a benomyl solution
•One week incubation in laboratory
Emerging colonies of Heterobasidion spp. are identified and counted through microscopic observation

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Diagnosis in laboratory: airborne inoculum
Heterobasidion spp. identification: Taxon-Specific Competitive-Priming (TSCP)-PCR
Depending on total number of observed colonies, up to 15 randomly chosen
colonies per trap are isolated.
Frequencies of different species of Heterobasidion are expressed (i) as
number of viable spores per square meter per hour (deposition rate – DR)
and (ii) as the percentage of infected disks (infection frequency – IF). We
assumed that each colony resulted from deposition of one viable spore.
•H. annosum s.s.: 230 bp
•H. abietinum: 195 bp
•H. parviporum: 185 bp
P F S

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Diagnosis in laboratory: multiplex-PCR
DNA extraction

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Infection biology and life cycle
By Jan Stenlid
primary infections
secondary infections

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The fungus produces both sexual spores and conidia, but it is largerly accepted that the
overwhelming majority of airborne infections are caused by sexual spores. There is an
active role of conidia only in short-distance transmission or when they are accidentally
vectored by root-feeding insects (Kadlec et al., 1992).
Spores of Heterobasidion spp. have been reported to travel long distances (hundreds of
kilometers - Rishbeth, 1959; Kallio, 1970)
Dispersal gradient of Heterobasidion spp.
only 3% of spores travels 10 m
Kallio, 1970; Stenlid, 1994
Epidemiology: spore dispersal range

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Epidemiology: temporal patterns of primary infections
Airborne inoculum in the Alps
Gonthier et al., 2005

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Epidemiology: factors affecting disease incidence in a stand
Heterobasidion spp. play a more subordinate role in natural forest ecosystems than in
managed forests (Shaw et al., 1994)
Site location, soil, and site history have all been reported to affect disease incidence
Management techniques adopted may play an important role

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Management strategies and tactics
Avoidance – Logging and thinning when risk of stump infection is low
Gonthier et al., 2005
Northern Europe – during the wintertime (when temperatures are below 0°C)
South-eastern USA – during the summer (when temperatures can reach 40°C)

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Management strategies and tactics
Avoidance – Logging and thinning when risk of stump infection is low
Case 1: uninfected stand
Case 2: infected stand
Case 2 Case 1

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Management strategies and tactics
Eradication – There is a little chance of eradicating the fungus totally once it is established
Stump removal is an expensive and time-consuming control method that requires the use of machines

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Management strategies and tactics
Eradication – There is a little chance of eradicating the fungus totally once it is established
Sanitation fellings

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• Urea
• Borates
• Phlebiopsis gigantea
In northern Europe the effects
are as good as winter logging
95-100% control
Management strategies and tactics
Protection – Stump protection with chemical and biological treatments

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Days after treatment
5 10 15 20
300 g/l
200 g/l
100 g/l
Susceptibility period of stumps
Dose
Urea applied to stumps is hydrolyzed by enzymes in the living tissues,
resulting in an increase in pH, which is toxic to Heterobasidion spp.
effectiveness dose-dependent
Management strategies and tactics
Stump protection with chemical treatments: urea (20% concentration)

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No stump treatment
Stump treatment
Management strategies and tactics
Stump protection with chemical treatments: urea (20% concentration)

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Stump protection with chemical treatments: borates (e.g. DOT, borax)
Borates are efficient against Heterobasidion infections as long as the concentration is at least 5%.
They act on the metabolism of basidiomycetes, but they are not registered as a pesticides in most
European countries and, consequently, they cannot be used everywhere in practical forestry.
Management strategies and tactics

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Stump protection with biological treatments: Phlebiopsis gigantea
Management strategies and tactics

42. Application of stump treatments
Gonthier and Thor, 2013
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Management strategies and tactics

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Management strategies and tactics
Resistance – Changes in tree species composition
Gonthier and Thor, 2013

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Management strategies and tactics
Integrated disease management – Combination of different management approaches
Gonthier and Thor, 2013

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Management strategies and tactics
Estimating and predicting disease incidence
Gonthier and Thor, 2013

46. for your attention
Dr. Luana GIORDANO
University of Torino, Italy
E-mail: luana.giordano@unito.it