Opportunistic fungal infections of trees

May 29, 2012

Pest control

Manion (1981) estimates that there are close to 100,000 species of fungi that each has special environmental requirements and occupy specific niches.  Fungi can fulfil many roles and for the most part, fungal activity is highly beneficial to both human beings and the forest community.  Their most important role is as saprophytes, breaking down dead organic matter.  The role as a parasite, dealing with the defences of the living host, is a minor role for fungi; these are often the pathogens (Manion, 1981).  Many fungi can also play a dual or alternating role of saprophyte and parasite (Sinclair & Lyon, 2005)

Each fungus has its specific nutrient requirements and optimum environmental conditions for development.  Most fungi require some free water to enable them to grow in or on a substrate.  They require a temperature range between freezing and approximately 43 °C, with most fungi preferring an optimum temperature of approximately 24 °C.

According to Rane and Pataky (1997) there are four elements required for the development of plant diseases.
These are:

  • Susceptible host – A plant must be able to be infected by a particular pathogen. Susceptible species or specimen with weakened or slow response mechanisms.
  • Plant pathogen – a microorganism capable of causing disease. Biotrophs (obligate parasite) are generally quite specialised in terms of host range and mode of attack. Opportunistic necrotrophs (facultative parasite) tend to be non-specialised in both respects (Sinclair & Lyon, 2005).
  • Favourable environment – Pathogens have defined temperature and moisture requirements for growth and entry into plants.  Without favourable infection conditions, a pathogen will not be able to cause disease even when present on a susceptible host.
  • Time – the interaction of the plant, the pathogen and the environment occurs over the passage of time (usually hours) before infection materialises. Time also influences the growth stage of the host and the development of symptoms on an infected plant. Symptoms also often change over time.

Disease is the result when all four elements are present and the process can be inhibited or reduced when one or more of the elements is removed or altered.

According to Manion (1981), biotic plant disease is the product of three interacting factors: the plant, the pathogen and the environment.  Abiotic agents such as high or low temperatures, nutritional deficiencies, soil aeration deficits, and moisture stress can also cause disease in plants.  Decline results from the interaction of a number of factors, either biotic or abiotic agents.

The Victorian Spring of 2011 was much wetter than usual with many Victorian weather stations receiving about one and a half times their usual rainfall amount.  Melbourne Regional Office recorded 834.6 mm, well above its long-term average of 649.8 mm. This was the second year in a row that Melbourne Regional Office recorded above average rainfall after 13 consecutive years of below average rainfall (Bureau of Meteorology, 2012).
Average maximum temperatures were generally close to average in the Melbourne area during 2011, however, Winter produced warmer than normal average maximum temperatures (most stations recorded temperatures within 1 °C of their long-term means) and Spring was also warmer than usual with most stations recording average maximum temperatures 1 – 2 °C above average. Bureau of Meteorology (2012) ‘Melbourne Metropolitan Area: Record breaking rain’, as seen: http://www.bom.gov.au/climate/current/annual/vic/melbourne.shtml.

There were also severe thunderstorms with hail on the 8th and 9th December 2011, which would have also aided certain fungal pathogens.
The increase of moisture during the Winter and Spring of 2011 would have aided the proliferation of fungal pathogens.  The increased opportunity for fungal pathogens combined with previous stresses present in certain trees, through extended drought or other abiotic agents, has lead to decline symptoms being expressed by a number of trees around Melbourne and Victoria.

The Turkish Pine (Pinus brutia) (otherwise known as the Lone Pine) located within the Shrine Reserve grounds, adjacent to Birdwood Avenue is in severe decline.  The Lone Pine has been found to be infected with the fungus Diplodia pinea (Desm.) Kickx.

Diplodia pinea (= Sphaeropsis sapinea) is an endophytic fungus and opportunistic canker pathogen of Pinus spp. Diplodia pinea kills current-year shoots, major branches, and ultimately entire trees.  The most conspicuous symptom is brown, stunted new shoots with short, brown needles.

When rainfall is above normal in late summer, unusually high numbers of pycnidia may develop on current-year needles and second-year cones.  Highly moist conditions are needed

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for infection.  Large numbers of spores are dispersed only during rainy periods and high relative humidities are required for spores to germinate and for germ tubes to grow and penetrate needles and shoots. If there is little rain when new shoots are highly susceptible infection levels are usually very low.  The thunderstorms and hail in early December 2011 may have aided the infection.

According to Hansen & Lewis, (1997) one of the predisposing factors are plants experiencing prolonged drought (dry soils) and competing vegetation.  Although the species descriptions of Turkish Pine would suggest a high tolerance of drought.  Severe symptoms can also develop on trees wounded and stressed by excessive pruning and on those wounded by hail.  The Lone Pine has either lost or had pruned off two main leaders in the past; which would constitute a significant proportion of live crown.  Altered nutrition, in particular excessive atmospheric nitrogen can also predispose to infection (Hansen & Lewis, 1997).

There is variation in susceptibility between different pine species as well as differences within host species.  This can be seen in the Shrine Reserve where a younger Pinus brutia (that would ultimately replace the existing mature tree) growing immediately to the west of the mature specimen, did not display any of the symptoms associated with the disease and was growing vigorously.  Although it should be noted that symptoms are often more severe on older trees (U.S. Department of Agriculture Forest Service, 2012).

Infection of new shoots can be reduced significantly by applying fungicide to pines during the 2-week period when shoots are highly susceptible to infection. This period, in early to mid-Spring, begins with the opening of buds. During this short period, two applications of Bordeaux mixture, copper sulphate, or hydrated lime (mixed to manufacturer specifications), approximately 1 week apart are more effective than one application (U.S. Department of Agriculture Forest Service, 2012).  This is a difficult operation for large, mature trees.  Removal of infected branches can improve aesthetics, but this procedure probably would not reduce the amount of infection significantly.

An opportunistic fungal disease was also found on a historical stand of Pinus canariensis (Canary Island Pine) growing adjacent to Old Calder Highway, Keilor, which were found to be infected with Botryosphaeria and Cytospora.

Cytospora (Valsa) is known to cause cankers on many host trees such as Prunus spp., conifers, poplars, and willows.  The disease is commonly called Cytospora canker or Valsa canker.

Botryosphaeria fungi are also associated with stem lesions/cankers and tip dieback of a wide range of woody ornamental and fruit tree hosts.  These fungi are considered to be weak pathogens and usually infect plants which have been stressed, weakened or injured.  Factors and conditions that could predispose plants to stress include drought, water logging, nutrient deficiencies, chemical toxicities, mechanical damage and frosts.

Control of these fungi will largely depend on management practices. Trees should be grown under optimal conditions and not unduly stressed, damaged or weakened.  Good hygiene, including removal and destruction of severely affected material will also assist in the management of these infections.

Healthy plants are rarely infected with Botryosphaeria as it is an opportunistic fungus. Consequently maintaining the plant’s cultural requirements is an important preventative measure.

An uncommon fungal disease on a tree was found on a recent project for Murray Shire to investigate the decline of a row of significant Brachychiton populneus (Kurrajongs).  Soil and foliar samples were taken and sent off for testing and analysis by the Department of Primary Industries (DPI) Crop Health Services.   The comments from Crop Health Services  (CHS) stated that “fungal culturing has consistently recovered the fungus Phoma. It suggests that this fungus to be a primary pathogen causing the problem.”

Credit
1. Photographer Robert L. Anderson  Organization: USDA Forest Service  United States
2. Photographer Information Name: Joseph O’Brien Organization: USDA Forest Service  Country: United States
3. Photographer Information Name: Joseph O’Brien Organization: USDA Forest Service  Country: United States

Comments from the plant pathologist at Crop Health Services suggests that while this fungus is a common soil fungi, a plant decline issue from this fungus is very rare and in fact it would appear that this is only the second time that CHS has found a link to a significant plant decline problem in Victoria and New South Wales.

It appeared that the decline linked to this fungal disease was compounded by other site issues to actually kill several of the trees on the site.  Recommendations were made to improve tree health for the remaining trees as well as for chemical treatment for prevention and/or control.

Phoma is a genus of common coelomycetous soil fungi. It contains many plant pathogenic species and the fungus can cause a condition known as Phoma blight, characterised by a withering and fading of the leaves of the plant and also causing necrotic spots on leaves and lesions on stems. The blight can eventually kill the plant, and it can spread to other plants and trees in the vicinity.

It may be beneficial to undertake a fungicide treatment at bud-break during Spring. Fungicides that may provide some protection against Phoma blight include those typically used for black spot prevention and control include copper hydroxide (Kocide®), thiophanate-methyl (Banrot®), potassium bicarbonate (Ecocarb®), copper hydroxide (Ridomil®), Mancozeb® and Iprodione®.

Banrot®, Ridomil® and Iprodione® are systemic products and therefore may be able to be trunk or soil injected. The other products would need to be sprayed on the canopy during and through bud-break and only typically have a protective-contact mode of action.

Note that pesticides used improperly can be injurious to humans, animals and other plants.  Follow manufacturer directions and heed all warnings.

In order to prevent the possibility of infection by opportunistic fungi it is best to cultivate our plants in optimum growing conditions.  This firstly involves appropriate plant selection and placement in the landscape so that plants are not predisposed to stress.  Choose trees that are best suited to your area and have the tolerances to deal with adverse growing conditions.  Ensure susceptible plants receive adequate moisture during extended dry periods.  Avoid compacted soil conditions by excluding people and traffic from root zones, cover the areas with approved woodchip mulch (to between 75mm to 100mm depth).  Avoid excessive or improper pruning and other mechanical injuries.  Do not over fertilise plants, undertake soil tests to identify deficiencies and apply appropriate amounts of supplemental nutrients.

Tree Logic can assist with all of your Plant Health Care requirements.

References
Hansen, E. M. & Lewis, K. J. (eds.) (1997) Compendium of conifer diseases. APS Press
Manion, P. D. (1981). Tree disease concepts. Prentice Hall
Rane, K., & Pataky, N. (1997) Managing infectious plant diseases. In Lloyd, J. ed. 1997. Plant health care for woody ornamentals. International Society of Arboriculture.
U.S. Department of Agriculture Forest Service. 2012. Peterson, G. W., Diplodia blight on pines. [Online] Accessed May 2012: http://na.fs.fed.us/spfo/pubs/fidls/diplodia/diplodiafidl.htm.
Sinclair, W. A. and Lyon, H. H. (2005). Diseases of trees and shrubs. Second edition. Cornell University Press.

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Stephen

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Stephen is the Manager Consulting and a Director of Tree Logic

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