Report: Sphaeropsis sapinea Diseases of Pines – A Review from a New Zealand Perspective
FHRC Project: 1998-03.
Date: November, 1999
Author: M. A. Dick
Publication: Forest Research Contract Report No. 8990
Project reference: 1998-03
Sphaeropsis sapinea is an opportunistic pathogen of Pinus species in the temperate regions of the world. Although it has also been recorded on other conifers such as species of Abies, Araucaria, Cedrus, Cupressus, Larix, Picea and on Chamaecyparis lawsoniana and Pseudotsuga menziesii, it is as the cause of disease and sapstain of plantation grown pines that S. sapinea has had an impact. The fungus has had many synonyms, and for many years was known as Diplodia pinea. The diseases that the fungus causes in New Zealand are commonly known as Diplodia dieback, Diplodia whorl canker, Diplodia shoot blight and Diplodia crown wilt.
Although S. sapinea has been present in Great Britain and western Europe for over 150 years it (with a few exceptions, localised in time–period and distribution) does little damage to conifers in that part of the world. In the United States S. sapinea does most damage in exotic plantations of P. nigra and P. sylvestris but the native P. resinosa, P. banksiana and P. ponderosa have also been severely affected in the central states. The most dramatic impact of the fungus has been in the Pinus radiata plantations of New Zealand, Australia, South Africa and Chile. Of the commonly grown plantation pines P. radiata is recognised as highly susceptible to attack by S. sapinea followed by P. nigra, P. sylvestris and P. ponderosa. Pinus caribaea, P. elliottii and P. taeda are more resistant and P. patula is least damaged.
Sphaeropsis sapinea has been associated with a wide range of symptoms including branch and stem cankers, dieback and stagheads, sap stain, shoot blight, bud wilt, seedling blight and seedling collar rot and root disease. Some symptoms and incidences of disease occur frequently in some parts of the world while being uncommon in other parts. Whorl canker of P. radiata, although common in parts of New Zealand is seldom recorded elsewhere. Collar rot of P. resinosa nursery seedlings has become a problem in the central United States but appears to be rare in other countries. Shoot infection followed by dieback is the most common symptom world–wide and occurs on seedlings and on trees of all ages.
Lesions on infected shoots can enlarge rapidly but resultant dieback seldom progresses down into tissue of the previous seasons growth. If shoot infection occurs in successive years trees develop a bushy multileadered form often with marked malformation in the logs. Whorl cankers are characterised by depressions above and blow the pruned stub and blue stain in the sapwood. Death of the whole tree may follow if the complete cross–section of the stem is invaded by the fungus. Crown wilt (death of the whole upper crown of P. radiata but not in association with pruned stub infection) has been recorded in New Zealand and Chile. Such mortality invariably occurs in locations where prolonged dry periods are common and occurs after a period of drought.
Most recorded instances of serious attack by S. sapinea involve predisposing conditions. These include physical injury which provides an entry point for the fungus such as hail, frost and animal damage or insect attack; drought stress and high nitrogen levels. Stem infections usually occur when there is a temporary breakdown in host resistance. Such infections may remain localised in the stem presumably because of restoration of host resistance when stress factors are relieved. If water conduction is not seriously hampered then obvious crown symptoms such as wilt will not be present though the wood may be extensively stained.
Production and dispersal of spores is correlated both to temperature and to rainfall. Moderate temperatures are required for production and the optimum temperature for spore germination is around 24–28oC. Spores remain viable for several weeks to several months if protected within pycnidia and if temperatures are not extreme. Dispersal of the spores is by splashing rain. The main period for shoot infection in New Zealand is therefore during late spring and summer with a minor wave of infection during the autumn flush if there is wet weather.
In the history of plantation forestry disease the most practical and effective management technique has been the replacement of susceptible species, cultivars or provenances with those showing a higher level of resistance. New Zealand has made a firm commitment to radiata pine as the predominant species for its exotic plantation forests but so far little useful resistance to S. sapinea within P. radiata has been identified. As such, some losses associated with this fungus will be an ongoing part of our forestry regimes.
An understanding of the factors influencing infection will enable the implementation of a number of measures to minimise losses. Restricting pruning damage will reduce losses from whorl canker and infection levels will be minimised if trees are pruned during periods unfavourable for spore dispersal and germination i.e. when conditions are dry and cool. Losses associated with stem infection and drought conditions (crown wilt) are more difficult to influence. Manipulation of stocking rates in drought–prone areas in order to reduce competition for moisture has been suggested as a means of disease management. Careful siting of plants in locations where environmental influences do not promote fungal development or dispersal, or where plants are not stressed to the point of decreased resistance to pathogenic attack can be a useful method for reducing disease impact. Sanitation measures to remove sources of inoculum, such as thinning and pruning slash has been proposed in some parts of the world as has removal of dead trees or parts of trees.
Many of the early reports on S. sapinea dismiss the fungus as a harmless saprophyte or as a wound parasite attacking tissue weakened by some climatic or edaphic factor. As records have built up it has often been speculated that conflicting results from independent investigations regarding the necessity of wounds for infection might be explained in part by the existence of different strains of the fungus. In recent years cultural and morphological differences between isolates of S. sapinea have been recognised. One form, designated the A morphotype, is present throughout the world and in the United States has been shown to be more aggressive than another clearly separable form, the B morphotype. In other countries the A morphotype is usually identifiable as part of the population but the remainder of the population shows a range of morphological characteristics.
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