Abstract:
Methods for detecting early stage fungal infection (and especially esca or esca-like infections) in woody plants are disclosed, using X-ray tomographic imaging. In preferred embodiments, axial X-Ray tomography is employed, with radiodense regions of the tomograms indicating the location of such infection. The methods are particularly relevant to detection of esca in grapevine and kiwi. Methods for controlling such infections using such tomographic methods and direct introduction of fungicides are also disclosed.

Description:
FIELD OF THE INVENTION 
     The invention relates to the diagnosis and treatment of fungal infections of woody plants, especially plants of the genus  Vitis, Actinidia  and  Citrus  and hybrids thereof, and especially fungal infections comprising Esca, Black Dead Arm (Botryosphaeriaceae fungi), or Esca-like infections, or infections with  Eutypa lata.    
     BACKGROUND AND PRIOR ART KNOWN TO THE APPLICANT 
     Wood diseases caused by fungi are presently the most serious threat to the lifespan of commercial fruit trees and vines throughout the globe. They endanger the profitability and sustained production by decreasing yield and fruit quality, mainly by inducing the decay and death of infected plants. Key species affected by such diseases include grapevine ( Vitis  spp. and their hybrids), kiwi-fruit ( Actinidia  spp. and their hybrids), although it is increasingly recognised that similar diseases affect many other hosts, as will be discussed below. 
     In grapevine ( Vitis  spp. and hybrids thereof), esca, Petri disease,  Eutypa  dieback, and black dead arm (BDA) are the main wood diseases inducing such decay and death of the vine plants (Luque et al., 2009, “Symptoms and fungi associated with declining mature grapevine plants in northeast Spain”,  Journal of Plant Pathology  91, 381-390). As no effective control measures are available against these diseases, they are dramatically affecting the durability of the wine-growing heritage (Larignon et al., 2009, “Esca et Black Dead Arm: deux acteurs majeurs des maladies du bois chez la vigne”,  Comptes Rendus Biologies  332, 765-783.). 
     Esca is a complex grapevine wood disease that is achieving an unprecedented and increasing worldwide importance. First believed as affecting only mature grapevine, the disease is now found at high levels in much younger plants. It is accepted that esca comprises a number of distinct diseases and that the main causal agents (mostly vascular fungi) invade the vines not only through wounds in the field but also as a result of nursery practices. When infection starts in the nursery, plants may develop different diseases, ranging from Petri decline (also known as “black goo”) to full-blown esca, with or without white decay of the woody tissues. 
     Esca is caused by the presence of at least three fungi,  Phaeomoniella chlamydospora, Phaeoacremonium aleophilum  and  Fomitiporia mediterranea , acting alone, in combination or in succession. The two first fungi are responsible for vascular necroses, which appear as dark wood streaking, sometimes with some areas of dark and hard necroses, while  F. mediterranea  (and/or other wood-rotting basidiomycetes) induces white rot, or “amadou”. 
     However, the pattern of symptoms and the initial onset of their expression are highly variable: severely infected plants can remain asymptomatic in successive years for a number of years without visible (leaf) symptoms of esca (“hidden” esca). This intermittent pattern of symptom expression leads infected plants to be indistinguishable from healthy ones. 
     It is of paramount importance to ensure healthy propagation of grapevine in the nursery, where only material without wood streaking should be selected. Once planted in the vineyard, infected propagating material develops Petri disease or decline, also characterised by wood dark streaking, slow dieback and “black goo”. Because these fungi grow inside the wood of trunk and large wooden branches, one cannot reach them by foliage spraying with fungicides. The best remedy still is digging up the infected vine and burning it. 
     The greatest problem of esca derives from its initial stages: the plants are usually externally asymptomatic, although occasionally fruiting bodies and spores could be present. However, they are capable of disseminating the disease through common vineyard practice, such as pruning. In other words, a typical vineyard machine may pass on the disease from one infected, but asymptomatic, vine to the next ones. Spores are also disseminated by the wind and rain. Thus, while pruning tools and vineyard specialized machinery are not important in spore dissemination, they may play a role in spreading fungal mycelia from diseased plants to healthy ones. However, in what spore dissemination and penetration is concerned, pruning wounds and other wounds caused by vineyard specialized machinery are particularly important. For all these reasons, it is of paramount importance to develop methodologies capable of nondestructive screening and detection of the initial stages of esca, either in nurseries or in vineyards. Improved strategies and methods for treating such fungal infections at an early stage whilst eradication of the disease is still possible are also required. 
     Plant species other than grapevine are also subject to wood affecting, esca-like diseases. In 1999, a wood decay of kiwifruit vines ( Actinidia ) was noted for the first time in Italian vineyards. A similar disease was subsequently reported in Greece and New Zealand. Such diseases were described under the names of “esca-like” in France and “Elephantiasis” in Italy. 
     Analysis of kiwifruit vineyards exhibiting visual symptoms of esca-like disease revealed the presence of two types of necrosis in the wood: a) hard, brown decay initiating in the medular tissues and expanding gradually in the entire trunk or branches, and b) white and soft decay like “amadou wood” of grapevine. Isolations from necrotic parts of wood revealed a number of fungi, including  Fomitiporia mediterranea, Phaeoacremonium aleophilum, Phaeoacremonium parasiticum , and  Cadophora malorum . At least two of the above fungal species ( F. mediterranea  and  P. aleophilum ) are also involved in esca disease of grapevine, and aspects in common between the decay of kiwifruit and esca of grapevine can be hypothesized. 
       Citrus  species (such as the orange cv. Washington navel, lemon and the common mandarin grafted on sour orange rootstocks) are experiencing a serious decline caused by  F. mediterranea , in southern Greece orchards. Affected trees exhibit leaf chlorosis, defoliation, and death of shoots and twigs. Cross-sections of the trunks and large branches reveal a light-coloured rot in the centre, which is surrounded by brown hard necrotic wood. Symptoms start from pruning wounds and extend to the rootstock wood, resembling the esca wood symptoms of grapevine. 
     As well as grapevine, kiwi and  citrus , esca-like diseases have also been identified in a number of other species.  Phaeoacremonium aleophilum  has been reported in  Prunus  species showing dieback symptoms in the Western Cape, South Africa. Serious dieback symptoms, caused by  Pm. parasiticum , have been observed in cherry trees in Greece.  Phaeoacremonium mortoniae  was shown to cause brown wood staining of  Fraxinus pennsylvanica  and  Salix  spp. 
     One of the key causative agents,  Fomitiporia mediterranea , is also found on  Corylus avellinus, Olea europaea, Lagerstroemia indica, Actinidia sinensis, Acer negundo, Olea europaea, Quercus  sp.,  Quercus ilex, Ligustrum vulgare  and  Robinia pseudoacacia.    
     A further plant disease, and similarly affecting woody species is Eutypiose, also known as “ Eutypa  dieback”, is caused by the fungus  Eutypa lata. E. lata  is a vascular pathogen and occurs on at least 88 species of woody dicots (Carter, 1991. “The status of  Eutypa lata  as pathogen”,  Phytopathological Paper  32) and is responsible for significant economic damage mainly to grapevine and apricot. 
     Many forest and ornamental species also host the pathogen, but it is not certain whether all isolates are pathogenic, although pathogenicity of  E. lata  has been confirmed for grapevine, apricot, sweet cherry ( Prunus avium  L.), chokecherry ( Prunus virginiana  L. var.  demissa ), sour cherry ( Prunus cerasus  L.), almond ( Prunus dulcis ), apple ( Malus domestica  Borkh.), pear ( Pyrus communis  L.), walnut ( Jugfans regia  L.), olive ( Olea europaea  L.), and  Ceanothus L . spp. 
     On grapevine, the most recognized symptom of  Eutypa  dieback is the stunted appearance of shoots at the early growth season, with small, chlorotic and distorted leaves, and short internodes. However, the symptomatic expression fluctuates markedly from year to year. Non-destructive techniques for diagnosis of  Eutypa  dieback are still unavailable. Furthermore, no efficient treatment is available, and infected plants will die within a few years. 
     A further disease of concern, especially in grapevine ( Vitis ) is associated with infection by Botryosphaeriaceae fungi. These have a wide distribution and are commonly associated with dieback and cankers of woody and non-woody host plants, including forest trees, e.g.  Pinus  spp.,  Eucalyptus  spp.,  Quercus  spp.,  Olea , stone fruit trees,  Protea  and grapevine. 
     On grapevine, several species in Botryosphaeriaceae are associated with wood necrosis and they are also able to infect both young and mature tissues as well as green shoots causing cankers, vascular discoloration, and/or otherwise dark streaking of the wood. 
     Some Botryosphaericaeae species are responsible for Black Dead Arm (BDA) disease, in which the wood symptoms include V-shaped necroses, similar to those caused by  Eutypa lata , and longitudinal brown streaking along the affected tissues. Other symptoms resembling  Eutypa  dieback, such as stunted chlorotic shoots, deformed leaves with necrotic areas are frequently observed. 
     It will be appreciated that all these diseases share common characteristics: (1) they are predominantly diseases of wood plants, and especially woody fruit-bearing plants of commercial importance; (2) they are fungal in origin, and (3) the pathogens invariably colonise the vasculature of the plant, especially the xylem. Furthermore, because (4) the disease symptoms are often localised—at least in the early stages—to the interior of the woody parts of the plant, non-invasive diagnosis of the conditions is difficult, if not impossible. In addition, once detected (5) treatment of the disease is difficult, because the internalisation of the fungal infection renders it barely susceptible to external application of fungicides. 
     In the wider context of diseases of the woody parts of commercially-important plants, one of the greatest problems facing agriculture is the fight against pathogens and insects that develop within the wood and so in a location where conventional fungicides/insecticides do not reach. It is therefore of great importance to develop methods to detect the existence, and the location, within infected plants, of wood diseases during the initial but asymptomatic and contagious phases of the infection (and thus, during a potentially treatable phase) and to develop treatment regimes that are effective against such diseases. 
     It is amongst the objects of the present invention to address these problems. 
     SUMMARY OF THE INVENTION 
     Accordingly, the inventors provide a method of detecting fungal infection in a woody plant comprising the use of X-ray tomographic imaging. A wide range of tomographic imaging methodologies are available, including X-Ray, PET (positron emission tomography), electrical resistance tomography and acoustic (including ultrasound) tomography. However, the inventors have found that the use of X-ray tomographic techniques including Computerised Axial Tomography (CAT), micro-computerised tomography, tangential scanning, simple radiography and multiple angle simple radiography allows externally asymptomatic plants to be diagnosed with a wide range of fungal infections. 
     In this way, the present method of the invention provides the possibility of detecting a fungal infection within such a plant, at a stage where it is possible to apply a treatment methodology at an early stage, i.e. where treatment is possible. If such diseases remain undetected, and therefore untreated, the plant will usually die, and will be a source of infection that may spread to other plants. 
     Whilst tomographic imaging has been used to image rot in wood caused by fungal decay of the dead woody material, and also the late stages of rot in dead areas of woody material in the trunks of growing trees, by contrast, the present invention relates to the early detection of fungal infection of live, growing plants susceptible to such infection by fungal plant pathogens. For example, U.S. Pat. No. 4,283,629 in the name of Habermehl describes a tomographic technique for detecting “red rot” in trees in which internal, dead woody areas within tree trunks can be detected by the decrease in radiodensity by comparison to surrounding healthy areas. The methods presented in this publication are directed towards commercial lumber production, to support commercial decision-making about when a wood-producing tree should be felled, and the extent to which its woody parts may be used in various manufacturing processes such as lumber or cellulose production. 
     In particularly preferred embodiments of the present invention said woody plant is a plant of the genus  Vitis  or a hybrid thereof, a plant of the genus  Actinidia  or a hybrid thereof or a plant of the genus  Citrus  or a hybrid thereof. 
     In any aspect of the invention, it is also preferred that said fungal infection is a fungal infection that causes accumulation or modification of fluid in the xylem of said plant. This fluid, such as the “black goo” seen in esca infection leads to a particular characteristic response in the tomographic image, allowing it to be differentiated from other internal features such as fungal rot. 
     Accordingly, it is particularly preferred that said fungal infection comprises Esca or an Esca-like disease. 
     In further preferred aspects of the invention said fungal infection comprises infection with  Eutypa lata , or  Botryosphaeria -associated canker (or generally infection with fungi of the Botryosphaeriaceae family), or infection with  Phoma tracheiphila.    
     In any aspect of the invention it is particularly preferred that said tomographic imaging comprises axial X-ray tomography. Axial X-Ray tomography has been shown by the inventors to be particularly effective at identifying regions of plant material, especially those not visible on the exterior of the plant, that are colonised by plant pathogenic fungi. 
     In particularly preferred aspects of the invention, the inventors provide such a method using X-Ray tomography, and especially axial X-Ray tomography or X-Ray microtomography, wherein said method comprises the steps of: obtaining a tomographic image of a stem of said woody plant, said tomographic image comprising spatially-distributed measurements of radiodensity; determining the presence or absence of regions within said image having a radiodensity larger than a first predetermined radiodensity; the presence of such regions of high radiodensity being indicative of the presence of said fungal infection. 
     Unusually, the inventors have found that such X-ray tomographic images reveal regions of living plant material that are colonised by fungal pathogens and, especially in the case where the fungal pathogens colonise the vasculature of the plant with subsequent production of fluid within the vasculature or chemical modification of fluid within the xylem (e.g. by production of polyphenolic material or gums), this is revealed by regions of increased radiodensity. The fungal pathogens are prone to induce the production of radiodense material within the vasculature (i.e. more radiodense than the fluid usually found in e.g. the xylem of the plant) that leads to a positive indication of such an infection. By way of contrast, previous use of X-ray tomography to image decay in dead wood (e.g. in building materials or in living trees in urban areas) detects the area of rot as regions of very low radiodensity, in fact producing little or no signal by comparison to the signal produced in the absence of wood. It is often not possible, therefore, to differentiate the presence of decay in dead wood (e.g. by soft rot fungi) from holes within the wood itself. 
     In such a method, it is particularly preferred that said first predetermined radiodensity is the mean radiodensity of a corresponding region in an uninfected control plant. In this way, the method may be readily adapted for application in a range of woody species by calibrating the tomographic images against images taken of healthy plants. 
     Preferably, in any such method, and especially in the case of grapevine said first predetermined radiodensity comprises 100, 120, 130, 140, 150, 160, 170, 180, 190 or 200 Hounsfield units. 
     Hounsfield units are well known in the field of X-Ray tomography, and are a linear transformation of the radio attenuation coefficient within the material, scaled with respect to the corresponding attenuation in air and water, as follows:
 
 R= 1000*(μ x −μ w )/(μ w −μ a )
 
     Where: R is the radiodensity in Hounsfield Units
         μ x  is the linear attenuation coefficient in the sample   μ w  is the linear attenuation coefficient of water   μ a  is the linear attenuation coefficient of air       

     In further preferred embodiments, the inventors provide such a method further comprising the step of: determining the presence or absence of regions within said image having a radiodensity less than a second predetermined radiodensity; the presence of both such regions of high and low radiodensity being indicative of the presence of an advanced stage of said fungal infection. Preferably, said second predetermined radiodensity comprises −200, −300, −400, −500, −600, −700, or −800 Hounsfield units. 
     In any aspect of the invention, it is preferred that said radiodense regions have a spatial dimension of less than 5 mm, 2 mm, 1 mm, 500 microns, 200 microns 100 microns or even 50 microns. The inventors have found that the radiodense regions correspond to accumulation of a “block goo”, predominantly of plant origin, and likely to comprise an excretion of polyphenolic compounds secreted by the plant as part of a host defense mechanism. The fluid accumulates primarily in the xylem vessels of the plant, and so the radiodense regions reflect the narrow, elongate nature of the vasculature. At least one of the spatial dimensions (width, height, and length) of the radiodense regions is therefore of a small dimension as recited above. The inventors have found that these regions may be imaged by the techniques described herein, and are particularly indicative of esca and esca-related infections. 
     Such a method is particularly applicable to fungal infections like esca or esca-like infections where accumulation of radiodense material in the vasculature of the plant is followed, during development of the infection, by the appearance of regions of rot of dead tissue within the plant structure, such as those referred to as “Amadou” in esca in grapevine. In this way, the presence of both regions of relatively high and relatively low radiodensity can detect a particularly advanced stage of such a fungal infection. This can aid with decision-making regarding remedial action to be taken, such as treatment of the infected plant, or a decision to uproot and replace the plant. 
     Also in any aspect of the invention, it is preferred that said stem has a diameter of between 1 and 10 cm. The rationale for the disclosed testing methodology is to be able to treat infected woody plants, and especially vines, and the use of the technique on relatively small stems is of assistance in identifying the disease at an early stage. 
     Also in any aspect of the invention, it is preferred that said woody plant comprises rootstock or graft material, and said stem has a diameter of less than 1 cm. The inventors have found that the techniques disclosed herein allow the rapid screening of rootstock and graft material, typically having stem diameters below 1 cm, as they leave nurseries. This is especially true in the case of vines, where it is vital to prevent infected material leaving vineyard nurseries providing a possible source of infection to otherwise healthy plants. 
     The availability of such methods to diagnose these devastating diseases in commercially-important species leads the inventors to be able to provide hitherto unavailable strategies and methods for management of such diseases in woody species. 
     Accordingly, the inventors provide a method of control of fungal infection in woody plants comprising the steps of: determining the presence or absence of said fungal infection using a method according to any of the methods disclosed herein; and treating said woody plant with a fungicide, and preferably a systemic fungicide if said fungal infection is so detected. 
     In any aspect of the invention, a range of fungicides are envisaged, and may be selected by the skilled addressee on the basis of their efficacy against the particular fungal pathogen to be treated. Amongst the families of fungicides available to the skilled addressee are Copper based products; Anilinopyrimidines Azoles and non-azoles (ergosterol biosynthesis inhibitors); Carboxamides; Phenylpyrroles; Phosphonates; pyridinyl ethylbenzamides and Qol—Strobilurins. 
     Particular examples of such fungicides include Cyprodinil, fludioxonil, pyraclostrobin, boscalid and mixtures thereof (cyprodinil+fludioxonil and pyraclostrobin+boscalid), tebuconazole, phosphonates. 
     There are also further compounds having fungicidal activity, but not specifically registered as fungicides, such as: baking soda (sodium bicarbonate, NaHCO 3 ) and potassium bicarbonate (KHCO 3 ) and hydrogen dioxide (hydrogen peroxide). 
     Fungicides having plant growth promotion activity are particularly preferred, such as blad. Blad is a polypeptide extractable from cotyledons of  Lupinus albus , or producible by use of recombinant DNA technologies. Details of Blad are given in co-pending International Patent Application WO2007010459. 
     Fungicides with a host defense-inducing activity, such as Chitosan, are also particularly preferred. 
     Also fungicides having a host defense-inducing activity, in addition to some direct effects on fungi, are particularly preferred, such as hyaluronic acid (an elicitor for induction of systemic resistance. See  World J Microbiol Biotechnol  (2008) 24:1153-1158) or acibenzolar-S-methyl. 
     Preferably, said fungicide is injected into said plant. By “injected”, we mean that such a fungicide is introduced directly into the internal region of the plant where the tomographic imaging has detected the fungal infection. As well as literally injecting such a fungicide, other means may be employed, such as drilling a hole in the stem or trunk of the plant, and dispensing the fungicide therein. A plug of slow-release fungicide is envisaged as being particularly advantageous. 
     In particularly preferred embodiments of the invention, the tomographic image is used to identify the region of the plant in which the infection is located (by the regions of relatively high radiodensity), and fungicide is introduced directly into these regions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, in which: 
         FIG. 1  shows photographs of cross-sections through esca-infected grapevine stems; 
         FIG. 2  is an optical micrograph of an esca-infected grapevine; 
         FIG. 3  shows photographs of “black goo” in the vasculature of esca-infected grapevine; 
         FIG. 4  shows a photograph and corresponding X-ray tomograph of an esca-infected grapevine; 
         FIG. 5  shows photographs and corresponding X-ray tomographs of grapevine stems in various stages of esca infection; 
         FIG. 6  is a series of tomographs and radiodensity graphs through healthy grapevine stems; 
         FIG. 7  is a series of tomographs and radiodensity graphs through esca-infected grapevine stems; and 
         FIGS. 8 to 12  are X-ray microtomographic images through esca-infected grapevine stems. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  illustrates, photographically, a typical progression (from A to D) of esca infection in grapevine wood along with some characteristic structural features of wood in its diseased state.  FIG. 1A  shows a section through a grapevine in its initial stages of infection, showing black spots (a) in an apparently healthy wood background.  FIGS. 1B , and  1 C show intermediate stages of esca infection, having black spots (a) and brown affected wood (b) as well as scars (c) and pith (d) that are normal features of wood.  FIG. 1D  shows final stages of esca infection with dead wood (e) and “Amadou” (f). It can be clearly seen that, in the earlier stages of infection ( FIGS. 1A and 1B ), the infection is localised within the interior of the woody parts of the vine. No external symptoms are present towards the periphery of the wood, and so no visible signs of infection are apparent on the plant. 
     Optical microscopy of a section of an esca-infected, asymptomatic grapevine reveals (in  FIG. 2 ) the presence of “black goo” within the lumen of tracheids and xylem vessels of the plant (black arrows). It is the presence of this comparatively radiodense material that may be detected using tomographic imaging, and especially X-Ray tomographic imaging. 
       FIG. 3  shows the presence of “black goo” within, and exuding from, xylem vessels in a cut section of esca-infected grapevine wood. In  FIG. 3(   a ) a peripheral region of the stem (B1) shows healthy, uninfected grapevine wood, whereas the dark region (B2) shows “black goo” within the xylem. In  FIG. 3(   b ), the “black goo” is seen to be exuding (B3) from the xylem vessels. 
       FIG. 4A  is a photograph of a cross-section of an esca-infected trunk of grapevine at an intermediate stage of infection, taken after previous CAT (Computerised Axial Tomography) analysis of an intact esca-infected grapevine trunk.  FIG. 4B  is a tomographic image obtained by axial X-Ray tomography according to the present invention. The tomographic image ( 4 B) shows radiodensity according to the scale presented on the right hand side of  FIG. 5D . Dark colours are indicative of regions of low radiodensity, with lighter areas indicating increasing levels of radiodense material. Black in the image represents a radiodensity of approximately −135 Hounsfield units, with white representing approximately +215 Hounsfield units. Images were obtained by use of a Phillips TOMOSCAN AV computer axial tomography apparatus. Images were obtained with the following parameters: 
     Scan: 120 kV, 90 mA, 2 s 
     Tilt: 0.0° 
     Depending on the size of trunk to be imaged, the following apertures were used: 
     250 mm/1.2 
     270 mm/1.1 
     160 mm/1.9 
       FIG. 4B  graphically illustrates the ability of axial X-ray tomography to detect signs of esca infection within the core of woody material, by virtue of the radiodense material “black goo” within the vascular system of the plant. The radiodensity values of two points within the image are given (184 and 246 Hounsfield units), indicative of regions having vasculature filled with the “black goo” characteristic of esca infection. 
       FIG. 5(A-D)  illustrates photographs of cross-sections (left-hand column) of grapevine wood with various degrees of esca infection, together with computer axial X-Ray tomographic images (right-hand column) obtained before the wood was sectioned for photography. The tomographic images show radiodensity maps of the wood, according to the scale adjacent  FIG. 5D , with dark regions representing low radiodensity and light regions a corresponding high radiodensity. Black regions have a radiodensity of −135 Hounsfield units or below, with white regions having a radiodensity of 215 Hounsfield units or above. 
       FIG. 5A  shows images of a healthy, uninfected stem;  FIG. 5B  is of a stem during the initial, asymptomatic stages of infection. Particularly evident are the regions of high radiodensity (white areas in  FIG. 5B  radiodensity map) that correspond with the black areas in the corresponding photograph, and comprising regions of vascular tissue having “black goo” within the xylem, as well as the areas of brown affected wood surrounding them. 
     The images of  FIG. 5C  correspond to a stem at an intermediate stage of esca infection. Again, the areas of high radiodensity shown as light-coloured regions correspond to the radiodense dark regions comprising “black-goo” filled regions of the woody stem shown in the photograph. 
     Finally, the images of  FIG. 5D  correspond to a stem at an advanced stage of esca infection. As well as regions of high radiodensity (the light-coloured patches in the tomograph) corresponding to filled vasculature, the images also show (on the left hand side) regions of low radiodensity (the dark, or black regions in the tomograph) that correspond to white rot affected areas or “amadou” shown in the photograph. 
     The images demonstrate the ability of computer axial X-Ray tomography to non-invasively identify early stages of esca infection through detection of regions of high radiodensity, as well as to identify later stages of infection through the detection of regions of low radiodensity, corresponding to “amadou”. 
       FIG. 6(   a )-( e ) show patterns of radiodensity obtained by X-ray based computerised axial tomography of in intact, healthy grapevine stem. The graphical representation of the radiodensity corresponds to the section illustrated on the 2-D tomographic image illustrated by a light-coloured line. The spot on the 2-D image is represented by the vertical reference line on each graph, with the corresponding horizontal line giving the spot radiodensity measurement expressed in Hounsfield units. 
     In each graph, the range of radiodensities illustrated is plotted on a scale of −478 to +173 Hounsfield units, and the “spot values” are as follows: 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Spot Value 
               
               
                   
                 FIG. 
                 (Hounsfield Units) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 6(a) 
                 +119 
               
               
                   
                 6(b) 
                 −46 
               
               
                   
                 6(c) 
                 −478 
               
               
                   
                 6(d) 
                 +125 
               
               
                   
                 6(e) 
                 +25 
               
               
                   
                   
               
             
          
         
       
     
     The very low radiodensity of the spot value of  FIG. 6(   c ) corresponds to a region of soft, spongy pith material at the centre of the stem. The larger values (e.g. +119 and +125) correspond to typical radiodensity values observed in highly hydrated regions of healthy wood tissue. 
       FIG. 7  ( a )-( l ) show patterns of radiodensity obtained by X-ray based computerised axial tomography in three sections of an esca-infected grapevine stem. The graphical representation of the radiodensity corresponds to the section illustrated on the 2-D tomographic image illustrated by a light-coloured line. Again, the spot on the 2-D image is represented by the vertical reference line on each graph, with the corresponding horizontal line giving the spot radiodensity measurement expressed in Hounsfield units. 
     In each graph, the range of radiodensities illustrated, and the “spot values” are as follows: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Y-axis 
                 Y-axis 
                   
               
               
                   
                   
                 minimum 
                 maximum 
                 Spot Value 
               
               
                   
                 FIG. 
                 (Hounsfield) 
                 (Hounsfield) 
                 (Hounsfield) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 7(a) 
                 −274 
                 +277 
                 −265 
               
               
                   
                 7(b) 
                 −274 
                 +277 
                 +277 
               
               
                   
                 7(c) 
                 −274 
                 +277 
                 +263 
               
               
                   
                 7(d) 
                 −998 
                 +333 
                 −170 
               
               
                   
                 7(e) 
                 −998 
                 +333 
                 +308 
               
               
                   
                 7(f) 
                 −998 
                 +333 
                 −998 
               
               
                   
                 7(g) 
                 −998 
                 +333 
                 +333 
               
               
                   
                 7(h) 
                 −998 
                 +333 
                 +255 
               
               
                   
                 7(i) 
                 +153 
                 +266 
                 +265 
               
               
                   
                 7(j) 
                 +153 
                 +266 
                 +162 
               
               
                   
                 7(k) 
                 +153 
                 +266 
                 +153 
               
               
                   
                 7(l) 
                 +153 
                 +266 
                 +233 
               
               
                   
                   
               
             
          
         
       
     
     The very high levels of radiodensity (typically above +250 Hounsfield in this particular example) correspond to regions where “black goo” has accumulated in the vasculature of the plant. The very low levels (typically the negative values in this example) correspond to regions corresponding to an “amadou”, or to a hole or pith in the stem. 
     Analysis of the radiodensity from a large number of samples of grapevine wood in which features have been identified by subsequent sectioning have revealed the typical ranges of radiodensity observed in this species for various botanical features, or features of esca infection. These values are given in Table 1. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Typical Radiodensity of Features in Grapevine 
               
             
          
           
               
                   
                   
                 Radiodensity 
               
               
                   
                   
                 (mean +/− S.D.) 
               
               
                   
                 Feature 
                 Hounsfield Units 
               
               
                   
                   
               
               
                   
                 Pith and Hole 
                 −900 ± 50 
               
               
                   
                 “Amadou” 
                 −840 ± 70 
               
               
                   
                 Dead affected wood 
                  −450 ± 123 
               
               
                   
                 Empty vessels 
                 −109 ± 48 
               
               
                   
                 Healthy wood 
                  80 ± 32 
               
               
                   
                 Knots 
                  195 ± 77 
               
               
                   
                 Black spots 
                  234 ± 56 
               
               
                   
                 Brown Affected Wood 
                  294 ± 71 
               
               
                   
                   
               
             
          
         
       
     
     By use of X-ray tomographic imaging, followed by subsequent sectioning and identification of features, the skilled address may readily determine the typical range of densities for features in other woody species, and for other fungal infections. In this way, the techniques disclosed herein may be applied across a wide range of species and fungal infections. 
       FIGS. 8 to 12  show images obtained using high sensitivity computerised X-ray tomography of esca-infected grapevine ( Vitis ) stems at an early stage of infection, before external symptoms were apparent. 
       FIG. 8  shows a cross-section (A) and detail (B) from a thin trunk of a two-year old grapevine infected with initial stages of esca. The scale bar represents 5 mm. The bright areas in the image represent regions of high radiodensity, indicative of the esca infection. The larger of the bright spots on the inset (B) has a diameter of approximately 250 microns, illustrating the surprising ability of the technique to visualise even early stages of esca infection. 
       FIG. 9  shows a longitudinal cross-section X-ray tomographic image of a thin trunk from a two-year old grape vine ( Vitis ) infected with initial stages of esca. The scale bar represents 5 mm. The bright areas in the image again represent regions of high radiodensity, indicative of the esca infection. High radiodense regions as small as 50 microns can be seen in the images. 
       FIG. 10  shows X-ray tomographic images of xylem vessels with various degrees of filling by the “black goo” that characterises esca infection. The images are taken in a grapevine trunk during the initial, asymptomatic stages of esca infection. The scale bar represents 100 microns, again illustrating that very small infection-related features may be identified. 
       FIGS. 11 and 12  are 3-dimensional axial tomographic reconstructions of a grapevine ( Vitis ) trunk, highlighting its irregular shape. The scale bar represents 10 cm in  FIG. 11  and 10.13 cm in  FIG. 12 .