STUDIES IN THE BACTERIAL DIE-BACK AND CANKER DISEASE OF POPLAR

The bacterial die-back and canker of poplar is characterized by dark necrotic areas in the leaves, die-back accompanied by cracks in the bark of 1–4-year-old branches, and by cankers on the trunk and branches of different ages. The cankers may be 'closed' or 'open' and when moist exude slime containing bacteria.
The disease is caused by Pseudomonas syringae f. sp. populea , a new forma specialis closely resembling Ps. syringae in bacteriological characters but differing from it in parasitism. The association of the slime with the bacteria is essential for the production of typical lesions.
Inoculations with the pathogen alone, or in association with other organisms produce atypical symptoms.
The bacteria attack the cortical parenchyma and occasionally invade the xylem vessels.     

Inhibition of virulent and hypovirulent Cryphonectria parasitica growth in dual culture by fungi commonly isolated from chestnut blight cankers

Chestnut blight cankers, caused by the fungus Cryphonectria parasitica, are prone to invasion by other microorganisms as the canker ages. This microbial community has the potential to alter canker expansion, which may influence the probability that the canker girdles the infected stem. Hypoviruses infect the pathogen mycelium directly and are known to decrease pathogen virulence (i.e. hypovirulent). These viral infections can slow pathogen growth, decreasing the rate of canker expansion and lowering the probability of girdling. Saprophytic fungi also invade the expanding canker and may antagonize C. parasitica leading to reduced pathogen growth. The combined effects of fungal antagonism and a hypovirulent pathogen could work in combination to reduce the probability of girdling the infected stem. We assessed the ability of different fungal taxa, isolated from low severity cankers, to inhibit the growth of virulent and hypovirulent forms of C. parasitica in dual culture tests on two cultural media. Percent growth inhibition of virulent C. parasitica by potentially antagonistic fungi ranged from 2 % to 34 %, while inhibition of hypovirulent C. parasitica ranged from 18 % to 54 %. Only one isolate, identified as Umbelopsis isabellina (UmbelopsisWS) inhibited the virulent form of the pathogen more than the hypovirulent form. All three Trichoderma isolates caused the greatest growth inhibition of virulent C. parasitica, but they, like all other fungal isolates tested, inhibited the hypovirulent form of the pathogen more than the virulent form. These results suggest that commonly occurring fungi in chestnut blight cankers, including Trichoderma, may inhibit the hypovirulent C. parasitica more than virulent C. parasitica. Thus, the presence of other fungi in cankers may not enhance the effect of hypovirulent C. parasitica to delay cankers from girdling a stem but instead intensify canker development.     

Epidemiology of Leptosphaeria maculans in relation to forecasting stem canker severity on winter oilseed rape in the UK

In the UK, ascospores of Leptosphaeria maculans first infect leaves of oilseed rape in the autumn to cause phoma leaf spots, from which the fungus can grow to cause stem cankers in the spring. Yield losses due to early senescence and lodging result if the stem cankers become severe before harvest. The risk of severe stem canker epidemics needs to be forecast in the autumn when the pathogen is still in the leaves, since early infections cause the greatest yield losses and fungicides have limited curative activity. Currently, the most effective way to forecast severe stem canker is to monitor the onset of phoma leaf spotting in winter oilseed rape crops, although this does not allow much time in which to apply a fungicide. Early warnings of risks of severe stem canker epidemics could be provided at the beginning of the season through regional forecasts based on disease survey and weather data, with options for input of crop-specific information and for updating forecasts during the winter. The accuracy of such forecasts could be improved by including factors relating to the maturation of ascospores in pseudothecia, the release of ascospores and the occurrence of infection conditions, as they affect the onset, intensity and duration of the phoma leaf spotting phase. Accurate forecasting of severe stem canker epidemics can improve disease control and optimise fungicide use.     

Certain aspects of resistance of plum trees to bacterial canker: Part II. On the nature of the bacterial invasion of Prunus sp. by Pseudomonas mors-prunorutn Wormald

A detailed histological study was made of the necrotic areas induced in the stem of a resistant and a susceptible variety of plum as the result of artificial inoculations of Ps. mors-prunorum Wormald. The pathogen was found to penetrate the tissues intercellularly, causing plasmolysis of cell contents, followed by disintegration of cell walls, and gradual invasion of the bacteria. The seasonal progress of infection was followed, the maximum cankers caused by winter inoculations in the susceptible host appearing in spring. Limitation of necrosis occurs in summer, generally accompanied by the production of new tissue composed of xylem elements. Viable bacteria were isolated in autumn from those cankers in which the area of necrosis had extended into the wood cylinder beyond the limiting periderm. The difference in the nature of the injury to the tissues of the resistant trees is quantitative rather than qualitative. The significance of the apparently more efficient host mechanism of periderm production in this variety is considered. Lesions similar in histological aspect were obtained as the result of the injection of cell-free filtrates of cultures of the causal organism.     

Histopathology of Susceptible and Resistant Capsicum annuum Cultivars Infected with Ralstonia solanacearum

The vascular colonisation of resistant and susceptible hot chilli (Capsicum annuum) cultivars by Ralstonia solanacearum was examined using transmission electron microscopy. Tap roots of artificially-inoculated plants, grown in sterilised soil were investigated to observe the morphological barriers involved in the restriction of bacterial spread. In the resistant cultivar, several responses induced in response to bacterial infection, were observed. First, a cell wall coating material developed together with swelling of the primary wall of the xylem vessels, limiting the bacterial spread. Second, formation of various types of vesicles in the vascular parenchyma cells, which enveloped the bacterial mass and also partly restricted the pathogen spread. Third, induction of hypersensitive reaction in the xylem vessels resulted in the distortion and lysis of the bacteria. In the susceptible cultivar, vascular coating, production of vesicle and induction of hypersensitive reaction were not observed and bacterial spread was not limited. Rapid vascular colonisation of the susceptible cultivar seemed to be generalised which resulted in the rapid wilting of affected plants. Other reactions involved in both resistant and susceptible cultivars include disorganisation of cytoplasm of parenchyma cells, disintegration of nuclei, and rupturing of xylem vessel walls. The restriction of pathogen spread associated with the resistance in C. annuum to bacterial wilt was mainly attributed to some induced, morphological and physical barriers.     

The ecology of Erwinia rubrifaciens and the development of phloem canker of Persian walnut

A requisite for development of bacterial phloem canker caused by Erwinia rubrifaciens is the presence of the highly susceptible Hartley cultivar; when interplanted with Hartley, the Franquette and Payne cultivars are sometimes attacked by the disease. The recently developed cultivars, Gustine and Howe, developed active cankers when inoculated, but not as extensive as those in Hartley. The age of the plant part is important in the disease. One-year-old Hartley trees and twigs of the current season on mature trees did not develop active cankers. The complete disease syndrome occurs only on trunks and primary (scaffold) branches. Extension of the cankers was most rapid during the summer when the temperature was high. This was correlated with the effect of temperature on growth of the bacterium in culture. At midday in summer the temperature of the cambial area of a shaded trunk was as much as 10d?F (5.6 d?C) below ambient temperature but it was commonly a few degrees above ambient at night. Breaks in the thick phelloderm of the trunks and branches are necessary for penetration of the pathogen to the inner bark. Of the several types of breaks commonly occurring, those produced by mechanical harvesting equipment and by sap-sucking birds were found to be infection sites. Bacteria occur in large numbers in a slimy substance which exudes through cracks and accumulates on the bark of infected trees. They survived for at least 123 days in the exudate and were disseminated laterally as far as 20 ft (6.1 m) in wind-blown rain. Bacteria were also transmitted from tree to tree in exudate contaminating the pads of mechanical harvesting equipment.     

Genetic control of Eucalyptus urophylla and E. grandis resistance to canker caused by Chrysoporthe cubensis

Chrysophorte cubensis induced canker occurs in nearly all tropical and subtropical regions where eucalypts are planted, causing losses in both wood quality and volume productivity, especially so in the warmer and more humid regions of Brazil. The wide inter and intra-specific genetic variability of resistance to canker among Eucalyptus species facilitates the selection of resistant plants. In this study, we evaluated resistance to this pathogen in five Eucalyptus grandis (G) and 15 E. urophylla (U) trees, as well as in 495 individuals from 27 progenies derived from crosses between the trees. In the field, six-months-old test seedlings were inoculated with C. cubensis. Lesion length in the xylem and bark was measured eight months later. The results demonstrated that xylem lesions could preferentially be used for the selection of resistant clones. Eight trees (7 U and 1 G) were susceptible, and the remainder (8 U and 4 G) resistant. Individual narrow and broad sense heritability estimates were 17 and 81%, respectively, thereby suggesting that canker resistance is quantitative and highly dependent on dominance and epistasis.     

Deciphering the Niches of Colonisation of Vitis vinifera L. by the Esca-Associated Fungus Phaeoacremonium aleophilum Using a gfp Marked Strain and Cutting Systems

Introduction

Esca disease has become a major threat for viticulture. Phaeoacremonium aleophilum is considered a pioneer of the esca complex pathosystem, but its colonisation behaviour inside plants remains poorly investigated.

Material and Methods

In this study, P. aleophilum::gfp7 colonisation was assessed six and twelve weeks post-inoculation in two different types of tissues: in the node and the internode of one year-old rooted cuttings of Cabernet Sauvignon. These processes of colonisation were compared with the colonisation by the wild-type strain using a non-specific lectin probe Alexa Fluor 488-WGA.

Results

Data showed that six weeks post-inoculation of the internode, the fungus had colonised the inoculation point, the bark and xylem fibres. Bark, pith and xylem fibres were strongly colonised by the fungus twelve weeks post-inoculation and it can progress up to 8 mm from the point of inoculation using pith, bark and fibres. P. aleophilum was additionally detected in the lumen of xylem vessels in which tyloses blocked its progression. Different plant responses in specific tissues were additionally visualised. Inoculation of nodes led to restricted colonisation of P. aleophilum and this colonisation was associated with a plant response six weeks post-inoculation. The fungus was however detected in xylem vessels, bark and inside the pith twelve weeks post-inoculation.

Conclusions

These results demonstrate that P. aleophilum colonisation can vary according to the type of tissues and the type of spread using pith, bark and fibres. Woody tissues can respond to the injury and to the presence of this fungus, and xylem fibres play a key role in the early colonisation of the internode by P. aleophilum before the fungus can colonise xylem vessels.     

柑橘溃疡病及其微生物防治研究进展

柑橘是我国第一大水果,柑橘溃疡病是导致柑橘产量和品质下降的最具破坏性细菌性病害之一,给柑橘产业造成了巨大的经济损失,严重阻碍了柑橘产业的可持续发展。微生物防治柑橘溃疡病具有安全、环保、高效等优点,是当前研究的热点。本文主要概述了柑橘溃疡病特征及其病原菌分类、分布,全面分析了柑橘溃疡病病原菌主要致病机理和协助致病机理;系统梳理了柑橘溃疡病生防微生物的多样性;重点总结了微生物通过产生活性物质、诱导激活植物免疫防御系统等防治柑橘溃疡病的作用机制;最后,我们提出了柑橘溃疡病微生物防治面临的挑战以及未来可能的解决途径,以期为柑橘产业的健康发展提供参考。     

A PCR-RFLP based diagnostic technique to rapidly identify Seiridium species causing cypress canker

Seiridium cardinale, S. cupressi and S. unicorne represent three distinct species of fungi that cause cankers on Cupressus species and the disease collectively known as cypress canker. These fungi cannot be distinguished reliably from each other using morphological characters or ribosomal DNA sequence data. Here we describe a RFLP assay based on digesting β-tubulin amplicons with a single endonuclease, HaeIII, which easily can be used to distinguish among these three species. This RFLP assay provides an inexpensive and simple means of identifying Seiridium species, which include some of the most serious threats to trees in Cupressaceae.     

Cocoa canker caused by Phytophthora palmivora

Canker disease of Theobroma cacao L. caused by Phytophthora palmivora (Butler) Butler gives rise to serious damage in Fiji and is common in the S.W. Pacific area. Fusarium decemcellulareBiick (imperfect stage of Calonectria rigidiuscula (Berk. & Br.) Sacc.) was constantly associated with P. palmivora in disorders of cocoa but was not found to be a primary pathogen. Most Amelonado cocoa trees were resistant to canker but there were differences in the reactions of other Forastero and of Trinitario types. The F₁ generation of a local Trinitario selection, itself resistant, showed segregation resulting in a proportion of very susceptible plants. Isolates of P. palmivora from black pod, canker and green twigs were equally pathogenic, both to pods and seedling stems. Sporangial dimensions, particularly the length/breadth ratio, differed from those reported elsewhere. It is uncertain whether the reason that cocoa canker is more serious in some countries than others is because of differences in the pathogenicity of P. palmivora strains to bark or the genetic differences of the cocoa grown.     

STUDIES ON THE BACTERIAL DIE-BACK AND CANKER DISEASE OF POPLAR

Artificial freezing caused leaf necrosis and twig die-back but did not produce cracks or cankers on living branches of three varieties of poplar. Freezing increased the damage caused by a well-established infection of the bacterial die-back and canker but checked new lesions. Freezing prior to infection has no appreciable effect on the establishment of the disease.     

OBSERVATIONS ON THE PATHOGENICITY OF CALONECTRIA RIGIDIUSCULA (BERK. & BR.) SACC. ON THEOBROMA CACAO L

Calonectria rigidiuscula has been found associated with an acute and a chronic form of dieback of cacao. It can also infect cankers caused primarily by Phytophthora palmivora , and the lesions following attack by the capsids Sahlbergella singidaris and Distantiella theobroma. The association with the capsid lesions is of great economic importance since it appears that capsids alone are capable of killing only green shoots, and that the severe damage caused to woody shoots follows C. rigidiuscula infection of the capsid lesions. Acute dieback occurs only in certain districts after exceptional drought and may be considered primarily the result of environmental conditions. The main importance of chronic dieback is that it prevents the recovery of cacao which has been seriously weakened from other causes. C. rigidiuscula infection retards the healing of P. palmivora cankers and also produces a lesion in the xylem tissue which may be extensive.
C. rigidiuscula has been established in wounded cacao stems of all ages without difficulty. The spread of the fungus is slow, but is more rapid in the xylem than in the cortical tissues. Spread is greatest in the unlignified tissues.     

ITS序列结合培养特征鉴定梨树腐烂病菌

对采自中国4个省份的9个梨树腐烂病菌分离株和7个苹果树腐烂病菌分离株的ITS序列进行了测定和分析,并结合GenBank的有性型Valsa ceratosperma、V.ambiens和V.mali的ITS序列构建了系统发育树。结果表明梨和苹果树的各分离株在ITS核苷酸序列上分化较小(p-distance=1.55%),均在V.ceratosperma聚类组,但二者又分别处于两个独立小分支。其与V.ambiens和V.mali处在不同的聚类组中,且亲缘关系较远,表明供试梨树腐烂病菌并非V.ambiens。培养性状和生物学特点的研究结果还发现,梨树腐烂病菌各分离株无论在菌落颜色、产孢特点、还是37℃高温的生长情况都和苹果腐烂病菌有一定差别。前者菌落始在PDA终为乳白色,而后者菌落初为白色后期变褐色;在20%ABA上,前者形成的产孢体较大而数量较少,在37℃高温下能正常生长,后者则形成的产孢体较小而数量较多,在37℃高温下不能正常生长。并未发现二者在子实体上有稳定明显的差异。因而表明梨树腐烂病菌应为V.ceratosperma,但可用培养性状和生物学特点进行区分其和苹果树腐烂病菌。     

Possible Mechanisms Limiting Movement of Ralstonia solanacearum in Resistant Tomato Tissues

The distribution and appearance of Ralstonia solanacearum in the upper hypocotyl tissues of root‐inoculated tomato seedlings of resistant rootstock cultivar LS‐89 (a selection from Hawaii 7998) and susceptible cultivar Ponderosa were compared to clarify the mechanism that limits the movement of the bacterial pathogen in resistant tomato tissues. In stems of wilted Ponderosa plants, bacteria colonized both the primary and the secondary xylem tissues. Bacteria were abundant in vessels, of which the pit membranes were often degenerated. All parenchyma cells adjacent to vessels with bacteria were necrotic and some of them were colonized with bacteria. In stems of LS‐89 plants showing no discernible wilting symptoms, bacteria were observed in the primary xylem tissues but not in the secondary xylem tissues. Necrosis of parenchyma cells adjacent to vessels with bacteria was observed occasionally. The pit membranes were often thicker with high electron density. The inner electron‐dense layer of cell wall of parenchyma cells and vessels was thicker and more conspicuous in xylem tissues of infected LS‐89 than in xylem of infected Ponderosa or mock‐inoculated plants. Electron‐dense materials accumulated in or around pit cavities in parenchyma cells next to vessels with bacteria, and in vessels with bacteria. Many bacterial cells appeared normal in vessels, except for those in contact with the pit membranes. These results indicate that R. solanacearum moves from vessel to vessel in infected tissues through degenerated pit membranes and that restricted movement in xylem tissues was the characteristic feature in LS‐89. The limitation in bacterial movement may be related to the thickening of the pit membranes and/or the accumulations of electron‐dense materials in vessels and parenchyma cells.     

Comparison of paint and gel formulations for the treatment of Nectria cankers on apple trees

A gel formulation containing carbendazim and triadimefon completely eradicated infection from scraped or unscraped cankers and none became re-infected. Paint formulations containing mercuric oxide or octhilinone completely eradicated infection from scraped cankers but 30–50% of unscraped cankers receiving these treatments became re-infected. Scraping alone encouraged callus tissue formation but did not greatly reduce infection. Fungicide moved from gels into the wood beneath cankers and persisted there for up to 6 months; movement of fungicide from mercuric oxide or octhilinone paints was minimal. Carbendazim was more effective than thiophanate-methyl and xanthan gum than sodium alginate or alginate ester as fungicide and gel component respectively. Rainfall shortly after treatment probably aided movement of fungicide from gels into the wood but salt formation, by incorporation of acid, did little to improve fungicide performance. Unequivocal assessment of efficacy of canker treatment requires data on canker area changes, movement and persistence of active ingredient in the wood and extent of re-infection after 12 months.     

Bacterial canker of plum trees, caused by Pseudomonas syringae pathovars, as a serious threat for plum production in the Netherlands

In the Netherlands, bacterial canker in plum trees (Prunus domestica) is a serious and recent problem in plum production. It is caused by Pseudomonas syringae pathovars syringae and morsprunorum. The trunks of the affected plum trees are girdled by bacterial cankers resulting in sudden death of infected trees in 3-4 years after planting. Disease incidences can be very high, and sometimes complete orchards have to be removed. Recently, plum cultivation in the Netherlands has changed from a relatively extensive into an intensive cultivation. However, due to the risks of losses of trees due to bacterial canker, growers are reluctant to plant new plum orchards. In general nurseries and fruit growers are not familiar with bacterial diseases and lack knowledge in order to prevent infections. Therefore, control strategies to manage plum decline have to be developed.