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Overview
“Bot canker” is the informal name of a disease which was identified, in 2010, as the cause of rapidly spreading oak death in California and Florida, and grapevine mortality in California, Texas, and Spain.(Dreaden et al., 2011; Urbez-Torres et al., 2010; Varela et al., 2011). That name comes from the sexual stage of the causative fungus, Botryosphaeria corticola A.J.L. Phillips, Alves & Luque, sp. nov.; however, because the genus Botryosphaeria was shown to be composed of “10 lineages, which correspond to individual genera,” it is used now only for two species, B. dothidea and B. corticis, with the anamorph names applied to the other species formerly placed in Botryosphaeria(Moral et al., 2010).The anamorph (asexual) stage of this fungus is Diplodia corticola A.J.L. Phillips, Alves & Luque, sp. nov.(Alves et al., 2004).
Although fungi in the genus Botryosphaeria cause disease on hundreds of plant genera, gymnosperms as well as angiosperms (Bush, 2009; DeWet et al., 2007), Diplodia corticola has been found only on oak and grapevine. D. corticola is an opportunist which enters plants through wounds, including leaf scars, or stomata open for gas exchange.The fungus may colonize dead tissue, then move into healthy tissue in the branch (Bush, 2009).It was first identified in the 1980’s in cork oaks (Quercus suber L.) in Mediterranean countries, where it has devastated the cork industry.(The word “corticola” is derived from the Latin root for “cork.”)The traditional--and environmentally “green”--enterprise of removing cork from oak trees[i] causes mild injury providing entry to the pathogen (Franceschini et al., 2005).
Researchers later deduced that D. corticola was the cause of mortality in other species of European oaks, as well, a malady called “oak decline” (Campanile et al., 2007).The pathogen was first reported in California in 1998 (Urbez-Torres, 2010).The disease can be spread by air, water splash, or contaminated pruning tools.The possibility that it may also be vectored by insects (the gold spotted oak borer in California (Lynch et al., 2010), and various gall wasps in Florida) is under investigation. In 2008, Erbilgin et al. (2008) reported isolating D. corticola from live oak trees, both living and dead, that had been colonized by bark and ambrosia beetles following artificial inoculations with Phytophthora ramorum.
D. corticola is an ascomycete which produces several phytotoxins, primarily one recently identified and named diplopyrone (Maddau et al., 2008), which is toxic to Q. suber at concentrations from 0.01 to 0.1 mg/mL.The production of phytotoxins appears to be common to pathogenic species in Botryosphaeria, such that these species are properly termed necrotrophic, although they are usually characterized as saprotrophic and the poisons characterized as metabolites. Other toxins produced by D. corticola are sphaeropsidins A-C and sapinofuranone B.Botryosphaeria obtusa--which causes black rot of apple fruit and frogeye leaf spot--produces a veritable smorgasbord of toxins:mullein, tyrosol, 4-hydroxymellein, 5-hydroxymellein, and 4-hydroxybenzaldehyde, interesting because this organism attacks fruit rather than systemic tissue (Venkatasubbaiah et al., 1991).In none of the literature reviewed by this writer was it stated what Diplodia corticola is metabolizing.
Biocontrol of D. corticola may be possible via antagonistic endophytic fungi, such as Trichoderma viride and Fusarium tricinctum (Campanile et al., 2007; Maddau et al., 2005), but at present no fungicides are available, nor is the use of chemical fungicides prudent in forests (Bush, 2009).
Details
Symptoms of Bot Canker.In the cork oaks (Q. suber), discolored bark, difficult to detect, is the first disease symptom, appearing 2-3 months after cork removal.After approximately six months, the tree develops branch necroses of varying lengths.Wilting follows, because vascular tissue has lost function, and fruiting bodies (pycnidia) are observed.Bark comes off the trunk easily and the tree dies usually between one and three years after symptoms are first noted (Luque and Girbal, 1989).
Infection in live oak trees (Quercus virginiana) infected by D. corticola in California and Florida may be more difficult to detect, since it is not preceded by cork removal.Clumps of dead branches suddenly appear in the crown, randomly distributed (Dreaden et al., 2011). Cankers (elongated cracks in the bark) are evident on branches, and cutting into the branch at these points reveals sapwood and phloem discolored to dark brown.Pycnidia, tiny black spots which contain conidia, erupt through the bark or cankers.
[i] “Cork harvesting,” http://www.youtube.com/watch?v=ztr-RP0XYd8 (accessed 9.17.12).
Morphology and Life Cycle.
In some reports, in culture on potato dextrose agar D. corticola initially appears fluffy white on top, turning to dark gray mycelia after about five days, with the underside becoming olive-green, turning to black (Dreaden et al., 2011; Alves et al., 2004).(Interestingly, a recent report of two pathogenic species isolated from grapevines in southern Spain assigns the description of “abundant gray mycelium with a diurnal zonation that gradually became dark olivaceous” to Neofusicoccum mediterraneum, while describing the Diplodia corticola recovered from the same sample as “whitish, dense, aerial mycelium [which] remained white up to 10 days on PDA and darkened to gray thereafter.” (Varela et al., 2011).[i]Neofusicoccum is a clade resolved by Crous et al. (2006) to contain anamorphs of five other Botryosphaeria species previously classified as genera.)The similarities make identification tough, especially because the Varela group, for example, through DNA sequencing found D. corticola together with N. mediterraneum and five other pathogenic species, which included a second Botryosphaeria and second Neofusicoccum species, in trunk cankers.
The sexual stage, or teleomorph, is rarely found in nature, and the characters of teleomorphs are often too similar to distinguish among them.(Jacobs and Rehner, 1998).Thus, conidia and conidiophores are used, since such features as size, shape, color, wall thickness, and septation are distinct in these structures.They are described for D. corticola by Alves et al. (2004) as follows:
Conidiomata pycnidial, separate or aggregated, globose, dark brown to black, immersed, multilocular, thick-walled; outer wall layers thick-walled textura angularis, inner layers thin-walled, hyaline; ostiole central, papillate.Conidiophores reduced to conidiogenous cells.Conidiogenous cells holoblastic, integrated or discrete, determinate, cylindrical, hyaline, smooth, forming a single, apical conidium, proliferating percurrently to form one or two indistinct annelations, or proliferating at the same level to form periclinal thickenings.Conidia smooth, unicellular, cylindric with broadly rounded ends, some with a large central guttule, smooth, with a thick glassy wall that remains hyaline even after the conidia have been discharged from the pycnidium, rarely becoming light brown and one- or two-septate after discharge (23.5–)26–35(–41) × (10–)11–17(–18.5) µm, average of 250 conidia = 29.9 ± 2.6 ´ 13.6 ± 1.4 µm.Length/width ratios in the range of (1.6–)2.2–2.3(–3.1) with a mean and standard deviation of 2.2 ± 0.3.
D. corticola asci and pseudoparaphysesBrown septate, and hyaline aseptate, ascospores
All images on this page and the next from:
http://www.crem.fct.unl.pt/botryosphaeria_site/botryosphaeria_corticola_2.htm
Conidiogenous cell with periclinal thickenings.Mature conidia. Scale bar = 10 µm Scale bar = 5 µm.
[i] This suggests cultures may look different depending on how long they are kept in the dark or the light.See:http://www.ncbi.nlm.nih.gov/pubmed/2386731)
Environmental and Cultural Significance.
The cork oak forests are a unique ecosystem which is home to many endangered and threatened species, including the Iberian Lynx, the Iberian Imperial Eagle, and the Barbary Deer.[i]The cork industry is sustainable, since cork renews itself and is harvested only every nine years.The harvesting is still done by laborers using handheld implements.Cork is also an excellent insulator, an evolutionary adaptation of Q. suber to fire. Unlike other species, which sprout from the ground after a fire, this oak’s branches resprout quickly to “recompose the tree canopy.”[ii]The lands occupied by the cork oak forests are, as might be expected, hungrily eyed by Mediterranean tourist resorts seeking to expand.
The loss of live oaks in California and Florida, and possibly other species of oak there and elsewhere, is a frightening prospect.In the words of one oak aficionado (Keator, 1998):
Oaks dominate many landscapes—scrublands, woods, and forests alike—and they are inextricably linked to a staggering number of other organisms, from fungi to bacteria, birds to bears, and wasps to ants.Humans, too, have always had a strong connection to oaks.They have figured largely in our knowledge and myths.Throughout history, oaks have served as signs of permanence, eminences of hardihood, and figures of enduring beauty.They have provided food, dye, shelter, and wood for implements, furniture, and fuel.… And oaks show no signs of stagnating; despite their ancient lineage, new hybrids are constantly cropping up in new places, as genetic combinations are tried and retried.Some of these natural, ongoing experiments will surely succeed in carrying oak evolution a step further.
He goes on to warn us that we cannot take the permanence of oaks for granted, however, since they are under siege already, from logging and overgrazing.Hopefully, they will be adaptable enough to overcome this latest pestilence.
[i] http://www.crem.fct.unl.pt/Research_Lines/Line_4/Line_4.htm
Phylogeny.
The fungus causing dieback of cork oak was originally identified as Botryosphaeria stevensii Shoemaker (anamorph:Diplodia mutila Fr.) (Luque and Girbal, 1989).Differences in the sizes and shapes of conidia and ascospores, as well as in the ITS region of the nuclear ribosomal DNA sequence, led to the identification of D. corticola as a new species by Alves et al. (2004).
The taxonomy of the “species formerly known as Botryosphaeria spp.” is still unsettled--and confusing.Alves et al. (2004) resolved the Botryosphaeria genus into two main groups, the Diplodia anamorphs and the Fusicoccum anamorphs, showing that they diverge from a common ancestor. Diplodia is then divided into five subgroups the members of which, many having similar names (e.g., six named B. corticola and four named B. stevensii), are denominated by unique GenBank accession numbers.Both B. corticola and B. stevensii are also names of two of the subgroups.
Crous et al. (2006) built on this work, among many other things noting that, although Diplodia conidia are dark, conidia in species of Fusicoccum become dark as they age, making it extremely difficult to distinguish the two.(Compounding the difficulty is that Alves et al. (2004) says Diplodia conidia are hyaline, “rarely becoming light brown… after discharge”!)To resolve the confusing morphology and evolutionary lineages of this group, gene sequences from additional regions of the genome will need to be compared.
Molecular investigations are underway to determine whether the Florida strains are identical to those infecting oaks in California, as well as the degree of similarity of those strains to isolates recovered in Portugal, Spain, Italy, Greece, and Hungary; host range studies are also being conducted to test the susceptibility of 23 species of oaks indigenous to Florida (Smith et al., 2011).It is presently unknown how the fungus arrived in the United States.
The classification of this organism is:
Kingdom:Fungi
Phylum:Ascomycota
Class:Dothidiomycetes
Order:Botryosphaeriales
Family:Botryosphaeriaceae
Genus: Diplodia
Species:corticola
[i] http://www.crem.fct.unl.pt/Research_Lines/Line_4/Line_4.htm
Symptoms of Bot Canker.In the cork oaks (Q. suber), discolored bark, difficult to detect, is the first disease symptom, appearing 2-3 months after cork removal.After approximately six months, the tree develops branch necroses of varying lengths.Wilting follows, because vascular tissue has lost function, and fruiting bodies (pycnidia) are observed.Bark comes off the trunk easily and the tree dies usually between one and three years after symptoms are first noted (Luque and Girbal, 1989).
Infection in live oak trees (Quercus virginiana) infected by D. corticola in California and Florida may be more difficult to detect, since it is not preceded by cork removal.Clumps of dead branches suddenly appear in the crown, randomly distributed (Dreaden et al., 2011). Cankers (elongated cracks in the bark) are evident on branches, and cutting into the branch at these points reveals sapwood and phloem discolored to dark brown.Pycnidia, tiny black spots which contain conidia, erupt through the bark or cankers.
Diplodia corticolaa is a species of anamorphic fungus in the family Botryosphaeriaceae.[1][2]
Diplodia corticolaa is a species of anamorphic fungus in the family Botryosphaeriaceae.
