Saturday, November 15, 2008

Coming back soon

It has taken a lot more of my time and energy that I thought it would to finish writing my dissertation. If all goes well, that will soon be over, and I am looking forward to returning to this blog and posting regularly. In the meantime, I have been saving all the notes from seminars I attended, and I will post them as soon as possible. Sadly, I was unable to attend all of those seminars. Blame it on the writing too! Exciting topics in are coming up, possibly as early as the first week of December.

Thursday, August 14, 2008

End of summer

The summer is almost over. I have not been posting as much as I would have like to. Currently I am trying to finish up my degree by December of this year. This poses interesting challenges. The first submission of my dissertation is due in the second week of October, but I still have experiments running until at least the third week of September. I have a set of backup experiments that will not be done until the first week of October. This means it will be extremely hectic while I try to write up my dissertation and do research at the same time.

For this blog that means that there will be almost no posting of original articles like I had planned. I have several articles in the pipeline, including one that would explain the title of this blog "The Hypersensitive Response."

The good news is that the Fall semester will start in a couple of weeks, which means that weekly seminar will return. I am still planning to attend every last one of those and post summaries on this blog as I have in the past. So stay tuned!

Saturday, July 26, 2008

Women and Cultural Diversity

Today, I attended my first committee meeting, and a lively event it was. I officially joined the "Joint Committee on Women in Plant Pathology and Cultural Diversity." That is a mouthful, isn't it? And indeed, a good deal of the discussion concerned the name and scope of the committee.

The committee finds its roots in two committees of the past. One was the Committee of Women in Plant Pathology, the goal of which (among other things) was to enhance the opportunities of women in the field, whose representation in APS leadership was not a true reflection of their composition of the organization. As you can imagine, women in plant pathology have faced similar problems as women in science, or women in society in general. The committee for Cultural Diversity had similar goals, although their target audience was minorities. The two committees came to realize that their respective programs served both target groups, and decided to join.

A large part of the discussion today was focused on the fact that the majority of the attendees of the meeting were, in fact, women. Culturally diverse women, to be exact, but largely women. Period. There was at least one person who was of the opinion that the overwhelming majority of women signified the downfall of the cultural diversity aspect of the committee.

There were 2 men there at the start of the meeting, a few more came trickling in as the meeting progressed.

One of the discussion points was that it is possible that the name of the committee is off-putting to men who want to join (even culturally diverse ones). If so, should the name be changed? In the end we decided to put the issue up for vote. Six new names were suggested for the committee, and APS members will be able to decide which one of the 7 names (the old one and 6 new ones) will prevail. The discussion is scheduled to continue on the APSnet discussion boards, which are open to APS members only by clicking on the "Interactive" link.

More importantly though, the point was made that the group might be losing track of its goals, and those might need to be redefined. No specific plan was made to address that issue, but I have high hopes that the committee will find a way to do that.

APS Centennial Meeting

The American Phytopathological Society (APS) annual meeting started today July 26, 2008. It is a very special meeting, since it is the centennial meeting, and I'm very excited to be here. This is the first APS meeting I've had the pleasure to attend. I'm hoping to write regularly about the happenings at the meeting, so stay tuned! Hopefully, I'll also get a chance to put in some pictures over time, although I haven't taken a single one yet, I've been too involved with the goings on.

Thursday, April 24, 2008

Xylella fastidiosa

The seminar disclaimer applies to this post.

The topic of seminar this week was: "Xylella fastidiosa: pathogenicity, host specificity, and disease management." The talk was given by Don Hopkins, faculty member of the University of Florida Institute for Agricultural Sciences Mid-Florida Research & Education Center (UF/IFAS-MREC)

Xylella fastidiosa is a bacterium that used to be classified as fastidious, because it was considered to be unculturable. Currently, methods do exist for growing X. fastidiosa.

X. fastidiosa causes economic losses in grape (Pierce's disease), citrus (citrus variegated chlorosis, CVC), almond, coffee, peach, and plum. It is also responsible for decline of many urban shade trees and shrubs. There appear to be 3 subspecies with different host-specificities, which is what part of Dr. Hopkin's research is focused on. He isolated X. fastidiosa from different plant species, and inoculated other plant species with those isolates. He found that isolates were most pathogenic on the plant species they were isolated from, but some isolates could cause disease on several plant species.

The disease
Picture credit.
The bacterium is spread by the glassy-winged sharpshooter, and infects the plant xylem, clogging it, and thereby slowing down the transpiration stream. The vascular obstruction causes symptoms of water stress. Bacterial toxins have also been proposed to be responsible for chlorosis and scorching symptoms, and growth regulators cold be the source of flattened dark green leaves, and shortened internodes which are among the symptoms of infected plants.

Research questions
Picture credit.
One of the questions Dr. Hopkins was trying to answer was whether the virulence of the bacterium is related to the rate of colonization of the xylem vessels, or the movement of the bacterium within the xylem. He found that the ability of X. fastidiosa to colonize the plant systemically as opposed to staying locally at the site of initial infection, determined virulence and host specificity. Since the entire genome of Xylella fastidiosa has been sequenced, researchers can use the hints provided by research results to look for specific genes that are likely involved in pathogenicity and virulence.

Disease management
Because there both the bacterium and the insect vector have a wide host range, and there are a number of plant hosts that show little or no symptoms, it is difficult to sustain efforts to exclude either X. fastidiosa or the sharpshooter. Systemic insecticides, especially within the confines of a vineyard, can to some extent be used to control the insect vector, but is made more difficult by the fact that there are so many plant hosts. Elimination of inoculum sources is another strategy, and involves removal of infected trees.

One approach in the grape industry in California involves application of the soil-applied systemic insecticide Admire in May, monitoring the vineyards, and removal of infected trees.

Plant resistance is of little value in grape culture, where the genotypes of the crop are of immense importance to the quality of the wine. Transgenic resistance does show some promise, and currently there are some field trial underway with grape vines that have be genetically altered to include a lytic peptide gene.

Of major interest is current research that involves the use of a weekly virulent strain of X. fastidiosa obtained from elderberry, which appears to offer cross-protection in the field. Very young plants are inoculated with this strain early on with this mild strain. In one experiment, plants are still healthy more than 10 years after the initial inoculation. Further experiments to test this biological control agent are being conducted in several different states. Interestingly, the treatment is more effective if the initial inoculation is performed with a highly diluted bacterial suspension. The procedure is currently in the patent process. So far there is no data to support the idea that the sharpshooter spreads the biocontrol agent, but this is hard to test, because the protecting strain maintains very low numbers in the plant, and is often hard to detect.

Future research
Current and future experiments focus on the testing of the effectiveness of cross-protection on other grape genotypes, expansion of testing in commercial vineyards in different areas, and the use of X. fastidiosa strains to control other diseases. Data is being collected to submit to the EPA, commercial interest is being evaluated, and experiments are performed to get more data on the efficacy of the treatment.


Further reading
Almeida, R. No year. Xylella fastidiosa-A scientific and community internet resource on plant diseases caused by the bacterium Xylella fastidiosa. Online at: http://www.cnr.berkeley.edu/xylella/

Mizell, R.F., Andersen, P.C., Tipping, C. and Brodbeck, B. 2008. Xylella fastidiosa diseases and their leafhopper vector. Document ENY-683 (INA174) Department of Entomology and Nematology, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Online at: http://edis.ifas.ufl.edu/in174

Tuesday, April 15, 2008

Liver cancer on citrus

The seminar disclaimer applies to this post.

Seminar was canceled two weeks ago. Last week’s seminar was presented by Mike Irey from the United States Sugar Corporation and Southern Gardens. Southern Gardens is the 3rd largest orange grower in Florida, with 21,000 total grove acreage, all planted with oranges. Over the years more than 21% of the acreage has been lost as a result of citrus canker and the citrus canker eradication effort.

However, if citrus canker was a hemorrhoid, citrus greening (also called huanglongbing, HLB) is the liver cancer of citrus

The disease
Picture credit.
HLB is a bacterial disease, caused by one of the two fastidious (up to now unculturable) bacteria Candidatus Liberibacter asiaticus or Candidatus Liberibacter africanus, the first one of which was found in Florida in 2005. The disease is vectored (spread) by the Asian citrus psyllid, an insect that doesn’t cause any major problems on citrus by itself. When 6 trees in Southern Gardens were identified to have HLB in October 2005, immediate inspection of all groves followed, and a control program to manage the vector, the Asian citrus psyllid was put into place.

Picture credit.
The initial HLB symptoms are misleadingly unimpressive, looking somewhat like nutritional deficiencies. Some leaves, usually near the center of the citrus tree show some mottling, but the trees themselves do not decline until much later. The spread of HLB has been quite dramatic over the years. It was first identified in Florida in 2005. In April 2006, 12 counties had positively identified trees, in January 2007 that was 14 counties, by June of 2007 there were 24 counties, and as of February 2008 all 30 citrus-producing countries had trees with HLB. The disease spreads very fast.

Impact
Crop losses include the immediate loss of removed trees, but also gradual tree decline and reduced production, fruit drop, reduced size of the fruit, and possibly juice quality issues, although the latter is still controversial. A small study with 10 pairs of trees comparing yields from symptomatic trees with non-symptomatic trees of approximately the same height, measured a 56% yield reduction in the symptomatic trees. This may be an over-estimate, but it is still a number to keep in mind.

Southern Gardens and US Sugar Corporation test samples from growers by polymerase chain reaction (PCR). They find that around 50-60% of the samples are positive. Some blocks of citrus have 80% of the trees infected, company-wide around 10% of the trees have HLB. Almost 300,000 trees (~1800 acres) have been removed so far, at a cost of more than 6 million dollars (roughly 1 million for tree removal, 2 million for tree replacement, 3 million for lost production). The costs of managing HLB disease has skyrocketed because of the ongoing tree removal, the increased insecticide applications, and currently 44 full-time scouts are employed, constantly surveying the groves in search of newly symptomatic trees.

Grower reactions have varied from doing nothing at all, to aggressive inoculum management, to abandoning groves all together. Those that wait for the silver bullet will likely be disappointed, as there will be no quick fix to the HLB problem.

Disease management
The main challenge is to make management decisions to deal with a disease when there is little knowledge, and a lot of data in the literature is anecdotal. In addition, the causal agent of HLB is a select agent, which complicates research.
Possible management options include:
- Vector control
- Tree removal
- Replanting disease-free material
- Disease-resistance plants (no know resistance to date)

Southern Gardens has opted for an open-door policy, opening their doors for hands-on training of scouts and managers, organizing grower meetings throughout the state, and field trials of experimental approaches.

Management of available inoculum through intensive surveys (full-time scouting) and aggressive removal of infected trees and control of the insect vector, have been the main focus of the approach so far.

The citrus industry has for a long time implemented an integrated pest management approach to control pests and diseases, using cultural practices, surveying, and careful weighing the necessity of chemical sprays. With the current intensive spray program to control the Asian citrus psyllid, the spray programs can no longer be considered IPM.

Management and administration costs have increased dramatically; disease testing for example, costs $6.50 per sample. If the psyllid is found in a nursery, the nursery managers risk have to worry about being assessed and quarantined, risking losses, and potentially even complete shutdown.

Even more troubling is that growers are becoming desperate, trying unproven methods. Often there is no data available from scientifically performed experiments with proper controls on products for which broad claims of efficacy are made. Meanwhile, the growers do not remove infected trees while they try out new products or approaches, risking further spread.

Long term strategies
At this point it is unclear what the best management strategy will turn out to be. One interesting observation to keep in mind in developing strategies is that it appears the disease is more prevalent at the edges of citrus blocks. Potential explanations are that the psyllid is coming from the outside, affecting the outer edges, or the psyllid is moving outward from the center of the block and is stopped by the lack of trees beyond the edge. Does it make sense to change the size and shape of blocks to reduce the edge/area ratio?

How effective are some of the alternative approaches and products that are currently available on the market? How can new strains be detected in a pathogen that is (up to now) unculturable? How does one optimize the PCR reaction used to detect the pathogen (where to set thresholds)? What time of the year is the best to sample? A particular tree may be negative if tested in one month, and positive in another. How effective (and economically feasible) is the strategy to intermingle citrus plants with guava plants, which seems to work in some countries? And can it still be considered a “citrus grove” if half the trees are guava?

Citrus greening is presenting new challenges to growers, nursery managers, plant pathologists, and lab technicians. A great deal of research is necessary to come up with answers.

Further reading
Halbert, Susan. Pest Alert. Florida Department of Agriculture and Consumer Services, Division of Plant Industry. http://www.doacs.state.fl.us/pi/chrp/greening/citrusgreeningalert.html

Langham, M.A.C. 2006. Citrus greening - The yellow dragon threatens Florida citrus. APSnet News and Views. http://www.apsnet.org/education/K-12PlantPathways/NewsViews/Archive/2006_04.html

Long, P. and Merzer, M. 2007. Florida citrus industry faces new peril. Miami Herald. Complete text online at Southern Gardens News and Press Releases: http://southerngardens.com/news/101607.html

Wednesday, April 2, 2008

Bacterial fruit blotch of cucurbits

The seminar disclaimer applies to this post.

Seminar this week was given by Dr. Ronald Walcott from the University of Georgia (UGA), Athens. He got his BS and MS degree from Iowa State University, and his PhD from UGA. While searching the internet for references related to this post, I found this interview with Dr. Walcott. Dr. Walcott studies seed-borne plant pathogens.

Picture credit
Bacterial fruit blotch (BFB) has become a significant pathogen over the years, in particular on watermelon. The first symptoms normally appear on the cotyledons, afterwards symptoms can sometimes be seen on the true leaves too. Under field conditions, it is really hard to distinguish these symptoms, because they are rather nondescript. The main problem occurs later in the season, when the fruits show irregularly shaped blotches, the fruits crack, and the insides of the fruit ooze out. Contrary to a NY Times report in May 1994, watermelons are not known to "explode." Still, these are not watermelons you want to buy or eat.

The disease is caused by the bacterial pathogen Acidovorax avenae subsp. citrulli (Aac), formerly considered to belong to the genus Pseudomonas. The disease cycle starts with infected seed, the most important source of primary inoculum. Since watermelons are typically seeded in flats, with many seedlings close together, maintained under high humidity, and watered by overhead irrigation, the pathogen can spread very easily from one seedling to the next, and a single infected seedling can infect the entire tray. The infection of the fruit actually occurs very early on, after the flower opens. The symptoms don't show up until much later. The infected fruit cracks open, seeds come out, can germinate in the soil (volunteer seedlings), and become a source of secondary inoculum.

BFB was first reported in Georgia in 1965, 4 years later it was observed at a research farm in Leesburg, Fl. In 1978 a strain was described that caused seedling blight, and did not show a hypersensitive response (HR) on tobacco or tomato. In 1987 there was an outbreak in the Mariana Islands, and in 1989 a strain was recovered from commercial plantings of watermelons in Florida and Indiana. Contrary to the 1978 strain, this strain caused seedling blight and fruit rot, and elicited an HR on tobacco and tomato. Since 1992, many US states have reported BFB, and there has been a significant amount of yield loss. In addition, growers have filed lawsuits against seed companies, the seed companies have in return restricted watermelon seed sales. Seeds now come with a disclaimer; the growers needs to accept the risk of BFB, and agree not to file any lawsuits.

In 1995, management guidelines were established in Georgia to control the pathogen, and these seemed to limit the problem initially, but then in 1999 and 2000 there were substantial outbreaks. Either the guidelines didn't work, or there was something else going on. Because of the increased movement of seeds, BFB currently occurs worldwide.

In 1999 there were also outbreaks on different crops. BFB was no longer restricted to watermelon, but was observed on canteloupe, melon, and pumpkin (oh no! What does that mean for Halloween?). There were often no obvious symptoms, other than a few small spots on the rind. However, below those spots, the rind doesn't develop, and the fruit goes bad. Some other hosts that the pathogen has been found on include cucumber, honeydew, hami melon, squash, bitter and bottle gourd. Seed transmission has been confirmed for several of these.

Dr. Walcott's lab investigated the genetic diversity of Aac. Using several different techniques, he discovered that at least two groups can be distinguished. Group I strains show less genetic variation than group II strains. Groups I strains have similar, moderate severity on a number of hosts, while group II strains is much more severe on watermelon, while being moderately aggressive on other hosts. The genome of a group II strain has recently been sequences, and annotation of the genome is currently in progress. Dr. Walcott is investigating the possibility of sequencing a group I strain too, so that differences at the genomic level can be correlated with differences in virulence.

Currently, the production of watermelon seed is not really conducive for Aac spread. The seeds are produced under cool, dry conditions, the seed production fields are visually inspected, and the seeds are tested for presence of Aac. But Aac continues to be a problem. Dr. Walcott therefore studied the mechanisms of seed infection. He figured there were 3 possibilities:
1. The bacteria penetrate through the ovary wall. This is unlikely, symptoms would be apparent.
2. The bacteria infect the plant systemically via the vascular system. However, there is no evidence of systemic infection.
3. The bacteria penetrate through the flower parts. Hmmm. This is promising.

Dr. Walcott considered that the bacteria may land on the stigma, move through the style, and end up in the ovary, and thought it the most likely possibility.

He took symptomless fruit, harvested the seed, and found that Aac can associate with seeds, without any symptoms on the fruit. He developed transgenic Aac bacteria expressing green fluorescent protein (GFP), so that he could track the bacteria easily. And indeed, he found the bacteria present in seed tissue, and he could track the bacteria following the path described above. He also noted that this type of infection is not unique to Aac, there are other pathogens that can do this. Using the GFP-tagged bacteria he also found out that the bacteria take about a week to travel from the stigma to the ovary via the pistil pathway. Once the bacteria reached the ovary, however, replication of the bacterium stopped. This explains the lack of symptoms.

Dr. Walcott discovered that bacterial motility was not important for colonization of the seeds, but that pollination was necessary. He also investigated the role of pollinating insects, and found in one experiment that bees did seem capable of spreading the infection, but these results need to be verified with additional experiments. The problem with this theory in practice is, however, that in commercial seed production fields, most pollination does not occur by bees, but by hand, so the bees may not be sufficient to explain the outbreaks seen.

Towards the end of seminar, Dr. Walcott discussed some of the experiments he has done to investigate the possibility of biological control. Although he found a good level of control with at least one biological control agent, this is not good enough for watermelon seed production with a zero-tolerance of Aac.

Further reading

Fessehaie, A., and Walcott, R.R. (2005) Biological control to protect watermelon blossoms and seeds from infection by Acidovorax avenae subsp. citrulli. Phytopathology 95:413-419.

Gitaitis, R.D. and Walcott, R.R. (2007) The epidemiology and management of seedborne bacterial diseases. Ann. Rev. Phytopathology 45: 371-397.

Lessl, J. T., Fessehaie, A. and Walcott, R. R.. 2007. Colonization of female watermelon blossoms by Acidovorax avenae subsp. citrulli and the relationship between blossom inoculum dosage and seed infestation. J. Phytopathology 155:114-121.

Walcott, R.R. (2005) Bacterial fruit blotch of cucurbits. The Plant Health Instructor. DOI:1094/PHI-I-2005-1025-02

Wednesday, March 26, 2008

The ins and outs of leaf scald of sugarcane

The seminar disclaimer applies to this post.

The topic of seminar for this week was the bacterial pathogen Xanthomonas albilineans, the causal agent of leaf scald of sugarcane. The speaker was Dr. Philippe Rott, from CIRAD UMR BGPI, Montpellier, France.

Sugarcane is the main source of sugar in the world. Sugarcane is propagated vegetatively from stalk cuttings. The cuttings germinate, tiller (produce new stalks), grow, and mature (final accumulation of sugar). The crop cycle is 3-10 years long. Sugarcane is grown in many countries, but Brazil, India and China are the main producers, the US is #10 on the list.

Leaf scald disease is caused by a bacterium, Xanthomonas albilineans, and the most typical symptom of leaf scald is a white pencil-line streak running parallel along the veins (Comstock and Lentini, 1991, 2005). Over time the leaf becomes necrotic and dies.
Picture credit.

Leaf scald of sugarcane was first described in 1911 in Australia, and is currently controlled to some extent by using resistant sugarcane crosses Saccharum officinarum x S. spontaneum. The genus Xanthomonas contains several major plant pathogens, including X. campestris pv. campestris (black rot of several plants), X. axonopodis pv. citri (citrus canker), X. axonopodis pv. vesicatoria (bacterial spot), and X. oryzae pv. oryzae (bacterial leaf blight of rice).

Some known pathogenicity factors (factors that the pathogen needs to cause disease) in these pathogens are the xanthan gum genes, the LPS (lipopolysaccharides that the bacteria secrete, rpf (regulation of pathogenicity factors) genes, and a type III secretion system (also called a hrp system).

X. albilineans invades the sugarcane xylem. It is an unusual xanthomonad in that is phylogenetically distinct from other plant-pathogenic xanthomonads, based on the sequences of the ITS region and gyrB. Based on PCR and Southern blot experiments, there is no evidence that X. albilineans has a hrp system like other phytopathogenic xanthomonads. The pathogen does produce a toxin, albicidin.

Albicidin is a specific molecule essential for the disease symptoms, and toxin(-) mutants show no symptoms on sugarcane. Albicidin inhibits DNA replication, chloroplasts, prokaryotes (which means it's an antibiotic), and bacteriophages.

In 1993, Dr. Rott joined the lab of Dr. Dean Gabriel in the department of Plant Pathology at the University of Florida as a visiting professor to further study X. albilineans. His goal was to characterize the biosynthetic pathway of albicidin, identify the genes involved, and study the relationship between biosynthesis variability and variability in disease symptoms.

Using Tn5 mutagenesis, he isolated 7100 mutants, 50 of which did not produce albicidin (assayed by the lack of an inhibition ring around the bacterial colonies). He identified 3 genomic regions that were involved in albicidin production. XALB1, a 55.8 kb region, a cluster of 20 predicted open reading frames, XALB2, a 2.9 kb region, predicted to encode a single protein, phosphopantheteinyl transferase (PPT), and XALB3, a 2.6 kb region, predicted to encode a single protein, a heat shock factor, HtpG (Vivien et al., 2007).

The XALB1 region contains genes that are similar to genes that encode non-ribosomal peptide synthases (NRPSs) and polyketide synthases (PKSs) in addition to predicted regulatory, modifying, and resistance genes. Mutations made in some of these genes and subsequent complementation confirmed they were necessary for albicidin synthesis (Royer et al., 2004).

NRPs and PKSs are large, multi-functional enzymes that are responsible for one specific elongation step of adding a peptide or ketide to a growing polypeptide/ketide chain. Certain domains within these proteins determine their substrate specificity (meaning they determine which particular peptide or ketide is added to the chain). The albicidin backbone is made by NRPSs and PKSs, although the complete structure has not been elucidated yet, because albicidin contains some unusual peptide residues.

Dr. Rott then set out to determine whether the diversity of the genes involved in the biosynthesis of albicidin are also involved in the pathogenicity variation observed for X. albilineans associated with different pathotypes, and disease outbreaks. Research found no structural differences in albicidin. There were differences found in the albicidin production levels, but there was no correlation with pathogenicity (Champoiseau et al., 2006). This implies that there is some other factor involved, which is responsible for the observed variation in pathogenicity. This hypothesis is supported by the fact that toxin(-) mutants are still able to colonize the sugarcane stalk, even though they do not produce symptoms.

Dr. Rott's team then set out to sequence the entire genome of X. albilineans. Because this research has not been published yet, he did not give many specifics about the sequence data. He did reveal that X. albilineans turned out even more strange than anticipated, with an unusually high number of NRPS genes, only 3 of which are required for albicidin synthesis.

Dr. Rott concluded by summarizing that X. albilineans
1) is an unusual xanthomonad that does not have high similarity in genome sequence to other published xanthomonad genomes;
2) produces no xanthan gum;
3) does not have a hrp system;
4) has a fairly small genome compared to other xanthomonads;
5) is phylogenetically distinct from other xanthomonads;
6) appears to specialize in the production of secondary metabolites, which can enter the plant cell, using NRPSs and PKSs.

References:

Champoiseau, P., Daugrois, J.-H., Girard, J.-C., Royer, M. and Rott, P.C. (2006) Variation in albicidin biosynthesis genes and in pathogenicity of Xanthomonas albilineans, the sugarcane leaf scald pathogen. Phytopathology 96(1):33-45.

Comstock, J.C. and Lentini, R.S. (1991, 2005) Sugarcane Leaf Scald Disease. Document SS-AGR-201, Agronomy Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. First printed March 1991. Revised March 2005. Online at: http://edis.ifas.ufl.edu/SC001. Last accessed: March 26, 2008.

Royer, M., Costet, L., Vivien, E., Bes, M., Cousin, A., Damais, A., Pieretti, I., Savin, A., Megessier, S., Viard, M., Frutos, R., Gabriel, D.W., and Rott, P.C. (2004) Albicidin pathotoxin produced by Xanthomonas albilineans is encoded by three large PKS and NRPS genes present in a gene cluster also containing several putative modifying, regulatory, and resistance genes. Mol. Plant-Microbe Interact. 17(4)414-427.

Vivien, E., Pitorre, D., Cociancich, S., Pieretti, I, Gabriel, D.W., Rott, P.C., and Royer, M. (2007) Heterologous production of albicidin: a promising approach to overproducing and characterizing this potent inhibitor of DNA gyrase. Antimicrob Agents Chemother. 51(4): 1549–1552.



Thursday, March 20, 2008

Late-season vine decline of melons

The seminar disclaimer applies to this post.

This week our departmental seminar was given by none other than the current President of the profession organization for plant pathologists, the American Phytopathological Society (APS), Dr. Raymond D. Martyn. Dr. Martyn is a homegrown graduate, who did his PhD research on biological control of waterhyacinths. He then moved to Texas A&M, where he was a professor for 20 years, and his research focused on soil-borne diseases of melon. He then moved to Purdue University as Head of the Botany and Plant Pathology Department.

Dr. Martyn is no longer involved in active research but this has given him plenty of time to digest the work he did over the years.

During the past 20-25 years a dramatic increase in late-season vine decline (LSVD) of melons has been observed. This involves all members of the cucurbit family of plants, for example watermelon, squash, and muskmelon. LSVD is a generic term used to describe a set of symptoms that include the sudden death of cucurbit foliage about 2-3 weeks before harvest. This period is critical for the accumulation of sugars during the final ripening stages of the melons, so the vine death results in major losses.

There is no single cause for LSVD, nothing that correlates exclusively with vine decline. Sometimes there are pathogens involved, other times not. Vine decline has reportedly been associated with presence of fungi, viruses, bacteria, application of herbicides, and more. The fungus Monosporascus cannonballus, is one of the major fungal pathogens found to be associated with LSVD.

M. cannonballus is a fungus that forms fruiting bodies called perithecia on the roots of the melons. These perithecia have structures in them called asci (singular: ascus). Fungi belonging to the ascomycota typically have asci with 8 ascospores, M. cannonballus is different in that each ascus only has one ascospore, but this spore contains 8 nuclei (it's a called a multinucleate ascospore). Sometimes 16, but mostly 8. Looking at the picture below, it's easy to see where the fungus gets its name from, the perithecium is almost perfectly round and smooth. The ascospores have very thick walls, and are hard to germinate: they are dormant spores. M. cannonballus was first reported in 1970 as a saprophyte (not a pathogen) on canteloupe, in 1983 it was first reported as a plant pathogen in Israel, and in 1989 it was first reported within the US by guess who... Dr. Ray Martyn.

Picture credit

In the greenhouse, M. cannonballus can infect many different plant species. When a fungus can infect many species, it is said to have a "wide host range." However, M. cannonballus is only a problem on watermelon and muskmelon in the field, so for practical purposes M. cannonballus has a narrow host range. Dr. Martyn studied potential biocontrol methods. He looked at ways in which the disease can be managed using organisms. In this case, M. cannonballus changed in culture, and became hypovirulent, meaning they caused a lot less disease than the original fungus. He investigated whether he could decrease the damage caused in the field by adding the hypovirulent fungus, which would "mate" with the disease-causing fungus, resulting in hypovirulent offspring.

Dr. Martyn authored an article for the APS Education website on Monosporascus root rot and late-season vine decline of melons.

A number of other pathogens were studied as possible causes of late-season vine decline, but all turned out to be fairly weak pathogens, that only caused damage when the plant was very young. In LSVD, the plant is mature, and much stronger, and these pathogens were not responsible for the disease symptoms. In addition, Monosporascus is not the cause of LSVD everywhere in the world. It is reported only in areas that are fairly warm.

Since LSVD is a fairly recent syndrome, Dr. Martyn considered the changes in melon production in the past 25 years. These are:
1. Melon production switched from open-pollinated plots to the use of hybrid melon cultures.
2. Instead of direct seeding into the ground, current production techniques almost exclusively use transplants (seedlings are grown in small trays, and transferred into the ground when they are older).
3. Planting used to be in bare ground, and irrigation systems used furrows, currently plastic mulch and drip irrigation are used.
4. Plants are planted much closer together than in the past.

Dr. Martyn proposed during the seminar that these changes in melon culture practices result in compromised and restricted root systems on the plants. When melons are seeded directly into the ground, the initial root that is formed when a seed germinates, the tap root, continues to grow deep into the soil, as deep as 6-8 feet. A large number of secondary and tertiary roots form along the length of the tap root, allowing the plant to extract water and nutrients from deep in the soil. When seed is germinated in little seedling trays, the tap root grows to meet the bottom of the tray and starts growing in a circle. Once the plant is transplanted into soil, the taproot never recovers, and dies off. Secondary roots that are formed after transplanting do not grow deeply into the soil, but remain about 4" or so below the ground level, and grow essentially horizontally. This drastically limits the availability of water and nutrients to the root system, because the roots do not reach deep enough into the soil.

The compromised root systems have a hard time keeping up with the demand for water of the mature plant, especially during the latter stages, when fruits need to accumulate sugars. Minor pathogens and additional stress, for example lack of water, cause rapid death of the plant in the final stages of fruit development, when the plant needs the resources the most.

Dr. Martyn investigated using cone-tainers, which are very long containers to start seedlings, and they allow for much more growth of the tap root before transplanting, and are so long that the tap root does not reach the bottom before transplanting occurs. After the seedling is planted in soil, the tap root can continue to develop.
Picture credit

In conclusion, LSVD correlates with cultural changes, resulting in vulnerable root systems that cannot handle multiple stresses.

Wednesday, March 19, 2008

Weekly seminar

The department of Plant Pathology at the University of Florida offers weekly seminars in the Spring and Fall semesters. I attend most of these seminars, and take notes. In the "weekly seminar" series I will summarize my notes on this blog.

It is important to note that the normal disclaimer applies to these posts (see top of the blog page). In addition, these posts are exclusively based on my notes, which are the result of my personal interpretation of what the speaker says. The speaker does not read or comment on my post before publishing, so what I write is not in any way endorsed or sanctioned by the speaker.

Be that as it may, most of the time, the topics addressed during the seminars are current, interesting, and a great source for discussion.

Plants get sick too

Welcome to The Hypersensitive Response! This blog is intended to provide information about the basics of plant pathology, the history of plant pathology, plant diseases, plant disease diagnosis, disease management, current topics, recently published papers, and much more.

For those of you who have never thought about it, plants are organisms that suffer stresses just like animals/humans do. Because of the nature of plants, the stresses they endure are in many ways different from those endured by animals, but they are remarkably similar too, as future posts will point out.

Plant stresses can be broadly divided in abiotic and biotic stresses. Abiotic stresses are those that result from the environment that the plant is in. For example, a plant cannot simply walk off to a cooler area when it is too hot, and low and high temperature can cause stress. Plants derive their mineral nutrition (mainly) through their root system, so availability of nutrients (either too much or not enough) can cause stress to a plant. Humidity, physical wounding, too much water or not enough, all are causes of abiotic stress.

Biotic stresses are those caused by other organisms. The term "organisms" is used broadly here, and includes viruses (which are not considered living organisms), bacteria, fungi, nematodes, insects, and even other plants.

The field of plant pathology mainly focuses on diseases caused by organisms, however, often interactions with abiotic factors are also considered. For example, a plant may normally not be attacked by a certain pathogen, but is a target whent he plant is already weakened by an excess water. There often are interactions between abiotic and biotic factors to plant disease, and hopefully these will be discussed extensively on this blog.

If you want to contribute as a guest author, please contact me at the e-mail address provided in the sidebar. I will gladly turn this blog into a group blog, and I invite everyone that has something to write on plant pathology to become a one-time guest author, or a regular contributor. The more people, the broader the scope, and the more interesting the blog becomes.

Welcome and enjoy!