Plant Diseases

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• Plant Diseases Bioterrorism
• Intro to plant diseases and plant pathogens
• - diversity of symptoms hosts
• - viruses, bacteria, fungi, protists, nematodes
• 2. Plant disease threats to US food and
  ecological
• security
• - emerging diseases escapes
• - exotic pests
• 3. Case Study Ralstonia solanacearum, a
• plant pathogen with enemies in high places

2
Plant diseases have diverse causes Viruses Nema
todes Fungi Oomycetes Bacteria Plant diseases
have diverse symptoms Leafspots and
cankers Wilting Tumors Rotting The
population, not the individual, is treated
emphasis on public health epidemiology
3
Potato late blight, Phytophthora infestans
(oomycete) caused 1846-8 Irish potato famine
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Tobacco Mosiac Virus ss RNA virus
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Tomato rust Puccinia pittierana (fungus)
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Citrus canker, Xanthomonas axonopodis pv citri
(bacterium)
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Citrus canker lesions on leaves
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Potato soft rot, Erwinia carotovora (bacterium)
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Botrytis rot of raspberries (fungus)
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Root knot nematode, Meloidogyne hapla
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Root knot nematode on bean, Meloidogyne hapla
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Potato cyst nematode, Globodera rostochiensis
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Corn smut, Ustilago maydis (fungus)
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Crown gall, Agrobacterium tumefaciens (bacterium,
and natural genetic engineer)
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• Plant disease can threaten ecological,
• agricultural even political security
• -Irish potato famine
• (1M died, 5M emigrated, toppled British govt)
• chestnut blight wiped out American chestnut
• Dutch elm disease transformed our cities
• - Southern corn leaf blight in 1970

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• However plant pathogens are not likely terror
  agents
• - They do not directly threaten human health
  (exception some fungal toxins like aflatoxin)
• So they evoke little emotional response
• Its difficult to get an epidemic started
  (environment
• must be right, crop must be at right growth
  stage,
• pathogen must be easily dispersed, stable,
• highly virulent)
• The few known attempts have failed biocontrol
  of coca in Peru, tobacco blue mold sugar cane
  rust in Cuba

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Dutch Elm Disease, Ceratocystis ulmi (fungus)
Imported in logs, originally from Asia
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Southern corn leaf blight Bipolaris maydis
(fungus)
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Southern corn leaf blight in Illinois, July 1970
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Where do our worst plant diseases come from? -
most crop species arent native to N. America -
arent adapted to local pests - may have escaped
pathogens from their origin
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Where do our worst plant disease problems come
from? -emerging pathogens pests from native
plants attack imported crops (cacao swollen
shoot virus) -exotic pathogens imported
pathogens new to the crop (chestnut blight,
Dutch elm, Sudden Oak Death?) -crops
inadvertently bred for susceptibility (S. corn
leaf blight, apple scab)
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Rice blast, Magnaporthe grisea (fungus) -
worlds single most important plant disease
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Sudden Oak Death, Phytophthora ramorum
(oomycete) new, broad host range, very
lethal imported exotic or emerging pest??
24
Coffee rust, Hemileia vastatrix (fungus)
why you drink shade-grown coffee
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Citrus variegated chlorosis, Xylella fastidiosa
(bacterium)
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Soybean rust, Phakopsora pachyrhizi (fungus) -
alarming exotic plant pathogen, came north from
Brazil but it has not caused big losses
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Citrus Huanglongbing - citrus greening disease
Extremely destructive emerging citrus disease,
causes yellow shoots and mottled leaves. Fruit
becomes bitter (useless) and trees eventually die
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Transmitted by phloem-feeding citrus psyllid
insects often a latent period before symptoms
appear. Pathogen is probably the Candidatus
Liberibacter asiaticus bacteria. It has not yet
been cultured. There are no good control
methods no resistant varieties, no fast
diagnostic methods
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HLB poses a very serious threat to Florida citrus
production Eradication efforts are on-going May
result in end of shipment of fresh citrus from
Florida in order to protect industries in Texas
and California.
30
The Agricultural Bioterrorism Protection Act of
2002 Listed Select Agents plant animal
pathogens that pose a severe threat to US
agriculture, mandated highest possible level of
security, exclusion, and eradication. Included
10 plant pathogens Plant pathology communitys
reaction Huh?
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• Select Agent Plant Pathogens (2008)
• Peronosclerospora philippinesis (sugarcane downy
  mildew)
• Phoma glycinicola (soybean red leaf blotch)
• Ralstonia solanacearum, race 3, biovar 2 (potato
  brown rot)
• Rathayibacter toxicus (ryegrass toxicity
  gumming)
• Scelrophthora rayssiae var. zeae (brown stripe
  downy mildew of corn)
• Synchytrium endobioticum (potato wart)
• Xanthomonas oryzae (bacterial blight of rice)
• Xylella fastidiosa (citrus variegated chlorosis
  strain)

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Thursday How the plant pathogenic bacterium
Ralstonia solanacearum made enemies in high
places earned a place on the Select Agents
list and destroyed the livelihoods of workers
in Central America and Africa. Or The
globalization of biosecurity anxiety