Learning from Past and Current

Plant Epidemics

Case Studies, ramifications, and emerging strategies

for combating plant diseases

This slide show was adapted from a December 2003 presentation by James Schoelz, Ph.D., at the Missouri Summit on Agroterrorism. Schoelz is Professor and Chair of the Department of Plant Microbiology and Pathology at the University of Missouri-Columbia.

Presentation Overview• Learning from Past Epidemics

• Case Study: The Potato Famine of Ireland 1845-1850

• Case Study: Southern Corn Leaf Blight 1970• Threats to the US

• Disease categories• New pests and pathogens• Scenario: Spodoptera

• State of the Art Strategies for Combating Plant Epidemics

Case History: The Irish Potato Famine

In the 1840s,

almost half of the population in Ireland depended on potatoes to survive.

Sources: Encyclopedia Britannica, 2002; Pictorial Times, 1846

The Irish Potato Famine--continued

In order to feed its people, Ireland relied primarily upon two high-yielding potato varieties.

Sources: Encyclopedia Britannica, 2002; Illustrated London News, 1849

When the potato disease struck, it resulted in a massive crop failure that lasted five years, 1845-1850.

Due to the crop failure . . .

…thousands resorted to begging for food…

…there were food riots…

… and many were evicted, and their homes were burned.

Sources: Encyclopedia Britannica, 2002; Illustrated London News, 1842, 1848; LLDB, 1847

Over a 15 year period in Ireland . . .

• A loss of 1 million lives due to starvation and disease.

• A loss of 1.5 million due to emigration.

Ireland’s 1845 population of 8 million dropped to 5.5 million by 1860.

Sources: Plant Diseases: Their Biology and Social Impact; Encyclopedia Britannica, 2002; Illustrated London News, 1847; 1851

What caused The Irish Potato Famine?

A disease, called

Late Blight of Potato

destroyed both the leaves and

tubers of the potato plants.

The cause was a fungus called

Phytophthora infestans

Why was the disease widespread?

Three factors:

Uniform susceptibility in the host plants

The introduction

of a pathogen

An environment conducive to

disease

Need a more recent case study?

Let’s look at the

1970

Southern Corn Leaf Blight Epidemic

In 1970, 80% of all hybrid field corn grown in the U.S. was susceptible toSouthern Corn Leaf Blight.

Southern Corn Leaf Blight Epidemic -- continued

The pathogen introduced was

Cochliobolus heterostrophus race T

The generation time for new inoculum?

Only 51 hrs

A. June 18

B. July 15

C. September 1

The path:

Progress of Southern Corn Leaf Blight Epidemic in North America (1970)

Source: Plant Diseases: Their Biology and Social Impact

Why did it happen?

Uniform susceptibility in the host plants

The introduction

of a pathogen

An environment conducive to

disease

The consequences?

• Over $1 billion in losses

• In many southern states, entire fields were lost

• Losses of 80 - 100% were common

Source: Plant Diseases: Their Biology and Social Impact

Part Two

Plant biosecurity threats to the U.S.

• Categories of Plant Diseases

• Recently-introduced pests and pathogens

• Scenario: Is a SpodopteraInfestation in our Future?

5 Categories of Plant Diseases

1. New Disease – introduced on new host within last five years in a new geographic area

2. Emerging Disease – increased incidence within last 10-15 years

3. Re-emerging disease - previously known in area but gaining importance

4. Threatening Disease – not reported or limited distribution in a new geographic area

5. Chronic/spreading disease -known for a long period and still causing out breaks.

Sources: SPDN; Damsteegt (1999)

Question:

Can you list an example of an

emerging plant pest or pathogen

that has been introduced in the

United States after 1990, but is

not yet eradicated?

Examples of new pests and pathogens introduced in U.S. since 1990

Pest or pathogen Susceptible Crops State

Citrus canker

Xanthomonas axonopodis pv. citri

Orange; grapefruit; key lime; sweet lime;lemons; orange; tangelo; kumquat; pineapple

FL1999

Plum pox virus (sharka)Plum pox potyvirus (PPV-D)

Peach; apricot; cherry; plum nectarine; almonds

PA1999

Glassy-winged sharpshooter, a new carrier (vector) of Pierce’sDiseaseXylella fastidiosa

Grapes (includes the raisin and wine industries)

CA1990

Source: APHIS/PPQ

Depending on the plant species and pest or pathogen, these outbreaks may be spread by:

• Aphids and other vectors (carriers)

• Propagation of diseased plants

• Planting of infected seeds

• Contaminated cattle manure

• Crops moved in contaminated equipment

• Wind, including hurricanes & tornadoes

• Human transportation of infected plants and fruits (intentional and unintentional)

• . . . and other methods

Losses associated with select pests and pathogens and their status in the U.S.:

Pest or pathogen Losses Status

Citrus canker

Xanthomonas axonopodis pv. citri

2.3 million commercial trees destroyed by Dec 2003642,219 residential trees destroyed Jan 2001$145 million spent on eradication in 2000

Eradicated in 1910 and 1986.Current FL eradication program started in 1999.Quarantines in 13 areas

Plum pox virus (sharka)Plum pox potyvirus (PPV-D)

900 acres of tress destroyed and burned

Quarantines in PA; ongoing surveys

Glassy-winged sharpshooter, a new carrier (vector) of Pierce’sDiseaseXylella fastidiosa

Multi-million dollar losses to grape industry.

CA statewide plan implemented

Sources: APHIS/PPQ and APS Society

Question:

Can you list a crop pest or

pathogen that has NOT YET been

widely introduced in the

continental United States, but for

which we are “at risk”?

Answer: Spodoptera

Spodoptera is a genus of insect that includes 25 species that attack many food and fiber crops.

Spodoptera is NOT widely present in the continental U.S. but is found in most other major agricultural areas in the world.

Image courtesy of Alton N. Sparks, Jr., The University of Georgia, and forestryimages.com

Spodoptera - continued

Spodoptera larvae feed on young, growing plants, gnawing holes which allow disease to enter or rot to form. The older insects chew large holes or wholly consume leaves or stalks.

Spodoptera frugiperda damageto corn ear and leaves.

Spodoptera frugiperda damageto cotton leaves.

Images courtesy of The University of Georgia, forestryimages.com, and Charles T. Bryson, UDSA ARS

Spodoptera - continued

▫ A small number of Spodoptera species were discovered in the U.S. the late 1800s, and S. littoralis lowered U.S. cotton yields by 75% in the 1930s.

▫ Some key species, however, are as yet absent from the U.S..

Spodoptera frugiperda Spodoptera frugiperda

Images courtesy of Alton N. Sparks, Jr., The University of Georgia, and forestryimages.com

S. littoralis and S. litura are totally polyphagus; that is, they eat a very wide range of crops. Main crop host species include corn, rice, soybeans, and vegetables.

Spodoptera - continued

Spodoptera litura larva

Image courtesy of K. Kiritani and EPPO, and the University of Georgia and forestryimages.com

Question: Could a Spodopteraepidemic occur?

Is there uniform susceptibilityin host plants?

Is a pathogen present?

Would the environment be conducive to disease?

If Spodoptera pests were introduced, the answer would be yes, an epidemic could occur. Image courtesy of The University of Georgia, and forestryimages.com

If an epidemic occurred in the U.S.,the potential ramifications are

Spodoptera - continued

• crop losses could be 10 - 75%

• trade losses could be in the billions of dollars

Image courtesy of The University of Georgia, and forestryimages.com

Spodoptera - continued

Question:

What strategies are being used

to keep Spodoptera out of the

United States?

Image courtesy of The University of Georgia, and forestryimages.com

Current Strategies to Manage Spodoptera

Immediate strategy: Identification, pesticides

Longer term strategy:Resistance

However, no commercial cultivars in the U.S. have resistance despite years of research.

Image courtesy of The University of Georgia, and forestryimages.com

Part Three

State of the Art Plant Biosecurity Strategies for combating natural and man-made epidemics

• Currently-available protection and control strategies

• Emerging plant protection and control strategies

Protection and Control Strategies

Depending on the plant pest or pathogen,strategies spread might include:

• Field, orchard, and vineyard quarantines

• Fungicide treatments

• Biological control such as insects, pathogens, and/or nematodes

• Destroy infected fields, orchards, and vineyards

• Disinfect equipment and storage facilities

Emerging Plant Protection Strategies

In the future, producers may also have access to

• Novel strategies for disease resistance

• Accurate and reliable sensing technologies

• Cultivars bred for resistance to threatening diseases

• Plants genetically-manipulated for natural resistance

Conclusion• Past epidemics teach us

strategies for combating new pests and pathogens:

Uniform susceptibility of host plants

Introduction pests or pathogens

Environments conducive to disease

• New pests and pathogens have been introduced; still others threaten the U.S.

• Several emerging and novel strategies for combating plant epidemics are on the horizon

Presentation ReferencesSchoelz, J. (2003, December). What can we learn from past and current epidemics of plants? Paper presented at the Missouri Summit on Agroterrorism. University of Missouri, Columbia, MO.Southern Plant Diagnostic Network. (2004). Disease categories. Retrieved from the University of Florida http://edis.ifas.ufl.edu/body_PP112

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