Learning from Past and Current

Plant Epidemics

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.

Case Studies, ramifications, and emerging strategies

for combating plant diseases

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: Soybean Rust

• State of the Art Strategies for Combating Plant Epidemics

Sources: Encyclopedia Britannica, 2002; Pictorial Times, 1846

Case History: The Irish Potato Famine

almost half of the population

in Ireland depended on potatoes

to survive.

In the 1840s,

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

The Irish Potato Famine--continued

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

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

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

• 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.

Over a 15 year period in Ireland . . .

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

What caused The Irish Potato Famine?

Why was the disease widespread?

An environment conducive to

disease

Uniform susceptibility in the host plants

The introduction

of a pathogen

Three factors:

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 to

Southern Corn Leaf Blight.

Southern Corn Leaf Blight Epidemic -- continued

The pathogen introduced was

Cochliobolus heterostrophus race T

A. June 18

B. July 15

C. September 1

The generation time for new inoculum? Only 51 hrs

Source: Plant Diseases: Their Biology and Social Impact

The path:

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

Why did it happen?

An environment conducive to

disease

Uniform susceptibility in the host plants

The introduction

of a pathogen

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

• Categories of Plant Diseases

• Recently-introduced pests

and pathogens

• Scenario: Is a Soybean Rust

Epidemic in our Future?

Plant biosecurity threats to the U.S.

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)

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?

Question:

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

Source: APHIS/PPQ

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’s DiseaseXylella fastidiosa

Grapes (includes the raisin and wine industries)

CA1990

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.:

Sources: APHIS/PPQ and APS Society

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’s DiseaseXylella fastidiosa

Multi-million dollar losses to grape industry.

CA statewide plan implemented

Can you list a soybean plant pest

or pathogen that has NOT YET

been introduced in the

continental United States, but for

which we are “at risk?”

Question:

Answer: Soybean Rust

Soybean Rust is NOT present in the continental U.S., but is found in every other major soybean-growing area in the world.

Source: Soybean Rust: Is the U.S. Crop at Risk,APSnet feature, 2003.

Soybean Rust is caused by the fungi

Phakopsora pachyrhizi and

P. meilbomiae

This chart depicts the exponential spread of a soybean rust epidemic in a field.

Soybean Rust-- continued

Once introduced, soybean rust could

spread rapidly as the generation time

for new inoculum can be as little as 9 days . . .

. . . and in the U.S., there is little variation in cultivars.

Source: SASPP.org 2002

The epidemic may readily spread across the U.S., as the rust spores are carried by wind to new areas.

The “Puccinia Pathway”

Stem rust of the spores survive the winter in Mexico and each year are blown northward to Canada. (It is likely that similar winds carried soybean rust from Asia to South Africa).

Soybean Rust -- continued

Source: SASPP.org 20002

Lima bean Butter bean Green bean

Cowpea Lupine Sweet Clover

. . . and Kudzu

Further, over 30 legume species may serve as alternate hosts for soybean rust, including

Soybean Rust -- continued

Question: Could a soybean rust epidemic occur?

Would the environment be conducive to disease?

Is there uniform susceptibility in host plants?

Is a pathogen present?

If the soybean rust fungi were introduced, the answer would be yes, an epidemic could occur.

• If established in main soybean production areas, losses could be 10 - 50%

• A 1984 economic risk analysis predicted total potential losses of $7.1 billion

Source: Soybean Rust: Is the U.S. Crop at RiskAPSnet feature, 2003

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

Soybean Rust -- continued

What strategies are being used

to keep soybean rust out of the

United States?

Question:

Soybean Rust -- continued

Current Strategies to Manage Soybean Rust

Immediate strategy:

Fungicides

Longer term strategy:

Resistance

However, no commercial cultivars in the U.S. have resistance, although a resistant germplasm has been identified in other parts of the world.

Part Three

• Currently-available protection and control

strategies

• Emerging plant protection and control

strategies

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

Protection and Control Strategies

• 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

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

In the future, producers may also have access to

Emerging Plant Protection Strategies

• 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:

Environments conducive to disease

Uniform susceptibility of host plants

Introduction pests or pathogens

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

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

Presentation References

Schoelz, 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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