Soybean RustLife Cycle, Spore Germination

and Survival

Dario NarvDario Narvááez. ez. NFRECNFREC--IFAS IFAS -- University of Florida.University of Florida.

Why was the disease widespread?

Three factors:

Uniform susceptibility in the host plants

The introduction

of a pathogen

An environment conducive to

disease

TAXONOMY

Phylum: Basidiomycota

Class: Urediniomycetes

Order: Uredinales

Family: Melampsoraceae

Full Name: Phakopsora pachyrhizi H. Sydow & Sydow

Synonyms: Phakopsora sojae Fujikuro

Phakopsora calothea H. Sydow

Malupa sojae (P. Hennings) Ono, Buritica, & Hennen comb. nov. (Anamorph)

Uredo sojae P. Hennings

(For additional synonyms, see the following: Green, 1984; Ono et al., 1992

Reproductive StagesAlexopoulos, Mims, and Blackwell 1996

Stage 0: Spermatia not observed

Stage I: Aeciospores not observed

Stage II: Urediniospores common

Stage III: Teliospores observed but not common

Stage IV: Basidiospores identified

Alternate host is not known

Phakopsora pachyrhizi

Obligate parasite

Urediniospores

Causes polycyclic disease

Teliospores observed

Unknown alternate host

Survives only in parasitic phase

Wide host range; legumes

C. STONE

First rust symptoms often along major leaf veins

Sporulating pustuleunder a hand lens

Uredinia and Urediniospores

Infected Soybean Leaves

Infected Kudzu Leaves

Look-Alike Diseases• One of the challenges of identifying Asian soybean rust is that the

early stage of the disease can look like other leaf diseases of soybean.

Brown Spot Bacterial Blight

Bacterial PustuleDowny Mildew

Look-Alike Diseases…continuation

Cercospora leaf blight

Frogeye leaf spot

Dissemination: wind

Dissemination

7-10 days occurs spore liberation

J.T.Yorinori

Reproduction

Symptoms show up 4-5 days after

inoculation

W.M.Paiva

One pustule releases spores

for 3 weeksJ.T.Yorinori

Germination

(optimum 180C – 260C)

Penetration

development

Infection

Free water on leaf surfaceMinimum: 6 hOptimun:12 - 14 h

Penetration

3-6 hr

Disease Cycle

M. IamautiG.N.Agrios

Survival ?

DepositionDry and wet

Source: Modified from EmbrapaTeliospore

B. Leite

T. Mueller

Direct &appressorium

Soybean Rust- continued

Once introduced to a new county, 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.

0

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Pro

porti

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f Dis

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d P

lant

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0 20 40 60 80Days After Planting

Disease Increase Over Time

20052006? This chart depicts the exponential

spread of a soybean rust epidemic in a field.

Source: SASPP.org 2002

The frequency and amount of rainfall is a major driving force for the incidence and

severity of soybean rust

Environment and Soybean Rust

Temperatures between 13o C and 26o CMaximum infection at 20 to 230 CNo lesions >28oC

Frequency of rain events12-15 rain days per month Late vegetative to late reproductive growth stages

Duration of leaf wetness 6-12 continuous hrs

Relative Humidity between 75-80 % required for spore germination and infection

Spore Germination

Spore germination pattern

020406080

100

0 5 10 15 20dias após a coleta

% d

e ge

rmin

ação

% G

erm

inat

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Days after collecting spores

Spore germination

Polystyrene coated slide

B. Leite

Percentage of Phakopsora pachyrhizi germ tube inhibition after exposure to four fungicideformulations: Chlorothalonil technical, Echo 720, Tebuconazole technical and

Tebuconazole 3.6 F.

Poster #47 Poster #47 BrenoBreno Leite, Leite, Jim Jim MaroisMarois and D. and D. Wright. Wright. In vitro and greenhouse In vitro and greenhouse trials of the fungicides trials of the fungicides chlorothalonilchlorothalonil and and tebuconazoletebuconazole..

Control line represents the % germ tube formation in deionized wControl line represents the % germ tube formation in deionized water. Germ tubes recorded in all fungicide ater. Germ tubes recorded in all fungicide treatments were in average 1/10 of the germ tube length observedtreatments were in average 1/10 of the germ tube length observed in the control. None of the test in the control. None of the test substances provided 100 % inhibition (or 0% inhibition), possiblsubstances provided 100 % inhibition (or 0% inhibition), possibly because y because urediniosporesurediniospores were placed in a were placed in a moist chamber for 48 hours prior to the experiment, therefore somoist chamber for 48 hours prior to the experiment, therefore some spores may have germinated before me spores may have germinated before they were exposed to test substances. Echo 720 and they were exposed to test substances. Echo 720 and ChlorothalonilChlorothalonil technical were the most inhibitory technical were the most inhibitory fungicide preparations.fungicide preparations.

Number of Phakopsora pachyrhizi appressoria on hydrophobic polystyrene coated slides after exposure to four fungicide formulations: Chlorothalonil technical, Echo 720,

Tebuconazole technical and Tebuconazole 3.6 F.

Poster #47 Poster #47 BrenoBreno Leite, Leite, Jim Jim MaroisMarois and D. Wright. and D. Wright. In vitro and greenhouse In vitro and greenhouse trials of the fungicides trials of the fungicides chlorothalonilchlorothalonil and and tebuconazoletebuconazole..

Control line (… …) represents the number of appressoria formed in deionized water. Undiluted fungicide solutions (1) were difficult to count. On polystyrene coated slides the absorption of excess of particles is not possible. In this graph zero appressoria formation was recorded. It is important to emphasize that an appressorium is an advanced stage in the pathogenicity process, compared to the formation of germ tubes. Echo 720 and Chlorothalonil technical again exhibited superior inhibitory capacity compared to Tebuconazole.

Do Do urediniosporesurediniospores survive survive long distance travel? long distance travel?

Dispersal

Mechanism:

• Soybean rust spreads primarily by wind-borne spores across regions depending upon prevailing winds and other environmental conditions conducive to disease development.

• The disease is likely to be restricted to parts of Florida and southern Texas during the winter in the frost-free areas or areas where the fungus could overcome short periods of below-freezing temperatures.

• Occurrence of rust epidemics within the U. S. soybean belt woulddepend on south-to-north dispersal of urediniospores (Pivonia and Yang, 2004).

Soybean Rust

R SchneiderR Schneider

R SchneiderR Schneider

B. Leite

Urediniospores (urediospores, uredospore, uredinospore) from infected host:

Size: Uredinospores are 18-34 to 15-24 microns (Ono et al., 1992).

Shape: Urediniospores are sessile, obovoid to broadly ellipsoid, and minutely and densely echinulate, and the walls are about 1 micron thick. The color of the urediniospores range from pale yellowish-brown to colorless (Ono et al., 1992).

B. Leite

B. Leite B. Leite

B. Leite

Figure 1 – A ) Typical urediniospore of Phakopsora pachyrhizi germinating on the surface of a PDA plate. In PDA urediniospores will rather form long germ tubes than form appressoria. B) Urediniospore forming an appressorium on the surface of a polystyrene covered slide (PCS). On PCS spore will preferentially form appressoria.

B. Leite

200 - 300 spores in this clump

“Pop corn and honey”Phakopsora pachyrhizi urediniospore clumps

Arrow points to an individual urediniospore

Weak adhesive material

Weak self-inhibition

Clumping Hypothesis

Leaf surface

Long

distances

Spores

germinating

pustules

Clumps will protect core cells against

desiccation, UV light and other environmental harms

High humidity

Leaf surface

Long

distances

Spores

germinating

pustules

Clumps will protect core cells against

desiccation, UV light and other environmental harms

High humidity

B. Leite

B. Leite

Germination of clumped urediniospores

Solar Irradiance

0

40

80

120

160

200

Pus

tule

s (#

/trifo

liate

)

Solar Irradiance

Water Control(0 MJ/m2)

Moderate(10 MJ/m2)

High(23 MJ/m2)

Experiment #1

0

40

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120

160

200

Pus

tule

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/trifo

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Solar Irradiance

Water Control(0 MJ/m2)

Moderate(10 MJ/m2)

High(23 MJ/m2)

Experiment #2

b

b

c

a a

a

• Proportion of P. pachyrhizi urediniosporesdecreased with increasing exposure to solar radiationExp ≥27.3 MJ/m2=no germ

• Urediniospores are sensitive to exposure to solar radiation, reducing viability

Isard et all. 2006. Plant Dis. 90:941-945

Wet Deposition: Rainfall Washout Simulation

• Preliminary Results indicate that spores deposited by short durations of simulated rainfall are distributed evenly throughout a soybean canopy.

A Portable Rainfall Simulator for Conducting Wet Deposition Field Studies ofPhakopsora pachyrhizi UrediniosporesA Portable Rainfall Simulator for Conducting Wet Deposition Field Studies of Phakopsora pachyrhizi Urediniospores

Survival of Phakospora pachyrhizi in USA

Description: P. pachyrhizi is still known as an obligate parasite. Urediniospores survive (in resting or dormant state) less than 2 days under natural outdoor conditions. They do not survive in dried or decayed tissues or in the soil (Ilag, 1977).

Survival Information: When soybean plants are not present in thefield the fungus is viable only when transferred to another living host (Ilag, 1977). One widespread host in the United States is kudzu, Pueraria montana var. lobata, that could serve as a reservoir for soybean rust. There are a variety of other leguminous crops or weeds that have shown varying degrees of susceptibility to both species of soybean rust.

Reservoir-- continued

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

Lima bean Butter bean Green beanCowpea Lupine Sweet Clover

. . . and KudzuKudzu is a common weed in the southern U.S. and is great concern that it could serve as a continual source of P. pachyrhizi inocula. Kudzu begins growth in the spring before soybeans are planted and thus, the weed could act as a "disease bridge" by allowing build-up of the fungus followed by movement into soybean.

Summary:• Phakopsora pachyrhizi will preferentially form

appressoria on polystyrene coated slides and germtubeson PDA and agar plates

• Viability of isolated urediniospores is frequently lost in a few days after being release from the pustule

• Germination of P. pachyrhizi urediniospores may occur in clumps.

• Clumps may protect urediniospores against desiccation, UV radiation and other environmental factors.

• Clumped spores are more likely to travel long distances and reach new areas.

Summary… continuation

• Information about clumping may help to properly design disease management programs.

• Proportion of P. pachyrhizi urediniospores decreased with increasing exposure to solar radiation

• Urediniospores are sensitive to exposure to solar radiation, reducing viability

• Rain simulator is an effective tool for examining the vertical distribution of spores in a soybean canopy

• Spores deposited by rainfall are distributed evenly throughout the soybean canopy – 20 inches high– 36 inch row spacing

Acknowledgments

• Dr. Jim Marois University of Florida

• Dr. David Wright University of Florida

• Dr. Scott Isard Penn State University

• Dr. Breno Leite University of Florida

• Mr. Tristan Mueller University of Illinois

• Mr. Kelly O’Brian University of Florida

• Mr. Nick Dufault Penn State University

Sources• Soybean Rusts Phakopsora meibomiae & Phakopsora pachyrhizi

– http://www.google.com/search?hl=en&q=phakopsora+pachyrhizi+spore+deposition– www.plantpath.wisc.edu/soyhealth/powerpoint/Soybean_Rust_technical.ppt

Additional alternative hosts of Phakopsora pachyrhizi , causal agent of soybean rust. Rytter, JL | Dowler, WM | Bromfield, KRPlant Disease [PLANT DIS.]. Vol. 68, no. 9, pp. 818-819. 1984.

Effects of duration, frequency, and temperature of leaf wetness periods on soybean rust. Melching, JS | Dowler, WM | Koogle, DL | Royer, MHPlant Disease [PLANT DIS.]. Vol. 73, no. 2, pp. 117-122. 1989.

Strategic plan to minimize the impact of the introduction and establishment of soybean rust on soybean production in the United States [ http://www.aphis.usda.gov/ppq/ep/soybean_rust/sbrplan12-03.pdfl ].

Status of Scientific Evidence on Risks Associated with the introduction into the Continental United States of Phakopsora pachyrhizi With Impported Soybean Grain, Seed and Meal [ http://www.aphis.usda.gov/ppq/ep/soybean_rust/sbr_riskevidoc2_23_04.pdf ].

NPAG data:: Phakopsora pachyrhizi: Australasian soybean rust[ http://www.aphis.usda.gov/ppq/ep/soybean_rust/UreMelPp502.pdf ].

(Poppler, 2004)