Asian Soybean Rust Gregory Shaner Dept Botany and Plant Pathology Purdue University Revised: 1/30/2006

Two species of rust fungi infect soybean Phakopsora pachyrhizi Asian Soybean Rust Extremely damaging First reported in continental U.S. in Nov 2004 Phakopsora meibomiae Known for many years from subtropical Americas Not as aggressive Not known in U.S.

Australasia through the 1960 Uganda Zimbabwe S. Africa Paraguay Brazil Argentina Hawaii 1994 Continental U.S

How did Phakopsora pachyrhizi get to the U.S.? Most likely hurricane winds in autumn of 2004 South America Central America Land bridge Gradual movement from South America to Central America, Mexico, southern U.S. Short-distance movement on susceptible plants

Soybean rust is a foliar disease Fungus infects mainly leaf blades Infections may also occur on leaf petioles and pods Most pustules are on the underside of the leaf

Tan and Red Brown reactions to Phakopsora pachyrhizi (USDA-APHIS Web site)

Premature defoliation Poor pod filling Smaller seed Rusts are obligate parasites, so they do more than just remove photosynthetic leaf area Rust can reduce yields 10-80%

Disease triangle Host Pathogen Environment

Soybean rust is a polycyclic disease Pustule from primary infection

Spores from primary pustules cause more infection About 9 days are required for an infection to mature into a pustule

These infection cycles begin whenever conditions allow spores on leaves to germinate and penetrate Pustule numbers increase exponentially

Under favorable conditions, pustules develop on all leaves

When infections become old, leaf tissue dies

Spore production in soybean rust pustules

Day 1 Day 27 Day 6 Day 13 Soybean rust can develop very rapidly

Winter survival of soybean rust This is the bottleneck Rust fungi do not persist in crop residue Soybean rust is not seed borne It survives on green host plants Unlikely to survive the winter in most U.S. soybean areas Probably will survive the winter on the Gulf Coast or Caribbean

The soybean rust fungus has a broad host range Phakopsora pachyrhizi is known to infect 35 species of legumes in addition to soybean Among these are several weeds and crops grown in the U.S. Most of these other host species occur in the South

Possible hosts of soybean rust in Indiana Cultivated Crops: Soybeans Korean and Japanese clover White clover Kidney beans Garden pea Ornamental plants: Hyacinth bean Wild hosts: Kudzu Crown-vetch Yellow sweet clover Kudzu with soybean rust

Soybean rust on kudzu

Epidemiology scenario Overwintering in South Annual migration of spores into Midwest Damage will depend on: How early spores arrive from South How favorable local weather is for spread

7.4% 5.2% 6.1% 7.6% 10.5% 36.8%

Weather that Favors Rust Summer climate in Midwest will probably be generally favorable for rust Temperatures from 59 to 82 °F Dew for several hours is sufficient for infection

Management of soybean rust Resistant cultivars This would be the desirable approach, but there are none Cultural practices Tillage, rotation, date of planting will not likely have any effect Fungicides There are several effective fungicides

Resistant cultivars USDA has been screening germplasm in a containment facility and in other countries So far, not much resistant germplasm has been found in Glycine max Some germplasm lines have partial resistance It’s not known if this resistance is sufficient to provide protection

Fungicides Fungicides will be the only control option for several years to come Effective fungicides currently fully labeled: Chlorothalonil (Bravo, Echo 720) Azoxystrobin (Quadris) Pyraclostrobin (Headline) Topsin M, also labeled for use on soybean, does not control rust

Fungicides approved under Section 18 Myclobutanil (Laredo) Tebuconazole (Folicur, Orius, Uppercut) Propiconazole (Tilt, Bumper, Propimax) Propiconazole + Trifloxystrobin (Stratego) Tetraconazole (Domark)

Systemic vs. non systemic fungicides Most fungicides that are, or will be, labeled for soybean rust are systemic They are absorbed by plant and move in transpiration stream (toward leaf tips) Products differ in degree of systemicity These products may show some curative activity Non systemic fungicides provide a protective barrier on the leaf surface

Types of fungicide action Protectant Applied before disease onset Kills spores as they begin to infect Curative Can kill established infections before symptoms appear Eradicant Inhibits development of an established, symptomatic infection May have antisporulant activity

Effects of fungicides on rust development Penetration Mycelial growth Sporulation Strobilurins Triazoles Highly effectiveLittle or no effect Pre- sporulation Chlorothalonil Spore germination

Strobilurin plus triazole: Twin biochemical modes of action Strobilurin Inhibits electron transfer in cytochrome bc1 complex of mitochondria. Therefore, disrupts energy production by the fungus. Powerful preventative action Fungal cell Triazole Inhibits sterol biosynthesis. Sterols are important components of the cell membrane. Good curative activity also of benefit for resistance management

Soybean rust fungicides: Biokinetics (bioassay) Folicur (tebuconazole) Untreated Alto (cyproconazole) Opus (epoxiconazole) 10 x 5 microlitre droplets (1.9 g ai/ha = oz av ai/A; assumed spray volume of 200 l/ha = 21.4 US Gal/A) applied to all leaflets in zone between the two white lines 1 day before inoculating lower leaf surface

An effective fungicide product is only part of the story Application must be timely Good coverage is important This includes getting fungicide down in the canopy By the time pustules break out on infected leaves, one or more leaves will have formed above

Application timing Fungicides work better as protectants than as therapeutics First spray should be applied before incidence exceeds 5% This means only a few pustules at mid-canopy on no more than 1/20 of the plants If application is delayed until most plants have rust, control will be unsatisfactory Incubation period is what can trip you up

Application technology Droplet size <220  m (“fine” to “medium”) 60 drops per cm 2 of leaf (387/in 2 ) Nozzles 1 ft above canopy 15 to 20 gpa spray volume 5 gpa for aerial application

Spraying conditions Don’t spray when: Temperature is above 86 °F Relative humidity is below 55% Wind speed is greater than 5 mph

Will more than one application be needed? Rust may start before plants have produced all their leaves A plant at R1 has developed only half its nodes New growth won’t be protected

Should growers scout their fields? Initial infections may be focal These may be overlooked Sentinel plots may be used Several plots in each state Sown 2 wk before normal sowing time Monitored twice weekly for rust Once rust is found, a general alert will be issued

Diagnosis and detection Early infections can be difficult to see Sporulation is on underside of leaf Several look-alike diseases Bacterial pustule Bacterial blight Brown spot

Rust on trifoliolate leaves

Five days after infection

Soybean rust lesions on leaves, seen with backlighting

Nine days after infection

Twenty-five days after infection

Bacterial pustule may be confused with soybean rust

Bacterial blight is another rust look-alike Lesions produced by Pseudomonas savastanoi pv glycinea. These may superficially resemble rust, but inspection with a hand lens will reveal the difference.

Brown spot Common in Indiana Appears soon after plants emerge Spots don’t reach upper leaves until late in the summer

Who is looking for rust? Scouting activity in Indiana will begin in the spring Plant pathologists at Purdue and county extension educators will be looking for soybean rust First detection of rust will also depend on others Soybean growers Crop advisors Seed company staff Crop certification inspectors Survey entomologists

First detectors Purdue has a program to train “first detectors” This is coordinated through the Plant and Pest Diagnostic Laboratory (PPDL)

What to do with a suspect sample Take sample to County Extension Office Submit sample to the Purdue Plant and Pest Diagnostic Laboratory (PPDL) County extension offices have submission forms Flatten symptomatic leaves between pieces of dry paper and double-bag in plastic Exact location of field is critical

Web site There are several Web sites about soybean rust The Purdue Plant and Pest Diagnostic Laboratory Web site has current information about soybean rust and links to other authoritative sites

Conclusions Rust will be a serious soybean disease in Indiana, but not necessarily every year Rust can be controlled Fungicides will be the first line of defense Ultimately, resistant cultivars will probably be developed Purdue CES is working with other governmental and commodity organizations to provide accurate and timely information about this disease