Melvin Newman, UT Extension Plant Pathologist

Background

Asian soybean rust is an aggressive fungal disease that, under optimal environmental conditions, can reduce soybean yield by as much as 80 percent.  It is a potentially damaging disease because the severity can double every 2 to 9 days depending on environmental conditions.  It is expected to quickly become endemic to coastal regions in southern Texas and parts of Florida (Pivonia and Yang, 2004) and could cost United States soybean producers $240 million to $2.0 billion annually (Livingston, et al., 2004).  A less aggressive species of soybean rust, Phakopsora meibomiae, also occurs throughout the world but is not considered yield limiting to soybean in the United States.

After Asian soybean rust was first recorded in Japan in 1902, the pathogen moved through Asia, Australia, and Africa before it was discovered in South America in 2000.  Asian Soybean Rust has been moving northward through South America and has now been confirmed in 9 southern states, including Tennessee, as of Dec. 1, 2004.  Since the growing season for soybean was already over, there was no yield loss from the rust infections.  However, it does give a good indication of how far and wide the rust spores can travel.  Soybean rust spores could overwinter in extreme southern areas where temperatures do not go below freezing and then be blown north into Tennessee and other states during the growing season.

The overwintering source(s) of primary inoculum for soybean rust in the United States is not known although at least 20 host species (including kudzu) are known to exist in the southern United States.  Current research suggests the pathogen will overwinter in certain areas surrounding the Gulf of Mexico and in the Caribbean.  Research also suggests that soybean rust will not overwinter in Tennessee or other northern soybean growing areas.  Rust spores will most likely be transported into Tennessee by wind or other mechanisms.  Rust spores must be introduced into Tennessee, be deposited on a suitable host, and be exposed to sufficient free moisture and moderate temperatures to initiate germination before yield loss will occur.

The most severe epidemics of Asian soybean rust occur when soybean leaves are infected early in the growing season; however, leaves are susceptible at all stages of plant growth.  Soybean rust pustules have been recorded on soybean as early as the V1 (first trifoliate leaf) stage of development and can reproduce on cotyledons, stems and pods.

Epidemics of soybean rust are greatly influenced by environmental conditions.  Weather conditions during spore dispersal, deposition, and germination greatly influence the success rate of lesion development as well as the time it takes for disease severity to double (i.e., from 2 to 4% severity or 20 to 40% severity).  Germination of urediniospores and subsequent host penetration and lesion development require a minimum of 6 to 7 hours of continual wetness with an optimum infection rate occurring if dew periods are ≥12 hours in duration with temperatures between 18 and 26.5^o C during the wetness period (Melching et al., 1989).  Spores can remain viable in the absence of moisture for durations of no more than 8 days (Kitani and Inoue, 1960; Melching et al., 1989).  However, Paltil et al., 1977, reported that soybean rust spores could survive 50 days in the shade.  Spore viability is negatively influenced by ultraviolet light.  Exposure to sunlight reduces viability of spores compared to spores exposed to cloudy conditions (Melching et al., 1989).

First Detectors

County Extension Agents, Certified Crop Advisors (CCA) and agronomists will be trained as First Detectors of Asian soybean rust, in January-March 2005.  First Detectors comprise the eyes and ears of the monitoring programs because they monitor the growth and development of Tennessee’s soybean crop on a daily basis.  First Detectors will be trained to accurately identify all foliar soybean disease symptoms that may be confused with Asian soybean rust.  Training will be provided by University of Tennessee Extension Specialists and the State Plant Diagnostic Network (PDN) experts.

Triage Personnel

UT Extension Field Crop Specialists and selected County Extension Education Directors will be trained as Triage Personnel.  Triage Personnel have advanced training in diagnosing symptoms of diseases with characteristics similar to Asian soybean rust.  Triage Personnel will act as the first screen of potential rust samples to keep the UT Plant disease clinic from being overloaded with avoidable samples.  Training will be provided by UT Extension faculty, PDN and expert soybean pathologists.

Identification of Asian Soybean Rust (The following material was reprinted from the Iowa Soybean Rust Response Plan)

Accurate and timely identification is the key to determining whether a response will be attempted and, if so, the extent, direction, and magnitude of that response.  It will also help determine program changes and failures.

Symptoms of soybean rust appear identically regardless if they are caused by Phakopsora pachyrhizi or Phakospora meibomiae.  Host plants infected with soybean rust first exhibit small lesions that gradually increase in size and turn from gray to tan or brown.  They become polygonally shaped restricted by leaf veins, and may eventually reach 2 to 3 square millimeters.

Infection begins on the lower first leaves of plants and appears as chlorotic or mosaic-like areas with uredinia observed usually at or after the plant flowering stage.  Lesions may appear on most above-ground plant parts, but are most common on the underside of the leaves. As the plant matures and sets pods, infection progresses rapidly under the right environmental conditions (i.e., moisture, high humidity and heat) to cause high rates of infection in the middle and upper leaves of the plant.  Clouds of spores have been observed within and above canopies of highly infected plant stands.

Plants show two different lesion reactions to infection by soybean rust.  Tan lesions consist of small uredinia surrounded by slightly discolored necrotic areas of leaf surfaces.  Early stages show an ostiole, or small hole, where urediniospores emerge.  As uredinia become larger, they release masses of tan colored urediniospores that appear as light brown or white raised areas.  Uredinial pustules become more numerous with advancing infection and often will coalesce, forming larger pustules that break open and release masses of urediniospores.

The other type of lesion that occurs with soybean rust infection is the reddish-brown lesion. These lesions have larger areas of necrosis that are reddish brown surrounding a limited number of uredinia.  A few urediniospores are usually visible on the surface.

Early symptoms of soybean rust are easily confused with bacterial pustules (caused by Xanthomonas campestris pv. phaseoli (Smith) Dye), or bacterial blight (caused by Psuedomonas glycinea Coerper), and brown spot (caused by the fungus Septoria glycines).  These diseases also occur often on the underside of soybean leaves, causing a raised light brown blister within a lesion.  These leaf lesions vary from small specks to large irregular brown areas that form when small lesions coalesce.  A hand lens or dissecting microscope is usually used to distinguish these disease symptoms from ASBR, but early stages of the disease are difficult to distinguish if no spores, conidia, or bacteria are evident.

Recovery from Asian Soybean Rust

The occurrence of Asian soybean rust will have a significant impact on the production of soybeans in the United States.  The impact on northern soybean growing areas is expected to be less severe than effects in the southern United States as the disease must re-infect the crop from inoculum coming from more southerly climates and regions.  Growers can expect an increase in production costs related to fungicides and their application to protect the crop.

The best long-term strategy for minimizing the effects of soybean rust in the United States is in the development of resistant/tolerant varieties.  There are thousands of plant lines of soybean in germplasm repositories, and screening for soybean resistance has been on-going for several years in other countries and in the United States in the containment facilities at the ARS Foreign Disease-Weed Science Research Unit in Fort Detrick, Maryland.  However, the availability of cultivars with good resistance and other characteristics desired in soybean for commercial production is still five or more years away.

Fungicides have been shown to be effective in controlling soybean rust in Zimbabwe, South Africa and Brazil.  With USDA assistance, South Dakota and Minnesota are making an effort to obtain a state quarantine exemption for seven fungicides.  Tennessee Extension has also submitted to the state department of Agriculture for the same exemptions.