Author: Espinosa, A. Unversity of Florida
Introduction and Distribution:
There are two species of soybean rust Phakopsora pachyrhizi and Phakopsora meibomiae but P. pachyrhizi is the most dangerous and aggressive. This pathogen is native to eastern Australia and eastern Asia. Apparently Asian Soybean Rust moved over the years from Asia and Australia to Africa (1997) then to South America (2001) and then to North America (2004). It was first discovered in continental U.S. in November 2004. Appearance of the pathogen simultaneously among several states suggested that the spores were introduced into the U.S. by hurricane Ivan. Since its arrival it has dispersed within the country and as of 2008 soybean rust was found in 392 counties within 16 states. This included 56 counties in Alabama, 66 counties in Arkansas, 82 counties in Georgia, 24 counties in Florida, five counties in Illinois, 32 parishes in Louisiana, 4 counties in Kentucky, 1 county in Maryland, 79 counties in Mississippi, 1 county in Missouri, 5 counties in North Carolina, 1 county in Oklahoma, 16 counties in South Carolina, 5 counties in Tennessee, 5 counties in Texas, and 10 counties in Virginia. In Mexico, rust was also reported in 14 municipalities (counties) within 4 states. Rust remains a threat especially under mild winters but it can be controlled when detected early.
Phakopsora pachyrhizi is an obligate parasite, which means that it must have live, green tissue to survive. It is mainly a foliar disease; eventually infection may also be seen on leaf petioles and pods. This pathogen is an airborne disease and it is capable of remaining airborne throughout large sections of the soybean growing areas. It is well adapted for long distance dispersal by wind. Soybean rust is a polycyclic disease; first symptom appears on the lower canopy. Spores from the primary pustules are the ones that cause the most infection. It takes about 9 days for an infection to mature into a pustule. Pustule numbers grow exponentially, under ideal conditions, pustules can develop on all leaves. Original, old infection eventually ends up killing the leaves so it is common to see soybean plantations that are affected by soybean rust with their lower leaves severely affected and shed on the floor. Because soybean rust is an obligate parasite, it is unlikely that it will survive over winter in most of U.S. soybean areas but it can survive on the southern states. Epidemics of this pathogen will vary from year to year. Damage will depend on how early spores arrive from the southern states and by how favorable local weather is for spread. Spores require dew for germination. As the spores germinate, a germ tube is produced. The germ tube penetrates the leaf and establishes an internal infection. Once infection is established external leaf moisture is not important. Ideal conditions for rust to develop are prolonged dew periods or frequent rains and temperatures between 59 and 84 o F. The most favorable period for soybean rust development may vary between different regions of the country but probably the greatest risk nationwide is in July.
Fungal spores, called urediniospores, blow into the fields and land on soybean leaves. Under favorable conditions leaf spots and pustules form on infected leaves. Spots are visible 4 days after infection and pustules can be seen within 10 days. Each pustule can produce urediniospores for about 3 weeks these spores are easily carried and dispersed by wind causing more infection. Disease incidence and severity increase when canopy closes and crop begins flowering. The cycle continues over and over until the entire crop is defoliated or when environmental conditions are no longer favorable for disease development. Premature defoliation can occur 4 to 6 weeks after initial infection. Under favorable conditions rust can infect a field very quickly because infection grows exponentially.
This fungus has a wide host range. Besides soybeans, this fungus can infest over 95 plant species; of these over 30 are legume species. Among these are: green beans, kidney beans, lima beans, and cowpeas. This pathogen has been confirmed in kudzu and Florida beggarweed. These weeds serve as alternative hosts allowing the pathogen to build up inoculum.
Description of Damage / Symptoms:
Lesions tend to appear in mid to late summer. Early symptoms appear as chlorotic and brown or brick red spots on the upper leaf surface on the lower leaves in the canopy. It is easier to see the spots when holding the leaf up to a light source so that the leaf is backlit. Developing lesions can be confused with symptoms caused by other foliar diseases, such as bacterial pustule, bacterial blight, downy mildew, cercospora leaf blight, frogeye leaf spot and Septoria brown spot. It may also be confused with abiotic factors such as burning herbicide damage. The key diagnostic features of soybean rust are the cone-shaped pustules that form mostly on the undersides of the leaves and the dusty, light-tannish colored spores that erupt from the pustules. Once the disease invades an area, there is only a 7-day period for effective rescue treatment. After one week plants are highly infested and yields are negatively affected. When large necrotic areas are present, it is an indicator that the disease is in an advanced stage and that the fungus is no likely to be controlled by fungicides. Among the most common symptoms are premature defoliation, poor pod filling, and smaller seeds. The premature defoliation and reduction in days to maturity cause plants to have lower seed weight and fewer pods therefore fewer seeds. Due to the leaf perforation of the pustules plants tend to lose a lot of water therefore are commonly moisture stressed. There are two types of reactions, tan and red-brown. The tan reactions are considered to be fully susceptible where as red-brown reactions are apparently partially resistant because there is not as much spore production as there is on the tan reactions.
Potential Economic Impact:
In many countries Asian soybean rust has decreased soybean yields and increased production costs over the years. When untreated, soybean rust causes yield losses between 10 and 80% due to premature defoliation, therefore fewer seeds per pod and decreased number of filled pods per plant. It is predicted that annual yield losses in North America will be at least 10% in the upper Midwest, Northeast and Canada and 50% or greater in the Mississippi Delta and Southeastern states. Effective management tactics are very important to control this pathogen. Costs for applying fungicides to control soybean rust are estimated to range between $10 and $35 per acre per application.
Constant crop monitoring plays a major role because within 10 days one leaf can be severely infested. When scouting, focus on early planted fields with early maturing varieties, low lying or protected fields that have prolonged dew periods and fields with early canopy closure. Make sure to check the lower canopy if scouting before flowering. After flowering look for rust half way up the plant. A long stick for lifting the leaves, a sharpie to mark suspected areas on the leaf and 10 to 20X hand lens help detect pustules on the underside of the leaves. It is recommended to place the hand lens close to your eye and bring the leaf towards the hand lens. Look at the leaf facing the sun, so that you have a light background. When sampling, a field select 20 locations and sample 5 plants per location, that way you will have 100 plants sampled. Before applying fungicides, it is important to watch the forecast and to monitor the movement of the disease because crop and field conditions differ from year to year. This information can be obtained at: http://sbr.ipmpipe.org/cgi-bin/sbr/public.cgi. Epidemiological models show that soybean rust may not be an economic problem every production year on every region. Timing of rust movement from overwintering locations to main production areas will vary year to year and depending on that the potential effect will be higher or lower. Management is very difficult because currently there are no resistant cultivars and because this is an airborne pathogen, tillage, rotation, etc. will not have any effect on disease development. Currently, fungicides are the only option to control this pathogen. Among the fungicides that are effective controlling this pest are: Chlorothalonil, Strobirulins, Triazoles along with others. It is very important to time the fungicide applications and to be sure that all plants are well covered. Because soybean rust develops in the mid and lower canopy spray penetration into the canopy is essential to achieve rust control. Fungicides work better as protectants than as therapeutics so it is recommended that fungicides be applied before incidence is over 5%. Studies in Brazil have shown that when 20 to 30% of the soybean leaves in the mid canopy are affected by soybean rust, fungicides are no longer able to protect plants and fungicide applications can’t recover treatment costs; basically it is too late. It is best to apply fungicides at higher gallon per acre, higher pressure and different nozzle tips than herbicides. A recommendation that has proven to b effective is to set the pressure at 15 GPA (gallon per acre) and to use a twin spray nozzle with 2 fans, one angled forward in the direction of travel and the other angled backwards in the direction they just sprayed from that will increase canopy penetration. The maximum threshold for fungicide application is when there is a 10% disease incidence. Do not spray fungicides when temperatures are above 86o F, relative humidity is below 55% and wind speed is over 5 mph. For detailed information on management of soybean rust please go to http://oardc.osu.edu/soyrust/.
If you suspect you have rust:
Contact your local extension agent. Submit a sample to your local plant diagnostic clinic (http://www.sbrusa.net/NPDN_Pathology.html). It is very important to remember that rust adhere very easily to clothing and boots so if possible put on a disposable spray suit and change and wash before moving to the next location.
How to submit a sample:
When submitting a sample, flatten symptomatic leaves between two pieces of dry paper towel and double-bag in a ziplock plastic bag. If the plant material is not going to be shipped right away, once double-bagged store it in a refrigerator. For shipping place the double-bagged ziplocks inside approved cardboard shipping boxes sealed with approved shipping tape. It is very important to send samples via overnight delivery or hand-deliver the samples to the diagnostic clinic. In the submission form, be sure to write your contact information and the exact location where the suspected soybean rust was found.
- APS. 2004. Plant Pathologists Offer Soybean Rust Identification and Management Tips. Found at: http://www.apsnet.org/online/feature/rustadvice/
- Dorrance, A.E., M.A. Draper, D.E. Hershman. 2008. Using Foliar Fungicides to Manage Soybean Rust CD. online found at: http://oardc.osu.edu/soyrust/
- Dorrance, A.E., P.E. Lipps, D. Mills, and M. Vega-Sanchez. Soybean Rust. Found at: http://ohioline.osu.edu/ac-fact/0048.html
- Draper, M.A., and L.J. Giesler. 2004. Recognizing and Identifying Soybean Rust. Found at: http://plantsci.sdstate.edu/draperm/SoybeanRustEducation/Recognizing_Identify_rust.htm
- North Central IPM Center. 2005. Pest Alert: Soybean Rust. Found at: http://www.ncipmc.org/alerts/soybeanrust/index.cfm
- Shaner, G. and G. Rhul. 2006. Asian Soybean Rust. Found at: http://btny.agriculture.purdue.edu/slideviewer/slides.cfm?sequence=/SlideSets/SoybeanRust/SoybeanRust.txt&slide=1&thumbs=0
- Shaner, G. 2006. Soybean Rust: What Happened in 2005? What’s ahead? Found at: http://btny.agriculture.purdue.edu/slideviewer/?sequence=SlideSets/SoyRustCurrentStatus/SoyRustCurrentStatus.txt
- United Soybean Board. Our Soybean Checkoff. Found at: http://www.unitedsoybean.org/about.aspx
- United Soybean Board. Our Soybean Check off: Plant Disease & Pest Management Guide. Found at: http://www.stopsoybeanrust.com/viewStory.asp?StoryID=79
- USDA-APHIS. Soybean Rust. Found at: http://www.aphis.usda.gov/plant_health/plant_pest_info/soybean_rust/index.shtml
- USDA-APHIS. Soybean Rust. Found at: http://search.usda.gov/search?q=soybean+rust&sort=date%3AD%3AL%3Ad1&output=xml_no_dtd&site=APHIS_MAIN&ie=UTF-8&oe=UTF-8&client=default_frontend&proxystylesheet=default_frontend&btnG.x=0&btnG.y=0&btnG=Search
- USDA-IPM Pipe. Soybean Rust. Found at: http://sbr.ipmpipe.org/cgi-bin/sbr/public.cgi
- USDA, University of Nebraska Lincoln; Ohio State University. Identifying Soybean Rust. Found at: http://www.oardc.ohio-state.edu/ohiofieldcropdisease/soybeans/soyrust_files/soy%20rust%20ID%20card%20nov%202004.pdf