April's Ag Advice by April Borders There has been a lot of talk and concern about soybean rust and what this disease holds in store for soybeans in South Dakota. So I thought that this would be a good time to take a look at this disease and how it might affect our soybeans.
There are two fungal species, Phakopsora pachyrhizi and P. meibomiae, that cause soybean rust. Phakopsora pachyrhizi is the more aggressive species and was first reported in Japan in 1903. It was confined to the Eastern Hemisphere until its presence was documented in Hawaii in 1994. It has been found in Africa, Asia, Australia, Hawaii, and South America. The less aggressive soybean rust species, P. meibomiae, is present in the Western Hemisphere, including Puerto Rico. To date, neither rust species have been detected in the continental United States as of May 2003.
This disease is spread primarily by wind borne spores that can be transported over long distances. At this point seed borne transmission has not been documented, although seed lots may contain contaminated plant debris that could be capable of spreading the pathogen.
There are over 90 plant species that can serve as hosts. This includes soybeans, dry and snap beans, and the weed kudzu. This broad host range increases the likelihood or rapid spread once introduced into the United States. The good news is that this is an obligate parasite, meaning that it needs a living host to survive in. Since it will not have a host to over winter on in South Dakota, the rust will have to be reintroduced every year. Thanks goodness for cold winter!
Symptoms of the disease begin on the lower leaves of the plant as small lesions that increase in size and change from gray to tan or reddish brown on the undersides of the leaves. Lesions are most common on leaves but may occur on petioles, stems, and pods. Soybean rust produces two types of lesions, tan and reddish brown. The tan lesions, when mature, consist of small pustules surrounded by slightly discolored necrotic area with masses of tan spores on the lower leaf surfaces.
Reddish brown lesions have a larger reddish brown necrotic area, with a limited number of pustules and few visible spores on the lower leaf surface. Once pod set begins on soybeans, infection can spread rapidly to the middle and upper leaves of the plant.
Environmental conditions that are favorable for germination and infection include prolonged leaf wetness combined with temperatures between 59 and 86 degrees Fahrenheit and relative humidity of 75-80 percent. Under these conditions, pustules form within 5-10 days and spores are produced within 10-21 days. High levels of infection in soybean fields results in a distinct yellowing and browning of fields and commonly, premature senescence in plants.
All commercial varieties of soybeans currently available are highly susceptible to soybean rust. Early detection is required for the most effective management. Monitoring soybean fields and adjacent areas will be a must this year. Fungicide applications may reduce yield loss, depending on the plant developmental stage, time when soybean rust is detected, and fungicide application method.
Because we have air mass movement from South America to North America we will be actively watching for the rust this summer. It is predicted that all South American soybeans and true bean production areas will be infested by the end of 2004 and we could be facing introduction of the disease into the United States.
In 2003, it was estimated that about 50 percent of the Brazilian soybean crop was lost to soybean rust. As of reports from South American in February, they were receiving a lot of rain and the EMPRAPA (the "Brazilian" USDS) is reporting that 75 percent of their soybean crop has some level of rust. If their weather continues they will suffer severe losses.
We will be actively watching for the rust disease in Clay County this year. If introduced into the United States, we will most likely see it move in from the South and our county could be an indicator county. As you are scouting in your fields this year be on the look out. If you find anything that you are unsure of please, give me a call or collect a sample and bring it to the office. I will make sure that it gets to the plant pathology department for diagnosis.
If you would like more information on soybean rust I do have a National Pest Alert that you can stop by and pick up. If you have any questions feel free to call the Extension Office at 677-7111.