Detecting a disease in its early stages of development is important to managing disease risk. Last spring, I wrote an article  on making a scouting plan for soybean diseases.
I received some comments and decided to redo the article by adding new disease problems and specifying regions where certain diseases have been prevalent in the past few years.
There are several reasons for making a scouting plan. First, early detection of a disease can help you to make management decisions to prevent disease problems before they take place. Second, the occurrence of different diseases varies in a growing season and certain diseases may be important only in fields that have the right disease conditions. A scouting plan tailored to crop stages and specific farms is helpful in efficiently managing your time and scouting efforts.
A preplant scouting plan may integrate insects, diseases, and other agronomic components. The table will help you integrate your scouting activities for soybean diseases into a scouting plan or schedule if you wish to make one. This table has scouting information for major soybean diseases in Iowa, the growth stages when the disease symptoms are most representative, the best time to look for them, and the area in a field where a disease is most likely to be found. The last point is useful for improving scouting efforts when a specific disease occurs in a field where the disease has not been a problem previously. If a disease is not found in a spot where it is most likely to occur, other areas would be less likely.
Basically, with four or five visits to your field you can cover all the major diseases. With improvement, we may be able to determine disease probability in four visits. In the table, scouting for seedling diseases is listed according to growth stage of the plant and the time of year. Cold soil temperatures (<60°F) favor seedling blights caused by Pythium and Fusarium, and warm soil temperatures (70-80°F) are necessary for severe damping-off by Phytophthora or Rhizoctonia. Therefore, times to scout for these seedling blights are different.
White mold can be scouted in a season at two different times. One time is when white mold apothecia (mushrooms) occur; some growers use the presence of apothecia to determine whether or not to apply a chemical (fungicide and certain herbicides) to reduce the disease. Another time is when diseased plants show up; information collected at this time tells us how severe the disease is and how the disease is distributed in a field, which is useful for subsequent soybean management.
I have added viral diseases to the table but do not specify the time in a growing season to scout for three reasons:
- responses of soybean to certain herbicides may resemble viral disease symptoms
- there is nothing we can do in a season after we find the diseases, and
- checking infected seed is more accurate and effective for identifying these diseases.
Scouting information for major soybean diseases in Iowa.
|Disease||Growth stage to scout||Best time to scout||Spots likely to find disease||Regions more likely to find|
|Seedling blight by Pythium/Fusarium||before V2||late May||low and wet spots||statewide|
|Seedling blight by Phytophthora/Rhizoctonia||before V2
|mid-June||low and wet spots||southern Iowa|
|Phytophthora root or stem rot||vegetative stages||July and August||various||southern Iowa|
|White mold mushroom||canopy close||last week of June,
1st week of July,
varies with row space
|high soil||north of hwy 30|
|White-mold-infested plants||pod setting||August-September||visibly dead plants||north of hwy 30|
|Sudden death syndrome||pod setting||after mid-August||high-moisture areas in
spring in fertile fields
|eastern and central Iowa|
|Brown stem rot||full pod||late August||not specific||statewide|
|Bean pod mottle virus||maturity||at harvest||check seed||western Iowa|
|Soybean mosaic virus||maturity||at harvest||check seed||western Iowa|
This article originally appeared on pages 53-54 of the IC-482 (9) -- May 10, 1999 issue.