Soybean Rust Update By Laura Sweets

During the 2005 season soybean rust was not detected on soybean, kudzu or any other host plant in Missouri. Soybean rust was reported from nine states during the 2005 season primarily on either soybean or kudzu. A summary of the positive reports is as follows:

State	Number of counties with positive reports of soybean rust
Alabama	33
Florida	23
Georgia	35
Louisiana	2
Mississippi	2
North Carolina	18
South Carolina	23
Texas	1
Kentucky	1

The Texas and Kentucky reports came in late in the season and were both on kudzu.

A brief comparison between the falls of 2004 and 2005 may help explain why soybean rust was found late in the season on plants in fields in Pemiscot and New Madrid Counties in Missouri in 2004 but was not found in 2005.

Fall Conditions: 2004 vs 2005

Fall 2004	Fall 2005
Unusually wet	Unusually dry
Mild, <28 degrees F by Dec. 14	Cold earlier than normal, <28 degrees F by Nov. 17 (14 degrees F in Columbia on Nov. 17)
Nov. 6, 2004 ~ 67 percent soybean crop harvested	Nov. 6, 2005 ~91 percent soybean crop harvested

The positive reports in Missouri in 2004 were from scattered green plants in fields that had not been harvested due to wet conditions. By early to mid- November 2005, most soybean fields in the state had been harvested and surveys to detect any late season infections of soybean rust were unsuccessful.

Drs. Robert Kemerait, Philip Jost and Layla Sconyers of the University of Georgia, had an excellent article entitled "Looking Back at Soybean Rust in the 2005 Season" in their Soybean Newsletter dated December 7, 2005. A summary of main points from that article would be that the initial find of soybean rust on volunteer soybeans in Seminole County in late April did not have any real impact on the 2005 soybean crop. The major, sustained outbreak of soybean rust was first detected in southwest Georgia in mid- July. This outbreak continued to spread across the state and by the end of the season was found in counties bordering Florida, Alabama, South Carolina and Tennessee. The movement of the disease within the state was relatively slow. An estimate of the rate of movement from the Tift County site to Oconee County was approximately 60 miles per day. The number of days between first observation of initial rust pustules on leaves using a microscope to examine the leaves and symptoms in the field that would be apparent from typical scouting and examination of leaves with a hand lens was a number of weeks. Fungicide trials did show benefits from properly timed applications of triazoles, strobilurins and triazole-strobilurin mixes.

The question now is how well will the soybean rust pathogen overwinter on hosts such as kudzu, how far north will it overwinter and what can we expect in 2006?

The fungus that causes soybean rust, Phakopsora pachyrhizi, is an obligate parasite so survives on living plant tissue. The period of December 2004 through January 2005 was unusually cold in the southeastern United States. The accompanying maps show the 28 degrees or moderate frost line for the Gulf Coast states during that time period. The unusually low temperatures may have reduced the overwintering level of soybean rust by killing back perennial hosts such as kudzu to a greater extent than normal. Unfortunately parts of the southeastern United States have not experienced these moderate frost levels this winter. See accompanying maps below.

Folks in the southern United States have not taken a break from soybean rust scouting. Scouting primarily of kudzu but also of volunteer soybeans has continued through December and into January. Results so far from southern states include:

Texas (Dr. Tom Isakeit)	December 22, 2005: rust-infected kudzu patch near Dayton, TX was scouted. Patch had died back ~90% but still had a few green leaves loaded with rust pustules.
Alabama (Dr. Ed Sikora)	December 20 and 21, 2005: Green, rust-infected kudzu near Evergreen, AL; two rust-infected patches that were barely hanging on in Baldwin County, AL; and a sizeable semi-green, rust-infected patch of kudzu in Clark County, AL. January 12, 2006: Soybean rust detected on two kudzu patches in the city of Montgomery, AL. Most of the two patches were dormant, but the remaining green foliage was infected with soybean rust.
Georgia (Dr. Layla Sconyers)	January 11, 2006: Fourteen county survey found kudzu 99-100% dead although at a few locations new buds were beginning to form on some vines.
Florida (Drs. Jim Marois, Glen Hartman and Scott Isard)	January 10, 2006: Kudzu in a culvert facing south located near Quincy, FL was developing new leaves and one rust-infected leaf was found. January 11-13, 2006: A survey of all kudzu patches that were reported positive for soybean rust in 2005 (that could be located again) found seven of thirteen sites positive for rust in 2006. One additional rust-infected kudzu site was also found. Most sites had appreciable dieback but a few sites in the south had little dieback.

Active soybean rust pustules are being found on kudzu in scattered locations in the southeastern UnitedStates. Current levels seem quite low but it will be important to see how this develops over the next few months.

The bottom line still remains that in order for soybean rust to develop and cause significant damage to soybeans in Missouri, the pathogen must overwinter in the south, that inoculum must be moved or transported up to the Midwest and weather conditions in Missouris during the growing season must be favorable for disease development. For 2006 it is important for soybean producers in Missouri to be aware of disease development and weather patterns in the southern United States; to scout fields and be aware of results from the sentinel plot system in southern states, states close to Missouri and in Missouri; and to be prepared to take prompt action if a risk of soybean rust develops.

Laura Sweets
Ag Ext-Plant Sciences
573-884-7307