August 10, 2005
From: ProMED-mail <promed@promedmail.org>
Source: StopSoybeanRust.com, 9 Aug 2005 [edited]

Kentucky soybean update: Rust, no. Sudden death syndrome: yes

As of August 3005, there is no confirmed soybean rust anywhere near Kentucky.

However, according to Donald Hershman, Plant Pathologist, University of Kentucky Plant Disease Diagnostic Lab at Princeton has received numerous samples of soybean sudden death syndrome (SDS). Hershman reports that SDS is developing rapidly in some fields in southern Illinois, suggesting that SDS may be a significant player this year.

By way of background, SDS is a root disease caused by the fungal pathogen _Fusarium solani_) f.sp. _glycines_. Roots of plants are infected and become diseased during the vegetative stages of plant development. Then, typically as plants enter the pod development stages, foliar symptoms are expressed as a result of foliar sensitivity to one or more plant toxins produced by the fungus in diseased root tissue.

In extreme cases, plants can die prematurely, with yields being dramatically impacted. In some years (such as 2004), plants will show symptoms early and later recover, with no impact on crop yield. In most years the response is somewhere in the middle of these 2 extremes.

SDS is characterized by a rotting of the primary and secondary root system and a subtle brown discoloration of the stem tissue immediately inside the green exterior. Leaf symptoms that start as yellow spots and blotches between the veins of upper leaves (usually), leading to large areas of brown tissue between the veins, and eventual defoliation of the leaflets, but not the petioles.

Abortion of flowers and young pods is common. Symptoms usually develop first in hot spots in fields of varying shapes and sizes. Entire fields may eventually become involved in severe instances, but even then, there are typically areas in fields where the disease is more severe than in others.

At this time, there is nothing that can be done to make the disease less or more serious. The die is cast, as it were. SDS is in a race against time with the crop. If the crop reaches the R5-R6 stage before the disease is severe, then the yield prognosis will be excellent. Serious disease prior to the R5 stage, however, can result in substantial yield reductions. Typically the earlier the onset of serious SDS, the greater the impact will be on crop yield.

Hershman was surprised to see so much SDS, considering the predominately dry conditions we had in June and July. He recalled the cool conditions that lingered in April and early May, and the fact that many growers planted earlier than normal in an attempt to escape soybean rust. Both the cooler soils and early planting dates may have resulted in more root infection by the SDS pathogen than is "normal". Then, the later season moisture stress conditions probably encouraged foliar symptom expression of SDS.

He said that there is nothing one can do now to ameliorate the effects of SDS; the disease will have to run its course. However, the disease can be drastically reduced next season by planting a resistant variety coupled with avoiding late-April and early-May planting dates.

[Byline: Donald Hershman]

--
ProMED-mail
<promed@promedmail.org>

[Initially, the cause of SDS was elusive, with mechanical, cultural, environmental and biological factors all considered possible. Early studies involving soil fumigation indicated that the origin was biological. Subsequent work in Arkansas and Mississippi proved that SDS is caused by a strain of the common soil fungus _Fusarium solani_ (FS-A). This result has been confirmed by work in Indiana and Missouri and is now generally accepted. Although FS-A is the primary organism associated with SDS, other pathogens may also be involved in disease development. The most studied of these is the soybean cyst nematode (SCN). Mississippi researchers found that while SCN is not required for severe SDS to occur, SCN at sufficient levels exacerbates foliar symptoms, leading to early and severe SDS. This finding is important, since the disease's timing and severity, relative to soybean development, determines how severely the yields are reduced. In addition to SCN, other soybean pathogens (foliar and root/stem infecting) are being studied for their potential role in SDS development. Preliminary data suggest that any stress factor (biological, mechanical or environmental) may magnify SDS symptom expression and cause SDS-affected plants to deteriorate earlier and die prematurely.

Other conditions that affect SDS severity are planting date and nematode injury. Early planted soybeans generally suffer more injury than those planted later. The positive correlation between soybean cyst nematode injury and SDS severity was noted by some of the first researchers to study the disease. While the presence of _H. glycines_
tends to exacerbate problems with SDS, it is not required for successful establishment of _F. solani_ f. sp. _glycines_ in its host.

The best way to avoid SDS in future cropping is to avoid early planting, plant early to mid-season varieties, and plant varieties in SDS-prone fields which are not highly susceptible to SDS. True resistance to SDS is not yet available. However, avoiding planting highly susceptible varieties will usually help by delaying disease onset until the later stages of crop development as discussed above.

Links:
<http://www.ca.uky.edu/agc/pubs/ppa/ppa37/ppa37.htm>
<http://www.ag.uiuc.edu/cespubs/pest/articles/200017c.html>
<http://www.uky.edu/Agriculture/kpn/kpn_01/pn010226.htm#soybeans>
<http://nu-distance.unl.edu/homer/disease/agron/soybean/SoSDS.html>
- Mod.DH]

[see also in the archive:
2004
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Sudden death syndrome, soybean - Argentina 20040824.2357
2003
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Sudden death syndrome, soybean - USA (MN) 20030328.0776
Sudden death syndrome, soybean - Argentina 20030412.0886
2002
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Sudden death syndrome, soybean - USA (DE & MD) 20020710.4721 Heterodera glycines, soybean - Europe: alert 20020802.4914 2000
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Soybean crop deaths - USA (Midwest) 20000827.1431
Soybean Fusarium infection, improved detection 20000920.1622]