Soybean Rust
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A trace amount of rust was detected June 10 in one field in Hidalgo county. This is not surprising, as rust was previously detected in nearby fields in Mexico.



The normal progression of hot, dry weather during June throughout much of Texas will not support rust epidemics. If rust is present in a field in south Texas, there will not be rapid disease development unless the field receives frequent rain showers.


FOR THE LOWER RIO GRANDE VALLEY ONLY: Based on current weather, it does not look like the appearance of rust in the Valley for the rest of the season would pose any significant risk to yield. Unless there is a period of rainy weather, growers could conceivably wait until they see rust in their fields before deciding whether to apply a fungicide. The factors affecting a decision to spray are the stage of growth, the future weather, and the yield potential. Spraying would be okay at this time, but it is not urgently recommended, unlike the situation with the fall crop of 2008, when there was heavy rust pressure.

If a grower decides to scout and wait for rust, the threshold would be the appearance of rust on the lower canopy leaves. Donít wait to see it on mid or upper canopy leaves. Once found in a field, the grower should use a triazole fungicide. Strobilurin fungicides (Quadris or Headline) should only be used when there is no rust in the field at all, since these fungicides will not affect spores that have already germinated.

In scouting for rust, you should look at mid to lower canopy leaves. The leaves should be green, because you can see all sorts of spots that look like rust - but are not - on leaves that are senescing (yellowing) or under nutritional (iron deficiency) or drought stress. Several fields in the Valley have poor growth and are showing either moisture or nutrient deficiency. It would not likely be worthwhile to apply a preventative fungicide to such fields.

In contrast, fields that have excellent yield potential would be good candidates for a fungicide application. Given the current weather conditions, the lowest effective rate of the cheapest systemic fungicide should provide ample protection. If a grower applies a fungicide, he should think of this as “insurance” in case the weather changes to rainy.

As always, fungicides should only be applied from R1 to R5 growth stages, not earlier or later than this. If the application is too early, the fungicide does not last for the time when it is needed. An application after R5 will not have an impact on yield.

FOR THE REST OF TEXAS: No fungicide applications in other areas of the state are recommended at this time.

Keep scouting. Send suspicious leaves to our lab for confirmation of rust.

SYMPTOMS More | Photo Archives

This is not soybean rust! It looks like it, except that it lacks the raised pustules that would be visible with magnification. It is probably a mineral deficiency. If you are unsure whether you have soybean rust, hold off spraying until you receive a diagnosis through our lab. With very dry conditions throughout much of the state, such rust-like symptoms may be common. This leaf was collected from Burleson County, June 8, 2008.


Sentinel plots are located in soybean production areas throughout the state. Additionally, ad hoc surveys are made of commercial fields and kudzu patches.


Samples should be submitted for diagnosis to your local county agent or to Texas Plant Disease Diagnostic Laboratory, particularly if the plant is something other than soybean or kudzu. TPDDL, 1500 Research Parkway, Suite A130 Texas A&M University Research Park College Station, Texas USA 77845
Phone: (979) 845-8032† Fax: (979) 845-6499

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