WHEAT LEAF RUST IMPACT ON YIELD AND FUNGICIDE TIMING

What impact does leaf rust have on yield? Based on Kansas State University data, wheat in the flowering stage with 10% of the flag leaf affected is predicted to cause 10% yield loss, 40% severity may cause 20% yield loss, and 100% severity may cause 35% yield loss. As wheat reaches later growth stages the impact of leaf rust on yield is reduced. For example, at the soft dough stage, 10% rust severity on the flag leaf is predicted to cause 1% yield loss, 40% severity may cause 4% yield loss, and 100% severity may cause 10% yield loss.

If leaf rust is present on 5% or less of the area on the flag leaf, applying a fungicide may help to protect the flag leaf from further infection. However, if the leaf rust severity on the flag leaf is greater than 5%, it is too late for fungicide application. The reason is that additional infection has occurred and you are unable to see the symptoms from the “hidden” infection sites. For example, if 5% leaf rust severity is present on the flag leaf today, an additional 10 to 20% severity may appear in another 2 to 3 days from the “hidden” infection sites. Also, the potential for positive economic returns rapidly decrease as fungicide application is delayed past the flowering stage of wheat (and the majority of the regions’s wheat is beyond the flowering stage). In addition, pre-harvest interval restrictions exist for fungicides that limit how late fungicides may be applied to wheat before harvest.

In summary, fungicide application for leaf rust (and other leaf spot diseases) likely is too late for most of our region’s wheat because of the previously-mentioned reasons.

DRY BEAN WHITE MOLD MANAGEMENT

Saturated soil conditions during the past several weeks in much of the region will increase risk of white mold problems in dry bean. The following are some general guidelines to consider regarding fungicide use as beans begin to flower.

Decision aids for fungicide use:

*1) seed yield potential of at least 2000 lb/acre; 2) white mold has affected crops in the area in previous years; and 3) wet weather maintains soil surface wetness for 10-14 days before flowering (necessary for sclerotia germination and spore production) and maintains plant wetness for 2 or more days during flowering (wet conditions down in the canopy are needed for the fungus to become established on dead blossoms and then move to living tissue).

*Work at the University of Minnesota determined potential for a fungicide to be profitable when the following total rainfall from June 1 until 10 days into bloom was recorded:

-3 to 5 inches = profitable 2 of 10 years,

-5 to 7 inches = profitable 6.7 of 10 years,

-over 7 inches = profitable 8.5 of 10 years

(calculations based on an assumption of bean value of $15/cwt).

Fungicides:

*Benlate, Topsin M and Rovral are labeled.

Fungicide timing:

*Application at or prior to 10 days after the onset of bloom is most effective.

Spray technology:

*Ground application - pressures of 100 psi or slightly greater have provided white mold suppression and better yield yield response than pressures of 40-60 psi. The use of drop nozzles between the rows has provided the most efficient white mold management, along with air assist sprayers.

*Air application - many pilots use 7-10 gpa rather than 5, and the higher spray volumes have provided good white mold suppression.

Details may be found in NDSU Extension Service circular PP 576 “Dry edible bean diseases” available at extension offices or at the website:

WHEAT MIDGE UPDATE

Orange wheat blossom midge continue to be found in our area, but currently no reports have been received of midge reaching economic thresholds (average of 1 midge/4-5 wheat heads). The following is a listing of degree day (DD) units accumulated on July 11, based on the ND Ag Weather Network, for selected south-central and southeast ND locations: Oakes = 1956; McLeod = 1918; Linton = 1816; Edgeley = 1750; Carrington, Dazey, Jamestown, McHenry, Robinson, and Streeter = 1668 to1706; and Harvey = 1605. Midge emergence peaks at 1600 DD (about 90% of the females will have emerged). Continue scouting wheat fields that are at risk until 1800 DD have been reached. Details on wheat midge scouting and management may be obtained from NDSU Extension Service offices.