Southern leaf rust confirmed

Southern Leaf Rust Confirmed

I had a corn leaf sample brought into me earlier this week that was covered with small orange lesions with yellow leaf tissue around the lesions. From the visual observation, I was 99% sure it was southern rust of corn and confirmed it by looking at the spores under a microscope. This is significant because southern rust can be more damaging to corn than common rust.

Rusts are obligate parasites meaning they have to be on a living host at all times

John Wilson

to survive. So rust diseases do not overwinter in Nebraska, they develop on crops in the southern United States. Then the spores have to blow up from these southern infestations and the disease develops here when conditions are right. Rusts develop most rapidly when we have moderate temperatures, low to mid 80’s, and moisture on the leaf surface from heavy dew or rainfall.

When scouting fields, it is important to be able to distinguish between common rust and southern rust. We always have some common rust and there is quite a bit of it out there this year. However, common rust generally does not cause as great of a yield loss as southern rust. The last time we had a significant outbreak of southern rust in Nebraska was in 2006.

We need to look for several things when deciding which species of rust is present in fields. The first is location of the pustules on the leaf. A pustules is the raised area in the middle of the lesion that releases spores. If you look at these under a magnifying glass or hand lens, they appear as small raised bumps with a small crack where the spores are released.

In common rust, pustules are found on both the upper and lower surfaces of the leaf and randomly scattered while southern rust pustules are found almost entirely on the upper surface and may be scattered, but often are found in dense clusters in some areas on the leaf. Also common rust lesions are generally larger and tend to elongate while southern rust lesions are smaller and round. Common rust lesions tend to be darker, brick red to darker brown while southern rust lesions are usually orangish-brown.

Some, but not all corn hybrids have a distinct reaction to the southern rust lesions and form a halo of chlorotic leaf tissue around the lesions creating yellow circles less than a quarter inch in diameter with a small lesion in the center.

Before we confirmed southern rust here, the 11 counties where southern rust had been found were farther south and west, starting at the Kansas border around Hebron and north to Columbus and then in counties west of that line. The one exception would be Gage County or the Beatrice area. Burt County is as far north and east as it has been identified in Nebraska so far this year.

The level of infestation around here doesn’t warrant control now. It has been found primarily in the rows along the edge of the fields and only on a few leaves of the plant. However, if we get more August-like weather with warmer nights plus rainfall or lots of heavy dews and it could develop to levels where farmers do need to treat with a fungicide. If they do, they will want to use a fungicide with as long as residual as they can get, keeping in mind pre-harvest intervals.

So even though conditions don’t warrant treating now, it is important to scout fields to see if it is present in fields and to monitor if the disease is spreading to more leaves on the plants. The weather will play a major role in how the disease develops. For more information on common and southern rust in corn, visit the CropWatch website at http://cropwatch.unl.edu or contact your local UNL Extension office.

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News Release

John Wilson

Extension Educator

August 16, 2013

$45 Million in Losses

Last year, soybean cyst nematodes, SCN, cost Nebraska soybean growers over $45 million, more than all other soybean diseases combined. If you have SCN in your fields and are not managing it, YOU contributed to that loss. Farmers and ag professionals can learn how to identify and manage soybean cyst nematode at a UNL Extension SCN Management Field Day.

SCN often goes undetected in the field. Yield losses of 20-30% have been documented in Nebraska fields with no above-ground symptoms. If SCN caused holes, lesions, spots, or other plant abnormalities, it would be much easier to convince producers to test for and manage it. However, infested plants usually look healthy. Often, the first indication of a problem is when soybean yields level off or even start to drop while corn or other crop yields in the same field continue to improve.

What would a six bushel yield increase on your soybean acres do for your bottom line? That’s the average yield increase at 25 University of Nebraska research trials when resistant varieties were compared to top yielding susceptible varieties on SCN-infested sites. In addition to direct yield losses, SCN has been linked to an increase in the occurrence and severity of sudden death syndrome, SDS, in soybeans.

At each SCN Management Field Day site, you will be able to:

• see SCN resistant and susceptible soybean varieties in infested fields,
• examine cysts on soybean roots,
• learn how to identify and manage SCN infestations,
• receive bags for free SCN analyses – a $25/bag value,
• get answers to your SCN questions,
• enjoy a complimentary meal & visit informally with experts, and
• CCAs can earn 1 FREE ceu in pest management.

A local SCN Management Field Day will be held on Wednesday, August 28, 6:30 p.m., at the Tim Gregerson farm. From Herman, go 2.5 miles north on Highway 75 (or 4 miles south of Tekamah), then 1.5 miles west on County Road “C”, and 0.1 mile north on County Road 34. The field will be on the east side of the road.

Originally identified in counties bordering the Missouri River, soybean cyst nematode has been identified in 54 counties in eastern and central Nebraska as far west as McCook. The Soybean Cyst Nematode Management Field Days are presented by UNL Extension with support from the Nebraska Soybean Board. A meal will be provided courtesy of Monsanto (DeKalb/Asgrow).

For more information, contact your local UNL Extension office at 402.374.2929.