In this Issue:  July 7,  2004

Based on samples received in the diagnostic laboratory, phone calls, and my personal observations on a recent trip to western Kansas, iron chlorosis is more severe than at any time in at least ten years. While samples have been received from a wide area of the state, the chlorosis is most severe west of U.S. Hwy 281 (Smith Center to Medicine Lodge). While soybeans are showing the most severe symptoms, they can be readily found in grain sorghum and to a lesser extent corn. Even some weeds are showing signs of chlorosis.

Symptoms of iron chlorosis on soybeans include a yellowing of the interveinal areas on the newest growth with the veins remaining green. Later, even the veins may become chlorotic. In severe cases, brown, necrotic spots may occur on the leaves, especially near the margins. Significant stunting can also occur. On sorghum and corn, an alternating yellow and green striping occurs on the newest emerging leaves, with significant stunting also occurring.

In western Kansas, symptoms are most severe in cool soils, low in organic matter, with pH’s ranging from 7.0 on up to 8.5. Symptoms are also more severe on eroded side hills or where soil has been removed in leveling operations. In eastern Kansas, wet, compacted, poorly aerated soils seem to favor chlorosis development.

Soybean varieties vary greatly in their tolerance to iron chlorosis. Some varieties that show tolerance in an average year, may show high degrees of chlorosis in year’s such as 2004 when conditions are highly favorable.

The good news is that all but the most severely affected fields will grow out of the problem once more normal soil temperatures and moisture conditions return. In past studies conducted in Smith and Rush counties, chlorosis ratings were taken in variety demonstration plots where there was a wide response among the varieties. When the chlorosis ratings were compared to end of season yields, there was virtually no correlation. Some of the most severely chlorotic varieties early in the season ended up being top yielders, while those tolerant to iron chlorosis ended up being poor yielders for other agronomic reasons. (Jardine)

The recent spate of cool, cloudy, wet weather across much of the state is ideal for the development of gray leaf spot on corn. In much of eastern Kansas, the crop has already developed past the stage when a fungicide can be applied. In western Kansas, however, the crop is still a couple of weeks away from full blister, the point at which fungicides may no longer be applied.

Gray leaf spot is most common and severe in hilly areas where cool air settles into low spots in the field, causing dew to form earlier in the evening and remaining later in to the mornings. These are also areas where fog may accumulate. The disease will also be more severe in continuous corn, especially where no-till or reduced tillage is employed.

Gray leaf spot causes sharp-edged rectangular lesions approximately 1/10" x 1-2" in size and tan to gray in color. While most currently grown hybrids have moderate to good levels of resistance, there are still a few popular hybrids that are fairly susceptible. No hybrids are currently immune. On

susceptible- to moderately-susceptible hybrids, if lesions are found on the ear leaf prior to tasseling, a fungicide will usually be cost effective. While fungicides can be applied up through full blister, the initiation of silking is still the best time for the most effective control. (Jardine)

The Department of Plant Pathology invites you to join us in bidding farewell to Dr. Ned Tisserat and Dr. Jan Leach on Friday, July 23, at 3 p.m. in 4031 Throckmorton Hall. Drs. Tisserat and Leach have accepted positions at Colorado State University. We are making a memory book for them to take with them so please submit any letters, pictures, stories, anecdotes you have to share with or about them to Diana Pavlisko, 4024 Throckmorton by July 15. Feel free to submit items via e-mail to dpavlisk@ksu.edu . We look forward to seeing you July 23. --Diana Pavlisko

Doug Jardine
Extension Specialist
Plant Pathology Program

E-mail: jardine@plantpath.ksu.edu

Phone: (785) 532-5810

FAX: (785) 532-5692
http://www.oznet.ksu.edu/path-ext

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