In this Issue:  July 28, 2005

Soybean Rust Update

Soybean rust continues to move very slowly in the southeast, but it is moving. The nearest known location as of July 27th is still 765 miles from southeast Kansas (George Co., MS). Soybeans that are currently at or near growth stage R5 are probably safe for this year.

At a recent field day here in Manhattan, I queried some co-op people and they indicated they had enough fungicide inventory on hand to at least deal with any initial problems. Scouting once a week in counties along the Missouri border may not be a bad idea, although it is probably not absolutely necessary at this point. If the disease makes any big moves or creeps within 300 to 400 miles of Kansas, we will raise the alert level on scouting. (Jardine)

Other Soybean Foliar Diseases

The two foliar diseases most often confused with soybean rust are still active in many Kansas fields. Samples with brown spot and bacterial blight continue to arrive in the diagnostic lab. Brown spot will almost always be on the lowest leaves of the plant. Small, brown spots will form on the leaves. Eventually the leaves will yellow and drop to the ground. There are no other characteristics you can use to readily identify it. It is fairly easy to identify under a microscope if we incubate the leaves in a moist chamber for a couple of days. The higher humidity levels this summer has kept the disease active longer than normal.

Bacterial blight is somewhat easier to identify. It will most likely be found in the mid to upper canopy leaves. The disease usually appears about a week after a rain event. The lesions are brown to black in color and usually have a fairly bright yellow halo around them. After several days, the dead tissue falls out giving the leaf a very ragged appearance. Hot, dry weather is the best cure for this problem.

Sudden Death Syndrome Again Appearing

The first case of SDS was diagnosed from Sedgwick County this week. 2005 could be another banner year for this disease. We again had wet conditions early as in 2004, and the recent hot weather has put a severe stress on plants that are setting and beginning to fill pods. To review, symptoms usually do not begin to appear until the early-to-mid stage of pod fill. Leaves will develop small cream colored spots on the leaves. Over the period of a few days, the spots will begin to coalesce and turn brown. After a week or so, the entire leaf, except for the veins will be brown. The leaf blades eventually drop, but the petiole remains attached to the stem. If the plants are dug up and the tap root is split open, there is usually a distinct gray to brown discoloration of the inner root tissues.

In Kansas, SDS is highly associated with soybean cyst nematode (SCN) infested fields. Most fields that have SDS will test positive for SCN. Those that do not, probably are infested, but have not yet reached the level of detection.

Control measures start with planting SDS tolerant varieties. Delaying planting of fields until the end of the normal soybean planting time helps by allowing soils to warm up and dry out a little bit. Research in Illinois indicates there is also a strong correlation between SDS and soil compaction, so deep tillage to eliminate compaction can reduce SDS severity. Managing to reduce the levels of SCN in a field will also help to some extent. For more detailed information visit the Plant Health Initiative web site http://www.planthealth.info/sds_basics.htm.

Gray Leaf Spot and Anthracnose in Corn

Some areas of northeast, north cental and south central Kansas have developed moderately high levels of gray leaf spot. A few fields have significant anthracnose infections as well. Where fields are beyond full blister, it is too late for management. Where fields have not reached the full blister stage of development, fungicides may still be applied with the expectation of getting a good return on the investment.

Gray leaf spot symptoms begin as small brown spots, often with a yellow halo when looked at with back lighting from the sun. As the lesions develop, they elongate into the more characteristic rectangular shaped lesions. If lesions are within one or two leaves of the ear leaf before blister, fungicides are generally economical. For a more complete discussion of stalk rot see K-State Ext. Publ. MF-2341 http://www.oznet.ksu.edu/library/plant2/mf2341.pdf

Anthracnose is characterized by small, irregular shaped, brown lesions, usually on the lower leaves. If the fungus enters the midrib, it can form a large brown lesion that will take out the entire leaf. The foliar stage of anthracnose is seldom a problem in Kansas. It becomes important late in the season when it can develop into a stalk rot. Anthracnose stalk rot looks very much like Fusarium stalk rot, with the lower stalk showing degradation of the inner pith material. What distinguishes anthracnose, is the development of shiny, black patches starting at the lower nodes and spreading to the internodal areas of the stalk. Fields with any stalk rot should be targeted for early harvest before lodging can occur. For more complete information on stalk rots see (K-State Ext. Publ. L-741 http://www.oznet.ksu.edu/library/plant2/L741.pdf (Jardine)

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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