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Vol. 17, No. 9
Article 1 of 10
May 18, 2007

Field Crop Disease Observations- May 14, 2007
By Laura Sweets

Wheat
This has been a difficult season for the wheat crop. Effects of the low temperatures are still showing up in fields across the state. As the crop begins to head, head distortion and sterile glumes may be evident.

Loose smut on wheat.

There have been a few reports of loose smut showing up on plants in the boot stage or beginning stages of head elongation. Loose smut is quite easy to see in the field at this point in the season. The kernels on infected heads are replaced with masses of powdery black spores. So the heads have a very obvious, black, powdery appearance. These spores are eventually dislodged by wind and rain, so later in the season the smutted stems are less evident and only the bare rachis will be left. Spores produced on smutted heads now are being wind carried to adjacent plants in the field and infecting through the flowers. The fungus that causes loose smut survives within the embryo of wheat seeds. If infected seed is planted, the plants growing from those seeds will be infected and develop smutted heads the next season. If seed from a field that has a "small" amount of smut in one season is used for seed, the field planted with that seed may have a substantially higher level of smut. Loose smut is best controlled by planting either disease-free seed or using a systemic fungicide seed treatment.

Weather conditions have been conducive for the development of Fusarium head blight or scab in many areas of the state-see accompanying article in this issue of the newsletter.

Foliage diseases with the exception of powdery mildew have not been particularly widespread or severe. Most of the foliage diseases of wheat are favored by periods of free moisture on the leaf surfaces so the weather conditions over the next 7-14 days will determine if diseases such as Septoria leaf blotch, leaf rust and/or stripe rust buildup in the crop. Descriptions of the wheat foliage diseases and a table of fungicides labeled for use on wheat appeared in the April 27, 2007 issue of the Integrated Pest and Crop Management Newsletter. Most of these fungicides cannot be applied after Feekes growth stage 10.5 (full head emergence), so if a field has begun to flower, it is too late to apply a foliar fungicide.

There have been some reports of light green to yellow-green flecking and/or striping on wheat leaves, especially the flag leaf. Some wheat varieties, including Bess and Truman, have a tendency towards a genetic flecking which produces light green to yellow-green flecks or specks in the leaf tissue. These are usually evident as distinct small, oval to oblong lesions in the leaf tissue. In some samples, particularly from central Missouri, the discoloration is occurring as light green to yellow-green longer streaks or stripes in the leaf tissue. These streaks run parallel to the mid-rib. The leaves may also have a slightly desiccated or dry appearance. Several samples showing the streaking symptoms were tested for virus diseases and were positive for wheat streak mosaic. This is the virus disease spread or vectored by the wheat curl mite. Damage from wheat streak mosaic is usually most severe during periods of hot, dry weather. No controls are feasible at this time.

Corn
So far there have been very few reports of any disease problems on corn. Delays in planting because of wet conditions and delayed emergence because of cool, wet conditions are the most evident problems. As corn begins to emerge seedling blights may be evident. Descriptions of corn seedling blights were covered in the May 4, 2007 issue of the Integrated pest and Crop Management Newsletter.

Soybeans

Brown discoloration of soybean stem due to Phytophthora seedling blight.

Relatively few acres of soybean have been planted thus far. My early planted soybean seed treatment trial in central Missouri (planted April 10) is showing a range of seed decay and seedling blight symptoms including Pythium seed decay, Phytophthora seed decay and seedling blight and Rhizoctonia seedling blight. If the current weather pattern continues soybean seed and seedling blights could be a more significant problem than they have been for the last few years. This might be a good year to consider the use of a soybean seed treatment fungicide. See the January 26, 2007 issue of the Integrated Pest and Crop Management Newsletter for information on soybean seed treatment fungicides.

The soybean rust situation has taken an interesting twist since May 8. Most of the southern states have been dry, so environmental conditions have not been favorable for rust development on kudzu, soybeans in sentinel plots nor soybeans in commercial fields. However, on May 8 a sample suspected of having Asian soybean rust was collected in New Iberia, Louisiana. The sample was examined microscopically and then tested using both ELISA (immunoassay test strips) and PCR. The sample was positive for Asian soybean rust. So far soybean rust has only been found at the one site in Louisiana and both incidence and severity at that site are low. But the presence of soybean rust so early in the season is of concern. At this point in the season, soybean rust is active on kudzu in six counties in Florida and the one site in Louisiana. No currently active sites of soybean rust are reported in Alabama, Georgia, Mississippi or Texas. Updates from southern states are available on the IPM PIPE Website (http://www.sbrusa.net).

Laura Sweets
SweetsL@missouri.edu