Field Crop Disease Update- July 24, 2006
By Laura Sweets

Corn

We have not had many reports of diseases showing up on field corn. Anthracnose, gray leaf spot and rust are present in low levels in some fields. For the most part these foliar diseases are primarily in the lower portion of the canopy and the ear leaves are relatively disease free.

Soybean

Soybean rust continues to spread very slowly in the southern United States with only a few new reports of finds on kudzu, still only three finds on soybeans in sentinel plots and no reports from commercial soybean production fields. The risk of soybean rust in Missouri remains low.

We have been receiving leaflet samples from the 25 sentinel plots throughout the state of Missouri. Soybean rust has not been found on any of these samples. Septoria brown spot, downy mildew and bacterial blight have been present on samples from most regions of the state. However, none of these foliage diseases appear to be present at damaging levels.

A reminder seems in order about the use of foliar fungicides for "Plant Health" or for yield boost in soybean production. Quadris, Headline and several products containing chlorothalonil (ex. Bravo Ultrex, Bravo Weather Stik, Echo 90 DF, Echo 720, Equus DF, etc.) do have full federal registrations and are labeled for use on soybeans for a number of diseases in addition to soybean rust. The section 18 fungicides such as Bumper, Domark, Folicur, Headline BSR, Laredo, Orius, PropiMax, Quilt, Stratego, Tilt and Uppercut, have quarantine exemption labels for use on soybeans only if there is a threat of soybean rust and only for management of soybean rust. These section 18 fungicides are not labeled for general plant health or for the control of other soybean foliage diseases. The use of section 18 fungicides in the absence of threat of soybean rust is a misuse of those products and could jeopardize future section 18 requests.

We have received numerous calls and samples related to "yellow beans". Patterns in the field range from scattered yellowed plants to distinct circular to elongate patches of yellowed plants. Yellowing may be occurring in the lower canopy or the upper canopy. Finally the pattern of yellowing on individual leaflets may vary greatly.

In some cases the yellowed plants have poor root systems. The roots may be very shallow or crooked which could be the result of low soil moisture, compaction or a similar environmental factor. Many of these plants are also showing symptoms of either Fusarium root rot or Rhizoctonia root rot. Fusarium root rot is more likely to cause a general rotting and deterioration of the main taproot while Rhizoctonia root rot causes a reddish discoloration of the stem at the soil line.

Phytophthora root rot may cause entire plants to turn off-color, yellow, wilt and die. A key symptom of Phytophthora root rot is the dark brown discoloration of the main stem from the soil line up the stem and even out the lower side branches. The high temperatures of the last week have caused many plants that were showing initial wilting due to Phytophthora to die. These dead plants are harder to find in the field as the canopy closes and hides the dead plants.

Initial symptoms of sudden death syndrome SDS are showing up in some fields in central Missouri. The light yellow blotches between the veins of leaves in the mid to upper canopy, is a key symptom of this disease.

Soybean cyst nematode continues to be a major problem on soybeans in Missouri. Damage from SCN can be more severe when plants are under moisture stress. If beans are slow to close, if plants seem stunted or unthrifty or if yields are below expectations, it would be wise to collect soil samples for SCN analysis.

Bean pod mottle virus has been more prevalent this season than for several years. This virus is spread by the bean leaf beetle so high populations of bean leaf beetle can lead to an increase in bean pod mottle. Symptoms of this virus disease are most evident on the upper leaves and include a mottling of the leaf tissue. Leaflets may also be smaller in size and have a bumpy appearance.

Laura Sweets
573-884-3707