Field Crop Disease Update- July 10, 2006
By Laura Sweets

Corn
Corn foliage diseases are evident in many corn fields although both incidence and severity of these diseases is relatively low. Common rust, gray leaf spot and anthracnose appear to be the most common foliage diseases this season. Stewart’s wilt of corn has not been particularly prevalent this season. The last issue of the Integrated Pest and Crop Management Newsletter contained descriptions of corn foliage diseases.

Common rust is evident as small pustules on the upper leaf surfaces. These pustules quickly rupture to reveal masses of rusty, red-brown spores. Common rust may develop on lower leaf surfaces and even on stalks and ears but right now seems to be most prevalent on the upper leaf surfaces on leaves in the lower half of the canopy.

Older lesions of Stewart’s bacterial wilt.

Initially, gray leaf spot appears as somewhat oval to elongate reddish-brown lesions. The causal fungus tends to be restricted by the leaf veins so as lesions increase in size they take on the parallel edges characteristic of mature gray leaf spot lesions. Lesions may range from less than an inch to several inches in length. Gray leaf spot is showing up on lower leaves in the canopy and lesions are just beginning to expand and take on the parallel edge appearance.

Anthracnose is most common early in the season on the very first leaves that develop on corn plants. This year anthracnose has moved up in the canopy and can be found on the lower quarter to third of the leaves on a plant. Anthracnose lesions are oval to elongate in shape and tend to be reddish-brown to brown in color. Frequently anthracnose lesions will be surrounded by yellow to pinkish-yellow haloes.

Foliage diseases tend to have the greatest impact on yield when they are present in high levels on the ear leaf. The earlier in the season that ear leaves become infected the greater the potential for yield loss. So far most of the corn foliage diseases that I have seen have been on the lower leaves in the canopy and the ear leaves are quite clean. The corn foliage diseases are favored by extended periods of free moisture on the leaf surfaces. Frequent light rains, heavy dews that stay late in the day and overhead irrigation can lead to an increase in corn foliage diseases.

Anthracnose leaf blight on upper leaf.

Most of the management recommendations for minimizing losses due to corn foliage diseases are preventative measures such as planting resistant hybrids, rotating crops so corn doesn’t follow corn in the same field or using tillage to reduce the amount of infected residue left on the soil surface. Several fungicides are labeled for use on corn to control foliage diseases. These fungicides tend to be most effective if applied at the first sign of disease when weather conditions are favorable for disease development. Check fungicide labels for application restrictions such as do not apply after silking, do not apply within 30 days of harvest, , do not apply within 40 days of harvest, etc.

Soybean
A number of different disease problems are beginning to show up in soybean fields throughout the state.

Stunting and yellowing of plants may be the result of Fusarium root rot, Rhizoctonia root rot or Phytophthora root rot. With Fusarium and Rhizoctonia root rot plants may showing yellowing of the lower leaves, general stunting and poor root development but the entire plant may not yet be dead. Fusarium root rot tends to cause a rotting of the main taproot. The taproot may be discolored (ranging from light brown to a dark blackish-brown) and deteriorated. Rhizoctonia tends to cause red to reddish-brown lesions on the stem at or near the soil surface. With Phytophthora root rot the entire plant may have an off-color, wilted appearance. Entire plants may be dead. One key symptom of Phytophthora is the dark-brown discoloration of the main stem from the soil line up the main stem and even out side branches. At this point in the season there are no control measures that can be taken for these soybean root rot diseases. If weather conditions are stressful, i.e. hot and dry, affected plants may die. If weather conditions are mild with adequate moisture, affected plants may survive but not recover.

Progression of symptoms caused by Fusarium seedling blight and root rot (plant on left shows initial discoloration and rotting of taproot; at right, taproot has broken at rotted area.

Stunted, yellowed or poorly growing plants should also be checked for soybean cyst nematode.

If plants are carefully dug up and soil gently removed from the root system, it may be possible to see the white to yellow bodies of the females on the roots. If SCN is suspected, it would be wise to submit a soil sample for SCN analysis.

Some soybean foliage diseases are also showing up in low levels in many parts of the state. Septoria brown spot is occurring but is not as widespread or severe as it has been the last few years. Bacterial blight has been evident on some samples coming in from soybean rust sentinel plots. Bacterial blight lesions tend to be small, black to blackish-brown lesions with a slight yellow to light green halo. One or two samples have shown early symptoms of downy mildew. The upper leaf surfaces have bright yellow, irregular blotches and the downy mildew fungus may be obvious on the lower leaf surface as a purple to gray mold growth. Cercospora blight is also showing up on some samples. In some cases it has been developing as the reddish-purple discoloration across much of the upper leaf surface. In other fields it is showing up as the distinct brown lesions in areas of the leaf margin that are showing yellowing similar to that caused by potassium deficiency. None of the samples submitted thus far have had high levels of any of these foliage diseases.

Soybean rust has not been detected on any samples submitted from soybean rust sentinel plots. See article containing an update on soybean rust in this issue of the newsletter.

Finally, A few scattered plants are showing what may be very early symptoms of SDS or sudden death syndrome of soybean. Leaves in the upper to mid canopy are showing yellow, irregular blotches in the interveinal leaf tissue. The few fields in which I have seen this symptoms were planted prior to the wet period the end of April the first of May.

Laura Sweets
Ag Ext.-Plant Sciences
573-884-7307