In this Issue:  April 26, 2004

  Watch for Problems in Recently Planted and Emerged Corn

  Wheat Disease Update

Watch for Problems in Recently Planted and Emerged Corn

Corn planting remains in full swing across much of the state ranging from nearly complete in the southeast to just getting started in the northwest. Over the next several weeks we can expect to see a variety of early season corn problems. One of the most common is seed rot and seedling blight. Seed rot, as the name implies, occurs when one or more species of soil fungi invade the seed and kill it before it can emerge. Alternately, the seed may germinate and emerge in good condition, but the young seedling becomes infected and dies within the first two or three weeks of growth. The visible symptoms are usually a wilting of the plant. Closer inspection will reveal a constriction of the seedling stem at or just below the soil line. This symptom is known as damping-off. In nearly all cases, the plant will wilt and die. If seed rot or seedling blight occur on a large scale, replanting may be necessary.

A second problem sometimes seen is Stewart’s bacterial wilt. This disease is transmitted by the common flea beetle, and is therefore only a problem in years when flea beetle pressure is high. Plants will wilt similar to seedling blight, but damping-off will not be present. Instead, there will be very dark discoloration of the crown tissue visible when the plant is split open. The bacteria in the crown block water conduction from the roots to the leaves causing the plant to wilt and die.

A third problem has symptoms very similar to Stewart’s wilt. In this case, the problem is not caused by a disease organism, but rather by the weather. Extended periods of cool, wet weather when corn plants are small can result in damage to the crown tissue. It is believed this damage is a combination of chilling injury and oxygen deprivation. The problem, commonly called cold

weather crown stress, causes plants to grow very slowly and various nutrient deficiency symptoms may also be present. The crown of the plant will be brown-to-black in color, however, unlike Stewart’s wilt, the plant does not usually die. Plants will cold weather crown rot will eventually recover, but they tend to be more susceptible to stalk rot later in the season.

The final problem that is frequently seen is wireworm damage. This small, soilborne insect larvae can tunnel into the side of the young corn plant destroying the growing point. When this occurs, the whorl of the young plant will wilt and die while older leaves continue to grow. Careful inspection of the plants will usually reveal a small hole bored into the plant right at or above the crown. (Jardine)

Wheat Disease Update

Samples submitted to the clinic: Powdery mildew (PM) and soil-borne mosaic virus were the predominant diseases diagnosed on wheat this past week. PM of wheat was severe on the lower leaves of the plants. Drechslera leaf spot on brome grass was also diagnosed from a digitally submitted sample.

Field Report: A short and targeted survey of wheat and alfalfa in south central Kansas (Harvey, Reno, Rice, & McPherson Counties) was conducted last week. Most fields were at Feekes growth scale stage 6-7; the tips of flag leaves were visible in a few fields but in no field was the flag leaf emerged. In approximately fifteen fields of wheat, the number one problem was powdery mildew. The most prominent symptom is the appearance of visible mold on leaf surfaces and tillers. Several factors contribute to severity of powdery mildew including high nitrogen fertility, moderate temperatures, and

high relative humidity. In some fields, the powdery mildew severity was high enough that clouds of conidia were evident upon disturbance by either wind or walking through the field. In three fields, leaf rust was evident on the lower leaves at low severity (3-5 pustules/leaf). In two fields, greater than 15 leaf rust pustules/leaf were observed. Most of these fields were planted to Jagger and Jagalene, which are susceptible to both powdery mildew and leaf rust. Fields should be monitored as the flag leaf emerges through full heading. If disease continues to develop and threaten the flag leaf in a seed production field or a field with high yield potential (greater than 45 bushels), a fungicide application may be warranted. Check the KSU variety ratings to determine the susceptibility of the variety grown.

Barley yellow dwarf virus (BYDV) symptoms were observed in at least three fields; at high severity in two fields. The most prominent symptom is yellowing at the tips of leaves; some leaf tips will turn red to purple. The virus is vectored by aphids. At this point, there are no management options for BYDV. Almost all varieties are intermediate to very susceptible.

Alfalfa maturity varied widely. The main problems observed were insect damage. Disease incidence was very low and not considered serious at this point. (Stack)

James P. Stack
Extension Specialist
Plant Pathology Program

E-mail: jstack@.ksu.edu

Phone: (785) 532-5810

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

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