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Soggy fields can cause a variety of diseasesPosted Monday, July 26, 2010, at 5:24 PM
Common rust is one problem farmers should keep an eye on in cornfields according to Laura Sweets, Extension plant pathologist. The biggest problem with rust is that it is such a rapid building disease that you really need to get your fungicide on early to be effective. Rust will appear initially as a few pustules and then seem to jump exponentially where you will see hundreds per leaf. If fungicides are to be effective, you need to spray them before you see that exponential growth.
Rust tends to be a problem in seasons when strong wind currents come from the south. Typically our inoculum comes from Texas and Oklahoma on these wind currents. With the various stages of corn that are out there, farmers need to evaluate whether spraying will pay or not. If the plants are infected at early stages of growth, before corn tassels, you could be looking at losses up to 5 percent with common rust, but it would be much worse if it is southern rust.
With the variability in stands and the fields with saturated soil conditions and questionable nitrogen status, one would need to evaluate the whole situation to determine if applying fungicides would be a good investment or not.
Symptoms of common rust are circular to elongate, golden-brown to reddish-brown pustules on both upper and lower leaf surfaces and as plants mature, the pustules become brownish-black in color. The pustules rupture, revealing powdery brown spores. Symptoms of southern rust are light, reddish-brown, circular to oval pustules, primarily on the upper leaf surface. The pustules eventually rupture to reveal powdery spores. Later a brownish-black spore stage often forms in rings around the initial pustules.
Diplodia ear rot is also showing up in a few fields. If infection occurs soon after pollination, Diplodia ear rot may be evident as a bleaching or light straw coloration of the ear leaf and husks on the ears of infected plants. Since the primary source of inoculum is diseased corn debris, fields with corn after corn are at higher risk. Wet weather after silking also increases the risk of this disease developing. Diplodia can also cause a stalk rot of corn. Several weeks after silking, leaves on plants infected with Diplodia stalk rot may wilt, become dry and appear grayish green as though damaged by frost. Plants may die suddenly. Stalks become spongy and are easily crushed. If stalk rot develops in your field, you will need to prioritize harvest accordingly.
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As our local University of Missouri Agronomy Extension Specialist, Crook has been writing a column for the print edition Agriculture page for the past three years and we will now be sharing it on our web version. Crook has a bachelor and masters degree in agronomy from University of Missouri and received his doctorate in Agronomy from Kansas State University. He was in soybean variety development research for 22 years for various seed companies and has been Saline County's agronomy specialist for 10 years.