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June 18 to June 24, 2001
C.O.R.N. 2001-18
In This Issue:
A) Wheat Head Scab at Low Levels; Stagonospora Blotch Increasing
Slowly
B) Did your Genes hold up? "Disease Resistance Genes"
C) Weather Conditions Favorable for "Green Snap"
D) Diagnosing "Rootless Corn" Problems
Relatively dry weather and cool temperatures during the first two weeks of June have helped the wheat during the critical grain filling period. Several days of hot weather last week pushed the wheat in southern Ohio to more advanced stages of maturity and some early varieties in northern Ohio are beginning to turn color. Wheat harvest should be near the normal date for most regions in the state.
In spite of the wet weather earlier, the cooler temperatures during late May and the first 12 days of June have limited the development of severe disease. Stagonospora leaf blotch is common in most Ohio wheat fields, but its spread upwards on the plants to the heads has been relatively slow. We expect Stagonospora to have a yield impact only in those fields with considerable damage to the flag leaves and heads by this time. As the plants advance to ripening any further development of Stagonospora leaf and glume blotch will have less of an effect on yield. Wet, warm weather will be necessary for further blotch development.
Head scab has not caused as much of a problem in wheat as expected from the
wet weather in late May. Many counties are reporting less than one percent of
the heads affected with scab. Counties in central Ohio have reported some fields
with up to l0 to 15% of the heads with scab. Scab appears to be low in south
west, west, north west, north central and north east Ohio so far. There is still
time for symptoms of scab to show up in north west Ohio over the next few days.
You can view an update of the reported levels of head scab on the Ohio Field
Crop Disease web site www.oardc.ohio-state.edu/ohiofieldcropdisease/.
This information will be updated through the harvest season as we receive information
from locations in the state.
Phytophthora sojae will begin to kill plants where the Rps genes are not effective
against a population of this pathogen or partial resistance levels are too low
(scores of 6 or greater on a 1 to 10 scale where 1 is no disease and 10 is dead).
The population shifts from P. sojae will occur slowly, a few individuals will
be able to infect soybeans with an Rps gene, the result will be a build-up of
oospores in the soil. The first signs that we see are a few plants that are
dying here and there. If you find dying plants, one to two weeks after the next
heavy rain, it may be Rhizoctonia or P. sojae. Rhizocotonia will have a brick
red girdling lesion at the base of the young soybean plant http://ohioline.ag.ohio-state.edu/ac-fact/0025.html,
while P. sojae will be turning brown all the way up the stem in these early
growth stages www.oardc.ohio-state.edu/ohiofieldcropdisease/soybeans/prr.htm.
If you do find a number of plants (>30%) dying from Phytophthora in your
field, make notes and choose varieties with Rps gene combinations plus high
levels of partial resistance in future plantings.
The strong winds which accompany thunderstorms this time of the year may cause stalk breakage in corn fields depending on stage of plant growth. Such stalk breakage is often referred to as "green snap" or "brittle snap". Corn plants are more prone to snapping during the rapid elongation stage of growth. According to studies in Iowa, Minnesota, and Nebraska, the V5 to V8 stages (corn approx. 10-24 inches in height) and the V12 stage through tasseling are the most vulnerable stages.
Breaks in the stalk usually occur at nodes (along nodal plates) below the ear. When soil moisture and temperature conditions are favorable for growth during this stage of plant development, plants elongate rapidly but stalks are unusually brittle. Stalk brittleness is greatest in rapidly growing corn under high temperature and high soil moisture conditions. There is speculation that rapidly growing plants are more susceptible to snapping-off for several days during the few weeks before tasseling because there has been little time for plants to develop lignified tissues at the nodes. Although we've observed green snap periodically in Ohio, it's a more serious problem in the western Corn Belt. In Nebraska, where wind storms are more common, green snap has caused major stand losses. Based on studies in 1993 and 1994, Nebraska researchers observed that it was often the most productive fields with the highest yield potential that experienced the greatest green snap injury. They concluded that cultural and environmental factors promoting rapid growth early in the growing season predisposed corn to greater green snap injury.
Vulnerability to green snap damage does vary among hybrids. However, all hybrids
are at risk from wind injury when they are growing rapidly prior to tasseling.
The use of growth regulator herbicides, such as 2,4-D or Banvel, have also been
associated with stalk brittleness, especially if late application or application
during hot, humid conditions occur. Once tassels begin shedding pollen. green
snap problems generally disappear.
As growers scout their late planted fields this year to troubleshoot and diagnose various problems, they may encounter young corn plants with poorly developed permanent root systems. "Rootless corn syndrome" (or rootless corn) has become one of the more common root related problems in recent years. When corn plants emerge from cloddy seedbeds and are subject to hot, dry windy weather, rootless corn problems may occur. Plants exhibiting rootless corn symptoms are often leaning or appear ready to lodge (aka "floppy corn"). Affected corn plants may only be anchored in the soil by a single nodal root or by seminal roots. This condition is generally observed in plants from about the V3 stage to the V8 stage of development. Before the problem is evident, corn plants may appear vigorous and healthy, but after a sudden storm with strong winds, plants will be scattered or fall over because there is limited or no support. Leaning and lodged plants may also be wilted. When affected plants are examined they are found to lack all or most nodal roots. The nodal roots present appear stubby, blunt, and unanchored to the soil. Under favorable growing conditions, 4 to 5 sets of nodal roots (crown or secondary roots) develop above the seed and comprise the permanent root system of corn. The nodal roots, not the seminal roots (associated with the seed), are important in providing the water and the mineral nutrients that the corn plant needs for normal growth and development.
Rootless corn problems are usually caused by weather related conditions (that coincide with development of the permanent root system) and environmental factors. These include hot, dry surface soils, shallow plantings, compact soils, loose, or cloddy soil conditions. Nodal root development is inhibited by hot, dry compact soils. The abrasive action of strong winds can break off secondary roots and inhibit establishment of a permanent root system. Excessive rainfall and shallow plantings may cause erosion and soil removal around the crown region that can result in rootless corn.
Although certain types of herbicide injury (e.g. 2,4-D, banvel) and insect feeding (e.g. corn rootworm) may cause lodging to occur in corn plants during vegetative development, there is generally little evidence of their involvement in these rootless corn problems. Nevertheless, there may be situations where insect feeding and/or herbicides may be a contributing factor to the problem.
Poor root development may also occur in no-till fields that were planted when soils were too wet. In addition to likely compaction-induced root problems, abnormal root development may also occur because the furrow walls may have been smeared during the planting operation, preventing good seed-soil contact. If conditions turn dry, the walls of the furrow become hard and act as a barrier to root development. Roots growing in such an environment often appear flattened and under extreme conditions may result in plants' breaking and falling over during wind storms.
Can rootless corn recover? Yes, after plants lodge, adequate rainfall will promote crown root development and plants can recover. Cultivation to throw soil around exposed roots will aid the corn's recovery. Of course, this is difficult to do in a no-till situation or when the soil is hard and dry. Since affected corn is likely to be vulnerable to potential lodging problems at maturity, it should be harvested as soon as grain moisture conditions permit.
Readers can subscribe electronically to this newsletter by sending an e-mail message to: corn-out-on@postoffice.ag.ohio-state.edu. A successful subscription message will receive by an automatic reply from the listserv. Contact your local Ohio State University Extension Office or e-mail labarge.1@osu.edu if you have problems subscribing.
Past versions of C.O.R.N. can be found on the World Wide Web at: http:/www.ag.ohio-state.edu/~corn/archive/
C.O.R.N. is a summary of crop observations, related information, and appropriate recommendations for Ohio Crop Producers and Industry. C.O.R.N. is produced by the Ohio State University Extension Agronomy Team, State Specialists at The Ohio State University and Ohio Agricultural Research and Development Center. C.O.R.N. Questions are directed to State Specialists, Extension Associates, and Agents associated with Ohio State University Extension and the Ohio Agricultural Research and Development Center at The Ohio State University.
Contributors to C.O.R.N. this week include:Editor: Clark Hutson Web Editor: Tom Rosati
Information presented above and where trade names are used, they are supplied with the understanding that no discrimination is intended and no endorsement by Ohio State University Extension is implied. Although every attempt is made to produce information that is complete, timely, and accurate, the pesticide user bears responsibility of consulting the pesticide label and adhering to those directions.
All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.
Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Keith L. Smith, Director, Ohio State University Extension.
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