Corn Stalk Rot, Lodging, and Indirect Yield Losses
Authors: Pierce Paul, Dennis Mills
Substantial reduction of the photosynthetic capacity (amount of green tissue) of the corn plant prior to physiological maturity may result in poor stalk quality. Among other stress factors, foliar diseases (leaf blights) are the primary causes of reduced photosynthetic area, which results in poor stalk quality, and consequently, yield reduction. The leaves of the corn plant contribute a significant portion of the carbohydrates for grain fill. When the green tissue area of these leaves is reduced as a result of blighting, carbohydrates are redistributed from the stalk and roots to the developing ear for grain fill. When this happens, stalk and root tissues are deprived of the sugars necessary for proper development. This leads to weakened stalk and roots. In addition to foliar diseases, extreme environmental conditions such as drought stress or any other stresses that reduce the total photosynthetic capacity of the plant at the time when the developing ear competes with the stalk for sugars also affect stalk quality. Weakened stalks are predisposed to colonization by stalk rot fungi. For more information on the influence of drought and other stress factors on stalk quality consult “Potential for Stalk Rot and Lodging in Corn: Drought Effects” by Peter Thomison. C.O.R.N Newsletter 2005-28 (August 29, 2005 - September 7, 2005) available online http://corn.osu.edu/story.php?setissueID=100&storyID=592.
Stalk rot is the most common and most important disease of corn in Ohio. It is caused by several pathogenic fungi and secondary colonizers. The most common members of the stalk rot complex are Gibberella zeae, Colletorichum graminicola, Stenocarpella maydis and members of the genus Fusarium. The extent to which these fungi infect and cause stalk rot depends on the health of the plant. In general, severely stressed plants are more greatly affected by stalk rot than stress-free plants. The stalk rot fungi typically survive in corn residue on the soil surface and invade the base of the corn stalk either directly or through wounds made by corn borers, hail, or mechanical injury. Occasionally, fungal invasion occurs at nodes above ground or behind the leaf sheath. The plant tissue is usually resistant to fungal colonization up to silking, after which the fungus spreads from the roots to the stalks. Following infection and colonization, leaves turn grayish-green in color, ears droop, the lower part of the stalk turns from dark green to light brown, and the pith inside the stalk becomes soft. When diseased stalks are split, the pith is usually discolored and shows signs of disintegration. As the pith disintegrates, it separates from the rind and the stalk becomes a hollow tube-like structure. Destruction of the internal stalk tissue by fungi predisposes the plant to lodging.
The level of stalk rot varies from one growing season to another and for location to location. In seasons favorable for disease development, early occurrence of stalk rot cause direct yield losses as a result of premature plant death and reduced grain fill. The greatest damage caused by stalk rot, however, is stalk breakage or lodging. Even though the level of foliar disease this season may not have been sufficient to cause direct yield losses, indirect yield losses may occur as a result of lodging. Plant with stalk rot lodge and cannot be harvested mechanically. In addition, when plants lodge, ears in contact with the ground develop ear rot, leading to poor grain quality. To minimize yield losses associated with lodging, growers are encouraged to harvest field with stalk rot problems early. Scout field early for visual symptoms of stalk rot and test stalk integrity by squeezing the lower internodes between the thumb and forefinger. Stalks that are likely to lodge are easily compressed between the fingers. Since the level of stalk rot varies from field to field (as a function of cropping practice) and hybrids vary in their susceptibility to stalk rot, each field should be scouted separately. For more information on the different type of stalk rot and stalk rot management please refer to the OSU Plant Pathology web site "Ohio Field Crop Diseases" at:
http://www.oardc.ohio-state.edu/ohiofieldcropdisease/.
Winter Annuals and Dandelions are Fair Game
Authors: Mark Loux
Recent field observations indicate that we again have a bumper crop of winter annuals and dandelions in many corn and soybean fields. Anytime between now and mid-November is ideal to apply fall herbicide treatments for control of these weeds. Control of dandelion can be improved if application is delayed until after a frost. However, a fall herbicide treatment is the single most important step that can be taken for management of dandelions, and getting dandelion-infested fields treated in the fall is more important than when the treatment is actually applied. For more information and reminders on fall herbicide treatments see “Fall Herbicide Treatments – How They fit into Overall Weed Management Plans” in issue 2005-29 of C.O.R.N. (Sept 6, 2005). The full article is available at:
http://corn.osu.edu/story.php?setissueID=102&storyID=601
Scouting for Weeds from the Combine Seat
Authors: Mark Loux
Scouting fields for weed problems during harvest can provide useful information with regard to next year’s weed management plans. The 2005 crop year was not one of our more weed-free ones based on our observations and discussions with producers and dealers around the state. We observed generally increased mid to late season infestations of lambsquarters, giant ragweed, pigweeds, and velvetleaf in many corn and soybean fields this year. Some of this can be attributed to the unfavorable weather during the first part of the growing season, and the late development of crop canopies, which allowed later-emerging weeds to flourish. Even Roundup Ready soybean fields, which were typically extremely free of weeds during the first several years following introduction, had significant infestations of these weeds in areas. We believe the management of Roundup Ready soybeans has become more important in recent years than when they were first introduced. It is possible to observe two Roundup Ready fields in close proximity that have significant differences in weed control due to differences in management. We also have evidence that populations of certain weeds, such as lambsquarters and giant ragweed, are becoming less sensitive to glyphosate. Obtaining adequate control of these populations will likely require more intensive management of Roundup Ready soybeans, and integration of glyphosate with other herbicides. The bottom line: 1) take time during harvest to make a note of fields where weed control was less than adequate or where one or more weeds are becoming more prevalent (yield monitors may also provide useful information on the impact of weeds); and 2) use this information later this fall or winter when making weed management decisions for next year.
Peter Thomison (Corn Production), Robert Mullen (Soil Fertility), Pierce Paul and Dennis Mills (Plant Pathology), Mark Loux and Jeff Stachler (Weed Science). Extension Educators: Steve Foster (Darke), Howard Seigrist (Licking), Roger Bender (Shelby), Gary Wilson (Hancock), Dusty Sonnenberg (Henry), Curtis Young (Allen), Ed Lentz (Seneca), Harold Watters (Champaign), Glen Arnold (Putnam), Greg La Barge (Fulton), Mark Koenig (Sandusky), Jim Skeeles (Lorain), and Andy Kleinschmidt (Van Wert).