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For Week of June 29 to July 5, 1998
C.O.R.N. 98-16
In This Issue:
A) Symptoms Now May Signal Problems Later
B) Be on the Look out for These Soybean Diseases
C) Corn Rootworm Injury Observations
D) Surprise!! Potato Leafhopper Outbreaks in Alfalfa
E) Watch for "Green Snap" Problems in Corn
Anthracnose leaf blight on the lower leaves of corn plants can be found now. This is a common leaf blight in fields where there is substantial amounts of corn residue. The rain splashes the spores of the fungus from the corn residues on the soil surface to the bottom leaves of young plants. These may be fields where Anthracnose stalk rot may develop later in the season. Anthracnose leaf blight lesions appear blotchy with dark tan centers and yellowish-orange borders. Lesions appear first near the leaf tip and at the mid-rib. Note fields where anthracnose leaf blight is present now, then check these fields for Anthracnose stalk rot a week or so prior to harvest.
This wet weather will favor Phytophthora root and stem rot development on young soybeans. Fields which have had extended periods of saturated soils favor infection of roots of susceptible varieties by the Phytophthora fungus, PHYTOPHTHORA SOJAE. Varieties with Rps resistance genes or partial resistance (tolerance) will have little damage. If varieties with Rps genes do develop Phytophthora root and stem rot this is an indication that a race of Phytophthora is present in that field can recognize that Rps gene. Producers should change to a variety with a different Rps gene or use varieties with higher levels of partial resistance. We should begin to see development of Phytophthora stem rot throughout July and August from these late spring/early summer rains. Plants will appear wilted in the field, and slowly turn yellow with the soybean stem turning brown from the base of the plant upwards.
Rhizoctonia, Fusarium and Pythium are other fungi which can also infect young soybean roots and crowns, and result in poor stands. Rhizoctonia can have a deep brick red lesion at the soil line. Pythium, which is also favored by wet soil conditions, can cause damping off. The damage looks very similar to Phytophthora. There are lesions at the base of the stem with little to no fibrous root development in soybean plants infected with Fusarium. Fusarium is generally confined to the lower roots and base of the plants. The plants will form some adventitious roots near the upper portion of the taproot. Soybean plants infected with these three fungi will appear stunted and yellow later in the season. They will have very few roots, the tap root may be missing, and lesions may or may not be present on the crown, but the stem of affected plants will appear healthy.
Root systems on slow growing corn currently exhibit severe injury where economic levels of rootworm have been actively feeding. Where corn growth has been delayed, exposed to economic levels of rootworm feeding, and lacking a soil insecticide treatment, significant rootworm injury has occurred which will likely result in severe cases of lodging. In our insecticide trials on continuous corn at the Western Branch, severe damage was observed during the past week. Rootworm larvae were detected in both late instar and pupae stages. Significant rootworm injury was also observed in our first year corn trials in Van Wert, where planting was delayed and corn development was delayed due to adverse growing conditions. In contrast, rootworm injury in an early planted adjacent corn field was minimal where the root systems were larger and able to sustain a greater degree of feeding injury.
Where questions exist regarding the presence or absence of the first year corn rootworm problem, now is the time to be sampling root systems and looking for significant signs of rootworm injury and the presence of rootworm larvae. However, given the past month's growing conditions, one may also find a range of other problems such as brittle corn and herbicide injury to root systems
A week ago, I was reporting that alfalfa growth was vigorous and potato leafhopper (PLH) impact appeared to be lacking due to a total lack of nymph activity despite a growing number of adults. However, upon a return to the fields this past week, the situation had changed dramatically. PLH nymphs were suddenly in abundance and the development of hopper burn symptoms was very evident. In addition, very hot weather conditions were adding an additional stress factor which may cause the alfalfa to be very susceptible to heavy PLH pressure. Thus, growers should maintain a close watch on alfalfa, since field conditions have changed significantly.
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Potato Leafhopper nymph. |
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Potato leafhopper damage to alfalfa. |
Strong winds which accompanied thunderstorms during the past week may have caused 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 between V8 and tasseling, especially during the two week period prior to tasseling.
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. Vulnerability to green snap damage does vary among hybrids. However, all hybrids are at risk from such wind injury when they are growing rapidly prior to tasseling. The use of growth regulator herbicides such as 2,4-D or Banvel has also been associated with stalk brittleness, especially if late application or application during hot, humid conditions occur. Once the crop tassels green snap problems generally disappear.
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 factors promoting rapid growth early in the growing season also predisposed corn to greater green snap injury.
For additional articles on weather injury to corn see previous two issues (Week of June 15-21 , Week of June 22-28 ) of C.O.R.N. and visit Effects of Flooding and Ponding on Corn, AGF-118-95 a related publication on flooding and ponding in corn.
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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: State Specialists: Anne Dorrance (Plant Pathology), Hal Willson (Entomology), Peter Thomison (Corn Production), and Mark Loux and Jeff Stachler (Weed Science). District Specialist: Curtis Young (IPMP) Extension Agents: Clark Hutson (Seneca), Dave Jones (Allen), Greg LaBarge (Fulton), Rodger Bender (Shelby), Larry Lotz (Fayette), Steve Prochaska (Crawford) and John Barker (Knox).
Editor: Curtis Young Web Editor: David Etzkorn
Information presented above and where tradenames are used, they are supplied with the understanding that nodiscrimination 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.
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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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