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Plant Pathology

Integrated Crop Management News

2002

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Questions On Soybean Rust, X. B. Yang Nov 2002

Questions On Soybean Rust, X. B. Yang

Integrated Crop Management News

Recently, we have received questions on soybean rust from many Iowa soybean producers and agronomists in private and public sectors. This article summarizes their main questions and provides answers.


Hybrid Reactions To Gray Leaf Spot, Gary P. Munkvold Nov 2002

Hybrid Reactions To Gray Leaf Spot, Gary P. Munkvold

Integrated Crop Management News

Gray leaf spot continues to be the most bothersome leaf disease in Iowa. In our research plots, susceptible check hybrids were noticeably more diseased this year than they have been during the past couple of years. But it also was very noticeable how much better the majority of hybrids in the Iowa Crop Performance Test looked compared with the check hybrids.


Choosing An Scn-Resistant Soybean Variety: It's Not Just About Yield, Gregory L. Tylka Nov 2002

Choosing An Scn-Resistant Soybean Variety: It's Not Just About Yield, Gregory L. Tylka

Integrated Crop Management News

Soybean cyst nematode (SCN) is a widespread pest of soybean in Iowa. The nematode is a serious threat to soybean production in the Midwest because it reproduces very quickly, survives in the soil for many years in the absence of a soybean crop, and can cause substantial yield losses. Resistant soybean varieties are a very effective strategy for managing SCN, producing acceptable yields yet suppressing nematode reproduction. The number of maturity group I, II, and III soybean varieties with genetic resistance to SCN has increased dramatically, from a few dozen in the early 1990s to more than 650 currently.


Corn Ear Molds And Mycotoxins, Gary P. Munkvold Oct 2002

Corn Ear Molds And Mycotoxins, Gary P. Munkvold

Integrated Crop Management News

There has been elevated concern this year about mycotoxins in grain, especially aflatoxins. Aflatoxins are not a common problem in Iowa, but the dry weather that occurred in some parts of the state caused stress on the plants, which lead to aflatoxin problems. Corn plants that experience drought stress are more susceptible than usual to the fungus that produces aflatoxins, Aspergillus flavus. Surrounding states also have been reporting aflatoxins in corn. In Iowa the reports seem to be limited to the southwestern corner of the state. Corn in the earliest harvested fields seemed to be the most problematic, because these ...


Timing Tillage For Disease Control, X. B. Yang Oct 2002

Timing Tillage For Disease Control, X. B. Yang

Integrated Crop Management News

This season, Iowa soybean producers have been challenged by several disease problems and some problems may revisit in the next soybean crop if proper management is not applied. Although tillage has been considered a measure to reduce disease risk, not all diseases (for example, viral diseases with insect vectors) are lessened by tillage practices. Effectiveness of tillage in disease control varies depending on the biology of a disease. Knowing how tillage affects the occurrence of a disease can help minimize tillage efforts and achieve maximum control. This article discusses effects and timing of tillage measures for major soybean diseases this ...


Anthracnose Top Dieback Is Back, Gary P. Munkvold Sep 2002

Anthracnose Top Dieback Is Back, Gary P. Munkvold

Integrated Crop Management News

Anthracnose top dieback became very noticeable in Iowa several years ago, but last year this disease was not seen in many fields. This year is shaping up differently, and there are many fields with tops dying back and showing symptoms of anthracnose. This problem cannot be diagnosed, however, from a distance. There may be other causes for tops of plants to dieback, particularly European corn borer.


Seed Quality-Realted Soybean Diseases, X. B. Yang Sep 2002

Seed Quality-Realted Soybean Diseases, X. B. Yang

Integrated Crop Management News

This growing season, Iowa soybean producers have seen several diseases, a few of which affect seed quality. These diseases include fungal diseases and viral diseases. At harvest, it is likely that soybean from some fields will show discoloration, which can be a concern when beans are used for seeds. For some diseases, infected seeds may be discolored but not important to the spread of pathogens. This article discusses seed quality-related diseases at harvest.


Gray Leaf Spot Development, Gary P. Munkvold Aug 2002

Gray Leaf Spot Development, Gary P. Munkvold

Integrated Crop Management News

Gray leaf spot has become more evident during the past 2 weeks, primarily in eastern Iowa. Lower leaves in some fields have numerous well-developed lesions. So far, ear leaves in most hybrid fields are free of disease or have a few small lesions. Current hybrids are generally less susceptible than they were a few years ago. During the past few years, few hybrid fields in Iowa have had gray leaf spot levels that would warrant a fungicide application. Criteria for fungicide applications in hybrids have relied on presilking disease levels. The corn crop is well past silking, but I have ...


Time To Start Scouting For Corn Stalk Rot, Gary P. Munkvold Aug 2002

Time To Start Scouting For Corn Stalk Rot, Gary P. Munkvold

Integrated Crop Management News

In the late part of the growing season, it is time to pay attention to stalk quality. Some fields will be at high risk for stalk rot this year due to drought stress or wind damage that has already occurred. Watching these fields closely could prevent further losses due to lodging. Scouting should be done before black layer, approximately 40-50 days after pollination. While scouting for stalk rot, look for visible symptoms (such as stalk lesions) and test stalk firmness by pinching the lower internodes with thumb and forefinger. Healthy stalks are firm and cannot be compressed.


Sds Prevalent This Summer, X. B. Yang Aug 2002

Sds Prevalent This Summer, X. B. Yang

Integrated Crop Management News

Numerous reports have been received on the occurrence of soybean sudden death syndrome (SDS) in August in eastern and central Iowa. Reports came from producers, Iowa State University (ISU) extension staff, and agronomists of seed companies. As usual, the earliest reports were from eastern and southeastern Iowa; however, unlike in the 2000 growing season, more infected fields were found in central Iowa this year, with some reports from as far north as the Fort Dodge area. In central Iowa, some fields with SDS-infected plants are visible along highways. Fields planted in late April and early May seem to have higher ...


Needle Nematode Damage To Corn, Gregory L. Tylka, Paula Flynn Jul 2002

Needle Nematode Damage To Corn, Gregory L. Tylka, Paula Flynn

Integrated Crop Management News

Plant-parasitic nematodes can seriously injure corn, and there are numerous species known to affect corn in Iowa. The Iowa State University (ISU) Plant Disease Clinic has received several samples in the past few weeks from fields with corn damaged by the needle nematode. Plants from these fields were stunted and yellow and many of the roots were swollen.


Disease Prospects For Lodged Corn, Gary P. Munkvold Jul 2002

Disease Prospects For Lodged Corn, Gary P. Munkvold

Integrated Crop Management News

Last week's storms flattened many acres of corn in parts of the state, and now there is much speculation about the future of these fields. There will be a variety of consequences to the corn's prostrate position, including an increased risk for some diseases. Most reports indicate that the lodging was mainly root lodging, not green snap, and so I will limit my comments to root-lodged plants. I expect that the physical injury the plants experienced will lead to increased incidence of common smut, both in the ears and other plant parts. Additionally, some of these fields will ...


Phyllosticta Leaf Spot On Soybean, X. B. Yang Jul 2002

Phyllosticta Leaf Spot On Soybean, X. B. Yang

Integrated Crop Management News

Eastern Iowa has received more rain than western Iowa, and some foliar diseases, such as bacterial leaf spot and brown spot, are developing in eastern Iowa soybean fields. A disease relatively new to Iowa is Phyllosticta leaf spot, caused by Phyllosticta sojicola. Limited information is available on Phyllosticta leaf spot; most reports are from southern states. It is considered a minor disease throughout soybean production areas in the United States. A certified crop adviser detected this disease in Muscatine County, and his finding was confirmed by the Iowa State University Plant Disease Clinic. This disease also was found in several ...


Corn Leaf Diseases Appearing In 2002, Gary P. Munkvold Jul 2002

Corn Leaf Diseases Appearing In 2002, Gary P. Munkvold

Integrated Crop Management News

During the past week, leaf diseases have become noticeable on corn scattered throughout the state. Field specialists from several areas reported anthracnose is common. In addition, I have seen eyespot and common rust in central and southeastern Iowa. Gray leaf spot is showing up in some inbreds in our research plots in eastern Iowa.


Early Summer Soybean Diseases In 2002, X. B. Yang Jul 2002

Early Summer Soybean Diseases In 2002, X. B. Yang

Integrated Crop Management News

In early summer, soybean seedling diseases are diminishing and foliar and root diseases are starting to show up. This article discusses some soybean diseases (excluding viral diseases) you may see during scouting in July.


Crown Rot Symptoms Common In Corn, Gary P. Munkvold Jul 2002

Crown Rot Symptoms Common In Corn, Gary P. Munkvold

Integrated Crop Management News

The seedling disease problems that have plagued some fields in Iowa have evolved into crown rot problems. Seedling pathogens can rot seeds, prevent emergence, and kill emerged seedlings, but some infections are not lethal and the surviving plants are stunted. As healthy nodal roots develop, many of these plants should recover. However, it has become clear that some plants are not recovering and remain stunted. Symptoms of crown rot are observed with many of these stunted plants. Plants were stunted and yellowed even with relatively mild crown decay.


Iron Chlorosis Shows Up, X. B. Yang Jul 2002

Iron Chlorosis Shows Up, X. B. Yang

Integrated Crop Management News

Cool early summer temperatures have promoted symptom development of soybean iron chlorosis in some parts of Iowa, according to reports from Iowa State University field crop specialists and samples submitted to the Iowa State University Plant Disease Clinic. Yellowing soybean caused by iron chlorosis was a problem last growing season, and it may be a problem again for some growers this season.


Asian Soybean Rust Found In South America, X. B. Yang Jun 2002

Asian Soybean Rust Found In South America, X. B. Yang

Integrated Crop Management News

Recently, the United Soybean Board released news on the arrival of Asian soybean rust in South America. This arrival has great impact on soybean production in the Western Hemisphere and on the decision making of agencies involving soybean production in the United States. I have been working on Asian soybean rust, and after the news release, I received questions on this disease. This article provides some background information on this disease.


Controlling Leaf Diseases In Seed Corn In 2002, Gary P. Munkvold Jun 2002

Controlling Leaf Diseases In Seed Corn In 2002, Gary P. Munkvold

Integrated Crop Management News

The time is fast approaching when seed producers need to be looking for early symptoms of leaf diseases in seed corn. Eyespot (Aureobasidium zeae), common rust (Puccinia sorghi), gray leaf spot (Cercospora zeae-maydis), and northern leaf spot (Bipolaris zeicola, also known as Helminthosporium carbonum) are diseases that can cause losses in seed corn production and sometimes need to be controlled with a fungicide application. In addition, Northern leaf blight (Exserohilum turcicum) seems to be making a "comeback" in some areas.


Callisto Injury To Corn, Micheal D. Owen Jun 2002

Callisto Injury To Corn, Micheal D. Owen

Integrated Crop Management News

Weed management faculty and staff at Iowa State University have received several calls concerning Callisto injury to corn. Many field visits have been made, and numerous samples have been assessed. Some of the situations involved insecticides applied previously to corn, some involved tank mix combinations with other herbicides (e.g., Steadfast), and some involved preemergence-applied herbicides (e.g., Balance) followed by a postemergence application of Callisto. In addition, numerous calls about injury from other herbicides have been received.


Corn Seedling Pathogens, Gary P. Munkvold Jun 2002

Corn Seedling Pathogens, Gary P. Munkvold

Integrated Crop Management News

After a slow start, corn seedling problems have become evident in many fields during the past couple of weeks. With much of the corn now out of the "seedling" stage at V5 or higher, those plants that are not thriving have become obvious. On most of the samples that I have seen, the mesocotyl tissue has been rotted to some extent, either mildly or severely. This condition has resulted in poor growth or death of the plants. On the surviving seedlings, nodal roots have developed and may be able to pull the plants through, although these stunted plants are of ...


Stratego Receives Corn Label, Gary P. Munkvold Jun 2002

Stratego Receives Corn Label, Gary P. Munkvold

Integrated Crop Management News

Corn producers who want to control foliar diseases with a fungicide have a new tool. The fungicide Stratego, from Bayer Corp., is now registered for use on this crop. Stratego is a combination of two active ingredients, propiconazole and trifloxystrobin. Propiconazole has been used on corn for nearly a decade as the active ingredient in Tilt fungicide. Trifloxystrobin is a new product in the strobilurin family, which also includes azoxystrobin, the active ingredient in Quadris.


Now's The Time To Scout For Scn, Gregory L. Tylka Jun 2002

Now's The Time To Scout For Scn, Gregory L. Tylka

Integrated Crop Management News

Soybean cyst nematode (SCN) is an important, widespread soybean pest in Iowa that often goes unnoticed. To date, the nematode has been discovered in all but five Iowa counties. The only consistent and reliable sign of an SCN infestation in the field is the presence of adult SCN females and cysts (dead females) on the roots of infected soybean plants. Adult SCN females and cysts are small, round, and white to yellow, each approximately the size of a period at the end of a sentence.


Stand Reductions In Corn Due To Fungal Pathogens And Insects, Gary P. Munkvold, Marlin E. Rice Jun 2002

Stand Reductions In Corn Due To Fungal Pathogens And Insects, Gary P. Munkvold, Marlin E. Rice

Integrated Crop Management News

During the past week, numerous calls have come in about stand reduction in corn, especially fields planted during the last days of April and the first days of May. In many parts of the state, fields planted at that time were inundated with rain for a whole week, and many seeds and seedlings succumbed to seed rots and seedling diseases. Samples of seedlings received at the ISU Plant Disease Clinic have shown typical symptoms of seed rot or seedling blight on the roots and mesocotyl. Seeds that failed to germinate were rotted, sometimes as a result of insect injury.


Alfalfa Problems Appearing, Gary P. Munkvold, Stephen K. Barnhart Jun 2002

Alfalfa Problems Appearing, Gary P. Munkvold, Stephen K. Barnhart

Integrated Crop Management News

In early May, the reports on alfalfa progress were optimistic, but development slowed because of the cool temperatures as the month progressed. During the past week, the Plant Disease Clinic has received several samples, and we have had other reports of struggling alfalfa stands. Samples have come from across central Iowa and range from stands seeded last year to those in their third year of production. Some show similar symptoms, but it is likely there are different factors involved.


More On Fungicide Resistance, X. B. Yang May 2002

More On Fungicide Resistance, X. B. Yang

Integrated Crop Management News

In the May 6 issue of the ICM newsletter, I had an article on resistance of Phytophthora to the fungicide metalaxyl. I discussed a possible treatment option if resistance occurs, which involved switching to a different seed treatment, such as mefenoxam. Alternating different fungicides is a common strategy in fungicide resistance management; however, experts in fungicide chemistry pointed out that the active ingredients of mefenoxam and metalaxyl are isomers. Isomers are compounds that have the same number of atoms but differ in their structural arrangement.


Identifying Soybean Seedling Diseases, X. B. Yang May 2002

Identifying Soybean Seedling Diseases, X. B. Yang

Integrated Crop Management News

The cool weather early in the planting season makes generalizations about seedling diseases difficult. Although cool soil temperatures are favorable for infections by Pythium and Fusarium, soil moisture was not good for these two disease-causing fungi until last week (week of May 6), when rainfall saturated the soil. If there is another large rainfall in the next week or so, some seedling disease problems may occur, and the types of diseases depend on the temperature this week and next week. This article discusses identification of the possible diseases.


Early-Season Leaf Diseases On Corn, Gary P. Munkvold, Paula Flynn May 2002

Early-Season Leaf Diseases On Corn, Gary P. Munkvold, Paula Flynn

Integrated Crop Management News

The cool weather is still limiting the speed of corn growth, but it won't be long before the plants begin putting leaves out quickly. That's when problems that cause leaf symptoms will first be noticed, including those caused by diseases. There are three leaf diseases that tend to appear early in the season: holcus spot, anthracnose leaf blight, and eyespot.


Spring Alfalfa Diseases In 2002, X. B. Yang, Gary P. Munkvold May 2002

Spring Alfalfa Diseases In 2002, X. B. Yang, Gary P. Munkvold

Integrated Crop Management News

It is time to check for diseases in first crop alfalfa because cool temperatures and frequent spring rains are favorable for the development leaf diseases in Iowa alfalfa. Knowing the level of leaf diseases in early May can help in making management decisions. Severe diseases can cause early defoliation, so scouts should watch for the following diseases that occur in spring.


Early-Season Management Of Bean Leaf Beetle And Bean Pod Mottle Virus, Rayda Krell, Marlin E. Rice, Larry P. Pedigo, John H. Hill May 2002

Early-Season Management Of Bean Leaf Beetle And Bean Pod Mottle Virus, Rayda Krell, Marlin E. Rice, Larry P. Pedigo, John H. Hill

Integrated Crop Management News

During the past three summers, bean leaf beetles have reached historically high populations. At the same time, we learned that the insects are transmitting a disease called bean pod mottle virus. The occurrence of both of these pests in Iowa soybean has dramatically affected yields. Because the beetles are spreading the virus in soybean, we must rethink how we are going to manage these two pests.