Posts Tagged ‘agronomic crop diseases’

Agronomic Crop Diseases

Thursday, April 22nd, 2010

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Wheat
The first disease of the season on wheat is usually powdery mildew. In general, powdery mildew has not been a problem for several years. Once wheat reaches jointing (Growth Stage 6) it should be scouted regularly for powdery mildew. As always, planting the highest yielding resistant varieties is the best control strategy, but if mildew threatens to rob yields later, fungicide control is the best control measure. Tilt, Propimax EC, Stratego, Quilt, Proline, and Caramba (the new Group 3 triazole from BASF) are suggested for control when and if fungicides are needed. These fungicides are also very effective for control of tan spot and Septoria leaf spot and glume blotch. It is common for powdery mildew to infect the lowest leaves and remain there for some time. The critical time to scout for powdery mildew is GS 8-10 (when the last leaf just appears until head emergence) to determine if fungicides are needed.

Barley
We have had reports of increasing amounts of powdery mildew on barley. ‘Thoroughbred’ looks to be the most susceptible variety but others should be scouted as well. Regionally we have no data to evaluate fungicides for control of barley diseases because barley rarely needs to be sprayed for diseases and the cost has been prohibitive. Times have changed, and if the heads are emerging and the top two leaves are infected there may be some benefit to controlling powdery mildew on a susceptible variety such as ‘Thoroughbred’ if the weather continues to favor powdery mildew. Stratego, Tilt, Quilt (10.5 – 14.0 oz/A) would be suggested for control if necessary. A beneficial non-target effect will be brighter straw if straw is being baled.

Powdery mildew on barley.

Soybean Cyst Nematode
It is still not too late to check for soybean cyst nematode, especially if susceptible soybeans are going to be planted. Soil test bags with the submission form can be purchased at the Extension offices. If you have a fax machine and need results quickly, test results can be sent via FAX if you provide the number on the Nematode Assay Information Sheet. This information sheet can be found on the web at the Plant Clinic Website http://ag.udel.edu/plantclinic .

Supplemental Label for Headline Fungicide for “Plant Health”

Friday, March 6th, 2009

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Headline and other related fungicides called strobilurins including Quilt, Quadris and Stratego have been applied extensively in the US in last few years to enhance yields of corn, soybeans and wheat. Locally, the majority of the use has been on field corn in irrigated high yield environments. The “Plant Health” claims in this new supplemental Headline® label initiated a vigorous discussion among Extension Plant Pathologists throughout the East and Mid-West concerning the claims on this new label. The following article addresses those concerns from a scientific, unbiased perspective of the authors. This is a thoughtful and well articulated article and I am sure this is not going to be the last word on this topic but it explains the issue and the concerns pretty well.

Supplemental Label for Headline® Fungicde for “Plant Health”: Will It Improve Corn, Soybean and Small Grain Health?
By Paul Vincelli, Don Hershman, and Chad Lee*
Departments of Plant Pathology and *Plant and Soil Sciences
Kentucky Pest News, Number 1187, February 24, 2009, online at: www.uky.edu/Agriculture/kpn/kpnhome.htm

A couple of weeks ago, we learned of a supplemental label for Headline® fungicide for use on several crops for “disease control and plant health.” The impacted crops grown in Kentucky are corn, small grains (barley, rye and wheat), and soybean, as well as other edible legumes. Headline® and related strobilurin fungicides (Quadris®, Quilt®, and Stratego®) provide excellent control of certain fungal diseases of the above crops. In Kentucky, for example, use of these products to control gray leaf spot and/or northern leaf blight in corn, frogeye leaf spot and brown spot of soybean, and tan spot and leaf rust of wheat makes sense when the risk of disease is high. However, this new supplemental label makes claims that go way beyond disease control.

Claims Made on the New Supplemental Label
The supplemental label indicates that, through preventive applications of Headline® to crops, the plant health benefits may include improved host plant tolerance to yield-robbing environmental stresses, such as drought, heat, cold temperatures, and ozone damage. The supplemental label also claims that Headline can improve plant utilization of nitrogen and can increase tolerance to bacterial and viral infections. These benefits often translate to healthier plants producing greater yields at harvest, especially under stressful conditions. The supplemental label also claims that additional specific benefits can occur, including:

● Improved stalk or straw strength and better harvestability (barley, corn, rye, wheat)
● Induced tolerance to stalk diseases (corn)
● Better tolerance to hail (corn)
● More uniform seed size (corn, soybean, and edible legumes)
● Better seed quality (soybean and edible legumes)

Will “Plant Health” Be Improved?
Based on publicly available research reports, we see very little evidence that Headline® or other strobilurin fungicides should be applied to any of the above crops for any reason other than disease control. To date, no data have been circulated in either the scientific or farm communities which suggest that any strobilurin product, including Headline®, can reliably live up to the claims made for stress tolerance under field conditions.

Claims of stress tolerance sound exciting but, based on the data we have seen, deserve to be viewed with cautious skepticism. There are certainly studies in the laboratory, the greenhouse, and occasionally in the field that show beneficial physiological changes in crops treated with strobilurin fungicides. But don’t assume that the beneficial changes observed in those studies result in increased yield under field conditions. When a greening effect and/or yield improvement is observed in a treated crop (in the absence of significant disease pressure), it is assumed that stress tolerance and/or improved plant health (apart from disease control) is at work. This isn’t necessarily true. In order for any real-world stress tolerance claims to pass muster, scientifically, it is necessary to conduct replicated field studies where the appropriate environment, plant, and crop measurements are made, and appropriate experimental controls are in place. We do not believe these data exist in sufficient quantity to support the above stress tolerance claims. Certainly, it is inappropriate to draw conclusions about stress tolerance based solely on crop appearance and yield. For example, we have observed the greening effect in field crops, but it often does not translate to higher yields. We have also observed occasional yield increases in crops (mostly soybean) following a fungicide application, when no obvious disease symptoms were present. But there are a large number of potential reasons why yields are improved in treated crops. Tolerance to one or more stresses is a possibility, but it is also possible that some soil-borne disease or disease complex is being controlled, but we cannot easily observe it. There are many other possible reasons and the only way to know for certain is to conduct the appropriate replicated, controlled field studies.

Let us look at an example from soybean from two replicated studies conducted at the Research and Education Center where disease pressure was minimal and late season moisture stress was significant (especially in 2007). If Headline® application improves tolerance to drought stress (as per the supplemental label), then the application should improve yield in treated crops. But as can be seen in Table 1, soybean yields were not improved by Headline® in either year. Table 2 shows the results of a similar field trial for corn conducted on a Kentucky farm under drought conditions. You can see that Headline® provided no yield bump.

Table 1. Results of Headline® application (6 fl oz + Induce at R3 stage) in soybean where disease pressure was insignificant, under late season moisture stress (UKREC, Princeton, KY, 2007-2008)

Treatment

2007
Yield (bu/A)

2008
Yield (bu/A)

Check 24.5 51.5
Headline 23.8 53.0
Statistical result (LSD, P=.05) No statistical difference No statistical difference

Table 2. Results of Headline® application in corn where disease pressure was insignificant, under late-season moisture stress (Logan County, KY, 2007)

Treatment

% Gray Leaf Spot*

Yield (bu/A)

Check 1.3 160
Headline 2.0 155
Statistical result (LSD, P=.05) No statistical difference No statistical difference

*Disease assessed on ear leaf at half milk line.

It is important to emphasize that the data in Tables 1 and 2 are merely examples. The above data are typical of what has been seen over and over in a large number of university-conducted trials conducted over the past several years in corn, soybean, and small grains. If Headline® regularly improves yields by imparting stress tolerance to crops in the absence of disease, then more complete and convincing proof needs to be made public. And in the world of science, claims based on evidence that has not been made public are treated with suspicion.

The claims about improved stalk health in corn are not unreasonable. Occasionally (and we stress the word occasionally), applications of strobilurin fungicides have been shown to improve stalk strength and/or reduce stalk rots in university-conducted field trials. However, in our experience, that improvement in stalk health relates to control of foliar diseases (gray leaf spot, for example). You see, if foliar diseases are aggressively attacking the plant during grain fill, then the corn plant will attempt to fill the grain by cannibalizing the reserves in its own stalk. That weakens the stalk and can result in more aggressive stalk rots as well as reduced stalk strength. So, if foliar diseases are killing the upper and middle foliage during grain-fill, then it makes sense that a fungicide like Headline® might sometimes improve stalk health, which it sometimes does. But note carefully: this benefit still relates to control of foliar diseases. And like we said above, strobilurin fungicides are very good for controlling foliar diseases like gray leaf spot and northern leaf blight of corn if these diseases are present.

What about a fungicide enhancing tolerance to hail? Actually, conducting a study that tests for this type of benefit is more complex than you may realize. You must have the right kind of experimental design or you could be misled by the results. The only study we are aware of that tests this claim with a valid experimental design is one conducted in 2008 by Dr. Carl Bradley and colleagues at the University of Illinois. In that study, researchers used a weed-eater to simulate hail damage. In that study, they found absolutely no yield benefit from Headline®, Quadris® or Quilt® when applied following simulated hail damage.

Is There a Downside?
Producers should be aware that sometimes the late-season “greening” effect observed with strobilurin fungicides can result in higher grain moisture and therefore additional drying costs and a slower (more expensive) harvest. Conversely, if crop harvest is delayed until the desired harvest moisture content is reached, there can be a yield and/or quality penalty, depending on the crop. For example, delaying wheat harvest will result in delayed planting of doublecrop soybean, which can lead to lower yields in soybean. In soybean, if harvest is delayed, pod and stem blight levels may increase, which can reduce the quality of grain destined for seed use. This may necessitate additional grain clean-out and/or the use of seed-treatment fungicides prior to planting next season. (Strobilurins, in general, do not do a good job in controlling soybean pod and stem blight). The bottom line is that fungicides applied to corn, soybean, and wheat will sometimes increase production costs.

Another concern specifically relating to the plant health issue is that the use of a fungicide when disease activity is too low to affect yield increases the risk of fungicide resistance. It is because anytime you expose a fungus to the fungicide, even when fungal activity is low, you increase the selection pressure on the fungus towards resistance. Resistance to strobilurin fungicides is an important concern worldwide, and the use of any strobilurin fungicide for plant health reasons increases the risk of developing strobilurin-resistant gray leaf spot. Use of strobilurins may also incite flares in certain insect and mite populations under field conditions, because fungicides can sometimes suppress fungi that kill these arthropod pests.

Bottom Line
The strobilurin fungicides are very good for control of specific crop diseases (see product labels for a list), if they are present at high enough levels (or the risk is high enough) to reduce yields. However, applying a strobilurin fungicide for plant health or stress tolerance reasons alone – with little or no threat from foliar diseases – doesn’t make sense to us, based on our extensive study of the best available information. Land-Grant University trials, thus far, generally do not support claims of reliable improvement in crop yield under stress conditions from an application of Headline®, or any other strobilurin fungicide. Nor have fungicide manufacturers provided sufficient field evidence in support of these claims. In fact, the vast majority of industry data show yield impacts (usually in side by side comparisons) associated with specific fungicide treatments, but provide no measurements of diseases or stresses. The upshot of this is that there is absolutely no way to know what the cause of apparent yield improvement is in the vast majority of industry studies. Thus, at this time, we do not feel there is a scientifically defensible basis for assertions of improved plant health/stress tolerance in the absence of the diseases the fungicide was originally developed to control.

Plant Management Network Publishes Latest Plant Disease Management Reports

Thursday, August 28th, 2008

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

The following article is a news release about the Plant Management Network and specifically about the Plant Disease Management Reports. This resource is valuable to all those who use fungicides and those that recommend them to growers.

The 2008 volume of Plant Disease Management Reports (PDMR), an online resource developed to give growers, consultants, pesticide applicators, and extension specialists the latest in disease management information, is now published.

This latest volume contains more than 560 searchable reports on the effectiveness of fungicides/nematicides, resistant varieties, and other biological controls that defend against diseases of agricultural and horticultural crops.

All volumes of PDMR and its preceding publications, F&N Tests and B&C Tests, contain 5,000-plus reports, covering more than 1,500 chemical and biological controls.

“Many professionals in agriculture and horticulture depend on PDMR to develop disease management recommendations or make better pest management decisions,” said Dan Egel, Ph.D., Extension Plant Pathologist at Purdue University and Editor-in-Chief of Plant Disease Management Reports.

Each one to two-page report consists of a summary outlining trial conditions and results. Test plot trial data, also in the report, includes treatment rates, application timings, and pertinent efficacy data for each product tested.

Users can search the reports by keyword or section. Keyword searches can include product names, active ingredients, host crops, and authors. Sections include cereals and forage crops; citrus, tropical, and vegetable crops; field crops; ornamentals and trees; pome fruits; seed treatments (for all crops); small fruits; stone fruits and nuts; and turfgrass.

PDMR’s efficacy reports were first published in 1946 as a section in the USDA publication, Plant Disease Reporter. In 1960, the American Phytopathological Society (APS) published these reports independently under the title Fungicide and Nematicide (F&N) Tests. In 1986, APS developed a new publication, Biological and Cultural Tests for the Control of Plant Disease, or B&C Tests. By 2001, both resources became electronically accessible. In 2007, they were merged into Plant Disease Management Reports. A full history is discussed in this volume of PDMR.

Users can have continuous access to all volumes of Plant Disease Management Reports, F&N Tests, and B&C Tests online for $45 yearly. This subscription also includes access to other Plant Management Network resources, which include Arthropod Management Tests, a similar publication covering the effectiveness of insecticides; applied crop science journals, webcasts, targeted extension searches, image collections, proceedings, and more. To subscribe or learn more, visit http://www.plantmanagementnetwork.org/pub/trial/pdmr/.

PDMR is distributed online through the Plant Management Network (http//www.plantmanagementnetwork.org), a nonprofit publisher of applied plant science resources. The Plant Management Network is jointly managed by the American Phytopathological Society, the Crop Science Society of America, and the American Society of Agronomy. The Plant Management Network’s nonprofit publishing mission is to enhance the health, management, and production of agricultural and horticultural crops.

Agronomic Crop Diseases

Thursday, July 3rd, 2008

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Corn
Gray leaf spot was identified this week from an irrigated field in Sussex County. The lesions were on the lowest leaves and not very numerous but a cause for concern since it is relatively early for this diseases to be showing up. The corn was just beginning to tassel so this field is a good candidate for a yield response from a fungicide application. If strobilurin fungicides (Quadris, Headline, Stratego) are used on corn be sure to wait until tassel emergence, there have been some problems in the Midwest with early fungicide applications causing some strange symptoms on corn especially “beer can ears”.

 

Early symptoms of gray leafspot – note rectangular lesions

Soybean Rust Update
Things are slow on the soybean rust front. However there was more rain last week in northern Florida and south Georgia so we may see some activity this coming week. Let’s hope not.

Soybean rust was confirmed in Taylor County, Florida, on kudzu on June 20th. This is the first find in that county in 2008. Since the beginning of 2008, soybean rust has been reported on kudzu in one county in Alabama; eleven counties in Florida (two of these counties had reports on coral bean and snap bean, and one county had a report on soybean); three counties in Louisiana; one county in Mississippi, and three counties in Texas. Reported infected kudzu sites in many counties have been destroyed. Rust was also reported in three states (5 municipalities) in Mexico on yam bean and soybean. These too have been destroyed or are no longer active, except for the find in Chiapas.

Agronomic Crop Diseases

Friday, June 13th, 2008

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Wheat
The wheat crop is made and now drying down. There was a late season disease that popped up that was not seen until now. Sharp eyespot is a disease that is caused by Rhizoctonia cerealis and causes stunting and “whiteheads” when it is severe. It can cause areas in the field that vary in size to be stunted and mature prematurely. As with take-all, the best control for sharp eyespot is rotation of at least one year out of small grains. The disease begins on the outer leaf sheath near the base of the plant. Lesions on the stems are light-brown to straw-colored with a sharply defined dark brown border. This disease could be confused with take-all but there is generally little root rot associated with sharp eyespot. Generally the disease is not severe enough to warrant control measures other than rotation with legumes or other nonhost crops.

Sharp eyespot on stems

Note sharp eyespot symptoms on lower leaf sheath.

Scab or head blight is present at varying levels in some fields and others have none. Fortunately for most growers, scab was not as bad as it could have been this year. There were some fields, mostly in Kent County so far, where scab is severe and growers should check for scab and adjust their combines accordingly. See the last issue for more info on scab.

Corn
Pythium root rot
has been the most common problem we have seen in the lab so far this season. With corn under water earlier in many places these conditions are extremely favorable for Pythium root rot. Seed treatments with metalaxyl or mefanoxam (Apron, Apron MAXX, Apron XL and others) should provide good control. Corn treated with Dynasty alone would not be effective under severe conditions for Pythium and should be combined with an Apron product for optimum disease control.

Seedling anthracnose was also diagnosed this week. Seedling anthracnose often occurs when corn follows corn especially in no-tillage systems. Initially, small watersoaked spots are seen and become tan with red or red brown borders as they age. Eventually the small hairs or setae of the fungus can be seen with a hand lens in the center of the lesions.  Anthracnose rarely causes any loss at this stage of growth and the plants grow out of the initial infections, which can recur later in the season if the weather is favorable for anthracnose leaf blight. Burying crop residues may be helpful in reducing these early season infections but do little or nothing in reducing the late season leaf blight and stalk rot phase of anthracnose.