Scout for Gray leaf spot and Northern corn leaf blight

July 2, 2015 in Corn, Corn Disease Management

Corn in many parts of the state is growing rapidly and tasseling or approaching tassel. Now is a good time to scout fields to see if there are any disease issues that may need attention. With recent wet weather diseases such as Grey leaf spot (GLS) and Northern corn leaf blight (NCLB) may flare up, particularly if you have a highly susceptible hybrid in your field. We have had reports of both diseases over the past week in grower fields. It is a little early for NCLB, but this season the disease has appeared earlier throughout many growing regions this year.

GLS and NCLB are similar in many aspects but can be distinguished in the field fairly easily (Table 1). Symptoms may be more pronounced in areas such as tree lines or low lying parts of the field, where humidity levels may persist for longer periods of time. Hybrid selection plays a key role in management of these diseases as does residue management and crop rotation. In some situations a fungicide application between VT and R3 may be profitable in high risk fields where these diseases are detected on the 2nd or 3rd leaf below the ear leaf at VT. Losses can occur when these diseases reach the ear leaf prior to grain fill and tend to be more problematic in late planted fields. If the diseases are restricted to the upper canopy or do not reach the ear leaf until 5-6 weeks post tassel, one can expect negligible yield loss.

Table 1.  Similarities and differences between GLS and NCLB

Gray Leaf Spot Northern Corn Leaf Blight
Source of disease

Corn residue

Optimal temperature 75-85°F 64-80°F
Favorable environment

Prolonged wet weather, heavy morning dew, overcast

Infection to symptom ≈14 days+ 7-10 days
Lesion shape Rectangular Oblong
Lesion length 1-3 inches 1-7 inches
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Stop sale on Nimitz nematicide lifted

July 1, 2015 in Vegetables

The stop sale that was briefly placed on Nimitz® nematicide due to potential label issues is over. The new label has removed tomatoes from the list of labeled crops for the short term. The link to the new label can be found here: http://www.adama.com/us/en/Images/NIMITZ%20Label%202015-06-01_tcm13-68006.pdf

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Explaining Late Blight Lineages

June 24, 2015 in Potatoes, Vegetables

Late blight is a disease that we deal with to some extent in the mid-Atlantic each year.  When reports come in you may see something mentioned about the lineage that was detected.  What exactly are we telling you when we mention this information?  Each lineage has unique genetic characteristics that may impact management practices and also level of concern in the region.  Some lineages prefer to cause disease on potatoes, some tomatoes, and some both.  In addition, some are sensitive, intermediate, or insensitive to mefanoxam fungicides.  As a quick reference, you can view the recent lineages reported in the US HERE.  I have simplified the information for you below.

 

Genotypea Host Mating type Sensitivity to mefenoxamc
US-8f Potato A2 I/R
US-11 Potato/Tomato A1 R
US-20 Tomato A2 I/R
US-21 Tomato A2 S/I/R
US-22g Potato/Tomato A2 S/I
US-23 Potato/Tomato A1 S/I
US-24 Potato A1 I

 

Source USAblight.org:  Summary of multilocus genotypes of Phytophthora infestans collected in the US and Canada, 2002-2009 (from Hu et al., Plant Dis. 2012 and Fry et al., APSnet Features 2012)

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Soybean Cyst Nematode Resistance in Soybeans

June 22, 2015 in Soybean, Soybean Disease Management, Soybean diseases

Soybean Cyst Nematodes (SCN) are the most damaging pest/pathogen of soybeans.  Iowa Public Television put together a short, easy to understand video explaining the basics of resistance development in soybeans to SCN, and how nematode populations can shift or change over time.  Click the following to view the brief video:  SCN Video

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Be on the lookout for seedling diseases

June 12, 2015 in Corn, Corn Disease Management, Soybean, Soybean Disease Management

Now is the time when you should be on the lookout for seedling diseases in corn and in full season soybeans. Seedling diseases can be categorized as either pre-emergent or post-emergent. Pre-emergent seedling diseases kill the seedling soon after germination or before it can reach the soil surface. Some potential indicators of pre-emergent seedling disease issues include poor stands in low lying or poorly drained areas of the field or overall poor seedling germination. Post-emergent seedling diseases can kill the seedling soon after emergence.

Sometimes plants can tolerate post emergent diseases and do not die, but may instead be stunted or yield less than healthy plants. Such plants may prone to drought related injury as a result of compromised root systems. Many times seedling diseases develop as a result of some other stressful event that predisposed the plants to infection. Good examples of such events include planting into cool soils, planting too deep, flooding, and misapplication of fertilizer and/or pesticides.

There are several fungi that can infect the roots of corn and soybean seedlings. Examples include Pythium, Fusarium, Rhizoctonia, and Diplodia (found on corn only). To distinguish plants suffering from root rot from those suffering from abiotic issues, such as poor nutrition, check the root systems. Plants suffering from root rot typically contain roots and root tips that are mushy, and brown to grey in color.  As mentioned previously, seldom are seedling diseases only the result of a pathogen. Many times other factors are the real culprit, allowing these fungi to take over. The best way to reduce seedling diseases is to plant at the correct depth into well prepared, warm soils (above 55°F). Treating seed with a fungicide seed treatment can provide some additional protection. There are innumerable fungicide seed treatments nowadays, and it can be difficult knowing what a given product contains. The University of Wisconsin published a nice guide showing many of the seed treatments for corn and the various active ingredients they contain.For a link to this click here

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Cucumber downy mildew present in South Carolina

June 7, 2015 in Vegetables

It is still very early for the onset of downy mildew of cucumbers and other cucurbits in Delaware and Maryland. These diseases cannot overwinter here, and must be reintroduced to our area each year from cucurbit crops in the South. This year, the disease has started to moving north quickly and has already been reported on cucumber and watermelon in South Carolina. The watermelon occurrence is very early. Downy mildew specific fungicides do not need to be applied yet in Delaware or Maryland. However you should scout your cucurbits after this storm has passed and keep a look out for downy mildew in the coming weeks.

-Information via Kate Everts, Vegetable pathologist UD and UMD.

For updates on CDM during the growing season, click here

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Scouting for scab in wheat

June 7, 2015 in Barley, Barley diseases, Wheat, Wheat Diseases

Wheat season is winding down and the window to see scab in fields started last week in many fields.  Overall we are looking at very low levels of scab in the majority of fields in Delaware.  However, the striking symptoms associated with Fusarium head scab can make it appear worse than it is.  Assessing scab in a field is important as it provides you with information about the current years management practices and environment, and it also gives you an idea of if you have issues that may require additional intervention.

To scout a field for scab you want to look at the field 18-21 days after flower.  A little sooner and you may miss symptoms, later and the heads may start to dry down, making it impossible to assess scab.

1) Select a minimum of 10 sites per field.  Choose sites at random-do not scout only field edges or along the road.  You want to ensure that the entire area is covered.

2) Take a gallon paper or plastic bag with you.  At each site keep your head up, and every 5 steps reach down, pick a head (don’t look down) and place it in the bag.  Do this 10-20 times per site.  You should end up with a bag of at least 100 heads, chosen at random.

3) Assess the incidence, or number of heads with any scab.  This is a simple number to obtain.  Simply count the number of heads with scab.  Then use the following formula to determine the incidence:

% Incidence = ((# heads with scab symptoms)/ total # heads collected)) x 100

In most fields this season we have observed 0-2% incidence, with most under 1%.  In 2013 we observed between 40-85%.

If you have a significant level of scab  can attempt to minimize scabby kernels (tombstones) by increasing the fan speed on your combine.  The majority of vomitoxin is found in tombstones and chaff, so the idea here is to remove as much of this from the bin as possible.

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Why the difference in Fusarium head blight fungicide timing in wheat and barley?

June 3, 2015 in Barley, Barley diseases, Wheat, Wheat Diseases

Timing of fungicides to suppress Fusarium head blight  varies between wheat and barley.  In wheat, the timing is just as flowering begins (Feekes 10.5.1) whereas in barley the timing is just after the head has emerged from the boot.  Why is this?

Wheat flowers after the head emerges from the boot.  Therefore, the head and any anthers produced are completely exposed to the environment and consequently, any spores that may land on the head.  Infection of anthers typically results in bleached heads and associated yield losses.  Barley on the other hand begins to flower just as the head emerges from the boot.    As a result, some of the flowering occurs in the boot with the sheath protecting heads from infection.  Yield loss from scab is not often an issue in barley although elevated DON can occur.

For an article explaining timings with illustrations I direct you to a post from Dr. Joel Ransom, Extension Cereals Plant Pathologist at NDSU: CLICK HERE

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Barley Yellow Dwarf-In Brief

May 29, 2015 in Barley, Barley diseases, Wheat, Wheat Diseases

Barley yellow dwarf (BYDV) is reported to be the most widely distributed virus in cereals. Under the right conditions, it can substantially reduce yield and grain quality. Losses upwards of 40% in barley and 25% in wheat are not uncommon in some areas where small grains are grown.  There are currently 8 “species” of BYDV known to exist.  Each species is variable in terms of its capacity to cause disease in various host plants. In addition to species effects, the amount and type of damage varies with the growth stage at which the plant is infected, plant species, cultivar, and environment.

Barley Yellow Dwarf can cause foliar discoloration.  Here you can see foliar discoloration that starts from the leaf tip down the leaf blade.  this is somewhat characteristic for some BYDV infections.  Visual symptoms are not diagnostic and can only be confirmed by specialized testing.  Photo by P. Sylvester.

Barley Yellow Dwarf can cause foliar discoloration. Here you can see foliar discoloration that starts from the leaf tip down the leaf blade. this is somewhat characteristic for some BYDV infections. Visual symptoms are not diagnostic and can only be confirmed by specialized testing. Photo by P. Sylvester.

Symptoms:

Plant viruses are often named for the symptoms they produce in their host.  Thus, one can expect that symptoms of BYDV include stunting and foliar discoloration.  Symptoms of BYDV are highly variable, and range from discolored foliage, to stunting and grain reduction.  In affected barley, foliage may be more golden yellow whereas whereas in wheat foliage may develop a orange/red/purple color.  Foliar discoloration typically develops from the tip downward. Black specks, puckered leaf margins, and erect leaves may accompany foliar discoloration. Typically it takes 2-4 weeks for symptoms to start to develop after infection.  Remember that many other causes for the aforementioned symptoms exist, including other viruses, abiotic factors, and other diseases.  The only way to know if you have BYDV is to do a specialized test, often what are known as ELISA tests or PCR.  Samples can be sent to specialized testing facilities such as Agdia for this purpose.  Some diagnostic clinics also may have the ability to conduct these tests in house.

Early infection of plants by BYDV can cause severe stunting as seen in this photograph.  Photograph obtained from Bugwood image database at www.Bugwood.org.

Early infection of plants by BYDV can cause severe stunting as seen in this photograph. Photograph obtained from Bugwood image database at www.Bugwood.org.

Stunting and reduction in tillering often is a result of Fall infections, whereas discoloration of the flag leaf without stunting indicates infection in the Spring. Plants can be found scattered throughout a field or found in round patches. Distribution of the virus is a result of the activity of aphids, which carry the virus and transmit it to new plants.

Role of aphids in transmission and spread:

The most important and common aphids that can carry and transmit the virus include the English grain aphid, the Bird Cherry Oat Aphid, the Corn Leaf Aphid, and the Greenbug.  Different aphids tend to spread different strains or species of BYDV.  When an aphid acquires BYDV it can transmit it for several weeks. Thus, a single aphid can be responsible for transmitting the disease to several plants within or even between fields. Infections of small grains in the fall, while the plants are small, may be more severe than when compared to infections that occur later in the growing season. Early infections can directly reduce yield by causing stunting and reducing tillering (Figure 2). Where do the aphids pick up the virus between crops? Well, they pick it up in volunteer wheat, oats, and barley. BYDV can also be found in numerous other grass species.

Management:

Management of BYDV is geared at limiting aphid activity and the potential for virus transmission. Planting small grains later in the season, after the Hessian fly free date can potentially limit the amount of time the crop may be exposed to aphids early in their development.  Good scouting and aphid management should be followed as well as solid weed management.  No resistance is available although some varieties are more tolerant than others.

For more information on BYDV, including pictures, I encourage you to read the following article from the American Phytopathological Society:Click here

 

 

 

 

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Soybean losses from diseases

May 28, 2015 in Soybean Disease Management, Soybean diseases

For several years we have been collecting data on soybean yield losses attributable to diseases and nematodes.  These data can be accessed online through the University of Illinois: click here

In 2014 Soybean Cyst Nematode again was the most damaging issue, followed by Sudden death syndrome and seedling diseases.

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