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7/21/2008 - 7/27/2008

More on Fungicide Application Questions

By XB Yang, Department of Plant Pathology

Near the end of July, a common time for fungicide applications, there are many questions about using fungicides to control soybean diseases. This article responds to questions producers are currently asking about soybean and corn fungicide use.

What diseases are prevalent in Iowa?  The most common soybean diseases are brown spot, bacterial blights, Rhizoctonia root rot, and a few cases of frog eye leaf spot.  Soybean rust, which is light in the southern U.S., should not be a concern in Iowa this season. 

Regarding corn, common corn rust is found throughout Iowa with one area reporting high incidence levels. Southern corn rust also has been found in southern Iowa but at a low level.  There are also reported findings of grey leaf spot.

When to spray?  If you determine that fungicide use is needed to control soybean diseases, now is the time to spray for many soybean fields. Over the years, we have found that sprays at R3-R4 growth stage provide the highest returns if one application is made. 

Because fungicides only protect plants for 2-3 weeks, in a rainy season like this year, one spray will not protect the plants from disease attacks after 3 weeks. For these who mixed a fungicide with the Roundup application, the applications were likely too early to achieve the best control result. 

For most growers, sprays after R4 should provide better coverage for the month of August, which should be the time later season foliar diseases develop.  However, we do not encourage people to spray after R5 or later because the plants grow high and dense, which increases the damage done by the tractor. Tractor damage  to plants may minimize the gains from disease protection.

It is important to note that fungicides have no effect on bacterial blight.

How to differentiate bacterial blight from brown spot? I have received more reports of bacterial blight this season than any other. Abundant rainfall promotes the development of this disease which is spread by splashing rain and favors cool temperatures. One of the common questions is how to tell the difference between bacterial blight and brown spot. This is an important question because symptoms of the two diseases are similar, and the fungicides have no effects on bacterial blight.

Brown spot starts in the bottom leaves. The lesions are numerous irregular, dark brown spots on both upper and lower leaf surfaces. Adjacent lesions frequently merge to form irregularly shaped blotches.

Bacterial blight is found in the upper portion of the plants, mostly in young and tender leaves.  Lesions of bacterial blight have a yellow halo that is lacking in brown spot.  Infected leaves often have ridged appearance, unique for bacterial blight. When temperatures get hotter and leaves become old in early August, severity of bacterial blight should decrease. But the disease may reemerge later in the summer when the temperature gets cooler again. 

brown spot and bacterial blight in soybean comparison

Bacterial blight (left) and brown spot (right).

 

There is no measure to control bacterial blight except for selecting a better variety, one less susceptible to bacterial blight.  If you find high levels of bacterial blight this season, you should not use the same variety for next soybean.

Which fungicides to use? It is natural that a company representative will promote his or her own products.  Basically, almost all fungicides on the market can provide a good control to the foliar diseases found this season.  Their protection periods are not greatly different.  Therefore, you should select a fungicide based on the price and availability.

Fungicides for hail injured corn?  There are discussions about applying fungicides to corn injured by hail. Keep in mind that fungicides cannot help plants recover from hail injury or cure the damaged plants. However, if the foliar disease risk is high for the rest of season, use of fungicides can protect undamaged or functional corn leaves from disease attacks. Such applications function as an insurance for yield production.  In a situation or season when disease risk is low, such insurance is unnecessary.  In this wet season, the potential benefit from disease protection seems greater than the cost in terms of dollars.
 

 

XB Yang is a professor Plant Pathology with research and extension responsibilities on soybean diseases.

Here Come the Bean Leaf Beetles!

By Jon Tollefson and Marlin Rice, Department of Entomology

The bean leaf beetle overwinters as an adult that feeds on young soybeans in May and June. They lay eggs that produce the first generation, which attacks beans in July. The subsequent generation, the second, is of most concern when protecting the plant from the beetles feeding because it attacks the pods and beans as well as foliage.

The first generation adult bean leaf beetles began emerging from the soil during the week of July 7 in central Iowa. To determine if the second generation will reach economic numbers and to avoid injury to the beans as the populations increase, the first generation can be sampled now. If the number of beetles present exceeds the economic threshold, the farmer should be ready to treat the field when the second generation begins to emerge in late August from the soil and the presence of new adults, which are soft and gray (see photo), in the field is confirmed.

The scouting methods explained in this article sample the first generation, which is occurring now, to predict the size of the second generation, which will occur in late August and September.

bean leaf beetle

Newly-emerged from the soil, a bean leaf beetle is soft and gray in color. Marlin E. Rice

 

Sample soybean fields now. When the number of beetles reaches or exceeds the threshold (Table 1 or Table 2), stop sampling. If the sample is below the threshold, sample the following week. If the sample remains below the threshold, sample a third and final week. If the threshold is not reached, an economic infestation of bean leaf beetles should not occur in your pod-stage soybeans during August and September.

The sampling procedures are explained below. The treatment thresholds for $7 to $15 control costs were taken from previous research and extension publications.  Because of the unusually high control costs and value of soybeans, the threshold values for $18 and $20 control costs were calculated from the earlier data assuming that the increase in the thresholds would be linear.

Drop cloth
• Walk 100 feet in from the field edge and scout each field and each variety separately.
• Place a 3-foot-wide strip of cloth on ground between the rows.
• Bend the plants on one row over the cloth, and shake them vigorously.
• Count the number of beetles on the cloth.
• Repeat the procedure four times for each 20 acres of the field.
• Determine the average number of beetles per 3 foot of row.
• See Table 1 for the number of beetles per 3 foot of row necessary to justify insecticide treatment for the second-generation adults in August or September.
• If the number of beetles is below the economic threshold, sample your fields again the following week, or a third week if necessary.

Sweep net
• Walk 100 feet in from the field edge and scout each field and each variety separately.
• Take 20 sweeps down the row, not across the row.
• Repeat the procedure four times for each 20 acres of the field.
• Determine the average number of beetles per 20 sweeps.
• Table 2 shows the number of beetles per 20 sweeps that justifies insecticide treatment for the second-generation adults.
• If the number of beetles is below the economic threshold, sample your fields again on the following week, or a third week if necessary.

The treatment thresholds are for the prevention of economic injury to the soybeans by the adult bean leaf beetle. They do not include prevention of disease transmission, which may accentuate the impact of the beetles on soybeans.

 

Table 1. Economic thresholds for first-generation bean leaf beetles (average number of beetles per 3 foot of row). If these numbers are reached in July, then spray the next generation in late August or September when soft gray beetle are first found in the field.

bean leaf beetle thresholds

 

 

Table 2. Economic thresholds for first-generation bean leaf beetles (average number of beetles per 20 sweeps. Note: sweeps should be taken the length of the row, not across the row). If these numbers are reached in July, then spray the next generation in late August or September when soft gray beetle are first found in the field.
bean leaf beetle table 2

(This article originally appeared on pages 90-91 of the IC-492)

 

 

Jon Tollefson is a professor of entomology with extension and research responsibilities.  Marlin E. Rice is a professor of entomology with extension and research responsibilities.

 

Is that Common or Southern Rust Showing up in Iowa Fields?

By Alison Robertson, Department of Plant Pathology

Is that common or southern rust showing up in Iowa fields?  Chances are it is common rust. It has probably been too cool and wet so far this growing season to see southern rust. Symptoms of the two rusts are very similar but there are subtle differences.

Location of pustules. Pustules of common rust (Figure 1) can be found on both the upper and lower leaf surfaces, compared with southern rust when pustules are usually only found on the upper leaf surface (Figure 2 and 3). Care should be taken though when using this characteristic for diagnosis since early symptoms of common rust may appear to be only sporulating on the upper leaf surface. 

Color of pustules. Common rust pustules are brick red in color; southern rust pustules are more orange brown in color (Figure 4).  Admittedly this can be difficult to distinguish unless you are familiar with the two color types.

Shape of pustules. Common rust pustules tend to be more elongated than the pustules of southern rust which are usually more round. In addition, common rust pustules are usually sparsely scattered over the leaf surface, while southern rust pustules tend to be more densely clustered.

Favorable conditions. Cool (61 to 77 degree Fahrenheit), wet conditions favor common rust. In Iowa we usually start to see common rust showing up on corn towards the end of June. Then as we really get into summer (hotter than 80 degrees Fahrenheit), disease development slows down. Southern rust is favored by warm (77 to 82 degree Fahrenheit), humid conditions and so it is usually mid- to late-August before we see southern rust in the state.   

Management. Foliar fungicides are effective against both common and southern rust, however it is rare for common rust disease severity to be high enough and southern rust to occur early enough in the growing season in Iowa to warrant an application. 

Additional information regarding rust diseases of corn is available in a publication from the University of Nebraska, Rust Diseases of Corn in Nebraska.

common rust in corn

Figure 1. Elongate, brick red pustules of common rust

 

southern rust in corn

Figure 2.  Under surface of a corn leaf infected with southern rust – note the absence of pustules.

 

southern rust in corn

Figure 3.  Circular, orange-brown pustules of southern rust on the upper leaf surface.

 

color of common rust and southern rust

Figure 4.  Comparison of common rust (right) and southern rust pustules (left) on a corn leaf.

 

 

Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases.



This article was published originally on 7/28/2008 The information contained within the article may or may not be up to date depending on when you are accessing the information.


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