COOPERATIVE EXTENSION 
INSTITUTE OF AGRICULTURE AND NATURAL RESOURCES         
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Entomology Evaluations
In this issue                                            
May 21 , 2002
Spring Black Stem and Leaf Spot  
Light Trap Counts
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Post Herbicide for Corn  

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SPRING BLACK STEM AND LEAF SPOT

Recent weather conditions have favored the development of spring black stem in the northern third of Nebraska. This disease is "sneaky" in that it can cause significant loss without being noticed. It is also often confused with insect damage. Following is a description of spring black stem.

Cause

Spring black stem and leaf spot is caused by the fungus Phoma medicaginis var. medicaginis. This fungus survives the winter as pycnidia or dormant mycelium in overwintered stem lesions or fallen leaves.

Symptoms occur primarily on stems and leaves but seedpods, crowns and upper taproots also may become infected. In spring small black spots develop on leaves, petioles and stems of new shoots. The leaf lesions are irregular, and enlarge and merge until much of the leaflet is covered Infected leaves yellow and drop from the plant. Lesions on stems and petioles turn black. As stem lesions enlarge and merge, most of the stem becomes black. If the stem is girdled by the advancing lesions, it will die. The fungus also can invade the plant base and cause the crown and upper taproot to rot.

Conditions Favoring Disease Development

Pycnidia form in abundance in infected stems during late fall and early winter. In spring during wet weather spores are produced inside the pycnidia. As they ooze from the pycnidia, they are splashed by rain onto the leaves, petioles and stems of newly emerged shoots. The new shoots become infected as they grow through the residue from the previous year's crop. Some spores may be spread by wind and insects. The foliage must be wet from dew or rain for infection and spread to occur. The disease continues to move upward in the canopy if cool, wet weather persists through spring.

The fungus also is carried in the seed. A seedling blight can occur when infected seed is planted. The disease will often spread from these seedlings to nearby healthy seedlings. Early seedling loss creates stand establishment problems.

Spring black stem and leaf spot is usually a problem only on the first cutting. Regrowth from this cutting may become infected but disease severity is generally light. If environmental conditions between mid-April and mid-June are favorable (cool and wet) for disease development, plants may become defoliated before cutting. When severe, as much as two-thirds to three-fourths leaf defoliation can occur which results in the first cutting being primarily stems. This not only reduces yield but significantly lowers the quality of the hay.

Control

Early cutting is recommended if spring black stem and leaf spot is prevalent on the lower portion of plants. Scout fields weekly in spring to determine the extent of disease development. Decide whether to cut early based on the results of scouting fields and weather forecasts. Don't delay the decision to cut early, otherwise most of the leaves may have been lost by harvest. Ideally, one-tenth bloom is the optimum growth stage for harvesting while maintaining forage quality and limiting premature leaf loss due to disease. If the field is recovering from winter injury, it may be necessary to delay the first cutting at the risk of foliage loss to spring black stem and leaf spot.

Alfalfa stands are sometimes burned in early spring to control alfalfa weevil. This practice also has been shown to reduce the inoculum of spring black stem. Grazing the aftermath after a hard freeze in the fall, which is frequently done in Wyoming and Nebraska, also should reduce pathogen inoculum.

Using certified seed produced in arid areas will insure maximum stand establishment and reduce the chance of seedling blight caused by the spring black stem fungus.

Confusing Black Stem with Insects

There is a tendency to confuse black stem with insect damage. Alfalfa weevils are usually the first suspect, but there is should be no confusion on what is causing defoliation. Alfalfa weevils are smallish green worms with a light stripe down the back and will feed on the top, newly emerging leaves, causing a shothole appearance. Heavy damage will cause the tops to look frosted as the damaged leaf tissue turns brown. Weevils should be easily found with a sweep net or bucket. Black stem starts on the lower portion of the plant and works its way up. Potato leafhoppers also work on the upper leaves as well as the bottom leaves, and will leave a characteristic "v" shape feeding pattern of the leaves.

Regardless of what is currently affecting alfalfa, at this time the best solution is to take a cutting and scout the regrowth. It is NOT necessary to treat with an insecticide immediately after the hay is removed. Give the field time for normal greenup and look for insects. If insects are holding back the regrowth, then spray. (KJ)

SELECTING THE RIGHT POST HERBICIDE FOR CORN

List of Post Emergence Corn Herbicides (pdf)

With improved growing conditions hopefully arriving this week, it won't be long until many producers are considering their postemergence weed management options. Consider several factors when choosing a postemergence herbicide. First, consider the efficacy of the particular herbicide on the weed species present. Obviously, some herbicides provide better control on some weeds than others. Choose a herbicide that will provide the control you desire. Second, make sure you consider crop safety and timing of the herbicide application. For example, a certain herbicide has good activity on many grass and broadleaf weeds but shouldn't be applied to corn over 12 inches. All herbicides carry some type of timing restriction and pushing that limit can easily result in crop injury or reduced weed control.

Often, efficacy is influenced by the rate used. Choose a herbicide that allows you to use the required rate for different weed sizes. For example, a rate of 24 ounces per acre of Roundup Ultra will do well on most velvetleaf plants in the 1-3 inch stage, however, if you are dealing with 4-8 inch weeds, the rate should be increased to 1 quart per acre. Use caution when increasing herbicide rates since this can also increase the potential for crop injury.

Finally, follow label recommendations for additives. Many labels will suggest adding crop oil or AMS to enhance herbicide uptake or movement into the plant cell. Most postemergence herbicides will call for an additive of some sort to enhance activity. As always, read and follow label recommendations and restrictions for maximum herbicide efficacy and crop safety. (See Table 1). Brady Kappler, Weed Science Educator

WHEN TO APPLY POST-EMERGENCE HERBICIDES

As the season progresses, not only your corn grows, but the weeds grow as well, competing with the crop for light, water and nutrients. The longer the weeds compete with the corn the greater yield losses expected. The level of crop yield loss will depend on environmental variables and (a) weed species composition within a given field, (b) weed density and (c) time of weed emergence relative to the crop growth stage.

In addition, to decide whether or not weed control is economically worthwhile, there is a need for understanding if a given weed infestation is likely to reduce yield if left uncontrolled. This establishes the rationale for introduction of the concept of critical period of weed control (CPWC). The CPWC is a period in the crop growth cycle during which weeds must be controlled to prevent yield losses. Weeds that emerge before or after this period may not present a threat to crop yields. This information is essential in making decisions on the need for and timing of weed control and in achieving an efficient use of herbicides.

Research at the University of Nebraska has shown that each crop has a CPWC during which weeds must be controlled to maintain maximum yields. However, we also concluded that the length of such critical period is influenced by the cropping practices, for example by the nitrogen level in corn.

CPWC in dry-land corn as affected by nitrogen: Studies were conducted in 1999 and 2000 at Mead and Concord. Predominant weed species at both locations/years were velvetleaf, common waterhemp and green foxtail, with the densities ranging from 80-120 plants per square yard. Nitrogen was applied immediately prior to planting as 46-0-0 and incorporated within one hour after application

CPWC in corn was affected by the level of nitrogen fertilizer. Generally, a reduction in nitrogen fertilizer resulted in a longer CPWC, thus corn was the less tolerant crop to weed presence. For example, at zero N level, CPWC ranged from approximately 1st to 11th leaf stage of corn, based on a 5% acceptable yield loss (Table 1). This suggests that when no N-fertilizer is applied, the timing of weed control measure should start early in the season (at the 1st leaf stage of corn) and needs to be maintained through the 11th leaf stage, approximately the time of crop canopy closure.

Table 1: Critical period of weed control in corn based on 5% yield loss expressed as crop leaf stage (eg.V1) and days after crop emergence as affected by the level of nitrogen fertilizer.
Nitrogen-Level Time to Control Weeds Time to Control Weeds
lbs / acre Corn leaf stage Approximated Days After Crop Emergence
N = 0 V1 - V11 8-45
N = 55 V3 - V10 10-42
N = 110 V4 - V9 15-39
N = 210 V6 - V9 20-39

This data implies that an increase in N fertilizer delayed the timing of weed control and increased the corn tolerance to weed presence. From a practical standpoint, an insufficient N can reduce corn tolerance to weeds and it can widen the window of a CPWC. Furthermore, from a nitrogen restriction-use and a regulatory perspective, anticipated restrictions on the level of N use in corn may require more intensive weed management programs.

Cost of delaying weed control in corn crop: A common question among producers is "how much is it going to cost me if I delay weed control". In order to answer such question we graphed the yield loss data against the crop growth stage at the time of weed removal (Figures 1 ). In a practical situation one may decide to select , for example, 2%, 5% or 10% yield loss to signify the beginning of the critical period (time of weed removal). This range will allow to adjust CPWC depending on the risk one is willing to take. In our study, an arbitrary level of 5% yield loss was used to determine the beginning of CPWC in both crops (see the 5% yield-loss-line at the Figures 1).

In order to determine the cost of delaying weed control, the curve above the arbitrarily selected point (the beginning of CPWC) should be used. For example, if an arbitrarily selected point of CPWC is 5%, the 5% yield loss will occur if the weeds are removed at the 2nd leaf stage in 0-N-level (Figure 1). Delaying weed control to the 3rd leaf stage will cause about 7% yield loss, in essence costing producer a 2 % of yield loss. Similar trend is observed for the later leaf stages at each of the four curves (Figure 1).

Therefore, we conclude that delaying the time of weed removal, after the starting point of CPWC will cost a producer an average of 2 % in yield loss per every leaf stage of delay . This recommendation is applicable up to canopy closure in corn (about 11 fully developed leaves).

To determine the actual economics of the cost of delayed control, the producer will have to convert the percentage yield loss of the actual target yield on his farm. For example, if a target yield for corn is 100 bushels per acre, delaying weed control for every leaf stage of crop will cost producers about 2 bushels per acre of yield (thus 2% of 100 bushels per acre). In terms of actual economic loss, it will be about $4 per acre for every crop leaf stage of delay, assuming a price of $2 bushel for corn.

Weed Size: Weed size at the time of weed control measure is another concern. In the corn study, the weeds were about the same size as the crop at the time of their removal except for the Mead site in 2000. If the weeds are taller than corn they will shade the crop so the control should be initiated 4-5 days (1-2 leaves) prior to the beginning of CPWC. If the weeds emerge 5-8 days after the crop they will not shade the crop that early in the season so the control can be initiated 5-10 days (2-3 leaves) after the beginning of critical period, as it is shown with the later start of the CPWC at Mead in 2000.

The size of weed species will effect the herbicide use rates too, especially the rates of Roundup in Roundup-Ready soybeans. It is well known that Roundup has much better activity on grassy than broad leaf species. Therefore the rates of 16 to 24 oz should provide control of most common annual grassy species (foxtails, barnyardgrass, field sandbur, woolly cupgrass, panicums) that are 3-8 inches tall. The same rates should control annual broadleaves (velvetleaf, lambsquarters, pigweeds, mustards) that are less than 6 inches tall. For taller grasses and broadleaf species a full rate (32 oz) will be required. Higher rates of Roundup (40 oz - 60oz) will be needed to control species such as ivy-leaf morning-glory, sweet clover, field bindweed, Venice mellow and various smartweeds (lady's thumb, Pennsylvania smartweed, wild buckwheat, etc).

Practical use of the CPWC and timing of weed control in herbicide tolerant crops: A generally sound strategy, for example in Roundup-Ready corn will be to apply Roundup tank-mixed with a residual herbicide at the beginning of the critical period, which will provide adequate weed control the entire critical period. In order to select appropriate herbicide mixtures for the weed spectrum at your farm, we suggest to consult the herbicide efficacy tables from the Guide for Weed Management in Nebraska (Extension Publication, EC-130). (SK)



Figure 1: Corn yield loss and beginning of CPWC as influenced by the timing of weed removal and N-rate

Keith Jarvi
Editor & IPM Extension Assistant  		 Charles Shapiro
Extension  Soils Specialist 		 Tom Hunt
Extension Entomologist
William Kranz
Extension  Irrigation Specialist		 Stevan Knezevic
Int. Weed Management Specialist		 Dick DeLoughery
Ext. Non-Pt. Pollution Program
Brady Kappler
Weed Science Educator
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