Posts Tagged ‘18:8’

2009 Soybean Cyst Nematode Survey Results

Thursday, May 6th, 2010

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

The soybean cyst nematode (Heterodera glycines) is the most limiting biotic factor of soybean production in Delaware. In 1993 and 1994 a major effort was made to survey the soybean acreage for the soybean cyst nematode (SCN) and determine the race composition of the SCN populations present at the time. The Delaware Soybean Board funded this project and the results demonstrated that roughly 60% of the populations that were race tested were race 3 and 30% race 1 and the remainder a mix of races 5, 7, and 9. Since that time Round-Up Ready soybeans were introduced with a single source of resistance to SCN derived from a soybean plant introduction referred to as PI88788. At the time of the first survey we demonstrated a significant yield reduction in one variety trial where race 3 resistant soybeans were planted in a field known to be infested with race 1. This was the first indication that not all race 1 populations could be controlled with a race 3 or 3, 14 resistant soybean variety. For the past 10 years SCN has not been identified as causing much yield loss because symptoms that were seen previously, namely severe stunting and chlorosis, only seem to be present when a susceptible variety is grown or high egg numbers combined with dry weather at planting occurs when a resistant variety is planted. During the 2008 growing season a small number of soybean fields had stunted plants, chlorosis, and SCN was present on the roots. All of these fields were planted with a Round-Up Ready variety with resistance to SCN. The difference in 2008 was that it was dry from planting through the first thirty days after planting. High SCN egg numbers and dry weather early are known to be very detrimental to early soybean growth and can produce stunting, chlorosis and yield loss.

Within the last 5 years there were indications that race 3 is no longer the predominant race. A small set of samples tested here and those sent to other institutions have tested as race 1. Since the majority of resistance in Round-Up Ready soybeans is from PI88788, which allows varying amounts of reproduction of race 1 populations, these varieties may have reduced effectiveness in suppressing current SCN populations. Other control measures may be needed if the current population structure is no longer predominately race 3. No surveys of SCN had been conducted in Delaware since last survey in 1993 and 1994.

The results of the 2009 survey were very informative. No SCN were found in New Castle County even though the previously reported areas were sampled intentionally. In Kent county most of the infested fields were south of Dover. 41% of the samples in Kent County were infested. It was no surprise that all but one sample in Sussex County was infested, resulting in 96% of the samples taken in Sussex were infested. Fifteen samples (43% of the 35 samples that had SCN) were sent to the University of Missouri Extension Nematology Laboratory for race and HG typing. The most significant finding was that no race 3 populations were identified. Race 1 was the most prevalent (47%) followed by race 5 (33%) and two race 2 populations (20%). These results confirmed our suspicions that the race structure in DE has indeed shifted.

 

Mature cyst of the soybean cyst nematodes with 200-250 eggs.

Since more than 90% of the current soybean acreage infested with SCN in Delaware is planted with glyphosate resistant soybeans with SCN resistance from PI88788, it was significant that 100% of those SCN populations could reproduce on PI88788 from 40- 80% (average 67%) compared to a susceptible variety. Thirteen of the populations from the 1993-4 survey had an average female index (FI) on PI88788 of 24%. This means that the current SCN populations that we sampled will reproduce and increase on varieties with PI88788 as the source of resistance to SCN — although more slowly than on a susceptible variety. At what level they will produce on these current varieties depends on the population of SCN present. Will you notice any symptoms on the crop? It will depend on the growing season and the initial number of SCN eggs present in the field.

Managing SCN requires knowing if fields have SCN and how many eggs are present. Growers with increasing egg counts in fields planted with SCN resistant varieties will need to rotate to a non-host crop like corn or vegetables (except snap beans, which are also a host). Soil sampling in the fall after harvest is probably the best time to monitor SCN populations but samples can be taken anytime the soil is not flooded or frozen. When soybeans are grown, do not plant the same SCN resistant varieties in the same field, rotate varieties as well as crops. Although most varieties have resistance from PI88788, there are 4 major genes for resistance and several minor ones. Varieties with resistance from that source can vary in the resistance genes they carry. So there are differences between varieties that have the same source of resistance. Planting high yielding varieties with resistance to SCN when SCN egg counts are low to medium can be expected to perform well under our conditions. The problem is that the nematode populations can be increasing on some of these varieties depending on the SCN population in the field and the crop’s full yield potential may not be met.

Beat the Pest-Take the Test
Soil sampling is highly recommended if you are planting soybeans frequently. Irrigation can mask SCN damage so irrigated fields should be sampled as well to make sure that populations are not increasing.

A workshop on soybean cyst nematode is planned for August 3, 2010.

Agronomic Crop Insects – May 7, 2010

Thursday, May 6th, 2010

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Alfalfa
If economic levels of alfalfa weevil are present before harvest and you cut instead of spray, be sure to check fields within one week of cutting for damage to the regrowth. If temperatures remain cool after cutting, there is often not enough “stubble heat” to control populations with early cutting. In some cases, damage to re-growth can be significant. A stubble treatment will be needed if you find 2 or more weevils per stem and the population levels remain steady. The first potato leafhoppers have also migrated to our area so be sure to sample within a week of first cutting. In addition, new seedings should be watched carefully since leafhoppers can quickly damage these plantings. Once the damage is found, yield loss has already occurred. The treatment thresholds for leafhoppers are 20 per 100 sweeps on alfalfa 3 inches or less in height, 50 per 100 sweeps in 4-6 inch tall alfalfa and 100 per 100 sweeps in 7-11 inch tall alfalfa.

Field Corn
Be sure to watch for cutworms and slug damage on newly emerged plants. Even though it has been warmer and dry compared to last season, the cooler, wet weather predicted this weekend in combination with the presence of slugs under no-till residue can still result in economic slug damage on newly emerging seedlings. So far, we only know of one field that was treated last week for slugs. Deadline M-Ps should be considered if the weather remains cool and wet and damage is present and/or increasing. The following link provides good information on slug biology, scouting and management of slugs on field crops. http://ohioline.osu.edu/ent-fact/pdf/0020.pdf

Small Grains
Be sure to watch for the movement of aphids into grain heads. In many cases, beneficial activity is still not high enough to take care of populations moving from the lower canopy of the plants into the grain heads.

Economic levels of Cereal leaf beetles continue to found in fields throughout the state. Since there were 2 periods of egg laying, we can often find larger larvae up on the flags leaves and in many cases higher numbers of smaller larvae still in the lower plant canopy. Depending on the temperature, newly hatched larvae will feed for up to 3 weeks. Research from Virginia and North Carolina indicates that the greatest damage can occur between flowering and the soft dough stage so continue to sample carefully for this insect. The treatment threshold is 25 eggs and/or small larvae per 100 tillers. Treatment is suggested when the egg threshold is reached and more than 50% of the sample consists of larvae, i.e. 50% egg hatch.

Wheat and barley should also be sampled for sawfly and armyworm larvae. Armyworm larvae are nocturnal so larvae are generally found at the base of the plants during the day. However, during cool, cloudy weather, you may also see them feeding on the stems during the day. As a general guideline, a treatment should be considered if you find one armyworm per foot of row for barley and 1-2 per foot of row for wheat. Since sawflies feed on the plants during the day, small sawfly larvae can often be detected early using a sweep net. However, there is no threshold for sweep net samples. Once sawfly larvae are detected, sample for larvae in 5 foot of row innerspace in 5-10 locations in a field to make a treatment decision. You will need to shake the plants to dislodge sawfly larvae that feed on the plants during the day. As a guideline, a treatment should be applied when you find 2 larvae per 5 foot of row innerspace or 0.4 larvae per foot of row. If armyworms and sawflies are present in the same field, the threshold for each should be reduced by one-half.

Timber Rot, White Mold, Sclerotinia Rot in Spring Greenhouses and High Tunnels

Thursday, May 6th, 2010

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

The fungus Sclerotinia sclerotiorum along with other Sclerotinia spp. cause disease on hundreds of plant species, including most vegetables. Diseases caused by Sclerotinia, such as timber rot or Sclerotinia rot are becoming very serious problems in vegetables grown in greenhouses and high tunnels. The diseases overwinter in the soil as large resistant sclerotia, which multiply during years of susceptible crop production. However, even when a high tunnel is moved between seasons, the disease can be severe because the fungus overwinters both in and around the greenhouses and tunnels. The primary source of inoculum may originate inside as well as outside these structures. In the spring when the soil is moist, the fungal fruiting bodies emerge and spores (ascospores) are released. These ascospores will be released continually throughout the spring and are carried throughout the house if originating inside the structure. If the ascospores originate outside the house, they are carried on wind through the doors, vents or raised sides of nearby structures. Ascospores are usually carried or dispersed less than 330 feet. Therefore, it is especially important to use sanitation within 330 feet of a greenhouse or high tunnel. No plants, leaf clippings, potting mix, or soil from the greenhouse or high tunnel should be discarded within this area.

Inside the greenhouse or high tunnel, improve air flow in and around the plants by increasing in-row spacing and trellising plants to reduce disease incidence. Conversely, the proliferation of leaves near the soil will increases disease. The biocontrol Contans has been effective in managing Sclerotinia diseases in the field. The active ingredient of Contans, Coniothyrium minitans, parasitizes the overwintering (or surviving) sclerotia. If Contans is sprayed on the area around the high tunnel and watered into the soil, it may help reduce ascospore formation in future years. Contans must sprayed long before disease development occurs (2 months) to be effective within a crop year. Because the product is living, handle it carefully prior to use. Contans would be a good choice to try in fields or areas around greenhouses and high tunnels that are used repeatedly for a susceptible crop. See the Contans label for additional information.

Other fungicide products labeled for Sclerotinia in the greenhouse are Botran and Terraclor. Maximizing fungicide coverage to plants during application is important. Apply fungicides prior to disease development for greatest efficacy. Keep in mind that the fungus becomes established on senescing tissue first and then colonizes the plant. See the Botran and Terraclor labels for information on individual vegetable crops.

The black sclerotina on the small tomato fruit will overwinter and result in ascospore formation in future years. The fruit should be either buried or discarded more that 330 feet from the high tunnel.

Managing Lima Bean Fields Infested with ALS-Resistant Pigweed in Delaware

Thursday, May 6th, 2010

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu and Gordon Johnson, Extension Fruit & Vegetable Specialist; gcjohn@udel.edu

Pigweed is one of the most wide-spread weed species in Delaware and the region, infesting vegetable crops as well as grain crops. Pigweed is capable of quickly becoming the dominant species in a field due to its high seed output, producing over 100,000 seeds per plant. A number of herbicides are effective on pigweed species if applied in a timely fashion. However, there are pigweed biotypes that are resistant to some of the most commonly used herbicides in commercial agriculture. One such group of herbicides is Group 2 (ALS-inhibiting) herbicides. This group includes the SU and IMI herbicides. SU and IMI herbicides refer to the class of herbicides, sulfonylurea and imidazolinone herbicides, respectively. ALS stands for acetolactate synthase inhibiting herbicide, since these herbicides inhibit the function of this plant enzyme. Herbicides in this group include: Pursuit, Raptor, Sandea, Accent, Matrix (or Resolve), FirstRate, and others.

In lima beans, Pursuit, Raptor, and Sandea are labeled and have been extensively used for broadleaf weed control. Preliminary surveys conducted by UD have found that many lima bean fields infested with pigweed at harvest often are infested with Group 2 (ALS) resistant pigweed.

A number of inter-related issues have resulted in the development and spread of Group2 (ALS herbicide) resistant pigweed in Delaware. ALS herbicides were commercialized in the mid-80s, and therefore have been used for over 20 years. ALS herbicides have been used extensively in soybeans. Prior to the development of Roundup Ready soybeans, Group 2 herbicides were the most widely used soybean herbicides. Also, they were commonly used as postemergence herbicides in corn. Use of these herbicides is again on the increase in Roundup Ready soybeans for control of glyphosate-resistant marestail. Research in vegetables showed good crop safety and improved weed control, resulting in Group 2 herbicide registrations for vegetables in mid-90s. As a result, Group 2 herbicides soon became the preferred option for broadleaf weed control in labeled vegetables. The use of cultivation has been reduced in the last 10 to 15 years with weed control relying more on herbicides. In fields with poor pigweed control, the fields were often re-treated with applications of group 2 herbicides, which did little to reduce seed production. In addition, there are no labeled postemergence herbicides in other herbicide groups that control pigweeds in lima beans. Fields with Group 2 resistant pigweeds produced seeds that were spread from field to field with equipment.

Soil-applied herbicides currently labeled for lima beans do not provide full-season control of ALS-herbicide resistant pigweed. Dual, Treflan, and Prowl will provide some early season control of resistant pigweed, but do not provide full-season control (control will last 4 to 6 weeks). Basagran is very inconsistent on pigweed, having activity only on plants less than 1 inch tall.

Control Strategies for Group 2 (ALS) Herbicide Resistant Pigweed in Lima Beans:
1) AVOID ALS-RESISTANT PIGWEED WITH CAREFUL FIELD SELECTION. If possible, do not plant lima beans in fields known to be infested with ALS-resistant pigweed. Sandea, Pursuit, or Raptor will not control ALS-resistant pigweed populations. Keep records of fields where ALS-resistant pigweed has been a problem.

2) DEVELOP ROTATIONS WHERE PIGWEED CAN BE EFFECTIVELY CONTROLLED. This is best done with corn or soybeans. Limit use of Group 2 herbicides in rotational crops and plan pigweed control programs around other herbicide groups. Plan rotations carefully since many vegetable crops do not have herbicides available to control ALS-resistant pigweed. While pigweed seeds remain viable in the soil for a number of years, the vast majority of the weed seeds will germinate within the first year.

3) USE RESIDUAL HERBICIDES WITH PIGWEED ACTIVITY. For lima beans, Treflan, Prowl, and Dual Magnum are labeled and have pigweed activity. While they may not provide full-season control, they will suppress pigweed growth and improve the effectiveness of cultivation.

4) PLAN TO CULTIVATE IN FIELDS KNOWN TO HAVE ALS-RESISTANT PIGWEED. Cultivate early, probably twice. Do not set cultivator too deep so that you minimize disruption of the herbicide layer.

5) REDUCE OR ELIMINATE SPREADING SEEDS WITH EQUIPMENT. If you suspect ALS resistant pigweed (or any other hard to control weed) is present in a field, limit seed production. Mowing, tillage, or non-selective herbicides should be used immediately after harvest to stop additional seed production. Be sure to clean the equipment before it moves from suspected field so that the infestation remains contained.

Harvest Aids
In fields with ALS-resistant pigweed escapes, one tool that is available is a wiper bar that applies glyphosate over top of the lima bean crop as a harvest aid to reduce weed biomass going through harvest equipment and reduce seed production. How effective is rope-wicking? Rope wicking (or wiper bars) is a harvest aid, not a viable weed control option. Rope wicking will only kill large weeds, those plants that do not come in contact with the wick will not be controlled. And those that are controlled will have already impacted yields. To improve the effectiveness of rope wicking, be sure to wipe in opposite directions and travel no faster than 2.5 mph for heavy weed densities.

To print this information off in factsheet form, go to http://www.rec.udel.edu/weedscience/FactSheetsWeb/WF20.pdf

Vegetable Crop Insects — May 7, 2010

Thursday, May 6th, 2010

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Asparagus
Asparagus beetles adults can still be found laying eggs on spears. As a general guideline, a treatment is recommended if 2% of the spears are infested with eggs. Since adults also feed on the spears, a treatment is recommended if 5% of the plants are infested with adults.

Cabbage
Continue to scout fields for imported cabbage worm and diamondback larvae. Larvae can be found and sprays will be needed before they move deep into the heads. As a general guideline, a treatment is recommended if you find 5% of the plants infested with larvae.

Melons
Economic levels of aphids can be found in the earliest transplanted fields. When sampling for aphids, be sure to watch for beneficial insects as well since they can help to crash aphid populations. In past years, we have been asked about ants being found near melon transplants. In many cases, if ants are present you should look carefully on the undersides of leaves for melon aphids. Ants are commonly found associated with melon aphids in fields. They are often present in fields to collect honeydew from the aphids and can even hinder predation by other insects. As a general guideline, a treatment should be applied for aphids when 20% of the plants are infested, with 5 aphids per leaf. Foliar treatments labeled for melon aphid control on melons include Actara, Assail, Beleaf, Fulfill, Lannate and Thionex. These materials should be applied before aphid populations explode. The Fulfill label states that the addition of a penetrating type spray adjuvant is recommended to provide optimum coverage and penetration. Admire and Platinum are also labeled at–planting for aphid control.

Peas
Continue to sample fields for aphids. On small plants, you should sample for aphids by counting the number of aphids on 10 plants in 10 locations throughout a field. On larger plants, take 10 sweeps in 10 locations. As a general guideline, a treatment is recommended if you find 5-10 aphids per plant or 50 or more aphids per sweep. When sampling dry land peas, you may want to reduce the threshold, especially if they are drought stressed. Be sure to check labels for application restrictions during bloom.

Potatoes
As soon as plants emerge, be sure to sample fields for Colorado potato beetle adults, especially if an at-planting material was not used. Low levels of the first emerged adults can now be found. A treatment should not be needed for adults until you find 25 beetles per 50 plants and defoliation has reached the 10% level. Low levels of corn borer moths have been caught in light traps. A corn borer spray may be needed 3-5 days after an increase in trap catches or when we reach 700-degree days (base 50). If you are scouting for infested terminals, the first treatment should be applied when 10% (fresh market) or 20-25% (processing) of the terminals are infested with small larvae.

Sweet Corn
Continue to scout emerged fields for cutworms and flea beetles. As a general guideline, treatments should be applied for cutworms if you find 3% cut plants or 10% leaf feeding. In order to get an accurate estimate of flea beetle populations, fields should be scouted mid-day when beetles are active. A treatment will be needed if 5% of the plants are infested with beetles. Also, begin to scout for corn borers in whorl stage plants, especially in fields planted under plastic.