In This Issue. Insects, Mites, and Nematodes. August 3, No. 20. Purdue Cooperative Extension Service. Insects, Mites, and Nematodes

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& August 3, 2001 - No. 20 In This Issue Insects, Mites, and Nematodes Late Planted and/or Maturing Corn, Watch Out!

1 & August 3, No. 20 In This Issue Insects, Mites, and Nematodes Late Planted and/or Maturing Corn, Watch Out! Monitor Soybean for Rootworm Beetles Now Black Light Trap Catch Report Weeds Poisonous Plants Reprise Plant Diseases Corn Anthracnose Diplodia Ear Rot Gray Leaf Spot Sudden Death Syndrome Weather Update Temperature Accumulations Insects, Mites, and Nematodes Late Planted and/or Maturing Corn, Watch Out! (John Obermeyer, Rich Edwards, and Larry Bledsoe) Earworm and corn borer moths are flying Late developing corn may attract moths Because of ear damage concerns, seed and sweet corn are at greatest risk Suddenly there s a surge of moth activity that should concern producers with corn that is late in development. Corn earworm and second generation European corn borer moths are becoming quite numerous in pheromone and black light traps. Both of these species are attracted for egg laying to fields that are actively pollinating. This is especially a concern in late maturing seed corn or late-market sweet corn fields. Monitoring for these pests in these fields should commence immediately. Though sampling schemes for these two species differ, they both involve monitoring for moths and eggs, which is tedious and time consuming. Waiting for larval emergence and damage to show up is too late! Though we have not heard much about fall armyworm as of yet, it too will be attracted to later developing corn. High risk fields are typically bottomland fields that have been planted extremely late. Fortunately, fall armyworm damage is almost always spotty in fields, which means it usually looks worse than it really is. Purdue Cooperative Extension Service

2 Monitor Soybean for Rootworm Beetles Now - (John Obermeyer, Rich Edwards, and Larry Bledsoe) - Western corn rootworm beetles are active throughout Indiana Sampling soybean fields now for beetle presence and densities could save money next year Sampling doesn t require the use of nets or traps, visual inspections can be effective in assessing relative beetle numbers Ron Blackwell, IPM Surveyor, is actively sweeping soybean fields to sample for western corn rootworm beetle populations. Stay tuned to future Pest&Crop issues as we report the catches by county. Pest managers have been reporting the western corn rootworm variant making its move from corn to soybean fields in many areas of Indiana. Knowledge of beetle numbers in soybean helps one to gauge the potential risk of rootworm damage to next year s corn. Few beetles mean low risk, thus little need for rootworm protection next year. Lots of beetles means higher risk, and the insecticide insurance will likely pay off. Several producers and agribusiness personnel throughout the state have initiated a sampling program for their soybean fields. Because of the variability of beetle numbers from field to field, those willing to inspect soybean now may reap the benefits next spring. Sampling for rootworm beetle in soybean fields does not require sticky traps or sweep nets, but they do make decision making more accurate. Visual inspections during the morning hours, while walking through the field and carefully observing the upper canopy, should help you reach a management decision. Soybean fields should be visited weekly until early September. A full color PDF fact sheet on monitoring soybean with yellow sticky traps is available on-line at < Black Light Trap Catch Report (Ron Blackwell) County/Cooperator 7/17/01-7/23/01 7/24/01-7/30/01 VC BCW ECB GC CEW FAW AW VC BCW ECB GC CEW FAW AW Clinton/Blackwell Dubois/SIPAC Jennings/SEPAC LaPorte/Pinney Ag Center Lawrence/Feldun Ag Center Randolph/Davis Ag Center Tippecanoe/P. J. Boeve 7 Whitley/NEPAC BCW = Black Cutworm AW = Armyworm ECB = European Corn Borer FAW = Fall Armyworm GC = Green Cloverworm CEW = Corn Earworm VC = Variegated Cutworm August 3, 2001 Page 2

3 Plant Diseases Poisonous Plants Reprise (Glenn Nice and Thomas Bauman) - We have been getting some phone calls about problems with toxic plants. Many of the plants regarded as weeds in pastures or roadsides can cause pathology in livestock and pets. Often the toxicity of a plant is only exhibited in certain conditions, or with certain plant parts. Below is a list of some of the more common toxic plants covered in Indiana Plants Poisonous to Livestock and Pets (WS-9) that can be classified as weeds. To access this publication I welcome you to visit this web address, < toxic/cover1.htm>. Weed Animals reported Weed Animals Reported Common burdock All types Poison - hemlock Cattle, dogs, goats, swine, horses, sheep, poultry Common Cattle, swine, sheep, Field horsetail, Horses, rarely cattle and sheep cocklebur poultry scouringrush Common tansy All types Buttercups All livestock Ergot infested Livestock Stinging nettle Shorthaired hunting dogs small grains Cultivated Oats Cattle, swine, goats, poultry, horses Redroot pigweed Cattle, goats, sheep, swine Brackenfern Cattle, horses, sometimes swine Wild mustard Cattle, horses, sheep, poultry Ragwort, groundsel Cattle, horses Pennycress Cattle, horses, sheep, poultry Milkweeds Cattle, goats, horses, Jimsonweed Cattle, swine, sheep, dogs, sheep, and poultry cats, poultry Spotted water- All livestock, especially Spurges Cattle, sheep, especially horses, hemlock, spotted cattle dogs, cats, caged birds cowbane Common St. Cattle, sheep, goats, horses Pokeweed Cattle, swine, sheep, sometimes Johnswort goats and horses Nightshades, Calves, goats, sheep, Wild black cherry Cattle, horses, dogs, goats, horsenettle swine, poultry swine, cats, caged birds August 3, 2001 Page 3

4 Weeds Corn Anthracnose - (Gregory Shaner) - Will anthracnose stalk rot be a problem this year? Last year, anthracnose stalk rot was widespread on corn in Indiana. Will we have a problem again this year? We can look to that much-loved metaphor (at least by plant pathologists), the disease triangle. This metaphor emphasizes that in order to have a serious plant disease outbreak, conditions on three sides of the disease triangle host, pathogen, and environment must be appropriate. We certainly have an abundant host corn. And, based on what was seen last year, many hybrids are susceptible to anthracnose late in the season. The pathogen side of the triangle is what may be quite different from last year. The anthracnose fungus (Colletotrichum graminicola) survives in corn residue. Evidently there was enough of the fungus around last year to infect a lot of corn. This year, the fungus may be even more abundant, because residue of all of that corn that had anthracnose stalk rot last fall will contain the fungus this year. It seemed to me that the early season leaf blight stage of anthracnose was more common this spring than usual, which would support the idea that there is abundant inoculum this year. The other side of the triangle is weather. Anthracnose is favored by wet, cloudy weather. We ve certainly been having that. Moisture favors production of spores and infection of plants. Cloudy weather seems to render plants less resistant to infection. Colletotrichum graminicola can infect stems of living plants. It is thought that spores of the fungus produced on leaf lesions may be washed by rain down between the leaf sheath and stalk where they can infect. Anthracnose leaf blight on young plants in the spring is not particularly damaging, but establishes the fungus in the plant, setting the stage for a stalk rot problem later. Wounds, such as caused by corn boring insects, are another important avenue of stalk infection. Infected stalks may show dark lesions on the surface of the rind. If a knife is used to shave into these dark areas, the discoloration will usually extend through the rind. Once through the rind, the fungus can invade the pith and cause stalk rot. Rotting of the pith may occur in the absence of rind lesions. If the fungus infected via a wound or roots, it can develop internally without conspicuous symptoms on the rind. Colletotrichum graminicola is also a virulent vascular pathogen. The fungus invades the xylem. Spores produced in this tissue may accumulate in xylem vessels at the nodes and plug them. The interference with movement of water to the top of plant can lead to the top kill symptoms seen in some years. The upper leaves will turn brown while the middle canopy is still green. I am not prepared to predict that we will have a problem with anthracnose stalk rot again this year, but I suspect that inoculum is abundant. If wet and cloudy weather continues during August that would seem to set the stage for the stalk rot phase of anthracnose. Diplodia Ear Rot - (Gregory Shaner) - Will 2001 be a repeat of 2000? Diplodia ear rot was another late season disease of corn that was widespread last year. Diplodia ear rot is caused by a fungus now known at Stenocarpella maydis (formerly Diplodia maydis), and like the anthracnose fungus, S. maydis survives on corn residue. Stenocarpella maydis produces spores on crop residue under warm, humid conditions. The fungus can infect stalks through the crowns or roots. Ear infection may occur via silks or via husks. Shanks may also become infected, from where the fungus can invade the ear. Last year, I saw severe Diplodia ear rot in a field on August 1 st. Many ears had been infected through husks. Infected husks had tan lesions near the base of the ear. Even at that time, when ears were in the milky ripe stage, the fungus had progressed through the husk tissue of some ears and invaded the kernels. A white mat of fungal mycelium was evident over much of the ear. The same argument that I made about anthracnose stalk rot would apply to Diplodia ear rot. The abundance of ear rot last year means that there is probably a lot of the fungus in corn residue this summer. Thus, the risk of Diplodia this year would seem to be higher than normal. The most vulnerable period for ear infection is for about 3 weeks after silking. It is thought that spores of S. maydis are dispersed mainly by rain splash, and many areas of the state have experienced rains during this period of corn development. Rain splash tends not to move spores over long distances. In contrast, dry spores that are dispersed by wind may be carried very long distances. Because of the limited spread of spores by rain splash, corn fields with corn residue in them or next to them are thought to be at greatest risk of Diplodia ear rot than corn fields with no local source of inoculum. However, I saw a field with severe Diplodia ear rot last year that had no corn residue on the soil surface. Ear rot was severe well into the field, at a considerable distance from the nearest corn residue. This hybrid was evidently very susceptible, but the situation there revealed that a field may be infected even though there seems to be no local source of inoculum. I don t know whether Diplodia will be a problem this year, but now is the time to start scouting fields. This August 3, 2001 Page 4

5 is especially important for fields in areas where Diplodia was a problem last year. Gray leaf spot is a polycyclic disease. Spores produced on corn residue, where the fungus survives the winter, infect new corn in the early summer. Spores on residue may continue to be a source of infection for several weeks. As lesions develop on leaves, these produce spores as well. As the season progresses, lesions on leaves become the main source of spores for infection of upper leaves. Even under favorable conditions, it takes about 2 weeks for a lesion to mature and produce spores. Initially, the increase in numbers of lesions on leaves will be slow, but lesion numbers will tend to increase exponentially, so at some point lesion numbers will increase dramatically. That point seems not to have been reached yet in most corn fields. During the next 2 weeks, we may see a dramatic increase in severity of gray leaf spot. The late development of disease will reduce its effect on grain yield and quality. Another reason for the late and slow development of gray leaf spot may be that corn breeders have indeed made substantial gains in genetic resistance to this disease. Improved resistance and low inoculum levels coming into this season may be what is holding an epidemic at bay, despite very favorable weather. Diplodia ear rot Gray Leaf Spot - (Gregory Shaner) - Where s the epidemic? Weather over the past few weeks seems to have been ideal for gray leaf spot. Hot, humid weather, warm nights, heavy dew, and frequent rainfall are favorable for production and dispersal of spores of Cercospora zeaemaydis and for infection of corn. Gray leaf spot is certainly present, and has been for some time. However, it does not seem to be developing as rapidly as might be expected given the weather we have been having. In the above articles about anthracnose stalk rot and Diplodia ear rot, I argued that the odds are probably more in favor of these two diseases this year than normal because inoculum levels are higher than normal. The same argument, in reverse, may explain why gray leaf spot is not more severe. It has been several years since gray leaf spot has been widespread and severe, so inoculum levels may be low this year. August 3, 2001 Page 5 Sudden Death Syndrome - (Gregory Shaner) - SDS is showing up in more fields In earlier issues of this newsletter, I discussed the likelihood that we would see sudden death syndrome in soybeans this year. Symptoms are now appearing in many fields. Based on my limited windshield survey, most fields do not show symptoms (at least not symptoms that can be seen from the road while driving by at 50 mph). However, a number of fields do have symptoms that can be seen during a fast drive-by. Closer inspection of these fields reveals that diseased plants have not quite reached the stage where leaflets are dropping, but the interveinal chlorosis and necrosis are evident. When I dug up symptomatic plants, I could see small patches of light blue on the surface of the taproot just below the soil surface. These are masses of spores of the SDS fungus, Fusarium solani f. sp. glycines. Cutting into the lower stem revealed the characteristic brown streaking in the cortex, while the pith remained white. At this stage of disease development, it is possible to see some plants in early stages of the disease (small, pale green spots on leaves). Over the next week or two, symptoms will become more severe on these plants. Thus, the number and size of patches of diseased plants in a field will increase.

At this stage SDS does not appear to be as extensive as it was during the 1998 and 2000 growing seasons.

SDS during 1998 and 2000 are probably in corn this year.

There are no remedial measures that can be taken for fields where SDS has shown up.

6 At this stage SDS does not appear to be as extensive as it was during the 1998 and 2000 growing seasons. Most fields that are in soybeans this year were probably in corn during the previous two outbreaks, and most fields that had SDS during 1998 and 2000 are probably in corn this year. Maybe, on average, these odd year soybean fields do not have the inoculum levels that the even year fields have. There are no remedial measures that can be taken for fields where SDS has shown up. Identification of problem fields, and mapping areas where the disease is most severe can provide information for future planting plans. SDS symptoms in field SDS symptoms on leaf Bug Scout How bad will the late season diseases be this fall? August 3, 2001 Page 6

7 Weather Update MAP KEY Location GDD(3) GDD(11) GDD(40) GDD(90) Temperature Accumulations from Jan. 1 to August 1, 2001 GDD(3) = Growing Degree Days from April 14 (3% of Indiana's corn planted), for corn growth and development GDD(11) = Growing Degree Days from april22 (11% of Indiana's corn planted), for corn growth and development GDD(40) = Growing Degree Days from April 28 (40% of Indiana's corn planted), for corn growth and development GDD(90) = Growing Degree Days from May 6 (90% of Indiana's corn planted), for corn growth and development 4" Bare Soil Temperatures 8/1/01 Location Max. Min. Wanatah Wanatah Young America Bluffton Bluffton Lafayette Tipton Farmland Perrysville Crawfordsville Greenfield Greencastle Franklin Terre Haute W Laf Agro Tipton Farmland Crawfordsville Trafalgar Liberty Terre Haute Freelandville Oolitic Milan Vincennes 83 Dubois Boonville August 3, 2001 Page 7

8 Pest&Crop Extension Entomology Office Department of Entomology Purdue University 1158 Smith Hall West Lafayette, IN /5/ Four Year Comparison (Jan. 1 to Date) 8/4/99 8/2/00 8/1/01 Growing Degree Days (GDD) Farmland Terre Haute Tipton Lafayette Wanatah Sand The Pest Management and Crop Production Newsletter is produced by the Departments of Agronomy, Botany and Plant Pathology, and Entomology at Purdue University. The Newsletter is published monthly February, March, October, and November. Weekly publication begins the first week of April and continues through mid-september. If there are questions or problems, contact the Extension Entomology Office at (765) DISCLAIMER Reference to products in this publication is not intended to be an endorsement to the exclusion of others which may have similar uses. Any person using products listed in this publication assumes full responsibility for their use in accordance with current directions of the manufacturer.

STALK ROTS. When to look for: Mid-August to Early October

STALK ROTS. When to look for: Mid-August to Early October Figure 19. Lodging of corn plants due to stalk rot damage. Photo courtesy of MAFRI. Figure 20. Shredding of internal tissues of a corn stalk by a stalk rot pathogen. Photo courtesy of MAFRI. STALK ROTS