AgAlerts 2007 From Griggs County
By John Swenson, Griggs County Extension Agent

Issue # 6, July 20th, 2007

• Barley Yellow Dwarf Virus in HRSW

• Potato Leaf Hopper in Alfalfa & Soybean

• Heat & Moisture Effects on Corn During Tasseling

• Timing of Pre-Harvest Roundup on HRSW 

Barley Yellow Dwarf Virus in HRSW

    Barley Yellow Dwarf Virus (BYDV) has been showing up in HRSW fields around the county.  This virus is transmitted only by aphids.  Symptoms in wheat include yellowing of leaves, stunting, and usually a reduction in the number of tillers.  These symptoms may appear as small sunken circles in the field due to not all aphids carry the virus.  These areas are where an infected aphid started feeding and producing more aphids.  As they spread out from the infected plant, they carried the virus with them and transferred it to their young.  In some varieties, leaf yellowing with bronzing of the leaf margins and tips can occur.  Leaf tissue at the midrib and sometimes along the veins may remain green longer than the interveinal tissues.  Pre-mature death usually occurs of the discolored leaves during hot, dry conditions.  Infected plant may have smaller heads then healthy plants.  Sterility of the apical and basal spikelets may also occur.  Kernel test weight, number and yield can be reduced depending on the severity of infection.
    Development of BYDV depends on the arrival of aphids carrying the virus from the southern states.  Perennial grasses can serve as an over-wintering reservoir for the virus.  This disease affects barley and is called "red leaf" in oats.  Controls rely on the control of aphids.  Management strategies include the use of tolerant varieties.  Only oats has resistant varieties.  Early seeding to allow for maximum growth prior to aphid migration.  Insecticide used to control early aphids populations, if timed, can reduce the spread of the virus and may increase yield and test weight.  Insecticides have given inconsistent results in trials.

Potato Leafhopper in Alfalfa & Soybean

    Monitor  alfalfa fields after cutting for potato leafhoppers.  Some fields in the county are showing leafhopper damage enough that controls are necessary.  Potato leafhoppers are wedge-shaped and pale green in color.  It is only 1/8 inch long.  Adults are very active, jumping or flying when disturbed.  Both adults and nymphs will run backwards or sideways rapidly.  Damage by leafhoppers is referred to as hopper-burn.  Foliage becomes dwarfed, crinkled and curled.  Small tirangular brown areas appear at the tips of leaves, gradually spreading around the entire leaf margin.  
    Thresholds are based on the number of leafhoppers per sweep when swinging a sweep net in a pendulumlike motion through the tops of the plants.

Insecticides approved for use to control leafhoppers in alfalfa include: permethrin*, Baythroid*, carbaryl, dimethoate EC, Furadan*, , Lorsban, Malathion 57 EC, Mustang Max, , and Warrior*.

Soybeans

    Soybeans are tolerant to leafhopper damage due to the moderate to dense pubescence, or plant hairs on the leaves.  The short plant hairs form a barrier that discourages leafhoppers from feeding and ovipositing eggs on plant tissue.  When feeding does occur, damage is referred to as hopper-burn.  Potential damage by leafhoppers is not fully understood.  Damage would be more likely when drier growing conditions occur.  Thresholds for basing spray decisions is when an average of 5 leafhoppers per plant are found in the vegetative stages, and 9 leafhoppers per plant in the early bloom stages.  Refer to the NDSU Field Crop Insect Management Guide for registered insecticides.
    When monitoring for soybean aphid, do not confuse potato leafhoppers as soybean aphids.  The following describes the differences.

Heat & Moisture Effects on Corn During Tasseling

    Tasseling, silking and pollination is the most critical stage in corn development.  Any type of stress during this stage can have an effect on yield.  Combined moisture-temperature stress during this time can substantially reduce yield.  Separating the effects of these two stresses is difficult.  Days when the mean temperature is above 77 degrees F and daily maximum is above 95 degrees F, regardless of soil moisture conditions, is stressful in corn reproduction.  The greatest yield reduction will occur with high temperature and moisture stress during the silking process.  Nutrient, pest or disease stress at this reproductive stage can also affect yield.  When moisture and heat stress is added, yield reduction can be cut up to 13% per day.  Nutrient stress often happens during drought and high heat conditions due to the soil profile where the nutrient has  been placed is dry and out of the active root zone.
    High-temperature damage to pollination almost always occurs in conjunction with drough stress, rarely by itself.  It is difficult to separate the two.  Temperatures in excess of 95 degrees, especially when accompanied by low relative humidity, can dessicate exposed silks, but not affect silk elongation.  The corn plant has a natural defense by shedding pollen during the early to mid-morning hours before temperature rise to dangerous levels.  Successful pollination can occur during lengthy periods of high temperatures if soil moisture reserves are adequate to meet the plant needs.  In short, if moisture is adequate, high temperatures will not severely impact the yield.   But, if water is inadequate, silk elongation will slow down resulting in delay or failure of the silks to emerge from the ear shoot.  Silks that due emerge, may dessicate rapidly and may become non-receptive to pollen.  Corn enters the period of grain yield determination and is most sensitive to drought stress about 2 weeks before silk emergence.  If you are seeing rolling leaves during the heat of the day, it does not constitute severe drought stress.  Effects on yield occur when leaf rolling begins early in the morning and extends throughout the day up to 12-18 hours per day.  
    At the time of this writing, soil moisture conditions are adequate.  But high temperatures are predicted for the following week.  We will have to see what effect it will have on eventual yields. 

Timing of Pre-Harvest Roundup on HRSW 

    Glyphosate is labeled as a harvest aid only in spring wheat and durum - not barley or oats. Glyphosate at 0.5 to 2 pt/A of a 3 lb ae/gal concentrate controls annual grass, broadleaf weeds, and quackgrass and suppresses Canada thistle in hard red spring wheat and durum. DO NOT apply to barley. DO NOT apply to wheat grown for seed as a reduction in germination or vigor
may occur. Glyphosate drift will injure or kill sensitive plants. 

Glyphosate should be applied after the hard dough stage (30% or less grain moisture) of the wheat and at least 7 days prior to harvest by air or ground in 3 to 10 gpa spray volume. See label for adjuvant use. Always add AMS at 8.5 to 17 lb/100 gallons of water. AMS increases control of annual and perennial weeds and especially control of weeds stressed by dry weather. AMS also eliminates antagonism from ions and carbonates in hard water. DO NOT use AMS in place of an NIS. Refer to label for addition of other adjuvants

Glyphosate can be tank mixed with 2,4-D for additional broadleaf control. A ND 2(ee) label interpretation has been granted allowing use of glyphosate at 0.75 to 2 pt/A + dicamba at 0.25 to 0.5 pt/A for a pre-harvest application to wheat and durum at the hard dough stage after green color is gone from stems. Allow a 14 day PHI. The tank-mix can be applied by ground or air.  (information from the 2007 NDSU Weed Control Guide, section B29)

Please Contact Our Office For Additional Information
E-mail: john.swenson@ndsu.edu
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