Cooler week ahead
Corn leaf striping
Pest monitoring cooperators needed
Arlene stirs the SBR pot

Introduction
A slightly cooler week is forecasted and will bring degree-day accumulations back to below average levels for some planting dates.  The corn and soybean crop benefited greatly from last weeks warmer temperatures, yet excessive rainfall in some areas has hindered field work and plant health in poorly drained soils.  Corn leaf striping has shown up in some corn fields again this year.  The striping is most likely from rapid growth rates in the corn.  Cooperators are needed for insect and disease monitoring projects; please contact me if you would like to help.  All eyes have been on tropical storm Arlene and its potential to spread soybean rust.  Only time will tell whether spores moved north or not.

Weather information
Growing Degree Day Last week northwest IA accumulated nearly 150 degree-days, 21% greater than the average of 124.  If the weather forecast holds true, we should see a slightly below average degree-day week ahead (130 vs. 139).  In the last six weeks, we’ve seen a see-saw of above and below average degree-day weeks.  For a short time last week, all planting dates were at or above normal with degree-day accumulations, but the late April planting dates will fall below normal again this week.   Accumulation and predicted plant phenology stages are shown in Table 1 and Figure 1 and is forecasted through June 12.  More detailed degree-day accumulations by planting date can be obtained at this URL:  http://www.extension.iastate.edu/nwcrops/degree-days-2005.htm

Crop Management
Corn development  Most corn across the region as entered the rapid stalk elongation phase, as is evident by looking area fields as some are close to a full canopy on 30 inch rows.  Corn across much of the region now ranges from the V6 to V9 leaf stage.  Corn planted during the cool April 20 – 30^th period should be at or approaching the V8 stage and past V9 by next weekend.  Corn planted in early April should be at the V9 leaf stage, approaching V10.   

Now that the majority of corn is at or shortly past the V-7 leaf stage, loss of the two lowest leaves may have occurred.  Ear shoots (potential ears) are visible upon dissection of a V-9 plant.  An ear shoot will develop from every above-ground node, except for the last six to eight nodes below the tassel. Only one or two ear shoots develop into a harvestable ear.  By V10, the time between the appearance of new leaf stages will shorten, generally occurring every two to three days. Nutrient uptake through the 8th leaf stage of corn is N = 10%, P = 8%, K = 18% of the years total.  Use the corn development chart (figure 2) to help predict timing of corn V stages.  If you want to figure you own crop development schedule, figure that corn requires about 75 degree-days (base 50) for each new leaf. 

Corn leaf striping has been visible in corn fields for much of the spring and early summer.  This can often be confused with nitrogen deficiency, but most likely is not.  Often times leaf striping occurs from rapid growth of the corn plant, which we’ve seen over the last seven to fourteen days.  Dr. John Sawyer (ISU Extension Soil Fertility Specialist) has indicated that leaf striping has shown up on nitrogen rate plots where nitrogen was applied in excess of 200 lbs/acre and is more prominent on continuous corn plots.  If you find leaf striping in your corn fields, monitor the situation and if the problem doesn’t correct itself, further investigation may be warranted.

Pest Management
Soybean disease survey cooperators needed  The ISU Department of Plant Pathology & ISU Extension has started to sample local soybean fields as part of a statewide soybean disease survey.  Three to five fields will be sampled in each county at stages V2/V3, R1/R2, R4/R5, and R6/R7.  The Extension crop specialists will be collecting most of the samples, but some help is needed.  Please contact me (712-792-2364) if you have a field I can sample or if you want to offer your time to help sample fields of your own.  This is going to be a very informative project as we get a better handle on diseases that are affecting soybeans across the state.

Western Bean Cutworm pheromone trap cooperators needed  The time is quickly approaching to begin setting up pheromone traps for the annual flight of the western bean cutworm.  Over the last several years ISU has been increasing its trapping efforts to determine the range and intensity of this potentially damaging pest to corn.  If you would like to learn more about the WBC in your area, then participating in this trapping project could be helpful.  To learn more, please contact me at 712-792-2364 or vagts@iastate.edu

Soybean Rust  The big question on soybean rust this week will be whether tropical depression Arlene carried rust spores to areas north and west of Florida.  The storm did not reach IA, but did impact areas as far north as Indiana and Ohio.  Here is Commentary from the USDA webpage sbrusa.net

…In the wake of tropical storm Arlene's path, the soybean rust model forecasts a swath of soybean rust spore deposition from Florida through all of Georgia, South Carolina, Alabama, Mississippi, most of Tennessee and Kentucky, and the southern half of Louisiana. Marginal spore deposition is possible in the border areas, including extreme eastern Arkansas, southeastern Missouri, lower Ohio Valley, and the central Appalachians. The model does not indicate any further transport of spores north of this area even though weather conditions are favorable due to the limited number of known source areas in Georgia and Florida.

The lack of knowledge of inoculum levels outside the country and weak spore production in the limited number of confirmed U.S. infected sites makes it difficult to assess the level of infection activity on soybean and non-soybean hosts in the immediate future. However, active scouting of potential soybean rust infection areas will commence during the first part of this week. Visitors to this web site are strongly encouraged to pay close attention to the Observation screen to monitor these scouting activities and to follow the status of soybean rust in their local area.

The USDA prediction assumes the source of spore production to be in the Caribbean, which the last APHIS scouting trip did not find any disease in the area, so this forecast may overstate the potential spread of spores.

Another important factor will be to monitor weather conditions in areas outside the current SBR infection sites.  Soybean rust will not develop even if spores were deposited if local conditions (warm and dry) are not conducive for SBR spore germination and infection of the plant.