August 13, 2008

In this issue

§ Vegetable insect update

§ Don’t miss the sowing window for your brassica cover crops

§ Regional reports

§ Weather

Next issue September 3

Vegetable insect update

Beth Bishop
Entomology

Several insect pests have been making a comeback in recent weeks. Corn earworm moth numbers increased, especially in southern and western Michigan, although they don’t come close to the hundreds of earworm moths caught during previous years. Weather conditions were conducive for migration of moths into the area and that is reflected in an increase in trap numbers.

European corn borer

The numbers of European corn borer moths caught in pheromone traps for the week of August 5 - 12 decreased slightly in most locations from those of the previous week (see chart), but this is most likely due to cooler weather reducing the activity. According to degree-day totals (base 50), we are in the midst of the second flight. We don’t yet know, however, how large this flight will be, but it will likely last for several more weeks.

Western bean cutworm

Western bean cutworm flight is winding down, according to field crops entomologist, Chris DiFonzo, (see article in the August 7, 2008 Field CAT Alert) some eggs have been found in sweet corn. We now have a resident population of western bean cutworm in the state and we can expect this pest to be a problem in future years.

Aster leafhopper

The percentage of aster leafhoppers infected with aster yellows has increased dramatically throughout the state. This is typical for this time of year. The number of infected aster leafhoppers depends on the number of infected plants, which depends on the number of infected leafhoppers, and this cycle causes an increase in infection in both plants and insects as the season progresses. The latest infectivity rate for aster leafhoppers from Mason and Oceana counties is very high, over 20 percent. Carrot fields in the area have many plants with aster yellows symptoms. The treatment threshold for this area is very low, one to two aster leafhoppers per 100 sweeps. Infectivity rates for aster leafhoppers collected from celery fields in southwest Michigan range from zero percent to six percent. There is no dependable regional pattern to the variation in infectivity rates. In general, the threshold in this area ranges from five to 25 aster leafhoppers per 100 sweeps. If growers are noticing aster yellows symptoms in their fields, they should use the lower numbers as their threshold. For the rest of the state, growers can use thresholds of five to 25 aster leafhoppers per 100 sweeps for celery, 10 to 40 aster leafhoppers per 100 sweeps for carrots, and three to 18 aster leafhoppers per 100 sweeps for lettuce. Again, if aster yellows symptoms are seen in the field, growers should go with the lower threshold. Remember, it takes about three weeks for aster yellows symptoms to develop, so growers should discontinue leafhopper sprays three weeks before harvest.

Aphids

Aphids are showing up in celery. The recent hotter, drier weather is conducive to aphid build-up. Aphid populations can increase suddenly, and often appear close to harvest. There are a number of insecticides registered for control of aphids on celery (See Bulletin E312). It is important to apply insecticides only when necessary, since aphids can quickly develop resistance. Two selective insecticides, Fulfill ® and Beleaf ® act to stop aphids from feeding. Because they are selective insecticides, they do not affect natural enemies. Although it may take several days for the aphids to die, they will not damage plants. Fulfill has a seven-day preharvest interval and Beleaf has a zero-day phi.

Don’t miss the sowing window for your brassica cover crops

Mathieu Ngouajio
Horticulture

Brassica or mustard cover crops offer multiple benefits in cropping systems. These include: erosion control, weed management, soil fertility improvement, pest and disease control and nutrient recycling. In several crops, such as celery, and onions, we have observed a yield increase of up to 15 percent in muck soils. Growers using brassica cover crops have also reported significant improvements in soil drainage and soil quality.

For growers in Michigan (and other regions with a temperate climate) who cannot afford to skip an entire summer to grow cover crops, there are two options for fitting brassica or cover crops into the rotation system (Figure 1). The spring planting usually begins in early April when conditions become favorable for field operations. The fall planting, (technically late summer planting) runs from late July to late August. In Michigan, success of brassica cover crops planted after August 25 has been variable depending on weather. Germination and early growth is severely affected under cool and dry conditions. Potential problems with late planting include low biomass production and overwintering, especially under mild winters, of young plants. The Brassicas are more susceptible to frost at flowering stage and growers relying on frost to kill the cover crops should avoid planting too late. For the above reasons, this is the time to plant your brassica cover crops in order to optimize the benefits.

The seeding rates of some of the Brassica cover crops are indicated in table 1. We have revised our seeding rates after several years of observations and results of a density study conducted this summer 2008. In general we have noticed that under high density, competition between plants (intra-specific competition) was very high and resulted in low total biomass production. We have also noticed that optimum seeding rate varies depending on the cultivar used. For example oilseed radish ‘Diakon’ and ‘Defender’ should not be seeded at the same density. Part of this difference is related to seed size. ‘Diakon’ seed is larger than ‘Defender’ seed. Therefore, with the same weights the number of seed planted is higher for the cultivar ‘Defender.’

Table 1. Suggested planting density of some brassica
cover crops for optimum biomass production.

Species	LB/Acre
Oilseed radish ‘Diakon’	15
Oilseed radish ‘Defender’	10
Brown mustard	6
Oriental mustard	6
Yellow mustard	8
Arugula	5

Regional Reports

1 - SW Michigan Research and Extension Center

Ron Goldy

Weather

Temperatures have been near to below normal for the period with lows from 49°F to 66°F and highs from 69°F to 82°F with most highs in the low to mid-70s. There was a half an inch of rain across the area for the week. Mornings continue to have heavy dew conducive to powdery mildew. Cool temperatures, especially at night, have slowed growth and maturity.

Crop report

Harvest activity continues from all crops with no new problems to report. Squash vine borer numbers have dropped to near zero, but squash bug activity has increased. Very little virus symptoms have been observed on vine crops. Many growers have commented on reduced yields on several crops. Reasons at this time are unclear.

2 – Grand Rapids Area

Bill Steenwyk

Weather

It has been dry in west-central Michigan.  Most areas in the counties surrounding Kent County have received only a few tenths of an inch of rain in the past week and the same for the week prior.  A few areas have received more.  The bright side of this is slower progression of many plant diseases.  Temperatures have been fairly cool, especially at night.

Crop report

On the muck, celery harvest continues at full speed.  Yields are variable, but improving as growers move to fields where the celery was smaller at the time of the early June floods.  Aphid numbers are climbing, especially south of Ottawa and Kent counties.  Aster leafhopper, and aster yellows disease is present in many fields, but usually not to a high degree. 

Onion harvest has begun, with early reports being generally positive.  However, fields subject to the heaviest early summer flooding are not likely to fare as well.  Bacterial soft rot, botrytis leaf blight and neck rot are common in fields that were flooded.  The more recent dry weather, along with protectant copper sprays, have helped prevent spread of the bacterial soft rot. Other foliar diseases such as purple blotch, smudge and stemphylium leaf blight were verified by samples sent to the diagnostic lab. Downy mildew was found in one field. Thrips are present to varying degrees. 

On upland soils, I have seen powdery mildew, phytopthora and angular leaf spot on vine crops.  Cucumber producers are applying preventive sprays for diseases.  At this point, no additional reports of downy mildew have occurred.  Most growers and consultants that I have talked to are positive regarding the cucurbits.  Cabbage harvest continues, with reports of an overall good crop, despite some injury on low-lying fields.  Cabbage looper has been found in some cabbage, broccoli and cauliflower fields. 

Sweet corn harvest continues, with generally good yields, despite some reduction on low-lying soils.   Twenty-four corn earworm moths were trapped  in Kent County.  This is rising from ten last week and nearly none before that.  Very few European corn borer moths were trapped.

5 – Macomb, Lapeer, St. Clair counties

Hannah Stevens

Weather and crop conditions

Field conditions continue to be wet in many areas depending upon soil type and topography. From half an inch to an inch (more in some areas) fell this week with heavy rain on Saturday August 9 and showers on four to five other occasions. Temperatures barely topped 80°F and the maximum temperature for the weekend at the Romeo Enviro-weather station was 68°F.

Crop report

The following are my observations this week in the three-county area.

While I reported last week that the second generation of European corn borers were flying, our sweet corn traps continue to have low numbers, which is good news for wet fields where access might be a problem.

On vine crops, melon and watermelon harvest is well underway and winter squash and gourds are beginning to color. Virus incidence has been low.

On tomatoes, bacterial speck, spot and canker are more troublesome this year than most and are visible on localized areas of fields. Nevertheless, a good crop is being harvested.

Prospects seem good for an excellent onion harvest this year.

I learned of a serious black rot outbreak in cabbage, which occurred several weeks ago. Southern transplants were affected.

6 - Monroe County

Sudeep Mathew

Weather

Weather continued to be dry except for isolated showers in some pockets. The Enviro-weather station in Dundee received a total precipitation of 0.47 inches during the past week. The total estimated evapotranspiration rate for this past week was 1.49 inches. That means loss of water from surface including plant and soil to the atmosphere has continued at high rates for the last three weeks. Soil temperatures ranged from 61°F to 81°F.

Crop report

Sweet corn harvest is continuing with later planted non-irrigated fields now looking dry due to lack of rains over the last three weeks. Although European corn borer moths are visible in the fields, I didn’t catch any European corn borers or corn earworms in the traps during this week.

Pumpkins and squash have made decent growth over the last two weeks, although it’s been very dry with fruits now showing in most fields.  Some color is also visible in earliest fields. Squash bug egg masses have been found in plants. Powdery mildew has increased again this week in many fields. There were no squash vine borers in the trap this week. Musk melon harvest is in progress and will continue during the following weeks.

Cabbage and cauliflower are looking good. Cauliflower is sensitive to water stress at all growth stages, and is responding to drought with slower growth than normal.

Green bell and banana peppers are looking good with uniform and good sized fruit. Green peach aphids have been found in green bell peppers. Blossom end rot has been found on green bell peppers (Photo 1). Sunscalded peppers were also visible in fields.

Processing tomato harvest began over last weekend. Market tomatoes are in sufficient supply with good quality. Etherel application is in progress in many processing tomato fields. Whiteflies have been found in cherry and heirloom tomatoes. Early blight symptoms were found in some fields.

Potatoes continue to be harvested for market and processing.  Alternaria early blight symptoms have been found in few vines. Potatoes are in good demand, especially for processing. There was one variegated cut worm on the trap.

Weather news

Jeff Andresen
Agricultural Meteorology
Geography

An upper air troughing pattern across the upper Great Lakes region will continue as the dominant feature in our weather for the next several days. A weak cool front will pass from north to south across the state Wednesday into early Thursday, (August 13 – 14) setting off scattered showers and a few thundershowers. Rainfall totals will generally remain at 0.25-inch or less where rain falls, with only scattered areal coverage. For a change, the best chances for rainfall will be across western and southern sections of Lower Michigan. Precipitation totals in much of this part of the state have remained well below normal since early July, in some cases less than 25 percent of normal, resulting in increasing levels of moisture stress.

After the frontal passage, high pressure will move into the state for the first half of the weekend, leading to fair and dry conditions Friday and Saturday with persistent northeasterly winds. Another frontal boundary is forecast to approach the state from the north by late Sunday or Monday with the next chance for precipitation. Temperatures Thursday through Saturday will remain at slightly below normal levels, with highs ranging from the low 70s north to low 80s south, and lows from the upper 40s to low 50s north to the upper 50s south. Warmer temperatures are likely by Sunday and into early next week.

Further ahead, the medium range forecast guidance suggests a general continuation of the current upper air pattern. The NOAA Climate Prediction Center 6 ‑ 10 day outlook, covering August 18 - 22, calls for mean temperatures to range from near normal levels across the Lower Peninsula to above normal levels over Upper Michigan and for precipitation totals to range from below normal levels in the western Upper Peninsula to above normal levels across southeastern Lower Michigan. During the 8 ‑ 14 day period, covering August 20 - 26, mean temperatures are forecast to range from near normal mean levels over southern sections of the state to above normal levels north. Precipitation totals during the time frame are expected to remain at near normal levels.