Wisconsin Fresh Fruit and Vegetable Newsletter
August 1, 2007

Contents:
Fruit Report
Vegetable Report
Peninsular Report
Drought Conditions
Degree Days
Contacts

Fruit Report

Sweet cherry harvest is nearly complete in Door County and Tart Cherry harvest is well underway.  The tart cherry crop this year was excellent.  Early apricots are finished while later maturing cultivars are still available.

The apple crop continues to develop.  Crop estimates around Wisconsin are variable, but most growers are reporting a full crop.  Some few growers have reported slight amounts of hail injury.

Managing orchards during drought conditions.

Well established trees will come through moderate drought conditions acceptably well.  Experience suggests, however, that during drought conditions fruit size (and thus yield) suffers. Following are some steps orchardists can take to reduce the effects of the drought on their farms.

Irrigate.  I grew up in the western U.S., so all the orchards I had seen in my early years were irrigated.  Research suggests that even during moderately dry years apple trees benefit from irrigation.  Further, growers with irrigation turn on irrigation too late.  Waiting until moisture stress is imminent is not good management.  Trickle irrigation is the most efficient and will be the least expensive irrigation to install.  Vendors can design workable systems.  The need to irrigate can be indicated with tensiometers placed at 12 and 18 inch depths in the tree row.

Maintain a vegetation free area in the tree row.  Reducing competition for moisture by weeds will maintain the soil moisture for trees. 

Mowing.  When the plant canopy in the alleyway is kept mowed short less water is transpired into the atmosphere from the soil.  If possible, consider blowing the clippings into the tree row to create a shallow mulch layer.

Young trees.  Trees that have been planted within the past year or two will require supplemental irrigation.  Ideally trickle irrigation was installed when the trees were planted.  If irrigation is not available then trees must be watered by hand.  Each tree should receive 5-10 gallons of water per week depending on the water holding capacity of the soil  Sandy soils will require more water than heavier soils.  If possible, consider mulching young trees to retain soil moisture.

Summer pruning apples. 
I am not an advocate of summer pruning apple trees.  However, for older trees of standard cultivars such as McIntosh fruit color can be improved by summer pruning.  Now is the time to summer prune if it is required or desirable in your situation.  Summer pruning consists of making a few smaller cuts in the upper portion of the canopy.  Also, pruning in early August usually results in less regrowth than when pruning is done in the dormant season or spring.  In my opinion, summer pruning is a last ditch effort to attempt to solve tree architecture problems that should have been resolved by proper training and pruning early in the life of the block.  Summer pruning (as defined here) is not required for trees on fully dwarfing rootstocks planted at high densities as these canopies are naturally shallow so light exposure is not a limiting factor to achieving good fruit color.

2,4-D on strawberries.
The best time to apply 2,4-D to strawberries is at renovation.  If an additional application of 2.4-D is required NOW is the time to make that application.  Applications later than early August can result in misshapen fruit the following year as fruit buds begin to form for the 2008 crop in mid-August as the days shorten.

Pre-harvest intervals.
Harvest of apples and fall bearing (primocane fruiting) raspberries will begin within the next few weeks.  As you consider the need to manage pests on these crops pay close attention to the pre-harvest intervals for these crops.  Tables showing the pre-harvest intervals for tree fruits are available in bulletin A3314 Commercial Tree Fruit Spray Guide and bulletin A1934 Strawberry and Raspberry Pest Management in Wisconsin.  However, be sure to read the product label to make sure you meet the pre-harvest interval requirements.

Teryl Roper, UW-Madison Extension Horticulturist

Vegetable Report

Vegetable Insect Report

Snap Beans - As reported in previous newsletters, problematic populations of potato leafhopper (PLH) continue to remain a threat in commercial snap beans. In the late planted snap bean crop, populations of adult PLH are increasing on these young plants and are nearing thresholds with an average of 0.8 adults / sweep at the Arlington Agricultural Research Station. In mid-season plantings, adult PLH numbers have exceeded thresholds in several areas with populations as high 1.8 adults / sweep in the central sands production area. Resulting injury from adult and immature feeding is described as ‘hopperburn’ with damage to the vascular tissues. Control of these populations can be achieved using a range of synthetic pyrethroids identified in the UW extension bulletin A3422. High populations of these insects continue to persist in the alfalfa crop and the prevailing warm and dry weather conditions are conducive to continued population growth and nymphal development. Crop scouts should continue to monitor for populations of these damaging insects in susceptible crops.

Increased adult captures of European corn borer (ECB) have been observed over the last week and the current conditions are again very conducive to increased dispersal as well as population growth. Elevated adult numbers were observed at several black light trap locations across the state during the last week averaging 10.2 moths / site during this interval. The first summer moths of the ECB will have emerged once 1400 DD50 have accumulated and the first eggs will have been laid at 1450 DD50. The treatment period for the summer generation is between 1550-2100 DD50. This means that the moths should be flying in an area southwest of a diagonal line that extends from just north of La Crosse down to Madison and east to Racine. Adult moths resulting from the second generation become more prevalent when the 1,550 GDD threshold has occurred.

Cucurbits & Melons – Increases in populations of cucumber beetles (striped and spotted) during late June have now resulted in a significant amount of bacterial wilt in susceptible vine crops. Adult beetles which have fed on the foliage and the developing fruit have been responsible for transmitting the bacterium Erwinia tracheiphila, to susceptible crops. In a very short period after an infectious beetle has fed, leaves and petioles will then begin to wilt, and the disease progresses through lateral shoots and eventually affects the entire plant. Cucumbers and muskmelons are affected most severely. Direct control of the insect vector (carrier) is the most effective form of disease control. Insect control should include practices that are not directly harmful to domestic or wild pollinator species of bees. Pest control products that contain the active ingredient carbaryl should be avoided when attempting to control cucumber beetles while conserving pollinators. To minimize the threat of bacterial wilt in the coming season, consider rotating affected vine crops to more distant areas of the farm or onto other management units where possible. Further, to limit the overwintering success of the striped cucumber beetle, practice clean culture with respect to unmanaged fence rows, grassed waterways, or noncultivated acres by mowing vegetation close to the soil before late fall.

Striped cucumber beetle

Spotted cucumber beetle

Tomatoes – Tomato fruit from field plantings are beginning to show up at farmer markets. While foliar diseases appear to be under control, it’s definitely time to be concerned with the management of fruit anthracnose. While the circular slightly depressed lesions with dark centers are diagnostic for fruit anthracnose on ripening fruit, it’s important to know that infection of fruit by the anthracnose fungus can occur anytime during the season. Infections on green immature fruit remain latent until the fruit begins to ripen and soften. Thereafter, the pathogen spreads through the tissues beneath the infection site and symptoms develop rapidly. Thus, control of fruit anthracnose is a season-long challenge. Rotation, mulching and fungicide sprays are all helpful in managing this disease.

Red Beets – Symptoms of Cercospora leaf blight are now common on the beet foliage as small gray lesions surrounded by distinct bright red borders. Often there can be hundreds of lesions per leaf, especially when there is an abundance of inoculum in debris from previous crops. Rotation is very helpful in reducing losses and only rarely are table beets sprayed with fungicide for management of foliar diseases.

Beets have a special need for boron. Insufficient boron in roots leads to a condition known as “black spot” where individual vascular elements in the roots turn brown to black. Roots may be disfigured and cracked and then invaded by soilborne pathogens such as Rhizoctonia solani and Phoma which can lead to decay of roots. Foliage may appear red, a signal of plant stress. This year, black spot symptoms have shown up in some production fields where what would normally have been sufficient amounts of boron had been applied. It appears that very dry soil conditions and lack of irrigation contributed to low boron accumulation in the growing plants and subsequent symptom development.

Cucumbers – Right on schedule, we are seeing samples of bacterial wilt where one or more runners suddenly wilt and die, the result of infection by the wilt pathogen. The bacterial pathogen is vectored by the cucumber beetle. In some areas of the state, there was heavy beetle feeding within the past month. Control of the insect vectors of this pathogen before feeding occurs is essential for control of this disease.

 Russ Groves, Extension Vegetable Entomologist, UW-Madison

Vegetable Disease Report

Cucumbers, Melons, Pumpkins and Other Cucurbits

Disease Alert – This past week, downy mildew was reported on processing pumpkins in central Illinois. In addition, Michigan State pathologists are reporting downy mildew on cucumber in central and southeast areas of their state. Wisconsin growers need to be alert for the first signs of this potentially devastating disease and treat accordingly. Remember that there are two distinct diseases we refer to as mildew – powdery mildew that occurs each and every year and downy mildew that has been a severe problem for the past few years in states to our east and south.  Let’s review the key symptoms - Powdery mildew is recognized by its own distinct signs and symptoms including white powdery growth that occurs primarily on the upper leaf surfaces.  Infected leaves are covered with the white powdery fuzz and eventually wither and die. In contrast, downy mildew appears as small yellow to brown somewhat angular lesions. Affected leaves have a mottled appearance with numerous yellow angular lesions best observed on the undersides of leaves. Also, on the underside of the downy mildew lesions, a dark purplish/gray fuzz often appears during wet or humid conditions. This is most commonly observed in the early morning. Both diseases are capable of rapid spread with downy mildew being the most serious and destructive of the two diseases. The control measures are quite different for the two diseases. For management of downy mildew, registered products include Previcur Flex, Forum, Ranman, or Tanos, each mixed with chlorothalonil or mancozeb. Gavel (already contains mancozeb) can also be used, but does not need a tank mixed companion product. For powdery mildew, consider a long list of materials that includes Flint, Pristine, Nova, Topsin-M, Procure, Quadris, Amistar or Cabrio.

While we have not seen downy mildew in Wisconsin yet this year, protective sprays are suggested at this time, particularly if we get into a rainy and cool period. Suspect samples should be brought in for diagnosis. We should also note that downy mildew did make an appearance in central Wisconsin late in the growing season last year on commercial cucumber fields. I fully expect that we’ll see downy mildew reappear in Wisconsin this year, given its appearance this early in the season in bordering states.

Curcurbit Virus Diseases – Stunted vines, leaf mottling and poor fruit set are all symptoms of virus infection. In the past few years, cucumber mosaic virus has caused widespread losses on a wide range of crops including pumpkins, squash and snap beans. We received the first reports of virus infection this week on late planted pumpkins. Cucumber mosaic virus is spread primarily by aphids in a non persistent manner. Insecticide sprays for aphid control on cucurbits haven’t shown much effect on virus incidence in the past since the aphid vectors carrying the virus are feeding and probing on both weeds and crop plants as they move through an area. Given the wide host range of cucumber mosaic virus that includes many common plants growing in field borders, control of virus transmission to crop plants is very difficult.  Snap Beans – Fields receiving intense rain and blowing soil are at risk from outbreaks of bacterial brown spot. Prompt reaction immediately after a storm by spraying a fixed copper material may help to reduce losses on these injured plants. Breeders have been making excellent progress in developing improved varieties with better resistance to bacterial brown spot. It pays to know the susceptibility of the variety you plant and to treat these fields accordingly.

Walt Stevenson, Extension Vegetable Plant Pathologist, UW-Madison

Drought Conditions

Degree Day Graphic

UW Fresh Fruit and Vegetable Contacts