WISCONSIN FRESH FRUIT AND VEGETABLE NEWSLETTER
June 1, 2007

Apple Scab Makes an Appearance

 Despite the relatively dry spring in many parts of the state, apple scab is showing up. Scab lesions on leaves at this time of year pose a large threat to fruit. Immature fruit are highly susceptible to scab, and this time of year, the weather is still cool enough that scab lesions are producing a lot of spores. Also, new leaves are still being formed, and these are susceptible to scab. Leaves are resistant to scab about 17 days after unfurling; fruit are never fully resistant, but they become less susceptible in mid to late July when a waxy cuticle develops. At this time of year, scab control should be focused on two things: 1. Protecting fruit and 2. Shutting down scab lesions. Captan is the best fruit protectant. Strobilurin fungicides such as Flint, Sovran, and Pristine are also very good, but they are prone to fungicide resistance and for that reason, they should not routinely be applied to sporulating lesions. Shutting down scab lesions was once accomplished with dodine (Syllit, Cyprex). This is still effective for some growers, but in some areas, scab is resistant to dodine. Likewise, the practice of combining a high rate of sterol inhibitor (SI) fungicide (e.g., Nova or Rubigan) with captan is also losing effectiveness in shutting down lesions because of fungicide resistance. Even if scab is not fully resistant to the SI’s, it is not good to spray SI’s to sporulating lesions, because this will lead to future resistance problems. Thus, where scab is resistant to dodine, it appears that using a full rate of captan is the best option to simultaneously protect fruit and shut down lesions. Hot, dry weather is the enemy of scab, but the near term forecast isn’t showing that. So, fungicides will be needed where leaf lesions are visible.

 Patty McManus, UW-Madison Extension Plant Pathologist

One Bad Berry Can Spoil the Whole Batch!

 Leather rot of strawberry is best known for the horrible bitter taste it gives berries and for the potency of that bad taste. A single bad berry can ruin a batch of jelly or jam, and it can really turn you off if you are unlucky enough to eat a bad berry straight up! The Cornell University Tree Fruit and Berry Pathology web site has a good fact sheet:

http://www.nysipm.cornell.edu/factsheets/berries/leather_rot.pdf

The disease is caused by a water mold, Phytophthora cactorum. Leather rot is not a problem where water drainage is adequate, but many strawberry farms in Wisconsin have a few low, poorly drained spots and/or heavy soil. This disease can develop over a wide range of temperatures, with the 60s and 70s F being optimal. The near term weather forecast is for rain and temperatures optimal for leather rot. Growers should familiarize themselves with symptoms, and if symptoms are detected, they should not sell berries from affected areas and take steps to prevent further spread of the disease. A clean straw layer reduces fruit contact with soil and also absorbs spores so that they don’t spread. Effective fungicides include Aliette or related phosphorous acid products, Ridomil, and strobilurin-based fungicides such as Cabrio, Pristine, and Quadris. Check the label for fungicide resistance management information and for pre-harvest intervals.

Patty McManus, UW-Madison Extension Plant Pathologist

Orchard Weed Control During Dry Weather

During dry weather such as is currently being experienced managing the vegetation on the orchard floor is critically important.  Weeds, especially grasses, compete with trees for available soil moisture contributing to water deficit in orchard trees.  Two factors are important to keep in mind regarding orchard floor management.

1.    Spring is the most critical time for fruit development.  Competition during May and June decreases growth and yield far more than weed competition in July and August.  Orchard weed control during August is relatively unimportant.  No vegetation should be allowed to grow within about a 3 foot radius of fruit trees during May and June.  If weeds are present you'll want to tank mix a post-emergent material such as glyphosate or paraquat with a pre-emergent herbicide.  Many pre-emergent herbicides are labeled for use on apples and cherries.  These are described in the Commercial Tree Fruit Spray Guide (A3314).  Now is the time to control vegetation in the tree row if you have not already done so.

2.    Mowing the vegetation between the rows reduces water loss from the soil.  With less leaf area to transpire water use can be significantly reduced simply by mowing the orchard.  Further, the clippings drop to the soil surface and create a mulch that will also reduce direct evaporation from the soil.

Teryl Roper, UW-Madison Extension Horticulturist

Cover Crop Management for Early Summer

Most of you are aware of the benefits of cover cropping and have incorporated or are considering incorporation of cover crops into your system.  Multiple publications have been produced discussing cover crop management including Managing Crops Profitably which was published by the sustainable agriculture network:  http://www.sare.org/publications/covercrops/covercrops.pdf

Considering the weather trends observed over the last month, I wanted to focus on water and irrigation management in fields managed with cover crops.  The lack of precipitation over much of Wisconsin has likely led to drying of soils where cover crops have been or are now growing. 

We have estimated the spring seeded cover crops at Arlington, WI,  used close to 1” of stored soil moisture by May 25 since planting April 20.  Winter cover crops have used close to 3” of stored soil moisture since spring growth began in mid-April.  This represents 25 and 75% of the allowable depletion level for vegetable crops with a 2 foot rooting depth for spring and fall seeded cover crops, respectively.  The allowable depletion level is the water available prior to drought stress occurring in crops.

Soil moisture will have to be managed in fields planted in late May or into June that had cover crops.  Management will depend on time of cover crop destruction, its relative size at incorporation, and date of vegetable crop planting.

Examine moisture content of surface soil closely.  Dry soils should be irrigated prior to or shortly after seeding to insure good germination of seeded crops and soil to root contact in transplanted crops.  In addition irrigation will have to continue to be applied to meet crop needs and replenish soil moisture lost due to evapotranspiration of the cover crop.

If irrigation is not available utilize deep tillage to bring up moist soil from deeper in the profile to insure adequate moisture for germination and early crop growth if less than 30 to 40% of the stored soil moisture has been utilized by the cover crop.  Alternatively, delay planting until adequate precipitation has fallen to replenish soil water to near field capacity.

On a side note, soil temperatures at Arlington over the past week have been in excess of 100 F.  New transplants should be monitored closely.  Light irrigations can help reduce soil temperatures through evaporative cooling if plants are stressed.     

Alvin J. Bussan, Vegetable production systems specialist
Department of Horticulture, University of Wisconsin-Madison

The European corn borer (ECB) flight should be starting this week.  Degree-days for ECB are at 380 as of Wednesday (5/16/07; base temperature 50°F).  First moth catch should occur at approximately 375 degree days.  However, with the rapidly changing temperatures we have been experiencing, keep in mind that ECB moths fly at night and the night-time temperature needs to be at least 55°F or higher for moths to fly. Eggs were expected to have been laid over Memorial Day weekend based on DD accumulations. Egg laying begins when 450 DD₅₀ have accumulated and this has occurred in southwest, south central, west central and parts of southeast and central Wisconsin.

Red turnip beetle - Adults were abundant on the leaves, stems and flowers of hoary alyssum, Berteroa incana, growing in alfalfa fields on sandy soils in Adams County. These distinctive, bright red beetles with three black bands were easily spotted due to their contrasting coloration (see image below). The larvae feed on vegetables and weeds in the mustard family Brassicaceae, including cabbage, turnips, radishes, shepherd's purse and wild mustard. The ½ inch-long adults appear in gardens from late May through June and feed for just two to three weeks. Damage occurs only sporadically in Wisconsin, usually when the beetles migrate into home gardens from nearby fields. Young seedlings and transplants are most susceptible to injury by this defoliator. Handpicking larvae and adults from foliage is the recommended method of control. From DATCP Cooperative Pest Survey Bulletin May 25^th.

Bean Leaf Beetle - This relatively new pest to Wisconsin can cause problems in peas and early snap beans. Warmer winters allow overwintering adults to survive in record numbers. However, overwintering adults is only part of the equation.  The summer generation of bean leaf beetles will contribute to the number of beetles entering the winter. This number may be reduced by pesticide applications used to control other leguminous insects.

Adult beetles cause damage by chewing small holes into the leaves, reducing vigor and yield. They can also cause damage to developing pods causing distortion as well as creating a point of entry for disease organisms. The larvae feed on the roots and nodules as well as the stems below the soil line and may girdle small plants.

For more information visit http://www.uwex.edu/ces/wihort/gardenfacts/X1131.pdf

Early migrants of the Potato Leafhopper have arrived in Wisconsin. The strong winds that blew from the southwest on May 23 & 24^th sped up migration considerably. Snap beans are particularly susceptible to potato leafhopper damage. These tiny wedge-shaped insects feed on plant sap giving the leaves a whitened, mottled appearance. The first symptom of injury is brown triangular lesions at the leaflet tips (hopperburn). Later damage includes stunting and yellowing of the leaves and premature death in severe infestations.

Snapbeans should be monitored for leafhoppers regularly. If you find 1 adult leafhopper per sweep or one nymph per 10 plants, insecticide treatment is necessary.

For more information on the potato leafhopper visit http://learningstore.uwex.edu/Potato-Leafhopper-P558C0.aspx.

Black Cutworms have been noticed doing damage to newly emerged sweet corn.  Cutworms are active feeders that clip many seedlings at or below the soil line in a single night. They prefer crops sown as seed rather than transplants. Susceptible crops include beets, carrots, cucumber, leafy greens, melons, peas, potatoes, pumpkins, snap beans, squash, and sweet corn. If not controlled, these pests can destroy approximately a third of a planting.

Newly hatched larvae feed on leaves but are unable to chew entirely through, creating a “window pane” effect. As they mature, they create small pinholes in the leaves. Larger larvae will cut the plant stems at or below the soil line. This type of injury is common during periods of dry weather. The heaviest injury occurs when 562-640 DD₅₀ have accumulated.

Cutworms are dark grey to black and have a greasy appearance with a grainy texture. Mature larvae grow to 1 ½ inches long. When disturbed, cutworms curl up into a tight C-shape. Adults are grey moths that have a series of distinctive dark markings on their forewings (including a dagger-shape on each outer forewing) and lighter colored hind wings.

For more information on monitoring suggestions and treatment thresholds, refer to http://learningstore.uwex.edu/Black-Cutworm-P1178C104.aspx.

Striped Cucumber Beetles are out and feeding on transplanted cucurbits. These insects can be a serious pest of vine crops. The beetles overwinter as adults in protected sites. They become active in mid- to late May. After mating, the female lays eggs in the soil at the base of cucurbits. The beetles are attracted to cucurbitacin produced by the plants.

Feeding damage can kill or stunt seedlings or transplants. Adult beetles feed on the aboveground plant parts and fruit. More importantly, these beetles spread the bacteria that cause bacterial wilt. This disease plugs the water-conducting vessels of the plant eventually resulting in plant death. The adult cucumber beetles are such efficient vectors of the bacteria that serious crop damage can occur if only 10% of the beetles are infective.

Beetles are usually localized in the field and spot insecticide treatments are effective. Some growers plant a row of vine crops near the upwind edge of the field on yellow or gold plastic mulch. This colored mulch attracts cucumber beetles to the crops in that row where they can be killed or removed.

For management information on cucumber beetles, visit http://learningstore.uwex.edu/Commercial-Vegetable-Production-in-Wisconsin2007-P540C104.aspx.

Onion Thrips (Thrips tabaci) were a serious problem in 2006.  Hot and dry conditions during mid summer contributed to elevated onion thrips populations.  So far in 2007 we have observed increases in daytime average high temperatures coupled with below average rainfall increasing the chance of early onion thrips populations.  A few adult onion thrips have been observed in the past 2 weeks upon recently emerged, direct-seeded onion.  Populations of onion thrips are moving in from overwintering hosts into onion at this time and are often initially highest along the field borders, near woods or in the vicinity of grain or field crops.

Damage results from feeding by both adult and immature thrips.  Thrips extract the contents of epidermal cells leaving them empty and necrotic resulting in ‘silvering’.  Chemical control of the onion thrips is presently the most consistent management measure.  Earlier this month, Wisconsin received approval from the EPA for a Section 18, Emergency Exemption for the use of Carzol SP.  Product in Wisconsin is available from the following suppliers:

1.)  Helena Chemical Co., Madison, WI, Dave Allen, (608) 577-7907
2.)  TH Agrichemicals, Plainfield, WI, Bob Zimple, (715) 335-6343
3.)  UAP, Plainfield, WI, Joe Kapral, (715) 335-4900
4.)  Wilbur-Ellis Co., Almond, WI, Tom Buchberger, (715) 366-2500

Yellow sticky traps can be used for monitoring the migration patterns of adult onion thrips from their over wintering hosts but are not informative regarding the decision to spray. Once the migration has begun, individual onion plants in several sections of the field should be inspected on a regular basis. Early in the season, surveillance and monitoring can be focused on the outer rows of plants with in the field where developing populations are first detected. Leaves must be parted to reveal the youngest emerging leaf in the center of the plant where immature thrips can be counted. Thrips prefer this area because it is the most succulent part of the plant and provides excellent protection from weather and insecticides. Adult thrips may also be found on older leaves, particularly where leaves have folded over, providing a sheltered area. Russell Groves, UW Department of Entomology.

Snap Beans - Populations of soybean aphid were first detected last week (May 21-25) by Dr. David Hogg, Department of Entomology, in an experimental soybean plot at the West Madison, AES.  Among 60 visually inspected plants, 3 had a single adult (alate) plus a few nymphs and another 5 had only nymphs resulting in13.3% (8/60) of the plants infested.  A total of 41 nymphs were counted, for a mean of 0.68 per plant. Most aphids were found on the unexpanded leaf tissue.  The soybean aphid has been implicated in past increases in Cucumber mosaic virus (CMV), Alfalfa mosaic virus (AMV), and other problematic viruses in processing snap beans and has more recently been documented as a competent vector of several nonpersistently transmitted viruses.  Unfortunately, insect vector control using insecticides has limited effectiveness in reducing spread of nonpersistent viruses into susceptible crops. Russell Groves, UW Department of Entomology.

Potato – Overwintering Colorado potato beetle (CPB) adults have been emerging over the last two weeks (May 13-26) and colonizing newly emerged potato fields.  The adult CPB overwinters in the soil along field margins near windbreaks and other wooded areas surrounding potato fields, as well as in the field.  These adults have begun to feed, mate and lay clusters of 10 – 30 yellow eggs on the underside of the newly emerged potato leaves. Females typically lay 350 or more eggs during their lives and they will last for 4-6 weeks during the early part of the season.  Egg masses at different stages of development have been observed ranging from recently laid, bright yellow egg clusters to darkened orange masses nearing hatch.  Egg hatch and emergence of 1^st instar larvae from the earliest egg masses is expected very soon at the Arlington, AES.

Dead adult beetles have been observed in furrows at both the Hancock and Arlington AES signaling the effects of the systemically applied, nicotinyl insecticides.  For growers who have not used an at-plant, systemic nicotinyl insecticide, plans should be made for the timely use of reduced risk materials targeting early instar larvae to include, but not limited to, spinosad (Spintor^® 2SC), abamectin (Agrimek^® 0.15EC), novaluron (Rimon^® 0.83EC), thiamethoxam (Actara^®), imidacloprid (Provado^® 1.6F), and Bacillus thuringiensis subsp. tenebrionis. Russell Groves, UW Department of Entomology.

Flea Beetles continue to be a problem on cole crop transplants in the field. This early season pest is commonly found on all members of the cole crop group as well as spinach, beets, potatoes and eggplant. All species of flea beetles have characteristically large hind legs that give adults the ability to jump. Larvae are delicate and thread-like with white bodies and brown heads.

 Adults feed on both leaf surfaces but usually on the underside where they chew small, circular holes through to the upper cuticle. This cuticle often remains in place for some time before drying and falling out. The circular holes give the plant a “shotgun” appearance.

 The best control is adjusting planting dates to avoid damage and to cover crops with floating row cover at seeding or transplant. For more information on flea beetles visit http://learningstore.uwex.edu/Flea-Beetles-P561C104.aspx.

 Finally, all growers with sweet corn in or near the tasselling stage should have their earworm traps out with lures in place as the storms May 24 & 25 have likely blown the moths northward.

No infections resulted for either cherry leafspot or apple scab with the overnight rains on the 24th and 25th. Although rainfall totals were great enough (>0.1 inch) to cause spore discharge, the length of wetting was only around 2.5 -3.5 hours and would not have resulted in spore germination.

Weather

Degree Day maps are available for several crops at the following URL:
http://www.soils.wisc.edu/wimnext/

Calendar Items

Apple IPM Field Day.  Wednesday June 20 at Green's Pleasant Spring Orchard north of Stoughton.  Pre-registration is required and attendance is limited.  Contact the office of the Wisconsin Apple Growers Association if you wish to attend. 920-478-3852 or acminc@verizon.net

Apple Field Day    Tuesday July 24 at the Peninsular Agricultural Research Station.  Registration forms will be sent to all known apple growers by the Wisconsin Apple Growers Association.  If you wish to receive information about this field data contact the WAGA at 920-478-3852 or acminc@verizon.net.

UW Fresh Fruit and Vegetable Contacts