WISCONSIN FRESH FRUIT AND VEGETABLE NEWSLETTER
May 15, 2007

Apple Thinning

             Now is the time when apple growers need to determine their potential crop load and begin to thin their crop if necessary.  Thinning results in larger fruit and greater return bloom. Once the king fruit is about 5-6 mm in diameter (about ¼ inch) it is time to begin cutting fruitlets and counting seeds.  By this time it should be possible to determine which seeds are viable and which seeds are not.  Viable seeds will be plump and cream colored and the cut surface will appear wet.  Non-viable seeds will be small and sunken and will appear dry if cut.  For the best size and shape of fruit each fruitlet should have at least 7 viable seeds.  Further, fruit with more seeds are harder to thin than those with fewer seeds.

            The thinning spray should be applied when the king fruit is about 10 mm in diameter (about 3/8 of an inch).  By this stage the king fruit is well set and the side fruit are still sufficiently small to respond well to thinners.

            Growers have several choices and combinations of chemical thinners to choose from.  These options are presented on page 44 and 45 of the 2007 version of bulletin A3314 Commercial Tree Fruit Spray Guide.  We also have a publication titled Plant Growth Regulator Use in Apples (A3524) that is available at: http://learningstore.uwex.edu/pdf%5CA3524.pdf

            Sevin is still the workhorse thinning chemical for apples.  The above cited publications offer suggestions for rates that are effective for different cultivars.  Tank mixes of Sevin plus MaxCel have also been very effective.

            Thinning Honeycrisp is critical.  If you have a large potential crop of Honeycrisp it is absolutely critical that it be substantially thinned.  Terence Robinson at the winter meeting suggested that Honeycrisp fruit load should not exceed 4-5 apples per cm² trunk cross sectional area.  This will avoid biennial bearing.

            The best environmental conditions for thinning are warm calm days with high humidity.  This promotes slow drying of the spray application, allowing the maximum time for uptake of the chemicals through the fruit cuticle.

Estimating crop load made easy

             Thinning young trees is challenging.  Within an orchard block there are discrepancies in tree size with some trees being larger or smaller than average.  Is there a good way to express crop load other than fruit per tree?  For years researchers have expressed crop load as the number of fruit per centimeter squared of trunk cross sectional area.  This metric is based on the correlation found between canopy volume and trunk cross sectional area.  Measuring canopy volume is hard and time consuming.  Measuring trunk circumference or diameter is relatively easy and repeatable. 

            To determine trunk cross sectional area (TCSA) you’ll need a caliper.  These are available at most good hardware stores.  Measure the caliper (diameter) of the trunk about a foot above the soil line in two directions, 90 degrees from one another.  This accounts for trunks not being fully round.  Determine the average caliper.  Look on the chart below to determine how many fruit should remain on the tree after thinning.

            It isn’t necessary to measure every tree and then determine crop load.  Once you have measure a couple of dozen trees you should have a feel for the actual TCSA and you can then figure out what the crop load should be.  This tool should assist you in managing crop loads of young trees and to assess if you thinning program was adequate.

Petal Fall Spray

             Of all insecticide applications made to apple trees during the course of a season perhaps the most important is the petal fall spray.  This spray should control populations of plum curculio, leafrollers, leafminers, and leafhoppers.  In addition to the traditional organophosphate insecticides there are a number of newer softer insecticides available that should provide control of early insect pests.  For details see bulletin A3314 Commercial Tree Fruit Spray Guide.

Strawberry Frost Protection

    Patchy frost is forecast for later this week in Wisconsin.  Strawberry plants are in bloom in southern Wisconsin and nearing bloom in northern Wisconsin.  At this stage they are most susceptible to cold temperatures.  The most common means of frost protection for strawberries is sprinkler irrigation.  As liquid water freezes into ice, latent heat is released keeping the local temperature at or slightly above freezing.  The principles are:

1.    Sprinkling must start before the temperature reaches freezing.
2.    Liquid water must always be present on the surface of the plants or ice.
3.    The rate of water application for effective frost protection depends on the wind speed and the temperature.
4.    Water application must continue until the air temperature rises above freezing in the morning.

Several excellent discussions of frost protection for strawberry growers (and growers of other tender crops) are available on the Internet. 

http://www.fruit.cornell.edu/Berries/strawpdf/strfrostprotect.pdf
http://www.omafra.gov.on.ca/english/crops/facts/frosprot_straw.htm
http://www.ces.ncsu.edu/depts/hort/hil/hil-705.html
http://www.ces.ncsu.edu/depts/hort/hil/hil-713.html
 

Rhubarb stems can be toxic.

    Because of the hard freeze in late April, there has been damage to rhubarb plantings. Because rhubarb emerges early in the spring, frost or hard-freeze damage is a common occurrence. In response to freezing temperatures, the oxalic acid in the leaves will translocate to the stalk. To determine if the plant has been frozen, the leaf tissue will initially appear 'water soaked' and then wilt, and eventually blacken along the edges or where tissue was damaged. Once the plant experiences a damaging freeze, the stalks should not be sole, eaten, or processed and should be removed from the plant.

    Eventually a new set of leaves will emerge, and no permanent damage is done to the plant. A mature plant should yield another crop of stalks within four to six weeks.

Colony Collapse Disorder

    The cause of colony collapse disorder (CCD) in honey bee hives is still unknown but it is suspected to be related to disease organisms, urban sprawl and the effects on forage plants, and finally pesticides. It is the latter, pesticides, particularly insecticides that may be the key culprit. Neonicotinyl insecticides are relatively new to the market and commonly used as a less toxic means of insect control. Neonicotinyl insecticides include Admire, Provado, Gaucho, Marathon, Calypso, Cruiser and Assail. This is one theory for CCD. Another includes the introduction of new pathogens that may cause problems in highly populated areas such as in hives and pollination areas. Whatever the cause, the immune and detoxification systems of the bees are compromised. To help bees survive you can become a bee keeper to increase the number of hives in your area which will help in the pollination of your crops that need it such as vine crops.

The University of Kentucky has a nice summary explanation of CCD on their website.
http://www.ca.uky.edu/agc/news/2007/may/honeybeedecline.htm

Periodical Cicadas

 The song of the periodical cicada or “17-year locusts” as they are often incorrectly called, attracts a great deal of attention when it is heard.  Brood XIII is expected to appear in parts of southern Wisconsin this spring.  In Wisconsin, this is one of two types of cicada expected to appear this season.  The other, more common species is the Dog Day cicada which appears annually in mid- to late summer.  Both types of cicada are similar in appearance; however the periodical cicada can be distinguished by the black ‘W’ on its front wing.  Most public fears arise due to the sudden and unexpected emergence of thousands of noisy insects in a short period of time.  Where they are abundant, periodical cicadas can do considerable damage to young orchards, nursery stock, or new landscape plantings.

Periodical cicadas cause damage as a result of egg-laying by adult females and root pruning by the larvae.  Egg-laying scars appear as roughened punctures on twigs of many woody plant species.  Damage to plants tends to be less severe in mature trees and shrubs due to their larger size and is often considered a form of natural pruning.  By contrast, injury to young orchards may result in the death of the trees, or more often, the destruction of form due to attack of the tender leader shoots. 

Apple and grapes are the key fruit crops commonly attacked by the adult cicadas. Epinasty, the curvature resulting from more rapid growth on the uninjured side of the branch, and the breakage of the current season’s growth are the most common symptoms.  Plant varieties vary as to their recovery capabilities.

They are abundant in late May, June, and very early July. 

The song of the cicada is actually made up of three separate sounds.  The first sound, which occurs early in the season, just after the males have emerged, is Pha-r-r-r-a-oh.  As the season progresses, the song changes to the loud, shrill, and characteristic tsh-e-e-E-E-E-E-e-ou made by many males singing in concert.  The song may last as long as 15 to 30 seconds but is usually five seconds or less in length.  Finally, the male cicadas also produce an intermittent clicking or chirping not unlike that of a cricket.

The periodical cicada requires 17 years to complete its life cycle and is located primarily in the northeastern Unites. Each race is further divided into broods.  Broods are various populations of cicadas which are separated geographically.  It is believed that originally there was a single brood.  Brood XIII is composed of the species Magicicada septendecim and is the predominant brood in the southern Wisconsin counties of Crawford, Dane, Grant, Green, Iowa, Jefferson, LaFayette, Milwaukee, Richland, Rock, Sauk, Walworth, and Waukesha and is expected to emerge in 2007.  Brood VI, a less abundant brood emerged in the year 2000, was more centrally located in Wisconsin than Brood XIII.  Historically, Brood VI has been sighted in Burnett, Columbia, Crawford, Dane, Fond du Lac, Green Lake, Marquette, Sauk, Sawyer, Washburn, and Waushara counties.

The emergence of the last nymphal stage during the last week in May marks the beginning of the above-ground portion of the cicada’s life cycle.  The nymphs burrow directly upward and emerge from the ground, leaving behind a small, round hole one-half inch in diameter.  In certain situations, such as shallow soil or saturated ground, immature cicadas may construct clay cones raised two inches above the ground surface in which they reside during the final stages of their development.

Once emerged from the soil, the cicadas climb the nearest tree, shrub, or post, split the nymphal skin down the back, and the adult emerges.  The cicadas remain attached to their supports until their bodies dry and wings harden.  The following day the adults take flight, feed, and begin to mate.  Feeding injury to trees and shrubs is very slight as only a small amount of sap is removed.  By contrast, it is the female’s egg laying that imparts irreversible damage to woody plants.

Within two weeks of emergence the females begin laying their eggs.  Each female cicada deposits from 400 to 600 eggs into slits made in the bark.  More than 80 species of plants are subject to attack.  The female cicada favors the tender twigs of one-year-old growth for egg laying.  In six to seven weeks the eggs hatch, the young fall to the ground and enter the soil for their long subterranean existence.  After forming a chamber adjacent to a rootlet, the nymphs penetrate the xylem vessels with their piercing-sucking mouthparts and begin to feed.  No apparent damage results from the nymphal feeding.  The nymphs remain in the soil for 17 years and only move from their original feeding site if adverse conditions arise. 

Prevention of cicada injury is aimed at the destruction of emerged adults.  Hand collection to remove and mechanically crush the adults provides adequate protection if the number of plants requiring protection is minimal, such as in most home landscapes.  Covering shrubs with mosquito netting to provide a barrier against the emerging adults may also provide adequate protection on a small scale. 

Many reptiles, mammals, and birds, including the English sparrow and robin, as well as predatory insects such as ground beetles and dragonflies attack the emerged cicada.  There are also several parasitic flies and wasps that provide natural control.

Severely damaged twigs should be removed and destroyed to prevent the eggs from hatching to produce another generation of cicadas.  Delayed pruning of ornamentals from the fall prior to a predicted cicada emergence until after damage has been done can reduce the amount of plant material removed.

Colorado Potato Beetles

     The first adults of the Colorado potato beetle have been sighted in southern Wisconsin. This sighting indicates that degree day accumulation should begin. The base temperature for degree day calculation for CPB is 52.

Life stages are as follows: eggs at 120 DD₅₂, 1^st instar larvae at 185 DD₅₂, 2^nd instar larvae at 240 DD₅₂, 3^rd instar at 300 DD₅₂, 4^th instar at 400 DD₅₂, Pupae at 675 DD_52.

CPB cause widespread defoliation of potato, eggplant and nightshade. Both adults and larvae are voracious leaf feeders. They can chew holes up to 1/8 inches in diameter. The larger the larvae, the larger the holes. Damage is particularly serious if it coincides with the flowering of the plant whereby yield will be reduced.

For more information on Colorado Potato Beetles visit http://learningstore.uwex.edu/Colorado-Potato-Beetle-P559C104.aspx

Flea beetles

     Flea beetle damage is apparent on plants in the brassicas genus that have been transplanted into the field. Growers should protect these crops from damage by the use of floating row covers. The larvae are delicate and threadlike with thread-like bodies and brown head capsules. All flea beetles have characteristically large hind legs that give the adults the ability to jump. The adults are tiny; about 1/10 inch in length. The most common types that feed on cole crops include the striped, western black and the crucifer flea beetles.

            Flea beetles overwinter as adults in leaf litter, hedgerows, windbreaks, and wooded areas. They emerge in late April when temperatures reach 50ºF and feed on cruciferous weeds and volunteer plants until new cole crops emerge.

            Adults feed on both the upper and lower leaf surfaces but they favor the underside where they chew small, circular holes through the leaf to the upper surface which often remains intact for some time before drying and falling out. The circular holes give plants a “shotgun” appearance.

            Protecting cole crops with floating row covers will prevent damage by flea beetles. It is also important to remove cruciferous weeds from the farm.         

            For complete information on flea beetles visit the Cooperative Extension website at http://learningstore.uwex.edu/Flea-Beetles-P561C104.aspx.

Onion Thrips

            As onions are transplanted into the field, onion thrips will become a problem. Adults and nymphs overwinter on plants, debris, and along weedy field edges.

            Thrips are tiny, pale yellow or brown insects about 1/12 inch long. Their wings have no veins and are fringed with long hairs. Nymphs resemble adults except they are smaller and lack wings.

            Onion thrips attack all members of the onion family, garlic, onions, leeks and other alliums.

            Damage is done when they feed on the plant with their rasping mouthparts. This produces whitish blotches on leaves. Both adults and larvae cause damage that appears as a silvery streaking on the leaves. As time passes, affected areas become dry and yellow. Heavy infestations can cause onion leaf tips to turn brown.

            Red onions are particularly susceptible to attack while sweet Spanish onions are more resistant.

            Onion thrips should be controlled early. Cleaning plant debris from the field and surround areas may aid in controlling thrips. Floating row covers on onion transplants will prevent thrips from damaging onion plants.

            For more information on onion thrips visit  http://learningstore.uwex.edu/Onion-Thrips-P562C104.aspx.

Asparagus Beetle

     The common asparagus beetle is a common pest in Wisconsin. It is bluish brown with cream spots. They are about ¼ inch long and larvae are cream colored with brown heads.

            Adults lay eggs or feed on spears, causing distortion thus making them unmarketable. Large populations of adults later in the season can cause defoliation of the ferns.

            Scout plants in the afternoon when beetles are most active. 5-10% of the plants infested with adults, 2% of the spears infested with eggs, 50-75% of the plants infested with larvae or 10% of the plants defoliated warrant control.

            Destroy crop residues to destroy overwintering sites. If beetles appear in the spring, spot treat infested sites with carbaryl, cholrpyrifos, malathion, methomyl, permethrin, pyrethrin, rotenone, or spinosad.

            For more information on the asparagus beetle, visit http://learningstore.uwex.edu/Asparagus-Beetle-P564C104.aspx.

Bean Leaf Beetle

             Bean leaf beetles are becoming more of a problem in Wisconsin that they have been in the past. Warmer winters may allow overwintering adults to survive in record numbers. Host plants include snap and lima beans, peas, soybeans, and leguminous weeds.

            Adult beetles chew small holes into the leaves while feeding from the lower leaf surface. Adult feeding on young plants can reduce vigor and yield. They can also feed on the 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.

            This year the overwintering survival is expected to be low so damage will be less than last year. To determine the overwintering survival rate, average temperatures are taken from a Minnesota model that uses the mean temperature from September through April to determine whether the populations are increasing of decreasing. For more information on the Bean Leaf Beetle visit http://www.plantpath.wisc.edu/pddc/publications.html 

Black Cutworm

             Flights are currently underway and are problematic in south central Wisconsin. This typically occurs when 300-350 DD50 is reached and when ≥9 moths per night for 2 consecutive nights. To determine the level of damage, check 50 plants in each of 5 locations in the field. If 5% of the plants show damage, treatment is warranted.

            Insecticides recommended for control include Ambush, Asana, Baythroid, Capture, Lorsban, Mustang Max, Pounce, and Warrior.

A Primary scab starts at green tip (0%) and accumulates degree days base 32. A total of 1000 DD (100%) signifies the end of primary scab.

University of Wisconsin Fresh Fruit and Vegetable Contacts