WISCONSIN FRESH FRUIT AND VEGETABLE NEWSLETTER

May 1, 2007

Inaugural Newsletter

            This newsletter is the first in what we hope will be a long running series of newsletters aimed at commercial fruit and vegetable growers.  We are attempting to disperse this newsletter as broadly as possible.  Articles will be written by UW-Madison Extension Specialists.  We will send the newsletter to the Ag Hort List.  It will also be sent to growers by the Wisconsin Apple Growers Assn., Wisconsin Berry Growers Assn., and the Wisconsin Fresh Market Vegetable Growers Association.  In the near future we will also set up a listserv so that the newsletter can be sent directly to growers and others if they choose to receive it that way. 

            Our goal is to provide short articles, but timely information to growers.  We may refer to other information that is available on the Internet.  If you have suggestions of topics that should be covered please contact us.  A list of contact information is provided at the end of this newsletter.  We will attempt to provide two editions per month during the growing season.  We may produce a newsletter or two during the winter.  We hope you’ll find this information useful as the season progresses. 

Teryl Roper, UW-Madison Extension Horticulturist

Easter Freeze of 2007

            One of the significant topics of conversation on the electronic discussion groups is the Easter Freeze of 2007.  Fruit crops in states just south of us were devastated by the unusually warm temperatures during the last week of March followed by unusually cold weather during the first week of April.  Missouri and Southern Illinois are predicting no stone fruits and perhaps a very small apple crop.  Raspberries and strawberries were also injured. Southern Indiana, Southern Ohio, and Kentucky would be in a similar situation.

            Because we are farther north, Wisconsin appears to have fared better than more southern locations.  During the last week of March southern Wisconsin enjoyed high temperatures in the 50’s, 60’s and even 70’s.  Madison broke records for high temperatures.  The following week the bottom dropped out of the thermometer as low temperatures plunged into the teens.  The warm temperatures had pushed apples through silver tip to the onset of green tip.  At Arlington the overnight low on April 7 was 16.6°F.

            Critical temperatures for various tree fruits are included near the back of publication A3314 Commercial Tree Fruit Spray Guide.  A summary of fruit crop critical temperatures including information for small fruits is available from Michigan State University. (http://www.canr.msu.edu/vanburen/crtmptxt.htm)  The temperatures we experienced at the stage of development suggest that some fruit buds were damaged, but not enough to destroy our crops yet. 

Unfortunately, for apples the king fruit was most advanced and would have been most susceptible to injury.  This year will require serious examination of flowers and fruitlets before thinning gets underway; at least in the southern parts of the state.

Is there any management that can be undertaken to minimize the effects of the freeze?  I don’t know of any management practice that will repair frozen buds.  Overall good management and selection of superior sites is the best way to avoid damage that I know. 

Teryl Roper, Horticulture

Door County Report

April 27, 2007

The wetting period that started yesterday on the 26th and is continuing through this morning is likely to result in the first scab infection period of the 2007 season. As of 8:00am Friday the 27th leaf wetness duration has been between 18 hours in the north and 28 hours south in Casco at an average temperature of between 40°F and 42°F.

Apple are starting to show green tissue throughout the peninsula. Where overwintering scab inoculum is heavy (those apple orchards that had late season scab in 2006) protectant fungicide sprays will be needed prior to any infection periods that occur.

Regular updates to the Door County report can be found at:

http://www.uwex.edu/ces/cty/door/researchstation.html

Vegetable Report

Karen Delahaut, Extension Vegetable Specialist,
Dept. of Horticulture, UW-Madison

Cabbage Maggot – Cabbage maggots are insects that damage cole crops (cabbage, broccoli, cauliflower, Brussels sprouts, kale, and radishes) by feeding on the plants’ roots and lower stems. Wounds produced by this feeding can create entry points for several cole crop diseases. Early season transplants, late-season seedlings and spring root crops are damaged most severely.

The adult cabbage maggot is an ash-grey, bristly fly that resembles a housefly but is half as long with black stripes on its thorax. The larvae are typical fly maggots – white with no legs and 1/3 inch long body that tapers toward the head.

Cabbage maggots feed both internally and on the surface of their roots. Their tunneling provides a point of entry into the plant for pathogens such as soft rot bacteria and blackleg fungus. Maggots can be especially damaging to seedlings, injuring the growing point of the root, and stunting plant growth. Affected plants appear stunted and off-color. Severely damaged plants may wilt during hot weather.

Cabbage maggots overwinter as pupae in the upper few inches of the soil. In early May, adults emerge and lay eggs on the soil near the base of cole crops. The eggs hatch in 3-7 days and the larvae immediately begin feeding on the roots of the plant. Feeding continues for 3-4 weeks before the larvae pupate in the soil. The second generation of adults emerges in late June and lays eggs. The second generation adults lay the third generation eggs, which develop into overwintering pupae by fall.

Fly populations can also be monitored using yellow plastic dishans filled with soapy water. Place the dishpans at 100-foot intervals along the field edge and check them every 4-6 days. Count and record the number of flies caught to determine if the population is building or dropping off. Growers can predict peak fly emergence by monitoring degree-day (DD) accumulations. Use a base temperature of 43^oF and begin accumulating degree days when the ground thaws. The first generation of adult flies appears after 300DD₄₃ have accumulated. The second and third generations appear when 1475DD₄₃ and 2652 DD₄₃ have accumulated.

Prevention is the best way to manage cabbage maggots. Plant crops in well-drained soils when soil temperatures exceed 50 ^oF. Late plantings (mid-June) generally suffer less damage than early plantings. If possible, time planting dates to avoid peak fly emergence. Transplants should be planted one week before peak fly emergence while seeds should be sown at least three weeks before, or one week after, emergence. Floating row covers are also effective in protecting plants during flight periods.

Root crops planted in sand are seldom attacked by cabbage maggots. Do not plant cole crops in fields where animal manure has been freshly applies. Crop residues should be worked into the soil immediately after harvest to reduce sites where cabbage maggots can overwinter.

Insecticides at planting time are recommended in areas that have historically had problems with cabbage maggots. If insecticides are used, direct the applications at the base of the plants to avoid disrupting soil-inhabiting beneficial insects.

The cabbage maggot is resistant to many insecticides. Therefore, select an effective material and rotate among pesticide classes to prevent the build-up of resistant populations. Chlorpyrifos (Lorsban) is the primary cabbage maggot control insecticide.

Blackleg – Once a very destructive disease, blackleg of cole crops has become less important with the introduction of effective control measures.

The fungus that causes blackleg affects most cole crops. Mildly susceptible plants include Chinese mustard, cress, rutabaga, and many turnips.

All plant parts are susceptible at all stages of development. The earliest symptoms occur in the plant bed 2-3 weeds before transplant time time. The leaf spots are inconspicuous and indefinite at first. The spots gradually become more defined and circular with ashen – grey centers speckled with tiny black dots. These dots are the fruiting bodies of the fungus. Leaf spots continue to appear, particularly during wet weather. Infected seedlings and young plants die quickly. On older plants, spots on outer leaves sometimes turn a reddish color, particularly near the margins. Blemished cabbage heads are unmarketable.

Elongated, sunken, tan lesions appear on the stems near the soil line. The spots are often surrounded by a purplish border. As the disease progresses, the stem may be girdled, resulting in the wilting and death of the plant.

Roots may also become infected and entire root systems may be destroyed. Often, the first above-ground sign of infection occurs when plants suddenly wilt. The leaves of wilted plants remain on the plant. Infected plants may topple over as they mature because of poor root anchorage. In crop that are harvested for their roots, such as radish and rutabaga, such damage makes them unmarketable.

The fungus overwinters on crop debris, soil, of infected seed. Spores are disseminated to susceptible plants by splashing rain, irrigation, or wind. The fungus can survive in the soil for at least 3 years and may also be present on seed. It usually kills plants rapidly and produces large numbers of spores on the dead tissue. Further spread requires moist periods and splattering rains or irrigation. The severity of the disease is in direct proportion to the amount of rainfall in early summer.

No cole crop varieties are currently resistant to blackleg. However, the following set of cultural controls are highly effective in preventing outbreaks of the disease.

• Plant certified disease-free seed grown in the Western United States.
• Grow seedlings in soil that is free of the blackleg fungus or sterilize the soil before planting.
• Carefully inspect plants at transplanting for symptoms of blackleg. Avoid transplanting any plants from a seedbed that shows even the slightest infection.
• Do not dip transplants in water prior to transplanting.
• Since the fungus remains alive in the soil for 3 years, practice at least a 4-year rotation with non-cruciferous crops.
• Do not plant seedbeds and fields in shaded areas where dew evaporates slowly. Dews that persist through the morning favor disease development.
• Eradicate susceptible crucifer weeds (wild mustard, shepherd’s purse, pepperweed) growing in and around seedbeds and fields.
• Do not compost infected plants and do not use manure from animals fed diseased plants.
• Manage cabbage maggots as they will produce wound on root tissue where the fungus can enter.

Flea Beetles – Flea beetles are an early season pest commonly found on all members of the cole crop group as well as spinach, beets, potatoes and eggplant. There are several different species of flea beetle that pose problems early in the season when they are considered occasional pests. Host plants of many of the flea beetles are easily identified by their common names. For example, the crucifer flea beetle attacks cole crops and mustards while the eggplant flea beetle is commonly associated with eggplant.

Common Wisconsin flea beetles include the crucifer, eggplant, horseradish, pale-striped, potato, spinach and striped varieties. All have characteristically large hind legs that give adults the ability to jump. Adult flea beetles range in size from about 1/10 – 1/5 in inch. 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 the upper cuticle. This cuticle often remains in place for some time before drying and falling out. The circular holes give the plant a “shot-gun” appearance. Heavy feeding on young plants may reduce yields or even kill plants in severe cases. Crops grown for their foliage such as kale, bok choy, spinach or mustards may be rendered unmarketable by flea beetle damage. Larvae feed on the roots and tubers of susceptible plants but don’t often cause economic damage. Larvae of the horseradish flea beetle also mine the stem and leaf veins. In addition, many are vectors of plant pathogens.

Flea beetles overwinter as adults in the soil or beneath plant debris. They become active in early spring when temperatures reach 50 ^oF, and begin feeding on weeds or early-planted crops. Adults lay eggs in the soil at the base of host plants in May. Eggs hatch in 7-14 days and larvae feed on various plant parts until fully grown. They pupate in earthen cells for 11-13 days before emerging as adults. Adult flea beetles are particularly active on warm, calm, sunny days. Depending upon the species, there may be 1-3 generations per year.

Adjusting planting dates to avoid damage caused by flea beetles may be useful in some situations. Enclosing seed beds with floating row covers protects plants from egg-laying adults. Removing alternate weed hosts, deep plowing of crop residues in the spring and crop rotation will help reduce populations.

Chemical control is recommended when flea beetle populations exceed threshold levels, particularly early in the season. Commercial fields should be scouted for adults with an insect sweep net. Because flea beetles can move into a field quickly, newly planted fields should be scouted for insects or damage every 1-2 days while plants are small and unable to withstand much damage. Soil applied insecticides at planting will provide season-long control. Foliar insecticides provide quick control of large populations of adult flea beetles. When selecting foliar insecticides, choose chemicals that will not disrupt the natural enemies of other pests such as Lepidoptera on cole crops. Insecticides with a short residual life are recommended.

• Beets – treat when beetles cause stand reduction on small plants
• Cole crops – undetermined
• Eggplant –
  • <3 inches = 2 beetles/plant
  • 3-6 inches = 4 beetles/plant
  • >6 inches = 8 beetles/plant
  • Horseradish – treat only if beetles are found in high numbers early in the season
  • Potato - > 2 beetles/sweep
  • Tomato - > 2 beetles/plant

Caterpillar pests – Growers should use synthetic pyrethroid insecticides (Asana, Proaxis, or Warrior) alternated with a reduced risk pesticide like Spintor, Avaunt, Proclain or Confirm.

Seed corn maggot – The seed corn maggot is a perennial pest of the seeds and seedlings of a wide variety of vegetable crops. In addition to corn, maggots will attack beans, beets, cabbage, cucumbers, peas, radishes, squash, and turnips. Seed corn maggots can decimate a crop stand if left untreated. They are more of a problem when susceptible crops are planted in succession.

The yellowish-white larvae are typical fly maggots: 1/5 inch long when fully grown, cream-colored, legless, and wedge-shaped. The maggot’s head end is sharply pointed. Pupae are brown, 1/5 inch long, cylindrical in shape, and rounded on both ends. Adults resemble miniature houseflies. They are dark grey, 1/5 inch long, and their wings are held overlapped over their bodies while at rest. The flies are often mistaken for adult cabbage maggots and onion maggots, but they are smaller is size. Eggs are tiny, oval, and white.

Seed corn maggots overwinter as pupae in the soil. Adult flies emerge in the spring with peak emergence in early to mid-May when swarms of flies are often seen over recently tilled fields. Adults mate within 2-3 days of emergence. Females lay eggs in soils with high organic matter or near seeds and seedlings of susceptible plants. Eggs hatch 2-4 days later. Larval feeding, development, and pupation all occur below ground and the next generation of adults appears within 3-4 weeks. This sequence of events is repeated for three to five generations that emerge each year.

Seed corn maggot larvae feed in the cotyledons and the below-ground hypocotyl tissue of seedlings, resulting in a variety of damage symptoms. Feeding damage in germinating seeds can kill the seedlings before they emerge. Poor germination or poor plant stands may indicate a seed corn maggot problem. To diagnose, dig up the seeds to look for damage. Plants that survive maggot damage to the seed often have holes in the first pair of true leaves or no leaves at al (snake-head seedlings.) Damage to the hypocotyls will leave the plant yellow and wilted.

Rescue treatments are effective for seed corn maggots. Once damage is detected, it’s too late to control the maggots. Therefore, there are no economic thresholds for this insect and any insecticides must be applied at planting as a protective measure.

Forecasting the appearance of generations can be accomplished by calculating degree-days starting when the ground thaws in the spring. Growers can predict peak fly emergence by monitoring degree-day (DD) accumulations. Use a maximum temperature of 86 ^oF  and a base temperature of 39^oF and begin accumulating degree days when the ground thaws. The first generation of adult flies appears after 200DD₃₉ have accumulated. The second and third generations appear when 600 DD₃₉ and 1000 DD₃₉ have accumulated.

Scouting can be helpful in determining the fly-free periods on a particular field. In early April, place three or four yellow plastic dishp0ans filled with soapy water along the edge of the field at 100-foot intervals. Every 4-6 days, remove and count the trapped flies and add fresh soapy water to the pans. Keep a record of the number of flies trapped will indicate when fly numbers are building up or tapering off.

Currently work is being done in Minnesota to evaluate the effectiveness of a pheromone trap for prediction the presence of adult flies.

Since adult seed corn maggot is attracted to decaying organic matter, do not plant susceptible crops in fields where animal or green manure has recently been applied. The faster the seeds germinate and grow, the less opportunity the maggots have to damage the crop. There are a few strategies to hasten germination: wait until soil temperatures are at least 50 ^oF before planting most susceptible crops. Peas and radishes may be planted when soil temperatures are above 40 ^oF. Plant seeds as shallowly as feasible to speed germination.  Soak untreated pea and bean seeds in water for 2 hours before planting to soften the seed coat.

Naturally occurring fungal diseases occasionally will greatly reduce seed corn maggot numbers significantly, particularly when flies are abundant and relative humidity is high. During a fungal epidemic, dead or diseased flies can be seen clinging to the highest parts of plants along field edges.

Predaceous ground beetles eat seed corn maggot eggs, larvae, and pupae and can be important in reducing maggot numbers. Because these soil-inhabiting beetles are susceptible to insecticides, avoid using broadcast soil insecticide treatments whenever possible.

If you have seed corn maggots one year you will likely have them the following year. To prevent damage, you have two options: plant treated seed or make a soil application at planting.  Gaucho, Thimet and chlorpyrifos are recommended for seed treatment.

Onion Thrips – are an important annual pest of onion. They may attack nearly all garden crops but serious damage is generally confined to onions, cauliflower, cabbage, snap beans, cucumbers, melons, and tomatoes.

Adult onion thrips are 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 that they are smaller and lack wings.

Onion thrips damage onions primarily by their rasping and feeding which produces whitish blotches on leaves or decreased pollen set. 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. Severe damage to onions causes bulbs to become distorted or undersized. Because thrips prefer tight spaces, cabbage varieties with extremely dense heads are most susceptive to damage. Thrips are often found several layers deep within developing cabbage heads. Red varieties are usually less susceptible. Thrips damage can be serious on kraut cabbage where there is a necrotic flecking of the internal tissues and white blisters on the outer leaves of the raw cabbage that show up as dark blotches on the processed kraut. Heavy thrips buildup inside the cabbage head may cause the head to become distorted. On cauliflower, thrips damage causes tan or brown streaks on the curd. Damaged curds are more susceptible to soft rot bacteria.  Onion thrips are also vectors of plant viruses such as the tomato spotted wilt virus.

Adults and nymphs overwinter on plants, debris, or along weedy field edges. Females can reproduce without mating and lay eggs beneath the leaf’s surface. Eggs hatch after 5-10 days and nymphs are full grown within 15-30 days. Development of the last two nymphal stages occurs in the soil, without feeding. After the fourth molt, adult female thrips return to the plant. Thrips produce about 5-8 generations per year. Hot, dry weather favors thrips outbreaks.

Thrips should be controlled early, before they become protected by plant tissue. Due to their small size and reclusive habit, onion thrips are difficult to monitor and control. No treatment thresholds have been established for onions or cabbage. Yellow or white sticky traps may be used also on field edges to monitor the initial migration of thrips into a field. Cleaning plant debris from the field and the surrounding area may aid in control. In general, onion cultivars with an open type of growth, circular leaf structures and glo9ssy green foliage suffer less damage than cultivars with leaf sheaths tight to the stem. Red onions are particularly susceptible to attack while sweet Spanish onions are more resistant.

Because of thrips’ protected location in the plant, control through the use of insecticides is difficult. Foliar insecticides should be applied in sufficient water with a spray additive to achieve penetration into the plant. Insecticide resistance is a primary concern, severely limiting the choice of chemicals.

Black cutworm - The first significant captures of 8-9 moths in two nights occurred near Janesville and Gratiot between April 23 and 26. Young corn plants will be susceptible to leaf feeding during the next 168 to 300 GDD (base 50F) and to cutting once 300 GDD have passed. Corn plants are most vulnerable to cutting by this insect 10-14 days after emergence. Wet, weedy fields more commonly develop black cutworm infestations because they are plowed late and damaging populations may have already developed by the time corn is planted.  From the DATCP Pest Survey Bulletin

European corn borer - Dissections of corn stubble continue to indicate winter mortality of the European corn borer, Ostrinia nubialis, due to climatic factors was very light. One hundred percent survival of ovewintered fifth instar larvae was noted in the Columbia, Iowa, Richland, Rock, and Walworth County fields checked. Sixty-six percent survival was recorded in one Walworth County field, although three others in the same county showed 100% survival. Despite what appears to be good survival of second generation corn borer larvae from 2006, a light first flight of moths is projected based the results of a larval abundance survey conducted last fall. The survey documented a relatively low statewide average of 0.29 borer per plant (29 larvae per 100 plants). From the DATCP Pest Survey Bulletin

Stewart’s Wilt Prediction – Because of the relatively high percentage of overwintering of corn flea beetles, the prognosis for Stewart’s wilt is high in Kenosha county and moderate to high in Broadhead, Dodgeville, Manitowoc, Milwaukee, Racine and Watertown.

Corn flea beetles are tiny (less than 1/16 inch), black beetles that transmit the bacteria that causes Stewart’s wilt. The pathogen overwinters in beetles, plant residue, and soil. During mild winters, enough flea beetles may survive in Wisconsin to lead to significant incidence of Stewart’s wilt in the southern tier or two of counties. To predict disease risk – and the need to scout for beetles – add the monthly average temperatures for December, January, and February. Scout seedling corn intensively if the total is greater than 90 ^oF. Treatment may be warranted if you find an average of six or more beetles per 100 plants. Beetles readily jump from plants when disturbed.

Off to a clean start:
Early season weed management
for vegetable growers

Jed Colquhoun, Extension Weed Specialist, UW-Madison Department of Horticulture

     Spring is a time to enjoy lush new green growth and a new beginning that will hopefully lead to a bountiful summer harvest.  Unfortunately, the weeds in your vegetable garden are also enjoying the abundant sunshine and soil moisture.  However, there are a few weed management strategies that can be implemented in the early season that will reduce the hours of hand-weeding and hoeing later in the growing season.  The old saying “an ounce of prevention is worth a pound of cure” holds true in the case of weeds.

     If planting timing is not absolutely critical or occurs later in the spring, such as with tomatoes and peppers, consider using the stale seedbed technique to eliminate a portion of the first weed flush prior to crop planting.  In the stale seedbed technique, till the soil to the desired final seedbed about 2 to 3 weeks prior to vegetable planting.  Allow weeds to germinate and emerge during the 2 to 3 week stale seedbed period, and then eliminate the first flush of weeds with a shallow tillage to prepare for planting.  Till the soil just enough to eliminate the young weeds and allow for crop planting – excessive or deep tillage will stimulate a fresh batch of weeds.  Finally, plant the vegetables into the stale seedbed, with a large portion of the first weed flush eliminated prior to crop planting.  In preliminary organic vegetable research trials that we conducted last year, about 40% of the season’s weeds were eliminated using the stale seedbed technique.

     Human activity is largely responsible for the introduction of new weeds in farming and gardening.  Consider all potential introductory sources of weed seed or vegetative matter, such as perennial root pieces.  Make sure that crop seed is not contaminated with weed seed.  Certified vegetable seed is inspected for weed seed and other potential contaminants, and this information is listed on the seed packet.  Consider the source of any soil or compost used in transplanting vegetables or amending the garden soil.  Keep in mind that creeping perennial weeds such as Canada thistle can reproduce not only by seed but also from vegetative structures such as roots.  Canada thistle, for example, can grow from a one quarter inch section of root, so tillage can actually “plant” a new crop of perennial weeds.  Finally, prevent weed seed production in areas surrounding the vegetable garden by mowing weeds prior to flowering.

Crop Progress:

Weekly crop progress reports for Wisconsin can be found at the following link.  You can also subscribe to receive the reports by e-mail.

http://www.nass.usda.gov/Statistics_by_State/Wisconsin/Publications/Crop_Progress_&_Condition/index.asp

UW-Extension Publications

UW-Extension has a wealth of publications related to the production of fruits and vegetables.  Most are available to view and print as pdf files.  To access the publications go to:

http://learningstore.uwex.edu/

UW Fresh Fruit and Vegetable Contacts