WISCONSIN FRESH FRUIT AND VEGETABLE NEWSLETTER
July 15, 2007

Contents

Potassium Deficiency in Apples
Apple Bitter Pit
Raspberry Renovation
Bird Management
Preharvest Intervals
Vegetable Report
Door County Report
Drought Monitor
Degree Days
Contacts

Potassium deficiency in Apple Leaves

 This week I’ve received a couple of photos of apple trees with leaves showing marginal necrosis.  Given the hot dry conditions that have prevailed around the state seeing potassium deficiency is not surprising.  Symptoms of potassium (K) deficiency are necrosis (browning) of the margins of leaves, with older leaves showing symptoms earlier than younger leaves.  K is mobile in plants so it can move from older leaves into younger ones.  That is the reason symptoms usually appear first on older leaves.

            K deficiency in apple trees does not necessarily result from having too little K in the soil.  Symptoms are worst in hot dry summers because water is required for movement of K within soils and into the rooting zone.  K also is carried into leaves in the transpiration stream and if trees are transporting less water they are also transporting less K.  K deficiency is very uncommon in irrigated orchards.

            K deficiency can be alleviated in two ways:  Foliar application of K and providing irrigation.  If you choose to make a foliar application of potassium use potassium sulfate and apply at the rate of 3 lbs/100 gallons dilute or 6 lbs/acre concentrate.  Using higher concentrations can cause leaf burning.

            Apple orchards benefit from irrigation.  In the Midwest few orchards are irrigated and the trees must suffer through hot dry summers.  This comes at a cost of yield, fruit size, and fruit quality.  K deficient apples are softer than apples with sufficient K.  Research at Cornell suggests that growers with irrigation often don’t turn it on soon enough so that trees experience drought stress prior to beginning irrigation.

            Young trees will require supplemental watering in non-irrigated orchards.  Each tree should receive 5-10 gallons of water per week to make good growth.  

            Keeping vegetation, especially grasses, away from tree trunks during hot dry weather will allow trees to obtain what moisture is available in the soil without competing with other plants for it.  Small plantings benefit from mulching to retain soil moisture.  Mulch will not replace the need for irrigation.

Bitter Pit of Apple

    This hot dry weather also is perfect for developing bitter pit in apple fruit.  Bitter pit is a calcium related disorder that causes small dark indentations on the surface of the fruit.  Some cultivars are more prone to bitter pit than others and popular cultivars like Honeycrisp are especially susceptible.  Once bitter pit appears it cannot be cured.  Management of bitter pit is purely preventative.

    Since bitter pit is caused by too little calcium in the fruit, the key to prevention is to provide external sources of calcium.  Spray applications of calcium beginning at first cover and continuing through 4-6 sprays is usually sufficient.  All sources of calcium are equally effective at preventing bitter pit when applied at the same rate of calcium.  Choose the product you use on price per pound of calcium along with ease of application and handling.  Calcium chloride is usually the least expensive material, but it is corrosive so sprayers must be carefully cleaned after use. Suggested rates of calcium chloride are 3 pounds per 100 gallons dilute or 6 pounds per acre, concentrate.  Don't re-apply calcium chloride unless rainfall is received between applications.  If you've not applied calcium this year don't delay.

    A photo of bitter pit is shown below.  I have also provided a link to a fact sheet that gives more information about bitter pit. 

Teryl Roper, UW-Madison Extension Horticulturist

http://wihort.uwex.edu/gardenfacts/XHT1055.pdf

http://postharvest.tfrec.wsu.edu/marketdiseases/bitterpit.html

Raspberry Renovation

    Once the summer harvest is completed for either summer bearing or fall bearing raspberries the planting needs to be pruned and renovated.  Canes that have finished fruiting will soon dry and die and should be removed from the planting.  Removing these canes retains space and resources for primocanes to grow unimpeded.  Old canes can also be reservoirs of insect and disease problems.  In addition to removing spent canes this is a good time to thin the stand of canes to six to eight canes per running foot of row.  This cane density will provide for a full crop of fruit without the canes competing for light and it promotes good air movement through the canopy.

    Renovating the planting also includes controlling weeds and narrowing the rows to the desired width.  Raspberries can be invasive and will spread widely without keeping them in the desired rows.  Row width after renovation should be about 15 to 18 inches.  Roto-till up the sides of the rows to remove canes that are growing out of the desired row.  With the row narrowed and canes thinned it is easier to get into the rows to hand weed or to use a hoe.  Once the planting has been pruned and renovated providing irrigation as necessary will encourage strong canes for either the fall crop on primocanes or the 2008 crop on floricanes.

Teryl Roper, UW-Madison Extension Horticulturist

Bird Control for Small Fruits

The Illinois Newsletter [ http://www.ipm.uiuc.edu/ifvn/index.html ] gave a reference to a very well done article about controlling bird injury to small fruits in the Cornell fruit newsletter.  You can read the article by going here:
http://www.nysaes.cornell.edu/pp/extension/tfabp/newslett/nybn66a.pdf

The bird article is several pages into the newsletter.

Pre-Harvest Intervals

    Harvest of many vegetables and fruit is underway or will begin shortly.  This is the time of year when paying careful attention to pre-harvest intervals is especially important.  Pre-harvest intervals for each product is listed on the package label or on the supplemental label.  Read these labels carefully before each application.  The purpose of the preharvest interval is to allow sufficient time for pesticides to break down so that any residues are below critical levels by the time the crop is ready for harvest and sale.  The 2007 pest management guides have tables showing pre-harvest intervals at the time of publication.  These may change during a season.  Read the label.  The label is the law.

Vegetable Report

Populations of Colorado Potato Beetle (CPB) appear to be 10-14 days ahead of their ‘normal’ schedule.  At both the Arlington and Hancock Agricultural Research Stations, we have recently observed the emergence of summer adult beetles.  These beetles will begin to feed aggressively in the coming weeks and will initiate the 2^nd generation of beetles once they initiate egg laying.  It will be important for growers to plan for the control of damaging populations of the 2^nd generation CPB in the context of insecticide resistance management.  If neonicotinoid compounds have been used at-planting or as foliar sprays against the 1^st generation of CPB, then effective CPB insecticide options need to be selected carefully for the control of the 2^nd generation.  A well-timed application of Indoxacarb (Avaunt^®) targeting the emergence of summer adults may be a first step towards the control of the 2^nd generation CPB population.  To increase the effectiveness of this control option, a synergist (piperonyl butoxide) may be added to the tank mix at suggested rate of 0.25 lb a.i. / acre.  Russ Groves – Extension Entomologist

 Also resulting from our continued warm weather, a generation (3^rd) of Seed Corn Maggot has recently emerged and adult female flies will be looking for suitable hosts on which to lay eggs. Seed corn maggot has a broad host range and will oviposit and feed on several economically important crop species including recently planted (or transplanted) cucumber, melon, or squash crops.  In general, a healthy and vigorously growing plant will withstand some pressure by this insect and can out-grow most damage inflicted by the feeding maggots.  In coarse soils, pressure from this insect is often less compared to heavier soils which contain a higher proportion of clay.  Further, incorporation of organic matter or green manures at, or prior to the time of planting should be avoided as the volatile emissions from this amendment will attract the adult flies.  Russ Groves – Extension Entomologist

 Increases in populations of Cucumber Beetles (striped and spotted) can also be expected with the initiation of flowering in several cucurbit crops.  The floral structures in these plants emit a volatile compound known as cucurbitacin which is highly attractive to the adult beetle(s) as both a good food source and also as an oviposition site.  Feeding damage by the immature cucumber beetle larvae can stunt or kill developing seedlings or transplants under high pest pressure. Adult beetles will feed on the foliage and the developing fruit, but are probably most damaging as a vector of the bacterium Erwinia tracheiphila, which causes bacterial wilt in several vine crop species.  In less than a week after an infectious beetle has fed, a wound made by a bacterially-infected cucumber beetle will result in dull green patches on the damaged leaf.  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.  Controlling the disease appears to be most effective through direct control of the insect vector (carrier), in this case the cucumber beetles.  Options for control can be found in Wisconsin’s Commercial Vegetable Production Guide (UW Extension Bulletin A3422).  Synthetic pyrethroids (e.g. Capture^® 2EC, Discipline^® 2EC, Decis^®, Pounce^®, Ambush^®, and Asana XL^®), carbamates (Sevin^®, Lannate^® LV, and Vydate^®), and a neonicotinoid (AdmirePro^® ) applied at-planting, generally constitute the range of registered products for the control of problematic beetles.  Russ Groves – Extension Entomologist

 Symptoms of Septoria Leaf Spot continue to appear on older tomato foliage as circular lesions with gray centers and dark brown borders.  This is particularly true in situations where there is minimal rotation and overhead irrigation.  Rainfall and irrigation will spread the pathogen so protective fungicide sprays are needed now for control if first symptoms are present. Walt Stevenson – Extension Plant Pathologist

 There are now reports of Downy Mildew on cucumbers in eastern Michigan, Ontario and Ohio.  Michigan State is now recommending fungicide sprays on cucurbits for downy mildew control.  See their web site at http://www.ipm.msu.edu/cat07veg/v06-27-07.htm#4 for the most current update.  The threat of downy mildew is still hundreds of miles away, but it does serve as a warning to be vigilant and to scout fields for symptoms of downy mildew.  Symptoms include diffuse somewhat angular yellow leaf lesions with a purplish fuzzy growth on the undersides of leaf lesions.  The disease moves quickly during cool and wet conditions so if you observe questionable symptoms, get a sample in to us for diagnosis.  Walt Stevenson – Extension Plant Pathologist

 Potato Leafhopper nymphs are on the rise indicating that reproduction is occurring in beans. Snap beans are very susceptible to potato leafhopper damage. These insects are very small (1/8 inch), wedge-shaped, green insects with whitish spots on the head and thorax (upper body). They have piercing-sucking mouthparts and are extremely active. They commonly jump, fly, or crawl both forward and backward when disturbed. Nymphs look like the adults; but lack fully developed wings.

 Both adults and nymphs suck plant juices fro green parts of plants, often giving leaves a whitened, mottled appearance. The first symptom of injury is a brown, triangular lesion at the leaflet tip. As symptoms develop, plants become stunted and yellow and the leaves curl upward at the margins. Premature death may occur in severe infestations. Unfortunately, yield loss occurs even before the development of obvious symptoms. The plants may show little evidence of hopperburn, and yet the yield losses can be substantial.

 For more information on potato leafhoppers, visit http://learningstore.uwex.edu/Potato-Leafhopper-P558C104.aspx.

 Onion Thrips have been identified on onion plants in parts of the state where conditions have been dry.  Adult onion thrips are tiny, pale yellow or brown insects about 1/12 inches 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 and 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 causes bulbs to become distorted or undersized. Onion thrips can also damage cabbage and cauliflower. Cabbage heads may be underweight and misshapen. On cauliflower, thrips damage causes tan or brown streaks on the curd. Damaged curds are more susceptible to soft rot bacteria.

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

 Botrytis Leaf Blight and Neck Rot of onions is a destructive disease that kills onion foliage so rapidly that it is often referred to as “blast”. All cultivated onions are susceptible to botrytis leaf blight. Botrytis neck rot also originates in the field but symptoms don’t appear until the onions are in storage. White onions are more susceptible than red or yellow varieties and leeks, shallots, and garlic are also susceptible to botrytis neck rot.

 Symptoms of botrytis leaf blight first appear as tiny, oval whitish or yellowish spots on the oldest leaves. If affected leaves are cut, you will notice that the lesions extend through the thickness of the leaf blade.

 Infected plants show little or no evidence of botrytis neck rot prior to harvest, but the disease becomes obvious after onions are topped and have been stored for a few days. Scales around the neck begin to soften and this softening progresses toward the base of the bulb. Infected tissue takes on a sunken, brownish, water-soaked appearance with a definite margin between healthy and diseased tissue. On red and yellow onions, the pigment of diseased tissue is destroyed.

 For more information on onion botrytis, visit http://learningstore.uwex.edu/Onion-Disorders-Botrytis-Leaf-Blight-Leaf-Fleck-and-Neck-Rot-P550C103.aspx.

 The first generation flight of the Cabbage Looper is declining and plants should be scouted for larvae. This worm-like insect has a greenish body 1 ½ inches long and tapers near the head. There is a thin white line along each side and two white lines along the back. The cabbage looper adult is a grayish-brown, night-flying moth that is monitored with blacklight traps. The mottled brown forewings are marked near the middle with a characteristic small, silver-white figure 8 or letter Y.

 Feeding damage caused by the cabbage looper is similar to that of the imported cabbageworm. Most of the damage appears in late summer and is caused by the second generation larvae. Head boring, is also common in early cabbage and can result in unmarketable heads.

 For more information on cabbage loopers, imported cabbage worms and diamondback moths, visit http://learningstore.uwex.edu/Caterpillar-Pests-of-Cole-Crops-P565C104.aspx.

 European Corn Borer is now in the pupation stage and the second generation adults will emerge when degree days reach 1400 which is currently being met in the southwestern part of the state. Since Corn Earworm is also becoming a problem, growers should now be monitoring populations of adults of both insects to determine treatment intervals. To view a chart on corn worm management, visit http://www.horticulture.wisc.edu/freshveg/Managing%20Sweet%20Corn%20Worms.doc.

 Tis the season to begin scouting for Powdery Mildew on vine crops. This disease becomes a problem late in the season on all species of vine crops but tends to be less common on watermelon. On susceptible crops, this disease is often severe enough to significantly reduce yields.  

Symptoms first appear as pale yellow spots on the oldest leaves. As the disease progresses, a white or brownish powdery growth covers affected plants. Eventually, affected plants wilt and die. Powdery mildew can be managed on cucumbers and muskmelons, by planting resistant varieties. On susceptible crops, fungicides must be applied every 5-7 days once the disease appears in the field to protect healthy plants.

For more information on powdery mildew, visit http://learningstore.uwex.edu/Vine-Crops-Disorder-Powdery-Mildew-P537C103.aspx. 

 Symptoms of Cercospora and Alternaria leaf blights are beginning to appear on commercial carrot plantings.  Treatment with chlorothalonil alone or mixed with one of the strobilurin fungicides will help to curtail spread and gain control.  Left untreated, these diseases can lead to premature defoliation, loss in yield and quality and weakening of the petioles that are needed for harvest. For more information on Cercospora and Alternaria, visit http://learningstore.uwex.edu/Carrot-Diseases-Alternaria-and-Cercospora-Leaf-Blights-P539C103.aspx. Walt Stevenson, Extension Plant Pathologist.

 I expect to see samples of pepper Sunscald within the next few days where the upper exposed surfaces of the earliest set peppers are brownish white and water-soaked.  Eventually these injured areas dry and are often colonized with secondary fungi giving the sunscald wounds a black color.  Fruit with sunscald injury should be removed as they will not be marketable.  Later as the plants produce more foliage, newly set fruit will be protected from direct exposure to the sun.  Walt Stevenson, Extension Plant Pathologist.

Light Cherry Leafspot and Apple Scab infections were recorded on July 9 th from Casco to Sturgeon Bay.

United States Drought Conditions

Note that most of Wisconsin is now considered abnormally dry to moderate drought conditions with Northern Wisconsin showing a greater need for water than southern Wisconsin.  The full description of drought conditions is available here:
http://drought.unl.edu/dm/monitor.html

DEGREE DAY ACCUMULATIONS

Degree-day maps for 2007 are available at:
http://www.soils.wisc.edu/wimnext/corn/corndd.html

UW Fresh Fruit and Vegetable Contacts