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2012 STRAWBERRY PEST MANAGEMENT SEASON BEGINS
Fighting Frost and Fungi
Note: Twilight Meeting on June 5 at 5:30 p.m. at Stutzman’s Farm in Sangerville
An early warm up in March and April got strawberry growth off to an early start, especially in fields that weren’t covered with straw. However, the trend reversed itself later and growers found themselves protecting buds and flowers from frost much earlier than usual. More recent cool, damp weather has slowed strawberry development, resulting in an extended bloom period. This stretch of wet weather may help keep spider mites and tarnished plant bugs in check, at least temporarily, as these pests tend to have less success reproducing under cool, wet conditions. However, gray mold, caused by the fungus Botrytis, and bacterial angular leaf spot can develop quite well under these conditions, and growers should be ready to apply preventative sprays as soon as possible. We will start scouting strawberry fields for major insect pests in earnest next week, including volunteer farms, in North Berwick, Wells, Cape Elizabeth, PolandSpring, New Gloucester, Dresden, Monmouth, Wayne, and Farmington, and will be reporting our findings through this newsletter on a weekly basis until harvest time. You can also get quick access to this information on the Pest Management web page at http://umaine.edu/ipm/. If you would prefer to receive this message via e-mail, please give us a call at 933.2100 or send an e-mail message to: email@example.com.
The best way to manage strawberry pests is to scout your own fields regularly and often. Start now, if you haven’t already. You should be able to identify the major pests and their damage, and be able to determine if control measures are necessary. To properly scout your fields you may want a copy of the Strawberry Production Guide for the Northeast, Midwest and Eastern Canada. This contains detailed information on strawberry pest identification and monitoring, and also provides information on all other aspects of strawberry production. It may be purchased for $45.00 per copy from the University of Maine Cooperative Extension. You should also have a copy of the 2010-2011 New England Small Fruit Pest Management Guide, which contains the latest information on management control options for the major strawberry pests as well as scouting information. Color pictures of the important strawberry pests are now also printed in the 2012-2013 New England Vegetable Management Guide. Call or e-mail us if you are interested in purchasing copies of these guides.
Situation: Early varieties are now showing some king blossoms in southern and mid-state fields. Plants that weren’t mulched or that had row covers over them are moving into full bloom or beyond. Development should speed up with some warm, dry days. We have seen some indication of winter injury in some fields, which is not surprising, given the past winter’s lack of snow cover and fluctuating temperatures. Frost injury is evident in early flowering varieties caught in the April freeze experienced in most of the state. There is a threat of frost injury for at least another two weeks, so pay close attention to weather reports and be ready to protect the buds and blossoms should the temperature dip below freezing. Bear in mind that fields that are irrigated for frost repeatedly during bloom face an increased risk of bacterial angular leaf spot.
Strawberry bud weevil or “clipper” is becoming active in fields this week. I spotted a few weevils and just a little feeding damage in three different locations. Expect these insects to become more active soon as temperatures rise and more flower buds emerge. The clipper is a small weevil, which girdles strawberry flower buds, causing them to dry up and fall off the flower stalk. Scout for damage by counting the number of clipped buds in two feet of row length at five different locations in a field. If the average number of clipped buds per two-foot sample exceeds 1.2, or if live clippers are found, control measures are recommended. Damage is usually first noticed at the edges of the field. Border sprays may be effective in keeping this insect from becoming a problem in larger fields. Fields with a history of clipper problems will typically exceed threshold nearly every year. Insecticide options for clipper include Lorsban®, Brigade®, Sevin® and PyGanic®.
Tarnished plant bugs adults have been seen on strawberry flowers this week, but the damp weather may have reduced their egg-laying success, as I have not found any nymphs (immature stages) yet. Once the eggs start to hatch, we’ll find the nymphs feeding in the flowers. The nymphs are small, active, yellow-green insects. It is important to scout for them regularly, as they can appear very quickly in warm weather. Tarnished plant bugs feed on the open strawberry flowers, causing the berries to have seedy ends. To scout for the nymphs shake 30 flower clusters (six clusters in five different locations) over a plate. If four or more of the clusters out of the 30 sampled have any nymphs, control measures should be taken. Be on the alert and scout your fields now! Insecticide options for tarnished plant bug include malathion, Brigade®, Danitol®, Thionex® and PyGanic®.
Cyclamen mites: Plants showing weak growth and yellow, pinkish or blackened, crinkled leaves may be infested with cyclamen mite. Cyclamen mites are very small, smaller than spider mites, and reside in the crown of the strawberry plant feeding on the developing leaves and flower buds. They are very hard to see, even with magnification. Infested plants have shrunken distorted leaves and flower stalks, and produce few, if any, marketable fruit. Miticides such as Thionex® or Kelthane® and Temprano® can be effective, but must be applied in lots of water to be sure that the material is carried down into the crowns where these mites reside. I haven’t found live cyclamen mites yet, but many fields were infested last year, so be on the look out for these symptoms.
Two-spotted spider mites have not yet been a problem this spring, but growers with plants under row covers should be alert. This is often where we first find mite problems. Spider mites will reproduce rapidly when warmer weather arrives, so it is important to scout for them regularly. Spider mites feed on the undersides of strawberry leaves, rasping the plant tissue and sucking the sap. Infested leaves will develop yellow flecking and a bronzed appearance. The plants become weakened and stunted. Fields that have had excessive nitrogen fertilizer and/or row covers tend to be most susceptible to mite injury. To scout for mites, collect 60 leaves from various locations in the field and examine the undersides for the presence of mites. Mites are very small – you may need a hand lens to see them. Chemical control options for two-spotted spider mites include Acramite®, Savey®, Zeal®, Vendex®, Oberon®, Brigade®, Danitol®, Thionex® and JMS Stylet oil ® (oils will cause plant injury if used in combination with captan or within 14 days of an application of sulfur).
Root weevil management
Given the relatively mild winter we should expect good survival of strawberry root weevil and black vine weevil in fields that were previously infested. Infested plants appear week and stunted, usually in somewhat circular patches in a field. Digging under the plants will reveal small (1/4″-1/2”) crescent-shaped legless grubs. Typically, the grubs begin to pupate when the plants are in bloom, but we have seen both small grubs and pupae already this spring. A soil drench of Platinum® (thiamethoxam) insecticide during the spring and/or fall when the grubs are active in the soil can provide control. However, Platinum® has a 50 day pre-harvest interval, so it is too late for applications in most fruiting fields this year. Platinum® may also be used as a pre-plant or planting treatment for root weevils. It is not too late to put on an application of nematodes to control the grubs (optimal timing is about mid-May). Two species of nematodes appear to offer the best control of root weevil grubs. Heterorhabditis bacteriophora (Hb) appears to be the best candidate for control of root weevils when the soil temperature is above 60 degrees (‘J-3 Max Hb’ from The Green Spot Ltd., http://greenmethods.com/site/; ‘GrubStake Hb’ from the Integrated Biocontrol Network, http://www.biconet.com/; ’Larvanem’ from Koppert Biological Systems, http://www.koppertonline.com/).
Nematodes are living organisms and they can be killed if they are misapplied. Order nematodes ahead of time and be ready to apply them through a sprayer or irrigation soon after they arrive, refrigerating if delay is necessary. Do not apply nematodes using a sprayer with a piston pump. Use clean equipment, removing all screens finer than 50-mesh. Apply nematodes in early morning or evening in a high volume of water to already moist soil, pre-irrigating if needed. Apply another ¼ inch of irrigation after application to wash them onto and into the soil. Researchers and suppliers recommended 250 (if banded in the row) to 500 million per acre, at a cost of about $100 per 200 acres depending on volume and source. Ironically, nematodes probably work best in the worst weevil-infested fields. High populations of weevil larvae allow explosive growth in nematode populations, while low populations of larvae may not permit efficient nematode reproduction. Strawberry plants can recover their vigor remarkably well if crown feeding has not occurred and diseases haven’t taken over the roots.
Once the adults become active in July, bifenthrin (Brigade®) will provide some control if used at the highest labeled rates. The best timing for this spray is at night during the peak feeding activity of adults, before they start laying eggs, or about the time harvest ends.
White Grubs: Weak growth noted in several fields this spring appears to be the result of white grubs feeding on the roots of newer plantings. These grubs are the larvae of beetles, most likely European chafer or Asiatic garden beetle. They differ from the larvae of black vine weevil and strawberry root weevil in that they have legs and a swollen anterior (rear end), and they tend to be larger. Their feeding weakens the plants by reducing the number of roots. The grubs can be found by pulling up weak plants and sifting through the soil that surrounded the roots. Controlling white grubs once they have become established in a field can be difficult. These tend to be more of a problem in new fields that have been planted following a grass rotation crop, because the adults prefer to lay their eggs in sod. Admire® insecticide is labeled for control of white grubs. It should be applied within two hours of irrigation or rainfall to be sure it gets into the root zone. It requires a 14 day to harvest interval.
Slugs are likely to be a problem in some fields this season. Moist conditions encourage the presence of these mollusks. Slugs usually feed at night, leaving large holes and tunnels in ripening fruit. Baits such as Deadline® and Sluggo® offer some control of slugs, but should be used prior to fruit ripening. Pay close attention to label instructions and precautions. Baits should also be applied to the fields in mid-September if slugs have been a problem, to reduce egg-laying.
Diseases: Bloom is a critical time to protect strawberry fruit against gray mold caused by the fungus Botrytis cinerea, especially when conditions have been damp. Two to three sprays of fungicide during bloom are typically required to provide good protection against this disease. If you tank mix insecticides and fungicides, avoid spraying when bees are active. Botrytis cinerea overwinters on old leaves and plant debris. Fruit infections take place almost exclusively through the flowers, so gray mold control efforts must be focused on the bloom period. If the bloom period is dry and/or good fungicide coverage is maintained, incidence of gray mold at harvest should be low.
There are several excellent fungicide choices for control of gray mold in strawberries. Elevate® (fenhexamid) has good to excellent activity against Botrytis. Captevate® is a pre-mix of captan and fenhexamid and has a broader spectrum of activity than Elevate® alone. Switch® (cyprodinil and fludioxonil), Scala® (pyramethanil) and Pristine® (pyraclostrobin and boscalid) are also excellent products for gray mold control. Topsin M® + captan is also a good fungicide combination, but remember that captan is strictly a protectant and can be washed off by rain or irrigation water. Thiram is similarly effective but also susceptible to wash-off.
The fungicides Cabrio® (pyraclostrobin) and Abound® (azoxystrobin) are NOT suitable for gray mold control, but are effective against anthracnose and other fruit rot and leaf spot diseases. All fungicides mentioned above have a 0-day pre-harvest interval, except Topsin M® (1 day) and thiram (3 days). Remember to alternate fungicides with different modes of action for resistance management purposes.
Leather rot (Phytopthora cactorum) may also become an issue when you have a lot of water in the fields during bloom and fruit development, especially if you did not get mulch on the fields last fall and the plants are on bare, wet soil. Leather rot is best controlled by growing strawberries in well-drained soil and applying straw mulch between the rows to prevent berries from touching the soil and reduce any soil from splashing up onto the berries. Foliar sprays of Aliette®, Agri-Phos® or Phostrol® (similar to Aliette®) may also provide control. Sprays should be applied during bloom and fruit development.
Red stele root rot
Although fall and early spring conditions were not especially conducive to red stele development, damp conditions this spring should make us alert for this root rot if any fields appear to be weak, stunted or dying. To diagnose red stele, pull up a few plants that look weak and scrape the roots of these plants to see if the center of the root, known as the stele, is rusty red in color, instead of the normal white. The red color would indicate an infection. Red stele is caused by Phytophthora fragariae, a soil pathogen that infects roots when soils are wet with temperatures around 50°F. The pathogen grows into the roots causing the plants to become weak, stunted and to eventually die. Symptoms are most evident in the spring, and can be mistaken for winter injury. Ridomil Gold®, Alliette® or Phostrol® are fungicides that can be applied in the late fall or early spring for control of red stele. Many varieties have some level of resistance to the disease, but the most effective management strategy is to plant only into well-drained soils, and/or plant onto raised beds.
Powdery mildew: This fungus disease has been starting to show up as purple or red blotches on the leaf petioles and flower stems in some fields. Most of us are more familiar with the later symptoms of upward curling of the leaves and white, powdery growth on the undersides of the leaves. Check your fields for pinkish purple leaf and flower stem lesion symptoms now. Abound®, captan, Pristine®, Cabrio®, Topsin-M® and JMS Stylet oil® are presently registered to control powdery mildew.
Angular leaf spot is a bacterial disease that is characterized by translucent leaf spots that may turn yellow and eventually black. The symptoms tend to start on the lower leaves but may move upwards as bacterial spores are splashed up by rain or irrigation water. Infection of the calyxes may result in a blackening of the berry stems and caps, reducing their marketability. Bacterial angular leaf spot is favored by extended cool, wet weather and nights with temperatures close to freezing. Frequent irrigation for frost protection can greatly encourage the development and spread of the disease, as will extended cool, damp weather. Susceptibility to this disease appears to vary significantly between varieties. Copper-containing chemicals, such as Kocide, Cuprofix, and Bordeaux are the only materials that have much effect on this disease. Some labels suggest adding lime as a “safener” to reduce the risk of crop injury. In susceptible varieties, start spray applications before bloom to prevent multiplication of the bacteria on the leaves before they jump to the berry caps. Application of copper sprays after bloom can result in fruit injury and is not recommended. Recent research suggests that hydrogen dioxide (OxiDate®) may also have some activity against angular leaf spot when used on strawberries as part of a gray mold management program.
Vegetable & Berry Grower Twilight Meeting on June 5
The University of Maine Cooperative Extension and the Maine Vegetable and Small Fruit Growers Association will hold a growers meeting at Stutzman’s Farm in Sangerville on Tuesday, June 5th at 5:30 p.m. Sid and Rainie have operated the farm for nearly 30 years and produce a wide range of vegetables and fruit for their stand and CSA. Sustainable practices have been an important part of their production strategy, and we’ll have an opportunity to see some of the work they have been doing with zone tillage, as well as new high tunnels, and their stand and bakery. We will have an opportunity to discuss the strawberry season and pest situation and have an update on GAP certifications and new regulations. We anticipate one pesticide applicator re-certification credit will be available for attending the meeting. Please join us! Stutzman’s Farm is located at 891 Douty Hill Road in Sangerville. You can visit their website at: http://www.mainehighlandsfarmers.com/StutzmansFarm.html. We’ll provide more detailed directions in a future issue.
David T. Handley
Vegetable & Small Fruit Specialist
Highmoor Farm Pest Management
P.O. Box179 491 College Ave
Monmouth,ME 04259 Orono, ME 04473
Maine Cooperative Extension, the Land Grant University of the state of Maine and the U.S. Department of Agriculture Cooperating. Cooperative Extension and other agencies of the U.S.D.A. provide equal opportunities in programs and employment. A Member of the University of Maine System.
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Any person with a disability who needs accommodations for this program should contact Pam St. Peter at 207.933.2100 or TDD 1.800.287.8957 to discuss their needs at least 7 days in advance.