Strawberry IPM Newsletter No. 3 – May 24, 2013
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COOL, WET WEATHER SLOWS STRAWBERRY DEVELOPMENT
Risk of Fruit Rot Diseases Increased
Situation: A very dry spring has quickly turned into a very wet spring for most farms, as nearly a week of cool, rainy weather has put more than two inches of water on the fields, and slowed plant growth significantly. Plant development has changed very little over the past week, with early varieties in southern Maine now moving into or just beyond full bloom, and later varieties coming into bloom. Fields that were under row covers have green fruit up to an inch in size. So ripe fruit should not be far off, if we see some warmer, sunnier weather soon. Insect activity has also been slowed by the weather, but these conditions create a high risk of fruit rot infections, and fields coming into bloom should be protected with fungicides when conditions permit.
Diseases: Bloom is the critical time to protect strawberries from developing gray mold caused by the Botrytis cinerea. The rain of the past few days has made conditions very conducive to fungal sporulation and flower infection. Two to three sprays of fungicide during bloom are often required to protect against gray mold. Fungicide choices for control of gray mold in strawberries include captan/Topsin M® tank mix, Elevate®, Captevate® (a pre-mix of captan and Elevate®), Switch®, Scala® and Pristine®.
Leather rot (Phytopthora cactorum) can be a problem when there is water standing in the fields during bloom and fruit development, especially if the fields were not mulched last fall and the plants are on bare, wet soil. Leather rot should be managed by growing strawberries in well-drained soil and applying straw mulch between the rows to prevent berries from touching the soil and to reduce soil splashing up onto the berries. Foliar sprays of Aliette®, Agri-Phos® or Phostrol® can be applied during bloom and fruit development to prevent leather rot when there is excessive moisture in the field, especially in fields with a history of this problem.
Anthracnose fruit rot is a potential problem when fruit ripens in fields that are wet from irrigation or rain. This fungus disease is favored by warm, humid conditions and can spread rapidly during rains or when fields are irrigated with overhead sprinklers. In cool seasons, it may appear close to harvest or may not show up at all. Anthracnose fruit rot is identified by black sunken lesions with wet, orange (and sometimes gray) spore masses in them. The fungus can survive and multiply on leaves without visible symptoms, appearing suddenly as a fruit rot when the conditions are right. Fungicides such as Cabrio® and Abound® can provide good control of anthracnose.
Powdery mildew: No severe symptoms of powdery mildew have been observed yet, but we’re starting to see some leaf cupping in fields, suggesting that this disease may become a problem when temperatures start to rise. Upward cupping of the leaves and reddish streaking or lesions on the leaf and flower stems are the most obvious symptoms of powdery mildew. White, powdery growth on the undersides of the leaves appears as the disease develops further.
Strawberry bud weevil or “clipper” is becoming active as flower buds emerge. I have found one field with clipper at levels over threshold this week, but most fields were showing very little, if any damage. Early blooming varieties may have developed beyond the point where clipper can do significant harm to buds, but later blooming fields should still be scouted for injury. Clipper is likely to become more active when conditions become a little drier and warmer.
Tarnished plant bugs adults are few and far between this week, which is typical in this kind of weather, but I still have not found any nymphs in blooming fields. It is difficult to find nymphs on wet plants, so we may see activity increase considerably when things dry out. But for now, no fields have been over threshold.
Cyclamen mites: We continue to see symptoms of cyclamen mite in strawberry fields this week, including weak growth, crinkled leaves and yellow, pinkish or blackened discoloration. Sprays for these mites must be applied with lots of water to carry the material down into the crowns where the mites reside. Portal®, Thionex® and Kelthane® are registered for control.
Two-spotted spider mites were not found over threshold in any fields this week. Cool, wet weather tends to significantly slow spider mite development in strawberry fields, but expect populations to rise as conditions become warmer and drier, and keep scouting.
White grubs: We have found white grubs in three different fields this week, most likely the larvae of Asiatic garden beetle or European chafer. These grubs have legs and a dark, swollen rear end. They are found in the soil around the roots of weak plants. Admire Pro® can be used to control white grubs and should be applied within two hours of irrigation or rainfall to get the chemical into the root zone. This product has a 14 day pre-harvest interval.
Spring fertilizer for strawberries
Heavy nitrogen fertilizer applications are not recommended in the springtime because excess vegetative growth at this time will result in a dense leaf canopy that will cover flowers and fruit, and encourage gray mold and two spotted spider mites. In the matted row system, most of the nitrogen fertilizer should be applied following harvest to stimulate new leaf and runner growth. For established beds, only 10-20 pounds of actual nitrogen should be applied in the spring. Calcium nitrate [Ca(NO3)2, 15% N] is a recommended source of nitrogen at this time because it is readily available, not volatile, and also provides calcium, which can help with fruit development. Boron is another nutrient which can help early spring growth, and is especially important in the pollination and fertilization process that helps determine fruit size and quality. However, excessive amounts of boron can be toxic to strawberry plants, so only one to two pounds of actual boron (B) is recommended per acre during the spring. This is often applied as a foliar spray in a material such as Solubor® (20% B). While foliar sprays are often an inefficient way to get nutrients to the plants, they are helpful when trying to evenly distribute a small amount of material over a large area.
What about pollination?
After a hard winter for bees and lots of rain during bloom, growers are concerned about good pollination in their strawberries. Cultivated strawberry flowers are self-fertile, meaning that they don’t require cross-pollination with other varieties like some other fruit crops, such as apples or blueberries. Some studies have shown that strawberries will produce a good crop in the absence of bees, with pollination being carried out by wind and small, native insects. Other studies have shown an increased fruit quality and size when bees help with pollination, so it is certainly good to have them working in the strawberry fields, but perhaps not essential for this particular crop.
2013-2014 New England Small Fruit Management Guides are available from the University of Maine Cooperative Extension at Highmoor Farm. This new, updated edition of the guide contains the latest information on management options for small fruit pests as well as cultural information. Cost of the guide is $10.00 plus $2.53 postage for a total of $12.53. To order a guide, please send your check made payable to UMaine Cooperative Extension mailed to: Highmoor Farm, P.O. Box 179, Monmouth, Maine 04259, atten. Pam St. Peter. For more information, contact Pam St. Peter at 933.2100 or firstname.lastname@example.org.
David T. Handley
Vegetable & Small Fruit Specialist
Highmoor Farm Pest Management
P.O. Box 179 491 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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