Strawberry IPM Newsletter No. 3 — May 16, 2025
STRAWBERRY BLOOM = TIME TO PREVENT FRUIT ROT
Spider Mite Populations Growing, White Grubs Found in Weak Plantings
Situation:
The mostly cool and damp weather has slowed strawberry development across the state. Early varieties such as Wendy and Honeoye are coming into bloom. Early fields under rowcovers are in full bloom in the southern part of the state. Fields continue to look good as growers are applying light applications of nitrogen to compensate for losses that may have occurred due to all the rain. There is no frost predicted in the near term forecast, but growers should be ready to protect flower buds and bloom from frost with irrigation or rowcovers from bud through bloom. Remember that bloom is the critical time to protect against Botrytis gray mold, as well as other fruit rots, including leather rot and anthracnose. Growers with bloom beginning in their fields should start applying preventive fungicide sprays.
Two-spotted spider mites:
We continue to find mites in most fields we’ve scouted this week. While fields that have been sprayed have seen pretty good control of adults, there are still lots of eggs and emerging young. Miticides such as Savey® and Zeal®, are very effective on eggs, less so on adults. You should be scouting for mites now; collect 60 leaves from various locations in the field and examine the undersides for the presence of mites. If 15 or more leaves have any mites (25%), a control should be applied. In addition to those mentioned above, control options include Acramite®, Portal®, Nealta®, Vendex®, Oberon®, Brigade®, Danitol®.
Strawberry bud weevil:
Early fields are now in the susceptible stage for clipper feeding and bud injury. However, we have not yet found any clipped buds or weevils in the field. Begin scouting for clipped buds when the very first flowers start to open. Small holes in the petals of open flowers indicate clipper feeding. Clippers can often be found within the flower clusters, laying eggs and feeding on pollen. The control threshold for clipper injury is more than 1.2 clipped buds per two feet of row. Clipper damage is often most common in later blooming varieties. Insecticide options for clippers include Brigade®, Bifenture®, Danitol®, Sevin XLR Plus® and PyGanic®.
Tarnished plant bugs:
We have not found any plant bug adults or nymphs yet, but Glen Koehler tells me he has caught a few in apple orchards, so I expect to see them in the strawberries soon. Adults usually appear when the first flowers appear and lay eggs in the flower stalks and leaf petioles. It is important to scout for tarnished plant bugs regularly, as they may appear very quickly. The threshold for plant bug nymphs is 4 or more flower clusters infested per 30 sampled. Tap flower clusters over a plate or tray to knock nymphs off. Insecticide options for tarnished plant bugs include malathion, Assail®, Brigade®, Bifenture®, Dibrom®, Danitol®, Closer® and PyGanic®.
White Grubs:
I have found white grubs in a couple of fields this week that were showing weak, stunted growth. These grubs are likely the larvae of Japanese beetle, European chafer and/or Asiatic Garden beetle. 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 surrounds the roots. These tend to be more of a problem in new fields that have been planted following a grass rotation crop. Admire Pro® insecticide is labeled for control of white grubs and should be applied within two hours of irrigation or rainfall to be sure the chemical gets into the root zone. Admire Pro® requires a 14-day harvest interval.
Cyclamen Mites:
I have seen limited examples of cyclamen mite injury in fields this week, but no large infestations. Infested plants show weak growth and shrunken, crinkled leaves. These mites are very small 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. Portal® miticide 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.
Diseases:
Bloom is the time to protect against fruit rot because, in many cases, this is when the infection occurs. Two to three fungicide sprays are recommended during bloom, one as bloom starts and a second at full bloom to protect strawberry fruit against gray mold. Additional sprays should be applied if weather conditions, e.g. an inch or more of rain has fallen, occur that would stimulate infection. Fungicide choices for control of gray mold in strawberries include: Topsin M® + captan, Elevate®, Captevate®, Switch®, Scala® and Pristine®. Remember to alternate fungicides with different modes of action for resistance management purposes.
Leather rot (Phytopthora cactorum):
With all the rain that has fallen this spring, leather rot may be a real threat this season, especially in fields where standing water is present during bloom and fruit development. Leather rot infections can be reduced by having a thick layer of straw mulch between the rows to prevent berries from touching the soil and reducing soil splashing onto the berries. Foliar sprays of Aliette®, Prophyt® or Phostrol® applied during bloom will also help prevent leather rot.
Angular Leaf Spot:
The wet spring weather may also stimulate infections of angular leaf spot this season. Symptoms of this bacterial disease include translucent spots between the veins on the leaves that turn yellow and eventually black. Symptoms start on the lower leaves and move upwards as spores are splashed up by rain or irrigation water. Infection of the calyxes causes blackening of berry stems and caps. Frequent irrigation for frost protection encourages the spread of this disease. Kocide® (copper), can reduce the spread of this disease but spray applications must be applied before bloom. Application of copper sprays after bloom can result in fruit injury. Hydrogen dioxide (OxiDate®) may have some activity against angular leaf spot when used on strawberries as part of a gray mold management program.
Highbush blueberries are showing varying degrees of winter injury ranging from severe to moderate. If you’re pruning out winter killed branches, sterilize pruning blades as you move from bush to bush with a mild bleach or alcohol solution to reduce the potential for spreading Phomopsis. Mummy berry spores are nearly done shooting from the overwintering mummies. These infect new growth causing the shoot blight phase of the disease. Recent weather has been very favorable for mummy berry development, so growers should be applying appropriate fungicides to protect against this disease. Registered materials include Orbit, Indar, and Quilt Excel. See the New England Small Fruit Management Guide for details.
Spring Growers’ Twilight Meeting
The Maine Vegetable & Small Fruit Growers Association will be having a Spring Twilight Meeting on Thursday, June 5, at 5:30 pm at Belanger’s Farm, 262 Cotton Road in Lewiston. The Belanger’s grow a wide variety of vegetables for the wholesale market and strawberries for both wholesale and PYO. They have recently invested in new packing and storage facilities.
2024-2025 New England Small Fruit Management Guides
A printed version of the new edition of the Guide is available through our office. The cost of the Guide is $25, including shipping. You can request a copy by sending a check made out to “UMaine Cooperative Extension” to our address below.
Free access to the latest edition of the Guide is also available online on the New England Small Fruit Management Guide webpage.
You can also get quick access to this information through the UMaine Highmoor Farm news blog or the UMaine Pest Management web page.
Sincerely,
David T. Handley
Vegetable & Small Fruit Specialist
Highmoor Farm
P.O. Box 179
52 U.S. Route 202
Monmouth, ME 04259
207.933.2100
UMaine Cooperative Extension Diagnostic and Research Lab
Pest Management Unit
17 Godfrey Drive
Orono, ME 04473
1.800.287.0279
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