Strawberry e-newsletter 2012-2

Wednesday, May 16, 2012

A. Frost control

Frost damage has been reported on several farms this spring and the danger is not over as we enter the sensitive bloom period. The damage reported earlier this month caught many growers off-guard and seemed to be related to wind and low dewpoints associated with the freezing temperatures. Wind coupled with low dewpoints creates conditions favourable for evaporative “cooling” when irrigation is applied and can actually cause more damage than good if appropriate adjustments in your usual procedures aren’t made. In general, irrigation needs to started earlier and more water is required under these conditions. A good article that outlines the physics of irrigation for frost protection and the adjustments necessary depending on wind speed and dewpoint can be found on the OMAFRA website.

B. Weed control

For fruiting fields, a residual herbicide should have been applied prior to mulching last fall or after mulch removal this spring. Post-emerge herbicides that are available to use prior to harvest are limited to Venture and Poast. These herbicides are both effective on emerged grasses, including volunteer cereals, and have pre-harvest intervals of 30 days and 25 days respectively, which means they have to be applied prior to first bloom. They have no activity on broadleaf weeds and should not be applied within 2 weeks (before or after) Sinbar without risk of Sinbar-type injury to the strawberry plants.

For new plants, I believe a good weed management program is built around herbicides as they give us the most cost-effective weed control. Hand weeding or hoeing is still required but will be much less if herbicides are used appropriately. A good herbicide based program is a challenge in strawberries because each herbicide controls only a select number of weed species and can only be applied at specific timings in the growing season. To use them effectively the grower must know his weeds and know which herbicides have the best efficacy at those timings. Herbicide options for use in planting year strawberries in matted row include:
• Dacthal – immediately after transplanting
• Devrinol – 2-4 weeks after planting, late summer (Labour Day), or late fall (only one application per year)
• Dual II Magnum – 2-4 weeks after planting
• Sinbar – 4-6 weeks after planting, late summer (Labour Day), and/or late fall
• Venture – anytime from 2 weeks after planting through to late fall
• Poast Ultra + Merge – anytime from 2 weeks after planting until late fall
• Goal 2XL – late fall (dormant application)
• Chateau – late fall (dormant application)
(Note: Read all labels carefully for application details and cautions.)

C. Root diseases and wet feet

The thing about mild winters and frosty springs is that you have a lot of free moisture in the root zone of the strawberries for extended periods and this is just not good for root health. Red stele, black root rot, and just plain old ‘wet feet’ develop under these conditions and plants can look pretty weak and lacking in vigor. What can we do? Well, fertilizer can be helpful and if your soil doesn’t have excessive levels of phosphate an application of diammonium phosphate (DAP) is very useful to help get some new roots started and to invigorate the plants before harvest. Be careful how much you apply as you can easily get too lush growth and fruit rot problems during harvest. Depending on your situation, somewhere between 100 and 150 kg/ha of DAP is appropriate and will benefit the most with application as soon as possible after mulch removal.

Another potentially beneficial treatment is an application of Aliette fungicide. Aliette is a foliar systemic fungicide that must be applied before bloom but has excellent activity on red stele and also has activity on Pythium species which are part of the black root rot complex. Closely related ‘phosphite’ foliar fertilizers have also been shown to be beneficial for managing these root diseases.

D. Fruit rot

Bloom is the most significant period for infection and the time when we need to apply protectants. Typically three applications of fungicide are recommended, corresponding with early (10%), mid- (50%), and late (90%) bloom, and these are usually about 7 days apart. It is always best to apply fungicides before expected rainfall although some products have some level of systemic activity and may control new infections. There are numerous options available for fruit rot management in strawberries and these can be reviewed in AgraPoint’s Strawberry Insect and Disease Management Schedule. A number of these can be applied up to harvest and should be considered if conditions are wet through bloom and up to harvest. Also, many of the fungicides are a high risk for resistance development with repeated use so it is always best to rotate among chemical families.

E. Mites and insect pests

I’ve been in a few fields where high levels of two-spotted spider mite are already evident so I encourage you to scout your fields for this pest. Pre-harvest miticide choices include Apollo (15 day pre-harvest interval, primarily for eggs and nymphs), Nexter (10 day PHI, primarily for nymphs and larva), Agri-Mek (3 day PHI, primarily for adult mites) and Oberon (3 day PHI, primarily for eggs and nymphs). Read labels carefully for appropriate rates and other cautions.

Also, fields should be monitored for strawberry bud clipper weevil and tarnished plant bugs as we enter the bloom period. To scout for clipper weevil begin by examining flower trusses for clipped buds or the adult weevil among the flower buds and blossoms. The adults will also often chew circular holes in the white petals of opened flower buds. A V-shaped sampling pattern incorporating 5-10 locations should be used and a 2-foot section of row should be examined at each location. An average of 0.6 clipped buds per foot of row is generally considered the economic threshold for treatment although new research has shown that a threshold of up to 5 clipped buds per foot of row would be more appropriate for many varieties.

For tarnished plant bug, tap 30 blossom clusters per field. The economic threshold to spray is if you exceed 15 plant bug nymphs per 30 clusters. Late fruiting varieties are most vulnerable as plant bug populations tend to increase through the season.

Be prepared – plan your monitoring and control strategies in advance! Let’s not get caught off guard as controls are limited during harvest.