Canola Insect/ Disease Watch – June 28, 2012

Brian Hall, OMAFRA, Stratford

 Diamond Back Moth now evident in Canola fields

Diamond back moth has occasional been an issue in Ontario canola, and is present in a number of fields that are in flowering and pod stage.  It will be important to watch fields because of the increased insect activity this season and wide variation in planting dates. Damage to canola is caused by larvae that feed on leaves, flower buds and pods.  Infestation is difficult to predict because the adult moths do not over winter inOntario, but arrive on storm fronts from theUSAin May-June. The diamond back normally takes about 32 days to develop from egg to adult, but this can vary between 21-52 days depending on weather. There can be more than one generation a year, but usually only one generation will cause a problem, because canola will be too mature for the next generation. The larvae can feed for 10-21 days. Initially the larvae mine into the leaf but shortly after emerge and feed on undersides of leaves causing shot holes. Later the larvae move to flower buds and pods where most of the damage occurs. On pods they strip green tissue, so pods are more prone to shattering and disease, and seed yield is reduced. Larvae are light green, and at end of their body have forked tail. They can be distinguished by the fact that they wiggle violently backwards when disturbed. On plants when they are disturbed they will hang by silk thread. Damage is usually first evident on ridges and knolls in the field.  Scouting involves removing plants from about a 1 foot square area, beating the plants on white cloth to dislodge the larvae and counting the number found. At the pod stage if 20-30 larvae per 1 square foot (approximately 2- 3 larvae per plant depending on plant density) are found, control is warranted. At flowering stage the threshold is 10-15 larvae per sq foot. There are several natural enemies and with bee activity in canola fields it is important to only spray if thresholds are reached. Matador/Silencer and Decis are registered for control.  

Tarnished Plant bugs

Tarnished plant bugs are another pest that could threaten the canola crop during pod fill. The pest can affect a number of crops, and we are finding higher numbers with the hot, dry weather of late. Plant bugs are attracted to canola during flowering and pod fill. Adult plant bugs feed during flowering and pod fill and cause damage by piercing the pod and feeding on individual seeds. This year we are catching more plant bugs, although no fields to date have been at threshold for spraying. The only practical way to scout for the pest is using a sweep net. Tarnished plant bugs have rarely been a problem in Ontario canola, but in some years in Western Canada has caused significant yield losses. The threshold for control is 2 bugs per sweep after pedal fall but prior to pod maturity.

Apply Fungicides Early to Beat Sclerotinia (White mould)

The hot, dry conditions recently and open canopies in many fields have not been favourable for sclerotinia to develop, but conditions can change quickly.

If a fungicide is going to be applied the most critical thing decision is to apply a fungicide early before sclerotinia gets established.  Fungicides only offer protectant activity. Correct timing is at 20 – 50% flower stage with optimum being 20 -30% flower. This is when there is near maximum number of flowers open and before pedal drop.

Predicting the occurrence of severe sclerotinia is difficult but good soil moisture leading up to flowering and showery weather coincides with inoculums production.  High risk conditions occur when several dry days follow that allow for spore release are followed by a day with light showers. Spores are spread easily by wind, and infected petals which drop and stick to wet stems and leaves are the food source for sclerotinia spores.  Petal stick is favoured by light rains and heavy dews, whereas heavy rains will wash pedals off.  Moderate temperatures (22⁰ C) and humidity in the canopy for 2-3 days in the canopy are important for the fungus to gain a foot hold.  A thin canopy or temperatures of over 28⁰ C reduce risk of infection.

It is important not to apply too early. The goal with a sclerotinia fungicide is to achieve maximum flower petal coverage. Petals are an essential part of the infection process, providing nutrients for sclerotinia spores to germinate.  The wide range in stages of the crop this year means it is important to assess each field on its own and for the weather at the time.  In some fields, there is considerable variation in canola plant stage, which will present challenge in timing fungicide application. Last year we had a lot of wet weather towards the end of flowering when early fungicide timings would have run out.

Keep water volumes up as good coverage is important to cover as many flower pedals as possible.

Identification of Flowering Stage of Canola

Pictorial Guide to flowering stage is available from Bayer CropScience or on line at Ontario Canola Growers website http://www.ontariocanolagrowers.ca/grower_info.html

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