Story Posted: August 04, 2011

Canola Watch: Late season insects, Harvest prep

Issues of the week: Diamondback moth, bertha armyworm, cabbage seedpod weevil and lygus bug are at or near thresholds in areas. Keep scouting to avoid losses due to late season insects.

High temperatures in previous weeks have really advanced canola growth and development. In the wetter areas, fields have dried out and crops are looking better. Some canola crops in driest areas are ripening prematurely.

Many fields have two or three distinct stages of growth due to hail, moisture stress, etc. The effects of weed control challenges and poor crop competition earlier in the season are becoming apparent as mature weeds begin to rise up through the crop canopy. Growers are beginning to ask questions about crop dry down and pre-harvest weed control vs. desiccation.

Many fields are podding with some fields beginning to show signs of seed colour change. This is a great time to observe patches of premature ripening and scout to determine why. This year it is easy to assume that soil moisture stress is the cause but take a closer look to rule out late season disease, insect, or soil fertility issues.

Crop and weather update
Peace: Decent rain events fell in the region where they were needed (in the north and central Peace). Crops in the north look much better than they did a couple of weeks ago. Canola on heavy clay soils in the wetter southern Peace are starting to deteriorate – where water was standing a few weeks ago, root rot is now starting to appear. Crop staging is variable with the later stages just coming out of flower. Swathing is still about a month away.

Alberta: Heat during recent weeks was welcome in central Alberta and the crop outlook has improved. That area still has adequate moisture to finish the season. Crops that were damaged by hail or were sheared-off by strong winds around Paradise Valley are beginning to reflower. Crop staging in the south is advancing nicely and is about three weeks from swathing. There is some irrigation happening around Lethbridge and the Medicine Hat area could use additional rainfall soon. Alberta crop report.

Saskatchewan: Extremely variable weather in parts of western areas recently. Thundershowers moved through the area with strong winds and heavy rainfall. There will be significant sclerotinia in parts of the northwest. Crops in the southwest (south of Rosetown and Kindersley) look very good and could use additional rainfall to fill. Canola crops in eastern areas look good. Generally crops in the province are coming out of flower and earliest swathing is about 10 days away. Saskatchewan crop report.

Manitoba: Rain for the week was variable (zero to 1 inch). Majority of areas need moisture after several weeks of extremely hot and dry weather. Thin, late seeded stands are not handling the heat/drought as well as the thicker stands either early or late seeded. Staging generally ranges from flowering to seed color change with some swathing occurring on very earliest fields. (No Manitoba crop report during election.)

Insect update
Lygus: Lygus is still being found in southern and central Alberta. Populations are higher in southern Alberta and northwest Saskatchewan and control decisions are being scrutinized on a field by field basis. Many fields are nearing the end of the susceptibility stage (when pod walls thicken/become leathery about 2 weeks after flowering). Below is a table outlining economic threshold for lygus.

Diamondback moth: Adult counts in traps are generally low — but higher in the Peace and eastern Manitoba. Spraying has occurred around Beausejour, Manitoba. As fields come out of flower and are podding, growers in areas with high counts need to scout aggressively to assess the threat of larval damage.

Cabbage seedpod weevil: The weevil is near thresholds in pockets of southern Alberta, but numbers are down from previous years. Click here for more information.

Bertha armyworm: Some spraying for bertha is occurring in the Peace Region with counts ranging from 2 to 13 larvae per square metre and in Manitoba's Interlake (around Teulon and Arborg) and Central Region (around Glenboro and Mariapolis). Higher numbers are being monitored in central Manitoba as well. The table below indicates the larval density (larva per square metre) at which an insecticide treatment in canola would be warranted. Twenty larvae per square metre in canola can reduce yields by 1.16 bushels per acre (Source: MAFRI). Click here for more information.

Grasshoppers: Some field edges have required spraying for grasshoppers in the Peace, notably when pastures or hay fields were adjacent. An economic threshold of 7 to 12 grasshoppers per square metre is recommended for canola. Click here for more information.

Root maggots: Plants are toppling over in some fields in central Alberta and Manitoba. Growers think disease is the cause, but when they pull up a plant they find the roots full of maggots.

Thrips: There have been reports of thrips stripping tissue off stems and the top layer of tissue on canola pods in Manitoba but this damage is not economic.

Pay attention to pre-harvest intervals
The pre-harvest interval is the number of days that must pass between the last application of a pesticide and cutting of the crop. Cutting is either swathing or straight cutting. In the table below, insecticides with the shortest pre-harvest intervals still require 7 days between spraying and cutting, so if the field is within a week of cutting, then spraying is no longer an option. In the case of lygus control, many pods are hardening and lygus will not be able to penetrate effectively to cause damage anyway.

Canola patches ripening pre-maturely? Take a closer look
As the canola crop nears the end of the growing season, signs of premature ripening are quite prominent. This year it might be easy to assume that soil moisture stress is the culprit. However, it is important to scout and take a closer look to determine what has happened in those areas. It might be moisture, but could also be disease or insect or fertility issues. Taking the time to assess the crop can help with future crop management decisions.

• Break open the stems of any bleached white plants and look for hard black sclerotia bodies forming inside the stem. High levels of sclerotia returned to the soil this fall can increase the risk of sclerotinia next year in canola and other susceptible crops such as sunflowers, field beans and lentils. If sclerotinia is present, assess the % of infected plants since half of this number will give a yield loss estimate. If fungicide was applied, evaluate that decision. Was a check strip used to determine if the disease was at least suppressed? Is it obvious infection occurred late in the flowering period, suggesting a later or split application may have been more effective? Was a fungicide not applied and perhaps should be under similar conditions in the future?
• Look for blackleg. Check for black rimmed stem lesions and basal stem cankers. If a high incidence (anything greater than 15% of plants) of blackleg is found, then this field should not be used in a tight rotation – consider rotations of one in three or one in four years. Also, the next time canola is planted on this field, ensure it is not the same variety, especially in tight rotations.
• Dig up some plants and check for galls from clubroot or root rot symptoms. If clubroot is confirmed, proper equipment sanitation to prevent spread will be critical. Click here for more on clubroot.
• Look for signs of insect damage that may indicate missed opportunities for control and a need for more thorough scouting in future years. Pod stripping and holes chewed in pods by bertha armyworm or diamondback moth larvae will be obvious. Less obvious are the tiny holes in pods and shriveled or devoured seeds beneath them, which suggest lygus bug and cabbage seedpod weevil feeding. Look for significant root maggot channels on the roots to determine if cultural control strategies should be considered in future years. Take note of fall flea beetle numbers. While high numbers in the fall do not always correlate with high numbers in the spring, these are the adults that will overwinter and emerge to feed on canola seedlings next year.
• Blank or missing pods are often found during this time. Click here to hear Murray Hartman, Oilseed Specialist with Alberta Agriculture and Rural Development, explain what causes this. Click here for a CCC factsheet.

Canola needs a month after flowering to mature
Good growing conditions in recent weeks have accelerated crop growth and development. Growing conditions play a major role in canola development, but as a rule of thumb, a crop requires 50 days to the start of flowering and another 50 days from flowering to swathing. Many fields across the prairies are coming out of flower. If they flowered for 20 to 25 days (as an average), then an additional 25 to 30 days are needed to finish off the growing season. At this rate, the majority of crops that are finished flowering should be swathed or ready to swath by the end of August.

Tips to prepare for harvest
Many crops are nearing the end of flowering or podding. Harvest decisions should be contemplated in the coming weeks. The following are tips to consider during the lead up to harvest.

1. Scout for disease prior to swathing. Assess levels to determine if premature ripening or pod damage is due to diseases like blackleg, sclerotinia stem rot, clubroot, or alternaria and may necessitate swathing earlier than normal – before seed shatter starts. It is important to determine which of these diseases is present in your field for future rotation planning. If blackleg or clubroot is found, then rotations should be lengthened to prevent the disease from building up and threatening neighboring fields and the effectiveness of our genetic resistance. If the disease is sclerotinia, then an assessment of the % of infected plants will help determine the yield loss and an estimation of whether fungicide applications would have been warranted. Click here for the disease assessment card. Fields with lots of diseased plants will be poorer candidates for straight cutting due to the increased shattering risk.

2. Hail damage can cause crops to mature unevenly. When swathing a hailed crop, assess the seed-colour change at the earliest and highest-producing parts of the field, especially if the delayed maturity creates a high risk for fall frost damage.

3. Check weed pressure. Green biomass in the swath will extend the curing time, and weed seeds and green plant material can be starting points for heating in storage. Fields with high weed counts may benefit from a pre-harvest spray to dry them down.

4. When to swath. The best timing for yield and quality is when 50% to 60% of seeds on the main stem are turning from green to brown. Click here for a time of swathing video.

5. Lay swaths parallel to the direction of prevailing winds to reduce the odds of cross winds flipping swaths. This reduces the risk of swaths blowing and pods shattering.

6. Set the roller low enough to anchor swath into standing stubble with slight pressure. Use a roller that tucks swath edges down into stubble.

7. Large dense swaths tend to take longer to condition and cure canola before combining. Thin swaths lying flat on the ground may also take longer.

8. Straight combining tends to work best on thick stands with plants meshed together to prevent whipping in the wind.

9. Tune up the combine with lots of tips from the recent combine clinic in Alberta.

Harvest aids for uneven crops
Fields with variable stages need to be scouted carefully to determine which stage will contribute most to overall yield. If later stages are the minority and the greatest yield contribution is from earliest stages, then maybe there is no need to hasten maturity.

If growers decided they need to spray to even out or dry down the crop, the choices are glyphosate or Reglone. Reglone is a contact herbicide (only kills what it contacts) and is registered in canola to dry immature green material to facilitate harvest. Reglone does not hasten crop maturity. It shuts the plant down quickly and basically STOPS it from maturing, which can lock in high green seed levels if applied prematurely. The recommended timing for application on canola is when 60 to 75% of the seed has turned brown, which is past the stage when swathing would typically be recommended. Applying Reglone earlier may result in higher green seed. The label indicates that it should be only applied to napus canola to facilitate dry down of lodged canola crops. Efficacy will be maximized with higher water volumes (91 to 222 L per acre ground or 18 L per acre aerial). Growers using Reglone on canola to be straight combined take note: Reglone can significantly increase both pod shatter and pod drop if harvesting is delayed, so be prepared to combine as soon green seed and seed moisture have reached suitable levels.

Glyphosate is registered for pre-harvest perennial weed control in canola. If a field can be sprayed during the longer, warmer days in August, then glyphosate can accelerate crop dry down. However, applications in September (when days are shorter and tend to be cooler) should be made with perennial weed control as the main goal. Any benefit to crop dry down should be seen as a bonus because at this time of year it can take weeks to see green material dry down. If the canola is glyphosate tolerant, then obviously no crop dry down will occur. Glyphosate is to be applied when the majority of seeds within the pod are yellow to brown in colour which is at least 30% seed colour change (which corresponds to when seed moisture is less than 30%).

Should I use a pod sealant?
Pod sealants are polymer sprays meant to reduce shattering when straight combining. Recent western Canadian research at a limited number of trial locations has to date not indicated significant benefits overall, but some individual trials and producer experiences have occasionally been positive. If farmers do use them, leave a test strip to assess whether the sealant made a difference on yield. Ensure application timing is optimal and water volumes are adequate to maximize the odds for success. Crops that are hail damaged or have a high disease incidence, such as alternaria, may not be good candidates for a pod-sealant. Lesions or hail scars on pods have damaged the outer waxy layer and the physical integrity of the pods and have increased the risk of pod shattering and have the potential to disrupt the pod-sealant's effectiveness.

Sunscald appearing
Sunscald has been appearing in some fields in many areas. Sunscald occurs when plants are ripening during periods of heat or other stresses. The main symptom is purpling on the stems and pods. The purpling is an abiotic stress response and does not cause yield loss. The purpling is likely due to higher levels of anthocyanin pigment and a lack of chlorophyll in the naturally senescing tissue and often shows up more in some varieties than others. Confirm sunscald by checking the underside of the pods or branches (areas not exposed to the sun) for normal pod colour.

	Sunscald

Blackleg appearing
Some canola fields in eastern Manitoba and northern Saskatchewan are showing signs of blackleg infection. There is no effective fungicide for canola at this growth stage (protective sprays had to be applied early in the season) but growers are encouraged to scout and assess the level of damage before they harvest the crop.

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