Heat blast and 6 other causes for missing pods

As canola starts to move from flowering and into pod formation, growers will often notice blanks up the raceme where pods did not form. Here are 7 possible reasons:

Heat. Hot days (28-30°C and up) and warm nights (16°C and up) from bud to mid-flowering stages can have a devastating effect on canola yield. Cool nights offer some recovery from hot days. Warm nights do not provide a recovery period, and more flowers are aborted, producing blanks along the stem. Even with a few days of heat, it can take a week for hormone balance and regular pod formation to return. Most years, seeding early so canola flowers during cooler parts of the summer is an effective strategy for higher yields. Recently opened flowers with shortened stamens that don’t protrude above the petals can be a sign of heat damage. Click here to see Murray Hartman explain how heat affects canola reproduction.

Drought. Moisture stress during reproductive stages can cause a hormone imbalance that disrupts pod formation and seed set. In a drought, canola plants will progressively drop later flowers at the top of the main raceme and on branches, and put energy into preserving the pods it has. In late season drought during pod fill, seeds that do form may germinate in the pod due to the hormone imbalance. If moisture returns, plants can start preserving flowers again, resulting in a stem with pods at the bottom and top but nothing in the middle. Additional symptoms of drought include leaf wilting, and flowers with smaller petals that may be off-color (pale or yellowish-orange), or flower buds that die before fully opening.

Late off-label applications of herbicides and spray tank residues or drift can damage buds and flowers, resulting in blanks. For example, glyphosate applied after the 6-leaf stage on Roundup Ready canola may result in pale petals of normal size and short stamens that don’t protrude above petals in recently opened flowers. There can be significant pod sterility and blanks as a result. Late applications may also use higher rates because weeds are bigger, which increase the yield loss. Look at sprayer misses or overlaps for differences to confirm this diagnosis.

Male sterility. The F2 generation (volunteer plants) will have the maximum amount of variability, resulting in up to 25% sterility. In some HT systems, there will be loss of herbicide resistance, and extreme variability of traits such as height and maturity. This is why it is not advisable to seed bin run canola that was grown from a hybrid (in addition to the legal ramifications). A very small percentage of plants in hybrid canola may be male sterile as its difficult with any system to have 100% hybridity at all times. Male sterile plants require cross pollination from neighbouring pollen-producing plants, and as a result will produce fewer pods and many blanks. Male sterile plants often tend to stick up above the rest, and are scattered throughout the field at low numbers, often making the situation look worse than it really is. Male flowers will not have evident stamens on recently opened flowers.

Sulphur deficiency can reduce pod and seed set. Look for stunted growth, light-coloured leaves on newer leaves vs older leaves, purpling and cupping on stem leaves during bolting, and pale, smaller flower petals, especially on hill tops where S deficiency tends to be more common. Stamens will protrude normally on S deficient plants.

Boron deficiency can reduce fertilization, leaving blanks up the stem. Plants with boron deficiency will often flower longer to compensate for poor seed set or may not flower at all, and may have leaf blotches ranging from pale yellow to reddish purple. Boron deficiency symptoms will show up in the younger leaves and growing points because boron is not mobile in the plant. Boron deficiency tends to be location specific, with only patches of the field (usually sandy areas) showing blanks. The challenge with boron is that application studies on the Prairies have not shown significant or reliable results. Boron may only cost $6 for the full rate, but third party studies, including UCC, have not shown a yield benefit.

Insects. Lygus bugs and diamondback moth larvae feed on buds, killing them. They may only kill a few buds per cluster, so the plant produces pods here and there up the stem. Dark stains on flower petals and shrivelled brown pedicels are signs of insect feeding on buds. (Pedicels will be intact after heat or drought damage.) Swede midge also damage flowers and bud clusters, resulting in missing pods and densely clustered pods on shortened racemes — a “bouquet” appearance.