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Heat blast in crops – check your canola

By: Matthew Bernard, AAg, MSc, Provincial Specialists, Oilseeds

High temperatures during crop flowering can cause stress. Symptoms resulting from environmental stress can be similar to those caused by nutrient deficiency and insect damage, especially as crops mature. Due to the substantial variability of maturity this year, even within the same field, the severity of heat blast may differ throughout a field. It can be alarming to scout your field and notice deformed flowers, abnormal pods or blanks along the stem. Before deciding on a management strategy, it is important to identify the cause and determine whether action is required.

Brassica (B.) crops such as canola (B. napus) and brown and oriental mustard (B. juncea) are cool-season crops that can be negatively affected by high temperatures (heat blast). High temperature stress can shorten the flowering period and reduce the amount of time the flower is receptive to pollen, as well as lower pollen viability. This will reduce the number of flowers that are pollinated, causing blanks along the stem. Plants subjected to high temperatures during flowering will also demonstrate abnormal pod and seed development, with fewer seeds per pod. Yield loss due to heat blast results from fewer pods and seeds per pod, as well as a lower seed weight.

Morphological symptoms on spring canola
Morphological symptoms on spring canola bud (top),
opened flower (middle) and mature pod (bottom).
Non-heat stress symptoms on bud, flower and pod
(20/15 C) (left) and heat stress symptoms (32/20 C)
(right). Photo used with the permission of Laxhman
Ramsahoi M.Sc. University of Guelph from M.Sc. thesis.
The most extreme symptoms will occur during early flowering (the first 14 days) when there are hot days, warm nights and dry conditions, and when the reproductive stages of the plant are most vulnerable to these environmental extremes. Although there has been some research indicating heat blast causes damage when temperatures exceed 30 C (Kutcher et al., 2010), negative effects are often observed at 28 C and above. There may be some variability in heat tolerance across varieties, but newer varieties are still susceptible to heat blast at that 28-to-30 C threshold. When nights remain warm (16 C and above), there will be an increase in heat stress symptoms, with more aborted flowers and blanks on the stem (Photo 1).

Crops damaged by heat blast might be capable of some recovery once temperatures come down, especially if the plant was past the critical reproductive phase of flowering when exposed to extreme heat stress. Recovery could be in the form of additional flower development after heat subsides, retention of flower and pods that would have been aborted, or compensation by pods on branches (Angadi et al., 2000). If seed formation occurs in these deformed pods, there is risk of deformed seed due to compression resulting from lack of space inside the pod (Angadi et al., 2000). Cool nights will offer some relief and allow the plants to recover from high daytime temperatures. There is little else that can mitigate the damage caused by heat blast.

During drought conditions, there may also be boron deficiency; although some research has investigated the effects of boron application to compensate for some damage resulting from heat blast, results show minimal and inconsistent positive effects and thus it is not a recommended practice at this time (Philp, 2015).

When compared to B. juncea, B. napus was less able to recover from heat blast, resulting in many deformed pods afterward (Angadi et al., 2000). Yellow mustard (Sinapis alba) is more tolerant to heat and drought than either B. juncea or B. napus (Brown et al., 2005).

Flower abortion and blanks in the stem can also be caused by other abiotic stresses such as poor fertility or herbicide damage. The important thing is to look at the entire plant. In the case of nutrient deficiencies and herbicide damage, the symptoms will not be isolated to flowers or missing pods, and there will also likely be foliar symptoms. 

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