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Environmental stresses – from heat blast to wet feet

By Barbara Ziesman, AAg, Provincial Specialist, Plant Disease 

Wet feet

Over the last few days, many areas across the province received high volumes of precipitation. The largest impact of the recent precipitation events will be on plant disease. Most diseases of field crops in Saskatchewan are caused by fungi and are favoured by humid, moist conditions.

The recent rain events coupled with dense crop canopies and potential lodging may increase the risk of diseases. Lodging within crop canopies may increase plant-to-plant spread of both monocyclic and polycyclic diseases. Crop types will also respond differently to high-moisture conditions. For example, pulses, such as peas and lentils, do not like wet feet and become stressed under wet conditions. As a result, when conditions are wet, the risk and occurrence of root rot diseases may increase.

The decision to apply fungicides after the rain events should be based on the crop canopy density, staging of the crop and fungicide label restrictions. It is important to follow label requirements and pre-harvest intervals. Applying products within the pre-harvest interval may result in fungicide residues remaining within the harvested grain and influence marketability. Crop canopy density will influence the efficacy of fungicide applications. Most fungicides on the market are contact or have limited systemic activity in the plant. As a result, adequate coverage of plant material is critical. In a dense canopy, it will be difficult to obtain adequate coverage of susceptible plant tissue and limit the effectiveness of the fungicide in managing the disease and preventing yield loss.

Heat Blast

Morphological symptoms on 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.

High temperatures during crop flowering can also cause stress. Symptoms resulting from environmental stress can be similar to those caused by nutrient deficiency and insect damage, especially as we move into crop maturity. 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.

Canola is a cool-season crop and can be negatively affected by high temperatures (heat blast), with sensitivity being highest during early flowering. 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. The most extreme symptoms will occur during hot days (above 29 C), warm nights (above 16 C) and dry conditions during early flowering. Cool nights will offer some relief and allow the plants to recover from high daytime temperatures. When nights remain warm, there will be an increase in heat stress symptoms, with more aborted flowers and blanks on the stem. 

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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