Several insects can be harmful to canola at various times of the year. The following pose the most serious threat:
Black sand mite/redlegged earth mite (Halotydeus destructor) and blue oat mite (Penthaleus major) – April to June
Adult mites are about 1mm long, with pear-shaped black bodies and red legs. Blue oat mites are also 1mm long, dark green to black, with a red patch on the front and back, and red legs. Sand mites feed on seedlings, causing silvery-white spots that eventually become withered and discoloured. Blue oat mites produce a scorched appearance to the leaves. Heavy infestations reduce stand, retard growth and lower yields. Quick-growing seedlings and adult plants can withstand moderate infestations.
Lucerne earth flea (Sminthurus viridis)
These insects are 1mm to 2mm long, greyish white and soft-bodied, with a strong, forked jumping organ under the abdomen. This enables them to leap long distances. They climb up the plants, eating tissue from the underside of foliage. Older nymphs eat through the leaves, creating ‘windows’. Heavy infestations are frequent in lands where canola succeeds pastures.
Cabbage aphid (Brevicoryne brassicae) – April to October
Aphids migrate from host plant species and populations can increase quickly when the temperature is high. Conversely, cold weather or good rain can reduce populations. Heavy infestations during flowering will prevent the development of flowers. Heavy infestations after flowering will adversely affect the seed fill of pods.
Diamondback moth (Plutella xylostella) – July to September
This is a small moth with a diamond pattern on its wings. Light green larvae create holes in the leaves and feed on pods, causing them to shatter more easily. Moths occur at the end of the stem elongation period, but can appear earlier during warmer spells.
Bollworm (Helicoverpa spp) – August to September
The colour of the larvae varies from light green to dark brown. Smaller larvae are harmless to canola as they feed on the surface of pods and leaves only. Larger larvae tunnel into the pods, leading to yield loss. Canola should be inspected regularly from the flowering stage onwards for infestations.
There are two diseases of major economic importance in canola:
A fungus spread by rain-splashed spores, windborne spores and infected seed. Spores land on the leaves of seedlings and then penetrate the leaf, causing lesions. These cause the stem to rot near the ground and impair the movement of nutrients to the pods. The plant then falls over and dies. In areas where blackleg is virulent, four-year crop rotation and resistant canola varieties are essential for controlling this disease. All plants that host blackleg should be destroyed. If possible, plant far from old, infected lands.
Sclerotinia stem rot or white mould can be destructive during periods of wet weather. The fungus survives for up to seven years in the soil and produces millions of airborne spores. Canola is especially susceptible during bloom stages and shortly after. Infections that start on the dead blossoms spread to adjacent tissue, resulting in dead branches or plants. The rotted stems usually have a bleached appearance.
A minimum rotation of four years is recommended for lands with a history of Sclerotinia infestations. During this rotation, avoid planting highly susceptible crops, such as sunflower, lupins and dry beans. Also avoid contaminated seed.
Source: Canola Production Guideline, compiled by the Directorate Plant Production in collaboration with ARC, published by the department of agriculture.