Brassicas are a large family of tasty vegetables which range from broccoli and cauliflower to kale and cabbage. They can be quite easy to grow in your backyard garden and to help you out, here are some common pests and diseases for brassicas in Western Australia and how to treat them organically.
Caterpillars
There are many types of caterpillars that can eat your brassicas. The most common species of caterpillars we have come across are the white cabbage butterfly, armyworm, cutworm, diamondback moth, heliothis moth, and white cedar moth. Left unchecked, they will eat your plants and leave you with no harvest.
To control caterpillars, we have found the most productive method for controlling them is to regularly inspect the plants. Check the underside of leaves for caterpillars and eggs, pick them off and squish them. For some caterpillar such as the armyworm, this is best done at night when they are active. Other methods to control caterpillar are spraying the plants with a soap spray or bacteria spray such as spinetoram or bacillus thuringiesis, often referred to as dipel or BT.
Black Hairy Caterpillar on a Cabbage Leaf 
White Cabbage Butterfly Caterpillars on a Broccoli Head
Whitefly
Whitefly is a small sap-sucking insect and can be quite harmful to your plants. In the cooler month, we have not had any issues with whitefly, but you may have some issues with them in warmer weather. A whitefly infestation will leave plants looking unhealthy because they sucking all of the nutrients out via the sap.
How you control whitefly will depend on the stage and scale of the infestation. If the problem is minor, you can just spray them with water as they don’t like to be disturbed. For larger infestations, our go-to use a vacuum cleaner to suck up the whitefly, then spray the foliage with a soap spray. Repeat this for several days for maximum effectiveness. Another option is to spray the plants with white oil or neem oil.
Slugs and Snails
Slugs and Snails are a huge pest in our backyard, consuming large quantities of our plants overnight. They tend to come out after the rain and love the cooler weather. There are lots of different suggestions as to how to control them, with varying degrees of success.
To control slugs and snails we regularly walk the garden at night, especially after rain with a bucket and gloves, picking them by hand. To kill the slugs and snails, we usually put some hot water in the bucket after collecting them. We have also found yeast traps to be effective, particularly in the aquaponics system. Aside from these, we have not found many other organic methods effective.
Slug Eating a Young Kale Plant 
Common Garden Snail Eating a Kohlrabi Leaf
Aphids
Similar to whitefly, aphids will deplete your plants of nutrients by sucking out the sap. As the weather warms up, aphids will become more prevalent. Fortunately, they are relatively easy to control, especially when treated early. To do this, spray plants with water at a high pressure repeatedly over several days. Be mindful that you want the stream of water strong enough to blast them off without damaging your plants. If you spot them when there are only a few, simply squish them with your fingers. Soap spray, white oil, and neem oil are also effective in controlling aphids in your garden.
Diseases
To date, we have not had any issues with diseases amongst our brassicas. However, you may encounter problems with root-knot nematodes, mildews, and root rot. These problems tend to occur more if grown outside of our preferred months or in poor soil. To improve your soil, here are 6 ways to boost your soil quality.
Most of the common pests and diseases for brassicas in Western Australia can be minimised by growing through the cooler months of March to September in rich and healthy soil. A healthy ecosystem will also attract natural predators that will help control pests. With regular care and management, you can prevent loss of your crops with minimal effort.