When growing broccoli there are a number of diseases and pests that frequently give backyard gardeners fits. The following list is not exhaustive, but it provides a general overview of some common issues impacting broccoli and other cole crops, as well as organic solutions to keep them under control.
1. Cabbage Loopers (Trichoplusia ni)
A major pest for broccoli and other crops, cabbage loopers are ravenous leaf consumers. cabbage looper is a small, smooth, green caterpillar with very thin white lines on its back and side, and it moves in a inch-worm like motion. This larvae matures into a medium-sized, gray-brown moth that lays eggs at night. Depending on the population of caterpillars in your garden, damage can range from purely cosmetic to lethal for broccoli plants.
For small-scale broccoli crops with a light infestation, hand-picking individual caterpillars is the best solution for the environment. For heavier infestations, the most effective treatment is an application of Bacillus thuringiensis (Bt) when the caterpillars are still small. Bt is also effective on a number of other caterpillar pests common to the brassica family, such as the imported cabbageworm and the cabbage webworm. For nearly complete exclusion, floating row covers can also be used to prevent the adult looper moths and other species from laying eggs among your broccoli plantings.
2. Aphids (Brevicoryne brassicae) and (Lipaphis erysimi)
Two species of aphids, turnip aphids and cabbage aphids, are commonly found on brassicas. As sap-sucking insects, large aphid infestations can kill young plants or significantly retard growth on mature plants. To determine if aphids are an issue, lift up several leaves and look for clusters of tiny white- to light-green insects on the underside of the vegetation.
The best organic treatment for aphid infestations on your broccoli crop is spraying the plants with soapy water followed by clear water. An alternative is using insecticidal soaps. You can help lower the population of aphids in your plantings by removing old vegetation and stalks from the garden immediately after the harvest and by encouraging beneficial insects, like lady bird beetles or green lacewings.
3. Harlequin Bug (Murgantia histrionica)
The harlequin bug is a cousin of the stink bug and an invasive species from the South that is currently working its way north across the United States. The pretty orange-and-black coloration belies the destructive nature of these shield-shaped, sap-sucking insects that can wipe out an entire crop when left uncontrolled. Symptoms of harlequin-bug infestation include visible adult insects and the wilting of leaves that turn brown and quickly die, which is caused by saliva injected into leaves, liquefying plant-based tissue so that the insects can feed on it.
Handpicking harlequin bugs early in the season will help limit population growth that occurs later in the season. One of the best organic controls is cultivating the ground in fall and early spring, which can kill a large percentage of the overwintering adult bugs. Another method that may be successful is planting a trap crop of mustard or kale to coax the insects away from the broccoli crop you wish to harvest. Floating row covers are also effective.
4. Downy Mildew
Gerald Holms/California Polytechnic State University at San Luis
A common issue with broccoli, downy mildew is caused by the fungus Peronospora parasitica and exacerbated by moist growing conditions. External signs of downy mildew include a gray mold on the lower surface of the leaves, while the upper leaves will often show signs of yellowing and will turn brown and die back. Internal damage isn’t visible until harvest, when you’ll find darkened florets.
The best ways to prevent downy mildew is to grow a broccoli cultivar resistant to downy mildew, such as Green Magic or Windsor. Cultural practices are also essential to fight downy mildew:
- Provide adequate spacing between plants to encourage good airflow.
- Water at the base of the plant to keep moisture off leaves.
- Remove all plant debris after harvest.
- Regularly rotate crops to lower the incidents of downy mildew.
Some organic fungicides are also available on the market, but effectiveness of these treatments varies compared to conventional fungicides.
Clubroot is a fungal infection that causes the root system to grow into poorly developed misshapen knots, which will eventually crack and allow secondary rot diseases to set in. This will either kill young plants or prevent mature plants from developing proper heads. Growers will first notice broccoli leaves starting wilt during warm days but bounce back at night. The daytime wilting will continue to worsen, and the leaves will likely start to yellow. To confirm the clubroot diagnosis, pull up a suspect plant and examine the root system for the characteristic root deformations.
Once your plot has been infected with clubroot, time is the only cure. Avoid planting any brassica crops, including cauliflower, cabbage and kale, in the area until the disease has disappeared. Take care not to transfer the spores to new plots via shoes, tools or irrigation water. Fortunately, clubroot can be prevented with regular crop rotation and amending your soil with lime to keep the pH slightly above 7.2, which is generally too alkaline for clubroot.