The top 4 diseases of zucchini & how to protect from them organically

Last week, we talked about powdery mildew which gets spread across your zucchini inadvertently by ladybirds, so this is an ideal time to look at the top four diseases that affect zucchini and – most importantly – how you can protect from them organically.

Zucchini can be plagued by Downy Mildew, Bacterial Wilt, Yellow Mosaic Virus and Botrytis Blight. These four diseases can develop rapidly and once they take hold, they can be pretty challenging to cure, so the best way to deal with these is prevention!

Disease #1: Downy Mildew

What is Downy Mildew and how does it affect my zucchini?

Downy Mildew is a common problem for zucchini from spring to early summer. The disease can stunt your plants’ growth or damage your plant and it is very hard to diagnose. This disease is not the same as Powdery Mildew, so best not to get these confused. Downy Mildew is caused by parasitic organisms that are quite closely related to algae. Downy Mildew requires the presence of water to spread and survive, so if you are in a dry area, you will be less likely to experience this disease.

Downy Mildew can appear as a kind of fuzzy growth that can vary in colour from yellow to grey, white or even purple. It usually starts on the lower leaves and can appear as spots on the leaves.

The difference between Downy Mildew & Powdery Mildew

How can I prevent Downy Mildew on my zucchini?

The best way to prevent this disease is to water your plants at soil level and not on the leaves, as this disease needs water to survive.

Disease #2: Bacterial Wilt

What is Bacterial Wilt and how does it affect my zucchini?

This disease usually appears early in the season and affects melons and pumpkins and squash. It overwinters within the cucumber beetle. When spring is upon us, the beetle starts to feed on the young plants, infecting the stems and leaves as it goes. It starts with the leaves and usually spreads downwards rapidly until the whole plant is affected. It will affect the fruits of the plants, which will either be of a strange shape or wilted. When touched, they can ooze a milky substance that is quite stinky.

How can I treat Bacterial Wilt on my zucchini?

The best thing to do if your plants are affected is to pull them out and dispose of. Do NOT compost as the disease will live on and keep reoccurring. To prevent, try to catch the cucumber beetles when you see them.

Bacterial Wilt

Disease #3: Yellow Mosaic Virus

What is Yellow Mosaic Virus and how does it affect my zucchini?

The Yellow Mosaic Virus is a seed borne disease, which causes slow production of fruit, defectively formed fruit – or even stunts fruit production completely.

How can I prevent Yellow Mosaic Virus on my zucchini?

The best way to prevent this is by purchasing true organically-certified seed. This disease really has no cure and can spread quickly to other plants, so put a bag over it and pull it out to prevent it spreading.

Yellow Mosaic Virus

Disease #4: Botrytis Blight

What is Botrytis Blight and how does it affect my zucchini?

Botrytis Blight is a fungus that really only attacks the tender young parts of the plant when there is high humidity, so when summer really kicks in. The flowers can change colour and wilt and the buds can fail to open. The outer petals of the flowers start to get a browning on the outer edges. Leaves and shoots with this disease have masses of grey spores or brown lesions. Sometimes the leaves can die and even drop from the plants. Fruit will rot and drop off.

How can I prevent Botrytis Blight on my zucchini?

Remove and destroy all infected parts of the plant. Botrytis blight can overwinter in the soil. This disease can be spread by water splashing, wind and high humidity. It can infect plants in their vulnerable areas of broken stems or where a plant has been cut.

Botrytis Blight

Prevention is the key, and the best way is to give your plants plenty of space to enable airflow. Make sure you use hygienic methods with cleaning and storing your tools as spores can easily spread this way too. Clean up dead leaves from around your plants and mulch them to prevent splashes from the soil. Remember – never compost infected plants.

Happy gardening!

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