Is Your Zucchini Plant Turning Brown? 4 Top Causes and How to Fix Them (2024)

Not much takes the wind out of our sails like a wilted plant. When it's zucchini that had big, bold, green leaves just yesterday you might wonder what caused this drastic change overnight.

Zucchini is a prolific, easy-to-grow vegetable that produces harvests nearly all summer long. When a pest or adverse growing condition affects your crop, symptoms show up quickly; plants wilt and the large, palm-shaped leaves develop yellow spots or turn yellow.

Here are four problems that crop up in the zucchini patch and how to prevent or fix them.

Under Watering

Zucchini leaves often wilt down in the hottest part of the day. Soil moisture is the first thing to check before you start looking for pests or other causes. Zucchini are not drought tolerant and underwatered plants develop yellow leaves that eventually turn brown and dry up.

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When to Water Zucchini Plants

If your zucchini plants look droopy in the morning do a soil moisture check. Meters are available that give you a reading or you can poke your index finger into the soil. If it comes out dry, you need to water.

Zucchini need at least an inch of water each week. A layer of mulch helps keep soil moist and cool. Water at the base of your plants instead of overhead to discourage foliage problems that lead to wilting and yellowing.

Squash Vine Borer

The biggest menace to squash is the squash vine borer, (Melittia cucurbitae). This day-flying moth has a distinctive, black-spotted, orange abdomen and emerges in late June after overwintering underground.

Squash vine borers lay eggs singly at the base of your zucchini plants. The egg hatches into a brown-headed, white grub that bores into vines hollowing them out and preventing water and nutrients from reaching the leaves. Starting out, your zucchini plant may wilt only in the heat of the day. But when leaves appear wilted in the morning, yellowing soon follows and the plant will succumb and die. Signs of squash vine borer activity include a sawdust-like build-up at the base of the plant called frass.

How to Treat Squash Vine Borer

Prevention is often the best approach to insect pests and environmental issues. Starting in mid-June, try spraying the center and base of your plants with a pesticide formulated for grubs and caterpillars. Organic alternatives include diatomaceous earth and the deterrent BT.

Follow label directions for the frequency of treatment. Use a floating row cover to prevent egg-laying activity.

When the plant already shows signs of infestation, removing the grub can sometimes revive it. Use a sharp knife to slit open the top of the vine. Be careful not to cut through the bottom. Gently open the vine until you find the white grub. Scrape it out with your knife but try to limit damage to the vine. Bury the opened part of the vine under the soil and check to see if the plant recovers.

Squash Bugs

Squash bugs, (Anasa tristis), are as prolific as zucchini plants, themselves. That's because adult females lay reddish-brown eggs in clusters of 15 to 40 on the undersides of leaves. Sap-sucking nymphs hatch out in numbers and can damage leaves quickly causing yellowing and wilt. Adults also feed on developing fruits.

How to Treat Squash Bugs

Once they establish squash bugs are hard to get rid of. Prevention is the best approach and organic sprays along with hand-picking are more effective than harsh chemicals which can kill pollinators necessary for a healthy crop.

1. Practice good garden hygiene by removing debris at season's end and tilling soil either in late fall or early spring.
2. Check the undersides of leaves regularly and scrape off any eggs.
3. Place wooden boards around the zucchini patch. Squash bugs shelter underneath at night and can be hand-picked in the morning.
4. Horticultural oils sometimes eliminate newly hatched nymphs but are ineffective on adult populations.
5. Plant blue hubbard squash as a trap crop and hand-pick pests that congregate.

Powdery and Downy Mildew

Powdery mildew looks like white chalk dust on zucchini foliage and usually appears first on the newest leaves. It's an airborne fungus that thrives in high humidity even in dry weather. Downy mildew causes yellow spots to form between leaf veins and a brownish-gray fuzz on the undersides of leaves. It's caused by a pathogen rather than a fungus and appears in wet, humid weather.

Zucchini are less vulnerable to these pathogens than cucumbers and melons but can still be affected.

How to Fix Downy and Powdery Mildew

1. Adequate air circulation is critical to preventing leaf infections. Plant zucchini the recommended distance apart. You can even remove some leaves at the base to improve airflow within the plant.
2. An appropriate fungicide can work to manage powdery mildew once it sets in. For downy mildew use neem or horticultural oil.
3. Remove infected leaves early and treat the remaining foliage.
4. Remove and discard severely infected plants from the garden to avoid disease spread.