Zucchini leaf insect damage?


Why are my zuccini plant leaves getting decimated. I can’t see any bugs. We are spraying with a enviro friendly bug spray. Doesn’t help.


Thank you for contacting Toronto Master Gardeners about your zucchini leaf problem. There are a number of diseases and pests that affect zucchini. One of best ways to start is choosing disease and pest resistant plants. Some varieties are “Tigress”, “Green Machine”, “Burpee Golden Glory” and “Yellow Fin”. Summer Squash “Dunja Hybrid Organic” proposes to be one of the most disease resistant plants, which produces until fall. Below is a link to a website with ten generalized growing problems with zucchini.

Zucchini Growing Problems

Your question is specifically about leaf decimation and it depends on exactly what that entails. There are several insects which attack zucchini leaves: aphids, cucumber beetles, cutworms, leafminers, spider mites, squash bugs, squash vine borers and thrips. The Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) offers advice on detection and treatment of pests in Cucurbit crops (cantaloupe, cucumbers, pumpkins, squash, watermelon and zucchini). Below are two OMAFRA links to Cucurbit Insects and Diseases/Disorders.

Cucurbit Insects

Squash larvae and bugs

Diseases and disorders that affect Cucurbit leaves are: downy and powdery mildew, bacterial wilt, anthracnose, angular leaf spot, gummy stem blight, viruses, Verticillium Wilt and Alternaria. 

Cucurbit diseases

Powdery Mildew

If you have checked your leaves for insects, then the problem could be either disease or a disorder. Look through the possibilities above and closely examine the leaf damage. Check the underside of leaves, stems and soil to see if insects are evident. If using any chemicals, follow the OMAFRA guidelines closely. It’s possible with a disease such as Verticillium Wilt your soil is contaminated. Removal and replacement of soil may be necessary. It all depends on the exact detection and cause. Best of luck with your diagnosis and if you have further questions, please send in photos of the damage.

Toronto Master Gardeners have previously responded to zucchini questions and below are the links:

Zucchini Question

Zucchini Blossom end rot

Cucumber and Squash beetle

June 28, 2023