Cucurbit Scab

Nobody wants scabby cucumbers, squashes, or melons, but sometimes it happens.

Cucumber infected with cucurbit scab (UGA Plant Pathology, Bugwood.org) CC BY 3.0 US

Cucurbit scab goes by several other names, some quite colorful: cucumber gummosis, cucumber leaf blight, cucumber spotting, cucurbit fruit blight, gray anthracnose of cucurbits (even though it’s not a type of anthracnose), and, my favorite, cucurbit pox.

I can hear it now as Renaissance Faire washerwomen throw insults and dirty undergarments at the crowd: “A pox upon thy cukes!” Of course, I would never wish a pox upon anything in your garden, so let’s see what we’re up against with this one.

The what and when of cucurbit pox

This disease affects nearly all members of the cucurbit family. So cantaloupe, gourds, honeydew, summer and winter squash, and pumpkins are all susceptible. For some reason, watermelon is rarely affected. I don’t know why.

Yellow summer squash infected with cucurbit scab (Clemson University, USDA CESS, Bugwood.org) CC BY 3.0 US

Cucurbit scab is a fungal disease of the squash family caused by Cladosporium cucumerinum. This disease thrives under cool, moist conditions. It becomes a real problem when temperatures are between 57°F and 77°F. Moisture can be from heavy dew, frequent fog, or light rain.

Zucchini infected with cucurbit scab (Rasbak) CC BY-SA 3.0

Cucurbit scab symptoms

Cucurbit scab occurs around the world. Symptoms can vary quite a bit, depending on location. Generally speaking, this disease presents as dark, olive-colored, velvety lesions on the surface of cucurbit fruits. Those lesions may also be brown or black. They are usually sunken and one-eighth of an inch in diameter. These lesions occur because of the pathogen’s ability to break down pectin and cellulose. A gummy substance may ooze from these lesions. As they mature, these spots get darker and sink into the fruit, creating holes loved by fruit flies and other pests. These pits can merge into rotted areas up to two inches across. In more resistant varieties, these lesions evolve into rigid, warty structures.

Scabby pumpkin (Scot Nelson) Public Domain

Infected leaves have pale green, water-soaked areas that fade to gray. These areas may have a yellow halo. From a distance, it may look like powdery mildew. Closer inspection shows these areas are distinctly triangular, hence the anthracnose assumption. These dead areas dry and fall out, leaving plants looking quite ragged. Cucurbit scab can also shorten the internodes, making plants look stunted.

Cucurbit scab symptoms on melon stem and leaves (Mary Ann Hansen, VPI and SU, Bugwood.org) CC BY 3.0 US

Cucurbit scab management

​Prevention is your first line of defense, as with any disease. Buying certified disease-free seeds, seedlings, and mature plants can help keep spores out of your landscape. Insects, tools, clothing, splashing rain, and the wind can spread these spores, so you still have to be on the lookout.

You can prevent cucurbit scab from occurring with some simple cultural practices:

• Remove and dispose of any infected plants.
• Do not leave old vines or seeds in the field where spores can overwinter.
• Use three-year crop rotations for all of your cucurbits.
• Apply common fungicides, such as fixed copper, when conditions favorable to this disease are present.

May your cucurbits be healthy and productive.