Tips to save squash plants from common ailments

Powdery mildew infects all cucurbits, including muskmelons, squash, cucumbers, gourds, watermelons and pumpkins. In severe cases, powdery mildew can cause the premature death of leaves and reduce yield and fruit quality.

Hello, Ashtabula County! We are two days from the county fair! This is the time of year that our office phones explode with home gardening questions and fair questions alike! 

During the past week, we received a number of calls on squash plant troubles. Today, I would like to share details on two of these issues, the squash bug and powdery mildew. 

Squash, pumpkin and cucumber growers may be seeing a spike in powdery mildew infection on their plants. In fact, as I was rummaging to pick a zucchini last Friday, I noticed the tell-tale signs of mildew showing up in my garden. 

Powdery mildew infects all cucurbits, including muskmelons, squash, cucumbers, gourds, watermelons and pumpkins. In severe cases, powdery mildew can cause the premature death of leaves and reduce yield and fruit quality.

Our vegetable disease specialist, Sally Miller reports the fungus that causes cucurbit powdery mildew does not overwinter in Ohio, so the disease does not appear until spores arrive on wind currents from warmer growing areas. Powdery mildew infections are favored by humid conditions with temperatures around 68-81 degrees Fahrenheit.

So what should I look for? Signs of infection are small circular powdery growths (mycelium and spores of the pathogen) on either side of the leaf. These spots enlarge and can eventually cover most of the leaf surface and kill the leaves. When the majority of the foliage is infected, the plant is weakened and the fruit ripens prematurely.

So how can I control it? Scouting is the main way! Don’t wait until the plants are dead to look at them! It is recommended that at least once a week examine 5 mature leaves for powdery mildew infection. 

Powdery mildew is primarily managed by using fungicides. Home gardeners can apply sulfur products to both the upper and lower surface of the leaves. It is important to apply fungicides when the disease first appears and incidence is low.

Gardeners should also select cultivars which have complete or partial resistance to powdery mildew. Provide good air movement around plants through proper spacing, stake up plants and keep weeds under control. 

Gardeners should also pull up and remove all plant debris at the end of the season. Our friends at the University of Minnesota have an excellent fact sheet on this disease and it can be found at: www.extension.umn.edu/garden/yard-garden/vegetables/diseases-of-cucurbits/powdery-mildew.

Keith Palmer from Cherry Valley also alerted me last week to an insect problem that gardeners should also be on the watch for. He sent me pictures of brown eggs he found on his squash. These eggs were of the Squash Bug.

Squash bugs overwinter as adults under plant debris, soil clods, rocks, log piles and buildings. Adults are sneaky and they love hiding on plants or in mulch. The adults lay numerous egg clusters primarily on the underside of the leaves.

Young nymphs in dense clusters will be easily detectable because of vivid red legs. These nymphs usually feed on shaded undersides of plants. They suck plant sap while secreting highly toxic saliva into the leaves, stems or fruit. 

Feeding on leaves produces small white dots, or stipples, and leaves will eventually appear tattered. Large numbers of squash bugs will cause leaves to yellow and die. In mid-summer it is common to see eggs, nymphs and adults all at the same time.

Excessive damage can affect plant growth and yield can be significantly reduced. So what can I do? Neem, horticultural oil and insecticidal soaps are effective when sprayed directly on nymphs. 

The adults may be difficult to kill as they hide. But gardeners can trap them by placing boards near host plants. Lift the boards and destroy bugs hiding underneath in the morning.

Gardeners should also remove all plant debris at the end of the growing season. Also, it is recommended that gardeners select cultivars of summer and winter squash that are resistant to squash bugs.

For gardeners looking for help on their gardening diagnostics, the Master Gardeners are conducting their hotline 1-4 p.m. on Monday afternoons and 9 a.m.-noon on Thursdays. They are glad to help you with your gardening problems! 

Our office will be closed during the Ashtabula County Fair but you can talk to the Master Gardeners in the Floral Building at the fair as they assist with the floral show.

To close, I would like to share a quote from the English Poet, Alfred Austin, who said, “There is no gardening without humility. Nature is constantly sending even its oldest scholars to the bottom of the class for some egregious blunder.” Have a good and safe day.

David Marrison is associate professor and extension educator, agriculture and natural resources, Ohio State University Extension. Marrison can be reached at (440) 576-9008 or marrison.2@osu.edu.