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How to Prevent Squash Vine Borer and Powdery Mildew on Squash, Organically

Many organic gardeners who have grown squash in the southeast US will think this must be a practical joke. It’s not! There are chemical-free ways to grow as much squash as your “conventional” neighbors. Then you can finally participate in Sneak Some Zucchini Onto Your Neighbor’s Porch Day. No really, that’s an honest-to-goodness national holiday on August 8th every year.

The best solution for powdery mildew is a spray of 1 part milk to 9 parts water coating the leaves every 1 – 2 weeks. Use the spray when there is full sun (morning is best) before the disease sets in and you’ll have a 90% prevention rate. I’ve already written an extensive article on this remedy (siting the scientific studies backing it). Here it is:

How to Spray Milk to Prevent Powdery Mildew Disease

Photo Caption: This is the perfect size to pick a ‘Tromboncino’ summer squash (C. moschata), though it is edible at any size or stage. If left on the vine it will harden into a butternut winter squash. We love the funny “spoon” shape.

Squash Vine Borer (SVB) is one of the most difficult pests for organic gardeners to control. As diurnal moths, they are active in the daytime instead of at night. SVBs are also a wasp mimic, but can’t sting you.

What they can do is decimate a squash plant almost overnight, leaving many gardeners confused about the culprit. SVB adults lay their eggs on the base of the plant and then the maggot-like caterpillars burrow inside the stems to eat. Symptoms of SVB include leaves wilting even though the soil is moist and wet brown frass (caterpillar poop) collecting at the base of the stems. SVB damaged plants are much more susceptible to other problems like Powdery Mildew.

SVB larva spend their time eating the plant from the inside out, which means they are notoriously difficult to remove. Sprays, powders, and other traditional methods are largely ineffective. Hand-picking can only be done by slitting open the squash stem and finding the tunneling caterpillar. This can have some success, especially if the damaged stems are then covered with soil so the squash can put out new roots.

However, it’s a pain! I don’t do it anymore.

Instead, I plant my summer squash and zucchini as early as possible each season. SVBs show up late enough in the summer that I can get a decent harvest before my plants start succumbing. I’ve already gotten enough squash and zucchini this year to make me feel like my handful of plants were worth it.

But I don’t give up on having squash all season, either. Squashes are in the genus Cucurbita and most of the summer squashes & zucchini humans have bred for the garden are from the species C. pepo. SVB moths prefer this species above all other squashes. They also like squash, pumpkins, and gourds bred from C. maxima. There isn’t much hope keeping SVBs away from these two, so I also grow summer squash and pumpkins bred from C. moschata to take over for me later in the season. C. moschata varieties have tight, narrow stems that the SVBs don’t seem interested in.

They also have some yummy fruits! Any unripe C. moschata variety can be eaten like a summer squash when it is young enough that the skin can be pricked with your fingernail. They tend to be pale green like a zucchini and have a nearly identical flavor and texture. If you’re more interested in winter squash, wait until the fruit hardens and the stem connecting it to the vine starts to turn tan.

My favorite C. moschata variety is an Italian heirloom that was actually bred to be eaten like a zucchini. I buy it from Pinetree Garden Seeds under the name ‘Zucchetta Rampicante Tromboncino.’ I’ve also seen it sold at places like Territorial Seeds under the simplified name of ‘Tromboncino.’ The flavor is fantastic and the vines produce like… a zucchini! They don’t have a bush habit, though — make sure to use a sturdy trellis for their long vines. Near the end of the season I leave some of the fruits to mature into giant, 4′ long butternut squashes.

Photo Caption: This mature ‘Tromboncino’ from last year is almost as long as me! At this stage it can be stored as a winter butternut squash.

I also have a favorite pumpkin. It is slightly flat and a beautiful muted pink, like something out of a fairy tale.

Photo Caption: This harvest from last year shows a mature (butternut) ‘Tromboncino’ squash on the left and a ripe, round ‘Long Island Cheese’ pumpkin toward the upper middle of the photo.

Even better is the pumpkin’s name of ‘Long Island Cheese.’ Perhaps it isn’t romantic to be named for looking like a wheel of coagulated milk, but it is certainly memorable. They’re delicious! I’d consider ‘Long Island Cheese’ to be one of the best flavored heirloom pumpkins. We especially love them in pies and curries.

Right now we have summer squash, ‘Tromboncino’ squash, and ‘Long Island Cheese’ pumpkins producing in the garden (though the pumpkins are far from ripe). I know I can count on the ‘Tromboncino’ variety to take my summer squash’s place when the SVBs really get going.

Now if anyone knows of some surefire solutions for squash bugs and pickleworms, we’ll be all set! (My great-uncle tells me that squash bugs are easier to hand pick early in the morning when the dew is still on the leaves). If you have any other suggestions please leave them in the comments.

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