Introducing the Squash Vine Borer Resistant Zucchini Project

It's a new year, and I'm rethinking what I do with this blog. It had gotten boring and tedious for me, but after taking a break for a while, I thought about some things I could write about that might make things more interesting.

One of my Christmas gifts was The Tao of Vegetable Gardening by Carol Deppe. I already had Breed Your Own Vegetable Varieties and The Resilient Gardener, so I was really glad to receive her newest book.

When I first started this blog, I was unemployed and gardening was a way to feel like I was doing something productive with my time. Now I have a full time job, but gardening is still very important to me for a lot of the same reasons Deppe talks about in her books.

She talks a lot about seed saving and plant breeding, and says that all gardeners should do it. I've been saving seeds for years, but I think it's finally about time I try my hand at a breeding project. If nothing else, it sounds like a lot of fun, but maybe it will also result in me getting a really useful new plant variety, or maybe even distribute to other people in my region.


A project like this should have a clear goal, so my goal is to breed a zucchini squash that can produce a good crop despite the bane of all people who attempt to grow summer squash in my area: the dreaded Squash Vine Borer.

Because of SVBs, the idea of having too much zucchini so you have to sneak some onto your neighbor's porch is a fantasy to gardeners around here. Squash Vine Borers are moths (actually really cool looking moths) that lay their eggs on squash plants. The caterpillars then bore into the squash plant’s stem and eat them out from the inside until the whole plant collapses and dies. They especially like Cucurbita pepo and Cucurbita maxima. (Cucurbita moschata and Cucurbita argyrosperma are resistant.)

Most summer squash is in the species C. pepo. Whenever I try to grow zucchini, pattypans, or yellow crooknecks, I might get a squash or two before they collapse from getting eaten from the inside by those darn borers. I haven't even tried growing any of the winter squash varieties of C. pepo like delicatas or acorns because I'm sure they won't make it to maturity.

But I have an idea for a solution! Tatume is the only C. pepo summer squash variety I’ve found so far that gives me a good crop before succumbing to the borers. I think that’s because unlike most summer squash varieties, it has long vines instead of a bushy growth habit. That allows it to outgrow the borers and root at the nodes. Borers still get into it, but I still get a decent crop of squash from it. Most other summer squashes die before I get any fruit at all because they have such short stems that the borers eat them up really fast.

People like bush squash because they take up less room, but I don't think that helps any when you don't even get any squash at all from them before the borers kill them. I know I'm kind of going backwards here, but I want more vigorous, viney summer squash varieties instead of compact bush varieties.

It also makes sense for Tatume to do well here because it’s a South Texas heirloom. Having long vines is only one of its advantages. It's also much more tolerant of heat and drought than other C. pepo squash. It probably has other useful genes that help it survive in Texas besides just the gene for a vine growth habit.

The fruits of Tatume are sort of like an oval shaped zucchini. They are green when at immature eating stages, and turn yellow when mature, looking a lot like a small spaghetti squash. They taste just fine, so if Tatume is just fine, why not just grow Tatume and that will be my one variety of summer squash I can grow?

Part of the reason is just for the heck of it, so I can try out a breeding project. But I also think it might be good to have more varieties of C. pepo I can grow here than just one. Crossing other varieties of C. pepo with Tatume could give me all kinds of different fruits on Tatume's drought tolerant, heat tolerant, squash vine borer tolerant vines.

I decided to start with crossing Tatume with a zucchini. Maybe later I can try with other varieties, but I think zucchini is a good place to start because zucchini isn't as different from Tatume as some varieties. I won’t have to deal with trying to change the fruit color that way, since Tatume is already green like a zucchini. They're also both good as summer squashes. Starting out crossing Tatume with a winter squash like a delicata might be complicated because then I'd have to select for fruits that are good as winter squash. I'll see how it goes with the Tatume/zucchini cross and then think about making other Tatume crosses.

The next step was to choose which variety of zucchini to use for the cross, since there are several. Carol Deppe and several other people consider Costata Romanesca to be the best zucchini, so last year I tried it out. I got the seeds from Bountiful Gardens. I planted six plants, and got three fruits, one on one plant and two on another plant. The other three plants all died from borers before producing fruits, though they did make some male flowers that may have contributed pollen. I ate one fruit off the plant that had two to see if it really did taste as good as people say, and it did. I let the other two fruits mature to see if that was even possible, and they did manage to mature before the plants died of borers, but just barely. I saved the seeds from the survivors, and I also have 16 of the original seeds left in the packet.

Costata Romanesca seems like a good zucchini, so now I have the parents for my new squash variety picked out. Next comes deciding which will be the mother (to get fruits and seeds from) and which will be the father (to get pollen from). The choice here is obvious. Tatume needs to be the mom and CR will be the dad. It’s easier for a plant to make male flowers and pollen than a fruit with mature seeds, so the mom should always be the better survivor of the two. I just barely got any fruits when I grew CR, so it makes a lot more sense to get pollen from CR and have Tatume make the fruits.

That means this year I should make my cross. I will need to grow both varieties, and get some female Tatume flowers pollinated by male Costata Romanesca flowers. Then the F1 seeds inside that Tatume fruit will have Tatume for a mom and CR for a dad.

After I get the F1 seeds is when things get tricky. Then what do I do for the next generation? Self the F1’s or cross back to one of the parents?

I want my new variety to have long vines, not bushes, so I needed to find out which is the dominant trait. I looked it up and, bad news, vines are recessive to bush. It’s a great example of how the “wild type” is not always the dominant gene. That’s going to make it harder than if vine was dominant. You’ll see why.

I don’t know what the real abbreviation for the gene for vine vs. bush, so I’ll use B and b. That means Tatume is bb and Costata Romanesca is BB. The F1 hybrid would then be Bb. That means my F1 generation will all be bushes since bush is dominant. Not good news, since bushes are the ones that are more susceptible to SVBs.

If I backcross the F1 to Tatume I’ll get more vines but also more Tatume-like fruit. If I backcross to CR I’ll get more zucchini-like fruit but also more bushes, so it looks like the best thing to do will be to self the F1’s. I’ll grow all F1’s that time around and let them cross with each other. I’ll have to give them extra pampering so I’ll get some fruits, since they’ll all be bushes being attacked by SVBs.

The seeds I get from that batch will be the F2 generation. That’s when things get interesting. When they are grown, if Gregor Mendel was right, they will be ¾ bushes and ¼ vines. Finally the vine trait will be back! In that generation, it will be easy to weed out the bush plants and only leave the vines. Carol Deppe says one way to do this is to sow the seeds thickly and then thin out the smallest plants pretty early on. The ones left will be mostly vines, since the viney plants grow faster than the bushes, long before their final growth form is obvious.

If I cull all the bushes early on I will just have F2 vines left to produce fruits, and then I can finally start selecting for fruit characteristics. The F2 vines should have a mixture of different fruit types, so I can start selecting for the most zucchini-like ones, and not saving seed from the Tatume-like ones.

Here’s my plan year by year:

Year 1: Grow Tatume and Costata Romanesca and pollinate female Tatume flowers with male CR flowers to get the F1 seeds.

Year 2: Grow out the F1 seeds, which will all be bushes with whatever fruit characteristics are dominant. Let them cross with each other.

Year 3: Grow out the F2 seeds. Pull out bush plants and leave only vines. See what kind of fruits the vines make. Save seeds from the most zucchini-like fruits.

Year 4: This depends on what the F2’s from last year were like. I might have to grow out some more F2’s, or I might try growing some F3’s. At this point space will be my main limitation. Since I can only grow out maybe a dozen plants at a time, I'll just have to see if I get lucky and find some that have the kind of fruits I want.

This means it will be at least three years before I even have a chance of having a vining zucchini like I want. However, since both Tatume and Costata Romanesca are perfectly edible summer squashes, I can still eat any of the squashes I don't want to save seeds from. There's not much to lose.

One thing that might speed things up is if I can grow two crops a year, since we really have two short growing seasons here rather than one long one. The spring summer squash crop is usually dead by July or August of either SVBs or heat and drought, but I might be able to grow a fall crop that might have time to mature before it freezes. It would be worth a try if I have enough seeds. That means I could get to the interesting F2 generation by Year 2 instead of Year 3. Then by Year 3 I'd already be at the stage where I have only vines and am selecting for fruit characteristics.

So that's the first breeding project I'm going to do. I've got some other ideas for things I'd like to try, but I'd better not get ahead of myself. This will be a good start.

Take that, Squash Vine Borers.