Adding more acid and pressure canning would be needed. Zuccini and yellow summer squash are high in water, and also are fairly low density, but would dilute the borderline amount of acid in tomatoes. Today, we can never know how much acidity is in tomatoes due to many variabes like climate, soil, and types of tomatoes. Thats the main reson citric acid is added to home canned tomato products. If you have a friend that is doing this without adding extra acid, and also cans in a boiling water bath, it can be dangerous to consume. Quite honestly, I find zuccini tasteless and most yellow squash also just too watery. About the only yellow type I like is crooked neck as it seems to have a bit more flavor. Home caning orange squash like butternut, pumpkin, etc, are just too dense for home canning. For a spagetti sauce, adding dried chopped onions, peppers helps to give it more texture as can tomato chunks that have only been peeled and deseeded.

How about if I puree the squash? And I would certainly add lemon juice or something to the same effect. I will go dig in my canning books again. Maybe I missed a good recipe.

Keep in mind that there are many canning books out there and some have not been updated for many years. Puree of the squash might help some, but then you lose that texture you were looking for. In any case, because its low acid, and is low in acid, more added acid is very necessary. The pressure canning of this may be the best way to go. For me, I grow my own plum type tomatoes which are meatier, they get cut open, the liquid inside is dug out along with most of the seeds. The rest is put through a food strainer like a Villaware/Weston, etc. This will remove all skins and any leftover seeds. The end result is a thicker sauce that needs less cooking down due to the fact the watery liquid has been removed before its strained. If I wish to add some tomato chunks, I will blanch some and cut them in pieces and add them to the sauce. Dried sweet peppers and dried onions are great to add as they soak up more of the liquids. I like thick, non watery sauces, and for texture, I will cook up italian sausages and/or meat balls and mix them in when serving. I recall someone adding cinnamon to a sauce, as well as shreaded carrots. So everyone has their own taste preferences. Its just that if its to be canned at home, some concessions and common sense most be considered.

And pureeing the squash would only compound the problem as it would change the density of the sauce. Change the density and a different processing time is needed and that processing time would be just a guess.

Not to mention that canning summer squash and zucchini are not recommended:

Why is canning summer squash or zucchini not recommended?

Recommendations for canning summer squashes, including zucchini, that appeared in former editions of So Easy to Preserve have been withdrawn due to uncertainty about the determination of processing times. Squashes are low-acid vegetables and require pressure canning for a known period of time that will destroy the bacteria that cause botulism. Documentation for the previous processing times cannot be found, and reports that are available do not support the old process. Slices or cubes of cooked summer squash will get quite soft and pack tightly into the jars. The amount of squash filled into a jar will affect the heating pattern in that jar. It is best to freeze or pickle summer squashes, but they may also be dried. NCHFP Guidelines

There are safer ways to provide the roughage her family needs. ;)

Sorry but this is just one more example of trying to force the safety requirements to change to fit the recipe instead of the other way around. ;)

And please note Ken's point that many books are badly outdated.

Dave

Please, stick with recipes that are tested for safety from Ball Blue Book or the USDA tested ones from NCFHFP, for example.
You have no idea if the ph level is correct, nor the density.
That is a cooking recipe and not for canning. You cannot safely make up your own recipe and can it, not even in a pressure canner.