"For vegetables, your best bet is to get some drip lines 'cause you don't want to get water on the leaves" said the helpful employee at a local farm supply store. I have heard this "advice" numerous times over the past few years, and it gets a little deeper under my skin each time I hear it. Like most advice handed out in this fashion, there's a kernel of truth hiding somewhere behind layers of indirection associated with such old wive's tales.
As I mentioned in my last gardening related post, I am certainly not an expert, but I'm also not a novice. Therefore take what I'm about to tell you as nothing more than a pile of supposition from lots of reading and years of experimentation.
The nugget of fact behind "don't get water on the leaves!" comes down to, at it's most basic level, avoiding scenarios which might promote fungal or mildew growth on plant leaves. That said, leaves are meant to get wet, in fact, most leaves helpfully channel water to the root system of the plant. This past season, I marveled at how perfectly the ridges in the okra leaves bowed and dripped water directly into the root zone. However, if leaves remain wet, that can promote the growth of crop-destroying fungi and mildews.
The most common affliction most home-gardeners will likely recognize would be the ever-spiteful powdery mildew. Powdery mildew can spread from leaf to leaf and demolish an entire crop. At the end of the summer season, my friend's pumpkin patch has been nearly entirely obliterated by powdery mildew. The blight has destroyed leaves up and down the vines and even spread to some pumpkins in the patch. Words cannot quite describe the nauseating sight of a 20lb pumpkin which been engulfed with the chalk-like green of the mildew.
I can state with confidence that my friend definitely was spraying water all over the leaves of that pumpkin patch. I can also state with confidence that simply spraying "water on the leaves" was not the cause of his mildew problem.
Some key contributing factors to fungal and mildew growth can be:
Much of gardening is simply providing an environment in which the plant you're growing will have it's best success. Unsurprisingly, most plants want to live. Your job as a gardener is to ensure the most suitable conditions for the plant to succeed, without enabling other naturally occurring organisms (fungus, mildew, weeds, etc) an opportunity to themselves succeed.
It's not just as simple as "don't get water on the leaves!" Which, said alone, is such simplistic advice you might as well treat it as a pleasantry like "have a nice day!"
Smile, nod, and on your way out the door respond with a hearty "you too!"