A number of problems can only be treated before they materialize. Peach leaf curl. Apple scab. If you've had these problems once and the environmental conditions are conducive to their development, it makes sense to treat preemptively.

This isn't the same as spraying with broad spectrum products for anything that might materialize.

I think the key is information - knowing as much as possible about potential problems and how best to address them.

I also think there are different cases and results. Weeds get pre-care because there is something called pre-emergent that will take care of the germination phase.

Snails and slugs also could be controlled before you see that they are a problem. Same for mice and rats (meaning to set out traps or bait before you see the tell-tale signs, if you are expecting them).

Certain things that can be prevented by mulching or watering in a certain way, can also be useful for prevention of said conditions.

But there are other things that you may not be able to deal with until you see them, such as tomato hornworms.

A good example for me is cutworms -- last year, I had problems, and they were taking out so many seedlings. This year, I don't (in fact I did not even see any evidence of cutworms in the same area). If I had pre-treated for cutworms thinking that they would ruin my seedlings again, I would have just wasted time and money.

Every time I wait I regret it, particularly for fungus. I start spraying my tomatoes with copper fungicide starting the time they're about a foot tall. I spray monthly from there on out.

I spray with Bt and neem as a preventative to control many caterpillars, worms and to hold back aphids as long as possible. Sluggo goes in as soon as I plant or slugs will mow down my sprouted seedlings in one night.

I haven't found anything organic that will kill cucumber beetles, though, before or after they show up. Same for grasshoppers. I just deal with them, or squash the ones I'm lucky enough to find.

Spraying cucurbits to prevent powdery mildew before it appears is essential. PM is a pain in the you-know-what once your plants have it.

Rodney

spraying for pests and/or diseases

I don't think you can lump the two together. Very different issues, goals, and consequences.

Proactive spraying of fungicides to prevent fungus diseases is required in some cases and has few negative consequences when done correctly and label directions are followed.

Proactive measures for preventing mammalian pests access to the gardens, things like fences and liquid repellents, also works and has few negative consequences.

But proactive spraying for insect pests is a different matter entirely. Not only because it doesn't work in most cases since the pesticide has to have some form of content with the pest so it wastes money, but because it has terminal negative consequences for the 90% of the bugs in a garden that are beneficials or at worst, neutrals.

Not to mention the issues of personal exposure, resistance buildup in the pests, ground water contamination, run-off, etc.

It's for those reasons that the first rule in Integrated Pest Management is #1 identify the pest. #2 is evaluate the damage done and only intervene when it is a threat to the survival of the plant. #3 choose the least toxic, pest specific form of intervention and read and follow the label directions.

Unfortunately, far too many gardeners feel the only good bug is a dead bug and even minimal damage to a few plant leaves should never be tolerated.

Dave

I'm in the "organic as much as possible" camp and there are a few things I hit proactively/preemptively. As mentioned, Powdery mildew HAS to be proactive. Once symptoms show, you're not getting rid of it.

I do the same as loribee with slugs. A little bit of the stuff really helpds until the little guys get enough foliage to fend for themselves.

I completely disagree with gardenper regarding hornworms. I know they're coming! And the only time I ever SEE them is AFTER they've stripped my plants of damn near all foliage -- they make quick work! Between the cabbage loopers and hornworms BT is a constant in my garden almost year round. If I had acres to deal with, I might find this expensive, but for the small backyard gardener, it's great peace of mind.

Cutworms -- simple! A couple toothpicks at the base of each seedling prevents them from wrapping around seedlings. Haven't lost a single seedling since I started doing this.

Kevin

I should also mention, I start spraying early, but I don't spray often. Once every few weeks tops. Because what I use doesn't harm beneficials, and I only plant about 200 sq ft of garden space, I'm not losing much by wandering around with a pump sprayer for a few minutes. I figure I'm not hurting anything, and when the bugs do show up (they always do), I'm catching them early enough so that they haven't completely taken over before I'm ready to pull the plants at the end of the season.

My goal isn't to keep my garden entirely pest free, but just to keep the numbers low enough so that we're all getting our fair share, LOL

After seeing numerous whiteflies and aphids, I have commenced a spraying sequence using these two products:

{{gwi:56515}}

So far, I have been pleased with the results.

Raybo

By the time you see cucumber beetles on curcubits they have probably already infected the plants with bacterial wilt, which can cause serious damage or outright death to the plants.
-Terry

I think proactive scheduled spraying for Powdery Mildew and tomato Blight are important to prevent it from happening I am talking about my own situation. Pest/insects control is a different issue. In most cases you have to identify it first and then fight it.
I have already sprayed my tomato plants with neem oil once. I will do it in another 2 weeks or so. With squash, cucumbers I will also do it bi weekly.

So it really depend on your situation and what you are growing.

Let's not forget to stay on top of resistant varieties, as the original poster mentioned. I won't grow a cuke that's not resistant to PM and bacterial wilt, and my tomatoes are evenly divided between OPs and the latest blight-resistant hybrids, which are amazing.