The best defense against insect pests and plant diseases is to make sure your plants are growing in a good, healthy soil so the plants can grow up strong and healthy and better resist those problems. If you have not already had a good, reliable soil test done contact your local office of the University of Connecticut USDA Cooperative Extension Service and inquire how to do that and then dig in with these simple soil tests to see what you have and what you need to do to create that good, healthy soil.
1) Structure. From that soil sample put enough of the rest to make a 4 inch level in a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and replace the lid, tightly. Shake the jar vigorously and then let it stand for 24 hours. Your soil will settle out according to soil particle size and weight. A good loam will have about 1-3/4 inch (about 45%) of sand on the bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above that, and about 1/4 inch (about 5%) of organic matter on the top.

2) Drainage. Dig a hole 1 foot square and 1 foot deep and fill that with water. After that water drains away refill the hole with more water and time how long it takes that to drain away. Anything less than 2 hours and your soil drains too quickly and needs more organic matter to slow that drainage down. Anything over 6 hours and the soil drains too slowly and needs lots of organic matter to speed it up.

3) Tilth. Take a handful of your slightly damp soil and squeeze it tightly. When the pressure is released the soil should hold together in that clump, but when poked with a finger that clump should fall apart.

4) Smell. What does your soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant odor? The more organic matter in your soil the more active the soil bacteria will be and the nicer you soil will smell.

5) Life. How many earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5, according to the Natural Resources Conservation Service, indicates a soil that is not healthy.
If problems arise you start with the least toxic means available to control that. Bacillus thuringiensis - Kurstaki is a good control for leaf eating larva of many insect and is of low toxicity to other insects. If Powdery Mildew is a problem start with either mixture, 1 teaspoon of baking soda in 1 quart of water, or a 50/50 mix of fat free milk and water, and spray as necessary. Neem products are of relatively high toxicity and should be used with care.

Here is a link that might be useful: UCONN CES