Overcrowding causes weak growth and subsequent insect and disease problems. Keep down weeds and grass. They often harbor pests and compete for nutrients and water. Use a mulch to reduce soil splash, which brings soil borne diseases into contact with lower leaves. Water in the morning so that plants have time to dry before the cool evening.
Drip irrigation prevents foliage from getting wet at all when watering.
Select disease-resistant varieties and plant crops that are suited to the soil and climate. If you do plant vegetables or fruits that are not normally grown in your area, do your best to provide necessary conditions. Use certified seed and plants if available. Remove infected leaves from diseased plants as soon as you observe them. Dispose of severely diseased plants before they contaminate others.