Moisture, temperature, wind, sunlight, nutrition and soil quality affect plant growth. If one of these factors is out of balance for the culture of a specific plant, that plant may have a greater tendency to become diseased. For example, lilacs growing in shade are more likely to be infected with powdery mildew than those growing in full sunlight. Environmental conditions also affect the growth and spread of disease pathogens. Very dry or wet weather will have an accompanying set of diseases that thrive under these conditions.
Moisture is critical to the spread of most plant diseases.
Familiar diseases, such as black spot, fireblight and apple scab require moisture to spread to and infect new host plants. Constantly wet foliage from overhead watering is a condition that promotes disease development. Seedlings grown indoors in soggy, unsterilized potting medium and pots are more prone to damping-off, a fungal disease. Each disease pathogen has a specific temperature range for growth and activity. Many powdery mildew diseases are late summer, warmer temperature diseases.