Pest problems are most often addressed by breeding resistance for diseases, insects, and nematodes. Climate and soil factors can affect potential development of pest problems; i.e. high relative humidity and temperature combining to increase disease potential, and poor drainage and low fertility or excessive acidity or alkalinity contributing to insect infestation of already stressed plants. Nematode problems can be affected by irrigation systems spreading nematodes from field to field or mediated by growing crops that reduce nematode numbers.