Humans help spread the disease when they transport infected plants, plant parts or infested soil.
Some plants absorb excess nitrates or oxalates from the soil and store them in plant tissues. Toxicity problems can occur in animals which feed on these plants.
Excessive summer irrigation of oak and madrone trees may promote fungal diseases such as the oak root fungus (aka armillaria root disease) and crown rot.
Plant hormones and growth regulators are chemicals that affect flowering, aging, root growth, distortion and killing of organs, prevention or promotion of stem elongation, color enhancement of fruit, prevention of leafing and/or leaf fall, and many other conditions.