- Botrytis: Wilting and death are common when infection of lower leaves occurs on tightly spaced, bed-grown snapdragons, leading to stem cankers that girdle the plants. Snapdragons grown as bedding or pot plants are also fairly susceptible to Botrytis lesions under conditions of poor aeration. Leaf spots and cankers are light tan in color.
- Downy mildew: Distorted foliage and/or leaf yellowing combined with easily visible patches of grayish sporulation on leaf undersides. Very damaging. Check incoming plugs carefully for symptoms.
- Powdery mildew: White fuzzy patches on leaves.
- Stem rot (Rhizoctonia): Brown canker at the soil line
- Stem rot (Pythium or Phytophthora): Generally root rot precedes stem invasion by these water mold fungi.
- Nematodes: In ground beds, nematodes may be responsible for symptoms of stunting or nutrient deficiency or for swollen, knobby areas on roots.
- Rust (Puccinia antirrhini): Dark brown spore pustules are strikingly visible on the undersides of leaves, while only a very faint chlorotic area may be seen from the upper surface. Symptoms develop first on the lowest foliage. Condensation or splashed water is necessary for infection, so environmental management can provide significant control of this disease. Systemic fungicides are also available.
- Impatiens necrotic spot tospovirus (INSV): Round, white, or tan leaf spots up to 1/4 in. diameter are caused by this virus on snapdragon. Stem cankers and wilting are also possible.