Anthracnose (fungus – Colletotrichum antirrhini): Large, light colored spots occur on leaves and stems. Spots frequently have black or brownish dots near centers. Entire leaves may be killed and often the whole plant dies from stem girdling. Spore bearing pustules appear as minute black pimples in the spots. Under a microscope, dark brown, hair-like outgrowths are visible on these pustules. Pull and burn affected plants as soon as disease appears. Spray with recommended fungicides to prevent spread. Water by irrigating surface of soil rather than wetting the foliage. Take cuttings only from healthy plants.
Rust (fungus – Puccinia antirrhini): Brown, powdery pustules appear on lower surface of leaves. Plants may be stunted and even killed. Propagate from healthy plants or grow from seed. Control insects that carry the fungus spores from one plant to another. Plants should be spaced so that they have plenty of air. Avoid wetting the foliage when watering. Keep greenhouse temperature above 70oF for several days and not below 60oF at night. Where foliage is wet when watering, spray at weekly intervals with recommended fungicide. Good coverage is necessary for best results. Some resistant varieties are available.
Blight (fungus – Phyllosticta antirrhini): Cream colored or light brown circular spots appear on the leaves, sometimes surrounded by circles of other colors. The small black fruiting bodies of the fungus develop on the upper surface of the leaves at the center of the spots. Ash-gray spots covered with the black pycnidia develop on stems. These spots enlarge rapidly and turn dark brown. If the spots entirely encircle the stem, the plants wilt and frequently die. All parts of the plants in diseased beds should be removed and burned at the end of the season. Spray with recommended fungicides. Keep leaves dry and space more widely.
Wilt (fungus – Verticillium albo-atrum): Infected plants wilt slowly. At first, only certain branches show wilting. Plant in disease-free soil or sterilize soil with steam. When only a few plants appear infected, remove these together with surrounding soil. Water as little as possible to obtain good growth. Cotton burs used for mulch in West Texas may carry the fungus and should be well composted.
Gray Mold (fungus – Botrytis cinerea): This fungus causes a wilting of the flower stalks. A development of light brown areas on the stems at the base of the flower clusters. Under moist conditions, the spores of the fungus may develop in light brown or gray masses. Avoid excess humidity. Cut off and burn infected flower-stalks as soon as they are detected. Keep greenhouse and garden plantings free from debris because the fungus can live as a decay fungus under such conditions and its spores may be blown to living plants. Spray with recommended fungicides after the plants have been cut back to force a second crop.
Powdery Mildew (fungus – Oidium spp.): Largely confined to greenhouse-grown snapdragons, this mildew forms a white, powdery coating on both leaf surfaces and young stems. Spray with recommended fungicide.
Leaf Spot (fungus – Cercospora antirrhini): Previously reported only from Guatemala, this leaf spot was found in Florida in 1958. Spots are circular, 0.5 to 5 mm in diameter, dingy gray to white. No control measures have been developed but fungicides that control Cercospora spp. on other crops might be effective.
Root Knot Nematodes: (See section on Root Knot Nematodes)
Southern Blight: (See section on Southern Blight)
Stem and Root Rot: (See section on Stem and Root Rot)
Cotton Root Rot: (See section on Cotton Root Rot)