Anthracnose – Definite yellow spots form on leaves and stems. These turn white and have a brown border. Leaves die as do entire plants if stems are girdled.
Colletotrichum antirrhini – Heat and ventilate to maintain low humidity. Water in a manner that keeps foliage surfaces dry. Apply azoxystrobin, mancozeb, or ziram to protect healthy plants.
Botrytis Blight – Petals turn brown and are covered with gray, fungal growth.
Botrytis cinerea – Space plants to insure good air circulation. Maintain low humidity. Avoid watering late in the day. Remove crop debris. Apply chlorothalonil, fludioxonil, iprodione, or mancozeb.
Downy Mildew – Seedlings and cuttings are stunted and leaves curl downward. Gray to white fungus lightly covers the underside of leaves. Upper side of infected leaves have yellowed areas. The fungus causes a systemic infection of all leaves of seedlings. Older plants yellow, may be stunted, and die from the top down. Plants fail to flower.
Peronospora antirrhini – Heat and vent to maintain low humidity, especially when weather is cool and cloudy. Water in a manner that keeps leaf surfaces dry. Spray azoxystrobin to cover all leaf surfaces. Before replanting greenhouse beds, steam the soil because the fungus survives in debris.
Phyllosticta Blight or Leafspot – Yellow spots on leaves collapse and become black. As these age, they become yellow or tan concentric rings. Small black fungal fruiting structures for within the spots.
Phyllosticta antirrhini – Keep humidity low. Avoid wetting the plants when watering. Remove and destroy crop debris. Apply a mancozeb fungicide.
Powdery Mildew – White fungal growth forms on either surface of lower leaves, also on stems of some cultivars.
Oidium – Heat and ventilate especially in the spring and fall to reduce night humidity. Apply Ampelomyces, triforine, triadimefon, myclobutanil, or thiophanate methyl + mancozeb.
Pythium Root Rot – Root tips turn brown and die. Plants are yellowed, stunted, wilt, and die. Older plants may survive if not placed under moisture stress.
Pythium spp. Plant in pasteurized raised beds. Do not recontaminate beds. Apply metalaxyl, mefenoxam, propamocarb, etridiazole, or etridiazole + thiophanate methyl to protect healthy plants.
Rhizoctonia Stem Rot – Stems of seedlings or recent transplants collapse at the soil line. A red-brown lesion forms at the base of the stem. Older plants wilt and die with lesions at the base as noted above.
Rhizoctonia solani – Plant in pasteurized raised beds. Apply fludioxonil, flutolanil, PCNB, or iprodione as a soil drench to protect healthy plants
Rust – Small yellow swellings form on leaves or stems and burst to release rusty colored spores. Concentric rings of spores form around the initial spot.
Puccinia antirrhini – Apply chlorothalonil, myclobutanil, mancozeb + thiophanate methyl, triadimefon, mancozeb, or triforine to protect healthy plants.