Leaf Spots (fungi – Ascochyta spp., Cercospora spp., Macrophoma spp., Phyllosticta spp., Ramularia spp., Septoria spp., and Stemphylium spp.): Septoria divaricata is among the most common and most destructive. It attacks primarily the lower leaves, producing dark brown, circular spots up to one-fourth inch in diameter, the centers of which are light gray, almost white. Infected leaves dry up and die prematurely. For control, spray periodically with recommended fungicides and burn infected leaves.
Powdery Mildew (fungi – Erysiphe cichoracearum and Sphaerotheca humuli): White or grayish moldy growth on upper side of older leaves. Leaves gradually die, beginning on the lower part of the plant. For control, spray with recommended fungicides as soon as the mildew appears.
Rust (fungi – Puccinia douglasii and Uromyces acuminatus f. spp. polemonii): Neither rust is important enough to warrant control. Both rusts appear on leaves as raised blisters or pustules covered with a brown coating that ruptures releasing the brown spores.
Cotton Root Rot: (See the section on Cotton Root Rot)
Crown Rot (fungi – Sclerotium spp., Rhizoctonia spp. and Thielaviopsis spp.): These fungi are destructive on seedlings. They cause a rotting of the plant at the soil line. Sterilizing potting soil is the best control.
Wilts (fungus – Verticillium albo-atrum): (See the section on Wilt)
Crown Gall: (See the section on Crown Gall)
Aster Yellows (mycoplasma): Flower petals often show white steaks and vein banding. Clusters of flowers or individual petals may turn into green, leafy structures.
Stem Blight (fungus – Pyrenochaeta spp.): Base of stem and roots may rot. They may be covered with a cottony mold. Avoid overwatering, overcrowding and planting in heavy, poorly drained soil.
Physiological Diseases: No organisms are associated with this condition. The disease is characterized by death of the lower leaves from the base progressively upward until the entire shoot is killed. The disease is most severe on old clumps and is entirely absent on seedlings or newly rooted cuttings. The trouble seems to be caused by excessive transpiration.
Root Knot Nematodes: (See the section on Root Knot Nematodes)