Leaf Spots (fungus – Aschochyta spp., Septoria spp., Cercospora spp., Alternaria spp., Phyllosticta spp.): Leaf spots may form in any stage of growth. Flower parts and stems may also be affected. Older leaves are generally affected first. Foliage may gradually wither and die starting with the lower leaves. Overhead sprinkling or watering will increase severity of disease. Spores of the causal organisms may be seed borne and use of seed treatment fungicides will reduce disease incidence. Planting in wet, poorly drained areas should be avoided. Foliar fungicides should be applied at the first sign of disease and continued at 7-10 day intervals.
Foot Rot (fungus – Phytophthora cryptogea): A rapid, permanent wilting of the leaves and a brownish-black discoloration appears on the lower portion of the stem. The stem may shrivel and collapse at or near the ground line. Roots may be affected by a soft, water-soaked decay. Adventitious roots may form close to diseased portions of the roots. Diseased plants are pulled easily from the soil, but outer bark of the roots sloughs off and remains in the soil. Plant only in well drained soils. This disease is soilborne and once the soil is infested, asters should not be replanted.
Root and Stem Rot (fungus- Rhizoctonia solani): The fungus may attack roots, stems and lower leaves. Symptoms on lower leaves first appear on those leaves in contact with the soil. Affected leaves are characterized by a well defined water-soaked area that progresses rapidly during cool, moist conditions. At first, the leaf is a dark green; it then wilts and turns dark brown. The stem and root may also be attacked directly. The infected area is first water-soaked in appearance; it later becomes sunken and turns a dark reddish-brown. Control is best obtained by planting only in sterilized beds. Beds should be well drained. Soil fungicides applied as a drench around infected plants may be effective.
Wilt (fungus – Fusarium oxysporum f.spp. callistephi): Both young and old plants may be affected. Young plants wilt and die rapidly. Older plants may exhibit two types of symptoms. One side of a plant may be stunted with a yellowing of the lower leaves on that side. When the stem is cut, a brown discoloration of the vascular tissue is found on the infected side of the plant. In the second type of symptom, lower leaves are yellow and wilt. This is followed by wilting and death of the entire plant. Generally this occurs when the plant is in full bloom. The lower portion of the stem may be dark brown when the stem is cut. Most effective means of control is with the use of wilt resistant varieties such as Gem mixed, or Dwarf Queen.
Powdery Mildew (fungus – Erysiphe cichoracearum): This disease is characterized by a white, powdery growth on the upper leaf surface. Older leaves may be affected with little distortion but young leaves become twisted and misshapen. Young growing portions of the plant may be completely infected resulting in dwarfing and curling of the leaves, stems and buds. Often infected buds fail to open. Effective control requires spraying plants with a recommended fungicide at the first sign of disease.