Sweet Alyssum (Lobularia)
Plant Health Problems
Diseases caused by Fungi:
Root rot and wilt, Rhizoctonia solani.
The above-ground symptoms of this are nonspecific and include a general yellowing, wilting, and collapse of the foliage and the entire plant. Black lesions are sometimes visible on the stem at or near ground level.
Control can be difficult once plants are infected so prevention is important. It is helpful to avoid overwatering, especially in heavy soils, and to avoid watering directly into the crown area of the plant. Highly symptomatic plants can be rogued and removed since recovery is unlikely.
Downy mildew, Peronospora.
Early symptoms of infection often go unnoticed and appear as slightly chlorotic areas on the upper leaf surface. Diagnostic symptoms develop as the fungus grows and breaks through the lower leaf surface and appears as a purplish-gray fuzzy growth. When infection is heavy, leaves shrivel and die.
Efforts to maximize plant vigor by fertilizing and watering are helpful. However, watering should be done early in the day to give the foliage a chance to dry before nighttime. This disease can be minimized by cleaning up plant refuse in the fall and by adequate spacing of the plants to promote good air circulation. Although not usually necessary, applications of fungicides can be made when new growth emerges in the spring. Among the compounds registered for use in Connecticut is mancozeb. Consult the label for dosage rates and safety precautions.
Aster leafhopper, Macrosteles quadrilineatus.
This insect feeds by sucking the juices from the plants and is responsible for the transmission of aster yellows from diseased to healthy plants. In the spring, the insects feed on diseased wild plants and then carry the virus to cultivated asters, marigolds, calendula, chrysanthemums, cosmos, dahlia and gaillardia. The adults are about one eighth of an inch long and greenish gray in color. Control of aster yellows is difficult because ornamental plants are continually reinfested by leafhoppers that have fed on diseased wild plants. Apply carbaryl, which is among the compounds registered for control of this pest in Connecticut, to manage leafhoppers. Imidacloprid applied as a systemic to be taken up by the roots will also provide season-long control. Consult the label for dosage rates and safety precautions. Discarding diseased plants throughout the season will also help.