The petunia, a popular annual, includes hundreds of varieties suitable for gardens and containers. Petunias can suffer physiological problems related to temperature and photoperiod, or daily light exposure. Flowering from spring to frost, these annuals can wither or die from diseases, lack of nutrients and pests.
Soil temperature for transplanted petunias should exceed 60 degree F, according to the University of Minnesota. Organic matter worked into the soil will allow more light exposure. Petunias require much light exposure, five to six hours or full sun. Increased shade can result in fewer flowers. While high temperatures and long light exposure can encourage flower production, a lower quality plant may grow.
If petunias lack calcium in the lower substrate, then symptoms such as smaller leaves, brown to purple spots, and deformed leaves can occur. The flowers can wither early. Deficiency in nitrogen can lead to greenish-yellow mature leaves in early stages and purplish lower leaves with dying on the leaf tips and margins. The lack of phosphorous can cause a purple appearance on small leaves and mid-vein. The smaller petunia plants may develop leaves with dark-purple patches and dying leaves resembling brown paper.
Damping-off diseases involve soil-borne fungi such as phythium, phytophthora and rhizoctonia. During greenhouse production, seeds may be soft, dark and decayed. Seedlings may show decayed or weakened stem tissue that can lead to a plant toppling and dying. Sometimes decayed roots hold up a stunted plant that eventually dies.
The fungus Botrytis cinerea can occur during wet or humid weather. Symptoms include masses of silver or grey spores on dying tissue. On petunias, Botrytis cinerea causes bud blast or flower blight.
The flea beetle can slow the petunia's growth. Symptoms include chewed leaves with shot-holes and leaf removal. Two species of aphids, green peach (Myzus persicae) and melon-cotton (Aphis gossypii) damage petunias. Aphids use their piercing-sucking mouthparts to suck plant sap and injure the plant. The honeydew they exude can aid in the growth of black sooty fungus that blocks sunlight.
White flies--such as greenhouse white fly and silverleaf white fly--can live on the underside of leaves where they feed with their mouthparts to extract plant tissue fluid. Adult and immature white flies feeding in large numbers can weaken or kill the petunia. Thrips have cigar-shaped bodies and fringed wings. They forage on petunia leaves and flowers, and can transmit viruses.
Although the petunia thrives in full sun, the heat can dry out the shallow roots. Symptoms include signs of wilting. Organic matter mixed into the soil to a depth of 8 inches will help retain moisture and nutrients. Watering once a week by soaking the soil to a depth of 6 to 8 inches should suffice. Petunias of the 'spreading' type may require more frequent watering.
In midsummer, this annual may appear leggy and bear fewer and smaller blooms. During August the flowers may droop. Cooler temperature in early fall can help revive the plant. Sensitive to cold temperatures, petunias will die during the fall frost.
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- North Carolina State University: Nutrient Deficiencies of Vegetative Petunia
- Alabama Cooperative Extension System: Greenhouse Production of Petunias
- Yardener: Caring for Petunias
- University of Minnesota Extension: Growing Petunias
- United States Department of Agriculture: Crop Profile for Petunias in Ohio