The hardiness of the petunia makes the many varieties a popular garden choice. Available in almost every color imaginable, with solid and variegated colors, single and double blossoms, a perfect petunia can be found for any hanging basket, container, or garden nook. Although easy to grow when planted in areas receiving at least six hours of daily sunshine, petunias can be susceptible to some pests and disease.
A purchased petunia bedding plant can sometimes be infested with a scale-like insect, greenhouse orthezia, which can kill the petunia. The first signs of damage will be small white bumps on the leaves. The leaves will drop off and new leaf growth will be discolored. Greenhouse orthezia can be light brown or dark green in color and measures about 1/3 of a inch long. The edge of the insect's body looks like it has a waxy white fringe. The wingless, dark green nymphs are about the size of a pinhead.
Nematodes are roundworms that can barely be seen by the naked eye. More common in southern areas of the United States, nematode infestation is not a common source of worry for growers of petunias, but a heavy infestation can damage the roots of the plant, causing them to die. The best prevention of nematode destruction is to dig up petunias at the end of the growing season so that the pest does not overwinter in the garden.
Barely visible to the naked eye, the spider mite multiplies quickly and sucks chlorophyll out of petunia foliage and injects a toxin into the leaves. Tiny yellow dots on the petunia plant are usually the first sign of infestation, followed by a covering of fine webbing. Spider mites are usually brought into the garden on purchased bedding plants. Inspect plants carefully before purchasing them to prevent a spider mite infestation.
Petunia seedlings in a greenhouse or window sill can often be attacked before they even emerge from the soil by damping-off that is caused by the fungus Rhizoctonia solania that lives in soil. Seedlings will rot at the base and fall over and die.
Snip crowded seedlings with a scissors to eliminate overcrowding and allow for good air circulation. Make sure the soil is not too wet, and water from the bottom.
A variety of fungi can cause stem rot in petunias. The fungi thrive when the petunia has limited air circulation caused by overcrowding and the soil is too moist. Prevent stem rot by planting petunias in well-drained soil, and keep mulch away from the base of the stems.
There is no cure for virus diseased plants, and they should be immediately removed from the garden and destroyed. Virus diseases that affect petunias include ring spot, beet curly top, mosaic, tomato spotted wilt virus and aster yellows.
Petunias that have yellow foliage, dead spots, pale areas or lines are probably affected by a viral disease. Deformed leaves, stunted and distorted growth and small flowers are other indicators of problems.