Normally, gardenias grown outside are attacked by only a few insects and one serious disease.
When gardenias are attacked by root-knot nematodes, the leaves become chlorotic.
These microscopic worms produce characteristic galls or swellings on the roots. Prevention consists of soil sterilization before planting but using steam or chemicals.
Care should be taken when using any chemical materials since they will kill plants (people too) when used too close. Once the plants are infested they should be destroyed as there is no cure.
Scale is the most common insect attacking gardenias grown outdoors. White flies and mealy bugs sometimes attack gardenias, too. The black vine weevil rarely causes trouble, but is serious when it does.
Thrips can also be bothersome, especially when gardenias are grown near a field of weeds.
Black scale can be a real pest. Each scale is about the size of a small split pea, dark brown to black and nearly hemispherical with a conspicuous H-shaped marking on the back.
The honey dew secreted attracts fungi which in turn give the leaves and stems a black, sooty appearance. This also attracts ants which help spread the scale.
In Florida and the Southeast the scale pest is the Florida waxscale, reddish or brown and the size of a pinhead.
Orthezia sometimes attack gardenias grown outdoors; they are small, long and white, with long stripes on their backs. You will need a hand lens to see them.
All the scales are sap-sucking and some inject a poison. The soft brown scale can be a serious pest at times. too. It is greenish brown, oval-shaped, rather flat and about 1/8 inch long. It infests leaves and limbs and produces a honey dew as does the black scale.
The treatment for all scale is a summer oil spray applied at two-week intervals using a 2 percent oil, Malathion can be used as well but I prefer oil.
Water plants well one half-hour before spraying, and again syringe with water an hour after spraying.
Thrips and the tobacco thrip are also common gardenia pests. Injury is manifested by a silvering of the leaf because the thrips suck the plant juices. You will need a hand lens to see them on the underside of the leaves as they are very small flying insects.
Clean up all weeds which harbor them.
The citrus mealy bug is usually found on greenhouse gardenias, but sometimes attacks those grown outside.
A hard, sharp syringing with a small spray of water usually keeps them under control. Otherwise, use a 2 percent summer oil and a teaspoonful of Malathion to each gallon of water.
White flies are quite small, about 1/16 inch long. The adults are pure white and when disturbed fly away from the underside of the leaf where they feed. The larvae look like very small mealy bugs. Both adults and immature stages are found together. For outdoor control use such sprays as Malathion.
Red spider sometimes attacks the plants in late summer, especially when the air gets – hot and dry. You will need a hand lens to see the reddish mites which can be controlled by regular hard syringing with cold water.
Do not become alarmed by our mention of all these insects – your plants may never be attacked by any of them but it is best to be aware of them if they do.
The one disease most often found is phomopsis canker, a fungus disease (ficus trees get this as well).
The first symptom of this disease will be a shrinking of the stem at the soil line. This gradually enlarges and the whole stem swells and becomes rough and cracked.
The leaves become pale green, then yellow and many fall. Since the disease enters plants only through wounds, use all possible care not to injure the stem when planting or pruning. There is no known cure so dig and destroy (burning is best) infected plants.