Trialeurodes vaporariorum This insect is a common pest of many garden and greenhouse plants. The four-winged adult lays eggs on the undersides of leaves. The pinhead-size larvae are flat, oval, and semitransparent. They feed for about a month before changing to the adult form. Larvae and adults suck sap from the leaves-larvae are more damaging because they feed more heavily. Neither can fully digest all the sugar in the plant sap, excreting the excess in a fluid, honeydew, which drops onto leaves below. Sooty mold may develop on the honeydew, causing gardenia leaves to appear black and dirty. In warm-winter areas, insect are active year-round, with eggs, larvae, and adults present at one time.
Treat plants with an insecticide labeled for this pest, following label instructions. Purchase only pest-free plants.
This is a common problem with acid-loving plants such as gardenia. The plant prefers soil with a pH between 5.0 and 6.0. The soil is seldom deficient in iron, but iron is often found in an insoluble form that is not available to the plant, especially in soil with a pH above 7.0. A high soil pH can result from overliming or from lime leached from cement or brick. Regions where soil is derived from limestone or where rainfall is low also have high-pH soils. Plants use iron in the formation of chlorophyll in the leaves. When iron is lacking, new leaves are yellow.
Fertilize with a plant food formulated for acid-loving lants. If your soil drainage is poor, add enough peat moss at a 50:50 ratio to native soil when planting gardenia. Never lime the soil around gardenia.
Abrupt changes in soil-moisture levels or air temperatures account for most yellowing leaves. A normal loss of foliage occurs in early spring just as bloom buds begin to appear and in the Southeast at the end of the rainy season. Most yellowing and dropping of leaves at other times are related to changes in weather or soil-moisture levels. Gardenia varieties differ in the severity of leaf loss, but healthier plants generally have less leaf drop.
Apply a fertilizer formulated for acid-loving plants. Avoid letting plants dry out excessively between waterings.
Nematodes are microscopic worms that live in the soil. They feed on plant roots, damaging and stunting them. The damaged roots can''t supply sufficient water and nutrients to the aboveground plant parts, and the plant is stunted or slowly dies. Nematodes prefer moist, sandy loam soils. They can move only a few inches each year on their own, but they may be carried long distances by soil, water, tools, or infested plants. Testing roots and soil is the only positive method for confirming the presence of nematodes. Contact your local county extension office for sampling instructions and addresses of testing laboratories. Soil and root problems such as poor soil structure, drought stress, nutrient deficiency, and root rots can produce symptoms similar to those caused by nematodes. Eliminate these problems as causes before sending soil and root samples for testing.
The worms can be controlled before planting by putting black plastic over the soil, which heats up and kills the pests.
This common black mold is found on a wide variety of plants in the garden. It is caused by several species of fungi that grow on the sugary material left on plants by aphids, mealybugs, scales, whiteflies, and other insects that suck sap from the plant. The insects are unable to digest all the sugar in the plant sap, and they excrete the excess in a fluid called honeydew, which drops onto the leaves below. The honeydew may also drop out of infested trees and shrubs onto gardenias growing beneath them. The sooty mold fungus develops on the honeydew, causing the gardenia leaves to appear black and dirty. Sooty mold is unsightly but is fairly harmless because it does not attack the leaf directly. Extremely heavy infestations prevent light from reaching the leaf, so the leaf produces fewer nutrients and may turn yellow. The presence of sooty mold indicates that the gardenia or a nearby plant is infested with insects.
Hose off sooty mold or wipe it from the leaves with a wet rag. Prevent more sooty mold from growing by controlling the insect that is producing the honeydew. Inspect the leaves and twigs above the sooty mold to find out what type of insect is present. Then apply an insecticide labeled for that insect, following label directions.
These mites, related to spiders, are major pests of many garden and greenhouse plants. They cause damage by sucking sap from buds and the undersides of leaves. As a result of their feeding, the plant''s green leaf pigment disappears, producing the stippled appearance. Spider mite webbing traps cast-off skins and debris, making the plant messy. Mites are active throughout the growing season but are favored by hot, dry weather (70°F and up). By midsummer, they have built up to tremendous numbers.
Apply an insecticide labeled for spider mites, following label directions. Mites are difficult to control because they reproduce so rapidly. Repeated applications may be necessary, especially during hot, dry weather. Hose down plants with a high-pressure nozzle every few days to knock off webs and mites.