Bay laurels (Laurus nobilis) are hardy plants that are widely used as ornamentals in warm climates, or as potted greenhouse plants in cooler regions. The leaves are used in cooking, massage therapy, and as a remedy for rashes associated with poison ivy. Though they are generally resistant to pests and diseases, occasionally yellow leaves may appear. This is often a symptom of an underlying problem, and may be related to environmental conditions, pests or disease.
The bay sucker, commonly known as jumping plant lice, is a somewhat common problem with bay trees. They are a sap-sucking insect that may be fatal to the tree if left untreated. An early sign of trouble is a yellowing of the leaves, which is usually accompanied by thicker than normal leaf tissue. As the pests become more prevalent, these leaves will turn brown and drop. At maturity, bay suckers are pale brown and are most active during late spring. Damaged leaves and branches should be removed and burnt while the rest of the tree is treated with an insecticidal soap. This soap should be focused on the underside of leaves and may require more than one dose to fix the problem.
A deficiency in iron, also called iron chlorosis, is among the most serious problems in the garden. Iron shortages are caused by a number of problems, particularly soil that is too alkaline or damaged roots. Iron is an essential part of chlorophyll production in plants, and without it food and energy production is greatly decreased. This decrease in chlorophyll causes leaves to turn yellow. This discolouration is characteristic in that it starts at the edges and moves inward, leaving the veins green. Other nutrient shortages will produce discolouration that is more uniform. Treating iron chlorosis may be done several ways, and will vary depending on the cause of the deficiency. If soil is too alkaline or wet, thoroughly mix well decomposed organic matter, sulphur or peat moss into the soil. This will correct the pH and, if necessary, improve drainage.
Like other nutrition problems, a nitrogen shortage may have severely damaging effects on the bay laurel. An imbalanced pH often results in soil that is nitrogen poor, and is the most common cause of nitrogen deficiency in plants. Early symptoms include yellowing leaves, which occur first in older, lower leaves and progresses upward. The discolouration is usually uniform and includes both the leaf and vein tissues. To correct the problem, growers should add a nitrate rich fertiliser to the soil. Follow directions carefully, as scorching may result if too much fertiliser is added too quickly. An additional option is to add a bit of decomposing organic matter. Though slower than fertiliser, it is safer.
Root rot is a fungi-caused problem that turns roots into an unusable, grey-brown mush. Early symptoms include wilting, leaf yellowing and dropping. Root rot is most prevalent in extremely wet periods, as the fungus requires cool, moist soils to thrive. In these conditions, the fungi will overrun the roots, making them unable to transmit water and nutrients to the rest of the plant. The best way to prevent this very serious problem is to avoid overwatering and standing water by providing adequate soil drainage. Proper drainage will remove standing water, reducing the chance for bacterial growth, and allow proper air flow to keep roots healthy. Prevention is the goal for growers, and this includes using disease-free soil that is well drained. Infected plants, if the fungus isn't too extensive, may be repotted or replanted if necessary. Prior to doing this, any infected plant parts should be removed. Generous application of fungicide may be effective against some fungi, but is not a guarantee.