Laurel shrub diseases
Laurel (Rhododendrons) are dense, evergreen shrubs that range in height from 1.8 to 4.5 m (6 to 15 feet). Some varieties of laurel grow as tall as 9 m (30 feet). The foliage of the shrubs is dark green and leathery and tends to yellow during winters.
Laurels are popularly used in landscapes as screens, borders or as specimen plantings. Laurels are susceptible to a number of insects and diseases that can lead to the death of the entire plant if left uncontrolled. Laurels grow best in partial shade and moist, moderately acid soil.
Laurel wilt is a highly destructive disease of the plants in the laurel family. The fungal disease is caused by the fungus Raffaelea sp.. The fungus infects the sapwood of the afflicted plants and restricts the flow of water through the plant, gradually causing the leaves to wilt. The fungus is transported to the plants by the redbay ambrosia beetles that carry the spores of the disease in their mouths. Management of the disease includes avoiding moving infected plant parts out of areas in which the disease is known to occur. Chemical-control options include injections of root-flare systemic fungicide containing propiconazole into infected plants.
Leaf spot diseases in different laurel shrubs are caused by fungi. The diseases are characterised by the appearance of spots and lesions of different sizes and colours on the foliage that range from small dots to large, irregular patches in shades of yellow to brown. In severe cases, leaf spot leads to excessive leaf drop. Leaf spot diseases are not damaging to the plant in the long run. Recommended management practice includes the immediate removal of fallen leaves and debris and avoiding watering from overhead sprinklers. It is also best to water plants early in the day so the foliage dries rapidly.
"Shot-hole" disease, also referred to as Coryneum blight, is a serious fungal disease that afflicts a number of host plants, including laurel. The disease causes the appearance of red or purple-brown spots on new shoots and foliage. As the spots grow larger in size, they develop brown centres often speckled with tiny dots which fall out when the leaves expand, creating "shot-holes" in the foliage. Management includes pruning and disposing of infected plant parts immediately. Avoid overhead sprinklers. Chemical-control options include the use of Bordeaux mixtures and fungicides containing copper as the active ingredient.