Leyland cypress or x Cupressocyparis leylandii are evergreen trees that grow to around 60 or 70 feet tall. They have tall, narrow crowns, reddish-brown bark and green or bluish-green needles. A number of different problems cause chlorosis, or yellowing, in Leyland cypress trees.
Spruce spider mites are tiny arthropods that cannot be seen without magnification. They have black bodies and yellowish-brown legs. They suck sap from the foliage, causing yellow stippling and defoliation. Scales are small rounded insects with no noticeable heads, legs or other moving parts. They drain sap from needles and stems. Infested needles turn yellow, then brown. Serious spider mite or scale infestations cause branch dieback or tree death.
The Botryosphaeria dothidea fungus causes Botryosphaeria (bot) canker in Leyland cypress trees, while the Seridium cardinale fungus causes seridium canker. The pathogens usually attack trees that are already suffering from water stress, heat stress or transplant shock, creating yellowed shoots and branches. Severe infections can result in significant dieback. Leyland cypress trees are also susceptible to phytophthora and annosus root rot. The pathogens attack the roots, causing yellowing, dieback or tree death.
Leyland cypress trees will yellow if they do not receive adequate water or if they are overwatered. In addition, trees that are planted in alkaline soils with high pH levels will turn yellow.