Question: Why are my Afghan (Pinus eldarica) trees' needles turning brown and falling off? The trees are deep watered on a regular basis. There are no signs of damage from insects or disease. What can I do about this problem?
Answer: During the summer months with high temperatures, many pine tree species will naturally shed some of their older needles. However, if the new terminal growth is turning brown, then some action needs to be taken to remedy the situation.
Control: Normal, proper watering should continue throughout the growing season. Some think that the monsoon rains produce enough water for good growth. This is usually not the case. Rain is inconsistent both in timing and amount. Another watering problem is with larger pine trees. When young trees are planted, a drip system or other watering system is installed to supply the needs of a young tree. Perhaps one or two emitters are placed near the trunk. This is sufficient while the tree is young, but not as the tree grows older. Emitters must be doubled up or replaced with higher flow emitters and spread out laterally to encourage extended root growth. I have seen mature evergreen trees blown over by a storm when the ground was wet. Always the tree has had an underdeveloped root system. The cause was poor water application that did not encourage extended root growth.
Question: I have a Golden Delicious apple tree with irregular large spots on the leaves. In some places the spots are turning brown and dying. Some of these affected leaves are falling to the ground. What is causing this and what can be done to stop it in the future?
Answer: The apple tree is experiencing necrotic leaf blotch, also called leaf spot or leaf drop. Golden Delicious is particularly susceptible and this condition is restricted to this cultivar and others with its "blood-line." This disorder is characterized by irregular necrotic blotches in mature leaves that develop rapidly, usually within 12 to 24 hours. The leaf first turns pale green and yellow and then darkens to a deep brown. The size of the affected area remains constant once visible symptoms appear. The size of the necrotic area is restricted by larger veins which are not affected. Mature leaves, in the mid-shoot region, are usually affected first. Young vigorous trees appear less severely affected than older, more slowly growing trees. Leaf fall can be rapid once symptoms occur, sometimes within 4 to 7 days. Severity of the disease varies greatly from year to year. Variation can be great among trees in the same orchard or yard. Necrotic leaf blotch is a physiological disorder related to air temperature, light intensity, and soil moisture. It is most severe when a cool, rainy period of 4 to 5 days precedes several hot sunny days. A hormonal imbalance is thought to be the cause since symptoms are enhanced by gibberellin (a plant hormone that is needed for cell enlargement) and inhibited by abscisic acid (another plant hormone that causes leaf and fruit fall).
Control: Zinc oxide applied every two weeks from bud break until harvest is effective in reducing disease incidences. Also, healthy, vigorous trees produced by good watering, fertilizing, fruit thinning, and pruning practices contribute to decreased incidences of necrotic leaf blotch.
Source: Compendium of Apple and Pear Diseases, 1991. The American Phytopathological Society, Page 88.