The name pecan is derived from the Algonquin word "pacane." The American Forests Organization states that pacane means "a nut so hard as to require a stone to crack." The botanical name for the pecan tree is Carya illinoensis; it is a tree that has existed in America for over 8,000 years. This magnificent tree is native only to North America. It is found in states from Illinois to Texas. The pecan tree is grown for shade as well as its nuts. Besides rosette disease, pecan trees are susceptible to scab, powdery mildew, blotch, wood or heart rots, internal breakdown of nuts and crown gall.
Rosette disease is a zinc deficiency, and it is a common problem with pecan trees. Zinc is one of the important nutrients that helps pecan trees to grow. It is not needed in as high concentration as other nutrients, such as nitrogen (25,000 ppm nitrogen and 50 ppm zinc). The pecan tree needs zinc to produce hormones. These hormones enable the cells to enlarge in both its stems and leaves.
Symptoms of Rosette Disease
If the soil is deficient in zinc, you will notice that the leaves of the pecan tree will be small, chlorotic and irregularly shaped. In extreme cases of Rosette disease, the pecan nuts will also be affected--they will be of low quality and there will be fewer pecan nuts. Rosette disease usually appears within two to three years after planting. The disease gets its name from the symptoms of the disease. As the disease progresses, there are an over abundance of smaller branches formed. This excess of smaller branches pulls the foliage together, which makes it look like a rosette--thus the name, "rosette disease."
The diseased pecan tree can be treated with a foliar application of zinc. In some pecan farms where there is a history of the disease, application of zinc compounds is done yearly. Foliar treatments should be applied as the leaves develop, early in the growing season. The treatment will then be done each week until the leaves are fully formed. (For young trees, the treatments generally continue until the end of July.) Mature pecan trees usually require five treatments. (The zinc compound is applied when the buds break and then every two weeks.) (Due to the height of most pecan trees, foliar treatments are almost impossible for the homeowner.)
Foliar treatment is more effective than soil treatment if the soil is highly alkaline. When treating the soil, zinc chelates are applied in early spring when the buds first appear. (The soil may be treated even when the trees do not have any leaves.) Soil application consists of spreading zinc from the trunk (outward) to the drip line of the tree. The rule of thumb when it comes to the amount of zinc that should be applied is ½ pound of zinc for every inch of the diameter of the trunk.
Rosette and Bunch Disease
Bunch disease has similar symptoms of rosette disease. However, with bunch disease there is no yellowing between the veins of the leaves, and they do not become crinkled. Scientists believe that bunch disease is caused by a phytoplasma. There is no treatment for bunch disease.