Relation of Soil Temperature to Chlorosis of Gardenia!

Normal leaves of gardenia are glossy and dark green in color. Immature leaves may be a lighter green, but they are glossy and quite green even as they open from the bud. During the winters of 1934-35 and 1935-36 a chlorotic condition of the leaves of gardenia growing under glass caused serious concern to Massachusetts growers. The malady in its early stages is characterized by a gradual decrease of green in the interveinal areas of the leaves. This decrease of green continues until a straw-yellow color has developed. The vein areas are the last to lose the green color. This type of chlorosis is first noted near the tips of shoots, i. e., newly developed and rapidly growing branches, and is followed by a simüar chlorotic development in the leaves of the longer terminal branches. In later stages growth slows down and new leaves, as they appear, are relatively small and lack green color. Frequently, brown areas develop at the tip of the leaf and also in the margin. Chlorosis is the visible effect of some cause, but it is by no means a criterion to establish a definite cause. Among the causes suggested for the chlorosis in question were the following: Deficiency in iron; soil reaction; unbalanced fertilizer ration; high concentration of calcium and sodium ions; nitrogen deficiency; lack of light; and mosaic diseases. A careful study of the environment and treatment of plants, some healthy and some chlorotic, indicated that none of these factors was involved. However, there did develop some evidence that a temperature factor was involved. Further investigation revealed that this temperature factor was localized in the soil.