Sulfur dioxide gas injury to fruit trees. III.

Leaf fall of citrus trees by Bordeaux spray before sulfur dioxide fumigation

Abstract

1. Leaf fall of citrus trees as induced by the sulfur dioxide fumigation particularly after the Bordeaux spray was studied with 3-year-old potted plants of Satsuma oranges (Citrus unshiu) and Hassaku oranges (C. Hassaku) respectively. Fumigation was conducted with 0, 1, and 5ppm of sulfur dioxide for 13 days from June 5 to 17 of 1964, 2 hours each day. On half the number of each plot, Bordeaux mixture of 6-6 had been sprayed just before the fumigation on June 5, and water was sprinkled for 3 days following the 7th day from the start of fumigation.
2. In Satsuma oranges, leaves fell down easily even by the single treatment of sulfur dioxide, the trend being more marked by the treatment after the Bordeaux spray. On the other hand, in Hassaku oranges leaf fall did not take place readily by the single treatment of sulfur dioxide. It was however much promoted by the treatment after the Bordeaux spray.
3. In either species, the Cu content of leaves and the rate of leaf fall were much increased with rising concentration of sulfur dioxide in fumigation. The Cu content of leaves was also increased by the Bordeaux spray, but it was not deeply connected with the leaf fall. Therefore, the leaf fall due to sulfur dioxide fumigation in addition to Bordeaux spray might be reduced both to the toxic function of SO₂ and that of Cu which was isolated from Bordeaux mixture by sulfur dioxide fumigation.