What is the first symptom of leaf gall formation?

The first symptom of leaf gall formation is that a part of the leaf becomes distorted. Galls are abnormal growths that occur in response to various stimuli, often involving the presence of insects, fungi, or bacteria. The initial reaction of the plant usually manifests as localized swelling or distortion of the tissue as the plant responds to the inducing agent. This distortion can create a visible change in the leaf's appearance, leading to the characteristic bulging or growth associated with galls.

In contrast, the other symptoms listed, such as the development of brown spots, yellowing of the entire leaf, or premature leaf drop, are typically associated with other types of plant stress or disease and may occur later in the plant's response to more severe damage or decline in health. These symptoms are not indicative of the specific initial process of gall formation, which is why the distortion of a part of the leaf is recognized as the primary and earliest sign of this phenomenon.