What Are Galls? Galls are unusual growths or deformities of plant tissue caused by an organism; be it insect, nematodes, fungi, bacteria or viruses. The presence of these organisms initiates an increased production of normal plant growth hormones and these plant hormones cause localized plant growth. The gall-making organism feeds inside the gall. The appearance of the gall is unique to the gall maker and … Continue reading Willow Leaf Gall Sawfly