Why is a special substance painted on the tree?

B
考查阅读技能，由第三段“One result of pruning is that an open wound is left on the tree and this provides an easy entry for disease”和“You should allow the cut surface to dry for a few hums and then paint it with one of the substances available from garden shops produced especially for this justification”可知，修剪树木留下的伤口给了疾病侵袭的机会，因此涂刷特殊物质是为了阻挡疾病通过伤口进入树木。故选B。