Symptoms of gummosis
Gummosis is evidenced by exudation of resin or gum from the wood which gets deposited on tree bark. The gum is produced in response to wounds on the stem due to insect attack, infection by plant pathogens or physical injury. On A. decurrens at MVNP, gum exudation was mainly due to wounds made by insects which were proved by boring dust and pitch tubes occurring outside the bark (Fig. 5a) and characteristic galleries under the bark. Our observations showed that most of the holes in the bark were caused by ambrosia beetles (i.e. Euwallacea sp.) which can bore into the xylem of the diseased trunk producing copious amounts of frass (Fig 5b). These beetles are known vectors ofCeratocystis spp.
Gum exuded from the stem turned dark or black in color after >1-week exposure (Fig. 6a) compared to the newly exuded gum (Fig. 6b). In contrast, gum exuded from the young branches, especially green branches, tended to be clearer in color (Fig. 6c). Most borer holes occurred on the stem, particularly on woody/ older stems, whereas, only limited holes were present on young branches.