Gumosis manifests itself by the presence of a discharge, on the bark of the trunk or carpenter branches, of a thick, amber-colored exudate, which is called gum. This secretion can be a simple localized natural defense reaction or be the expression of an attack by microorganisms, this one more worrying for the health of the tree. It is above all fruit trees, of the Rosaceae family, of the genus Prunus in particular, but also citrus fruits of the genus Citrus, of the Rutaceae family, and some other plants which are victims of this phenomenon.
Gum, a natural defense substance
Trees of the genus Prunus, cherry, peach, apricot, almond and plum, naturally secrete a thick carbohydrate substance by specialized cells. It is this gum, different from the sap, which accumulates on or under the bark of branches at the level of a crack caused by wounds of various origins. This is a defense of the tree because the gum engulfs the aggressors, thus prevented from penetrating the deep tissues. However, if this defense mechanism may seem not very dangerous for the subject, it is always important to suspect the action of an opportunistic pathogen.
It suffices to observe a possible sign of weakening of the tree, yellowing of the leaves, wilting of the ends of the twigs, an imperfect flowering and fruit setting, phenomena associated with gum flows, and the presence of 'a responsible microorganism is thus confirmed. For example, moniliosis, mycosis of fruit rotting, can be accompanied in some fruit trees by large gummy discharge. Likewise, the development of a gummy canker at the base of the trunk in citrus fruits leading to dieback of trees, has as a responsible agent a Phytophtora, from the family of Oomycetes, close to brown algae. We still know the dreaded bacterial canker caused by Pseudomonas, which eats away at the bark and deep tissue in Prunus and apple trees among others, with spectacular gum flows, and which seriously compromises the future of the tree.
Prevention and possible treatments
The integrity of the bark must first be guaranteed, so be wary of overly severe pruning and injury during maintenance work. Protect trees from severe frost and the consequences of hail. Injuries or cuts can be disinfected and chewed. Watch for leaf scars when the leaves fall in autumn and limit the power of penetration of fungi or bacteria by treatments with Bordeaux mixture, in late summer, on the still green foliage. We are still debating whether to treat gummy cankers by curettage with antifungal (Bordeaux mixture) and healing putty. Perhaps more useful is to delete the branch concerned if possible. This can go as far as uprooting, then burning, of an overly affected tree. Finally, remember that any crop involves thinking about the excessively high rate of soil moisture (often a precursor to pathogens) and its possible drainage. Especially the versatile action of copper from Bordeaux mixture can justify its use by spraying twice a year, as a major prevention technique. By C. Schutz Croué