Nursery Notes: Gummosis affects plants as well as man
Note: MTNA will not meet this month.
I have needed this written documentation of this common problem for years. I finally found an easy-to-understand explanation.
I have always felt lacking in my explanation of gummosis when visiting a good grower with a nice block of cherries. Last fall it was a block of 2-inch weeping cherry. They looked great from a distance. Closer observation revealed 10 to 15 percent with severe gummosis. They will likely die this summer after leafing out.
Reading this may not reduce the amount of gummosis you see, but you may understand it better or realize the cause is complicated. Stress is a killer in plants as well as in man and other animals.
Gummosis is a general, though nonspecific condition of Prunus trees (both fruiting and flowering) in which gum is exuded through the bark. Gum is produced in response to any type of stress, not necessarily a wound, regardless of whether it is due to insects, mechanical injury or disease.
Gum production (gummosis) is often associated with fungal (Botryosphaeria canker) and bacterial cankers of cherry and the peach tree borer. However, gummosis can also be associated with drought stress, certain viruses, adverse growing sites, winter damage, herbicide damage, wounds, bruising of stem tissue and many other factors. The canker fungi overwinter in active cankers in living or in dead wood.
Infection occurs where the bark is damaged or injured. Infection following cold injury frequently occurs in the crotch angles of affected trees. Other ports of entry for these fungi include pruning wounds, mechanical damage, insect damage and leaf scars.
Moisture is required for spore production, spore dissemination, and infection. The rate of canker development following infection depends on temperature and the species of fungus involved. When temperatures do not favor fungal activity, calluses form.
Canker activity resumes when temperatures again favor the fungus. This back and forth battle between the tree and the fungus usually ends with the fungus winning. The horticultural myth with gummosis is that it is an indication of borers.
This is seldom the case. Peachtree borers will attack cherry trees and other members of the Prunus genus. One symptom of borers in Prunus can be gum.
However, peachtree borer is not the only thing that can cause the gummy sap to appear on cherry and Prunus trunks. If the gum is emerging from rather high up on the trunk, rather than from near the root crown, the bleeding is not likely caused by peachtree borer.
Look for frass mixed in with the exuding gum. Frass is a combination of wood boring dust and the body excrement of borers. The presence of frass, with or without gum, confirms a borer. Peachtree borer injury occurs low on the trunk, usually within 3 feet of the soil. Use a sharp knife to slice behind the gum into the bark. The exit hole of a borer will be revealed if a borer is involved. No hole and no frass indicates the condition referred to as gummosis.
The exuding of gum from plants in the genus Prunus, including the cherries, is so common it has been given the name gummosis. The name gummosis does not define a cause, only a response. Peachtree borer injury and simple mechanical injury can result in gummosis. There are also several infectious diseases that can result in gummosis.
There are three groups of organisms that can cause cankers on cherries and result in a gummosis response. One is a bacteria in the genus
Pseudomonas which causes a disease known as bacterial canker. Another is a fungus in the genus Leucostoma (Cytospora) that causes Leucostoma canker of Prunus. The third is usually called fungal gummosis and is caused by the fungus Botryosphaeria dothidea. It is not important to identify the specific organism involved, but, it is important from a diagnostic point of view to differentiate between insect infestations, mechanical injury, and infectious disease.
A canker is a necrotic (dead), often sunken lesion on a stem, branch or twig of a plant. It will become more evident with time as the healthy tissue grows. The dead area appears to have sunken. The dead bark will slough off in time, exposing the dead wood.
In most cases where a canker and its casual disease is concerned the infected tree is, or has been, under some type of environmental stress.
This has frequently been cold injury or drought stress in the past.
Man does not always recognize that sufficient stress occurred to be a factor.
Cherry trees stress easily compared to many other landscape trees and this opens the door to an invasion of one of the canker causing diseases and the resulting gum bleeding.