Pruning during dormant season certainly has its place in the management of fruit trees, shade trees, and woody plants. It is a generally accepted concept: Prune in winter. But it is not that simple. In one particular example, it can be harmful: A common disease of olives can spread into fresh pruning wounds.
Olive knot, an infection caused by the bacterium Pseudomonas savastanoi pv. savastanoi spreads in water and infection occurs through any opening in the tree, such as fresh leaf-drop scars, frost wounds or fresh pruning or scuffing wounds. Olive knot bacteria can also spread systemically.
The disease causes the formation of galls or “knots”, rough swellings on branches and trunks, which can kill stems. Infections can also ruin the flavor of the olive oil as well as affecting fruit yield.
According to Paul Vossen, Technical Editor of the “Organic Olive Production Manual”, severity is “generally positively correlated with annual rainfall” — more rainfall leads to more infections.
Vossen notes that the disease is “almost completely absent from coastal areas of California”. But the climate in Napa County ranges from coastal to inland. Looking at the Sunset Climate Zone map we see the southern tip of the county in Zone 17 “…Coastal Strip”, the valley floor in Zone 14 with “…some ocean influence”, the surrounding hills in Zone 15 including “chilly winters and thermal belts”, and farther inland, Zone 7 with “hot summers and mild, pronounced winters”.
Several years ago, I witnessed one severely disfiguring outbreak of the disease on olive trees in Calistoga – an inland portion of Zone 14. Then a couple years later, I saw an outbreak in oleanders, also in Calistoga. In oleanders, the disease is caused by a closely related subspecies of the bacteria: Pseudomonas savastanoi pv. Nerii.
The bacteria cause and accumulate in galls—rough, irregular swollen tissue—found on trunks, and branches. Small, roughly spherical galls encircle and kill young branchlets. That mode of infection seems to be less common here in the Valley. It may occur when it rains soon after older leaves are shed, or when a hard freeze damages one-year old branchlets and the wounds are still fresh.
The foremost control measure for homeowners is to prune infected olive trees, and those near infected trees, in dry summer weather. Alternatively, if the pruning must be done in winter, it should be done during a dry spell. Branch wounds may be susceptible to infection for 20 days while leaf scars remain susceptible for about 10 days.
To further avoid wound infections, shears should be continually disinfected while pruning. Traditional pruning shear disinfectants include 70% rubbing alcohol, 10% bleach solution, or Lysol, which all have their pluses and minuses. Checking into the subject, I noticed something new to me: quaternary ammonium compounds. Research reported by Dr. James Adaskaveg found excellent results with a quaternary ammonium formulation. Unlike bleach and alcohol, this class of disinfectant is non-corrosive to tools and effective at a low concentration. Checking online and with a local garden center, I found no such products available and labelled specifically for this use. Still, it sounds interesting for the future if it gets labelled for this use in California.
Where galls exist on stems too large to remove without disfiguring the tree, the galls can be painted with Gallex, a thick, pink liquid that smells like creosote. That is probably a material for use only by licensed applicators.
Lastly, protective copper compound sprays can be applied to kill bacteria and protect stems from infections. The copper compound needs to be on the trees throughout the rainy season.
Though the copper compounds may be allowable on certified organic farms, they do present some risks and practical problems. Copper can cause eye injury and it can be phytotoxic i.e. damaging to plants. Another problem is the copper compounds are difficult to remove from the fruit, which is typically harvested after the rainy season begins.
“You absolutely do not want any copper residue in the oil,” Vossen says. So, even though it is less than ideal with respect to disease control, copper sprays must be delayed until after harvest.
Here are a couple links providing details on the disease and management.