Like many diseases we encounter in agriculture, bacterial canker falls under the category of a ‘predisposition disease.’ That is, the blossom blight, gummosis and canker formation blamed on the bacterium Pseudomonas syringae are preceded by events which weaken the host, making it easier for the pathogen the colonize affected tissues. Consequently, a program aimed at minimizing bacterial canker must focus on reducing the onset of those predisposing events.
Much of the initial work on bacterial canker was conducted by two of the finest plant pathologists I’ve had the privilege of being associated with, Dr. Harley W. English and Dr. James De Vay of U.C. Davis. Their pioneering work defined a close association between nematode damages to the root system and bacterial canker. Furthermore, they found that late pruning (i.e. spring pruning as opposed to winter pruning) and the use of copper sprays helped to reduce disease incidence. James De Vay also characterized the culprit bacterial toxins, syringomycin and syringotoxin.
Careful study has found that Ps. Syringae instigates freezing damage by raising the freezing temperature of plant tissues by 5 to 6 degrees.
In 1978, Dr. Steven Lindow of U.C. Berkeley, found that the canker bacterium, Ps. syringae, caused the surfaces of objects it colonized to freeze at 32 degrees Fahrenheit. While this may sound a bit mundane, he found that once freed of Ps. syringae, the same tissues would not freeze until temperatures were lowered to 26 or 27 degrees. Dr. Lindow characterized the bacterial canker organism as ‘ice positive.’
This was an interesting correlation, as previous work by English and De Vay had shown that bacterial canker was more prevalent in low-lying, cooler areas of fields, and that dormant copper sprays (which helped to reduce bacterial populations on the surfaces of tissues) helped reduce disease incidence. It is well known that bacterial canker strikes predominately following a cold winter or a series of freezing temperatures.
Pseudomonas syringae is a ubiquitous epiphyte, which can be commonly isolated from the surface tissues of various plants. That is, it lives on the surface of plants, gaining its nourishment from exudates. As the polysaccharide coat of this bacterium contributes to raising the freezing point, its body acts as a nucleus for ice formation. This is the reason for the name term ‘ice nucleation bacterium.’ Freezing damage to tissues creates fissures and cracks. The bacteria, whose presence contributes to this freezing and damage, is consequently well positioned to invade the very cracks and fissures which it helps to create.
Indeed, freezing events followed by rain are most conducive to disease, as the moisture aids in the movement of bacteria into the strategically incited freezing wounds.
Combating bacterial canker must begin by improving tissue hardiness and resilience to freezing.
A plant that has been grown outdoors and acclimated to increasingly cold temperatures will withstand a freezing event better than the same plant species exposed to freezing shortly after being taken from a greenhouse. This common knowledge has oftentimes led growers to cut water and nutrition in the late summer or early fall to induce tree dormancy. However, herein lies one of the most common errors. Premature induction of dormancy in the presence of warm temperatures can reduce the effective period of postharvest photosynthesis and food accumulation, creating undue nutrient and water stress on tissues.
Cold hardiness is in part imparted by the biochemical constitution of the cell sap. In most cases, plants which have more tolerance to freezing damage have a higher concentration of sugars and related molecules within the sap. Secondly, and perhaps playing a larger role, is the ratio of polyunsaturated to saturated fatty acids. The elasticity of membranes and their ability to adjust to sudden cross flows of liquid between the inside and outside of the cell, which occurs during repeated fluctuations above and below 32 degrees, is facilitated by a higher fraction of polyunsaturated fatty acids. These healthier levels of polyunsaturated fatty acids are promoted by maintaining the healthy status of the tree. Thus, prematurely inducing dormancy by cutting irrigation water too early in the season can undermine resistance to cold.
Proper nutrition—avoiding excessive nitrogen and providing adequate boron and carbon—is key to mitigating the effects of bacterial canker.
Maintenance of sound plant nutrition and overall health also coincides with another important fact: cell membrane integrity is largely influenced by sound calcium and boron nutrition. A common practice which contributes to bacterial canker is excessive nitrogen fertilization. A key physiological response to heavy nitrogen fertilization is rapid growth, and an inability to harvest and assimilate calcium and boron. Tissues resulting from excessive nitrogen generally have low carbon and boron, and low tissue integrity.
Bacterial canker is a complex disease, the occurrence of which is influenced by various factors which predispose the host, weakening its physiology such that it sustains freezing damages. Freezing cracks serve as a port of entry to the bacterial pathogen Ps. syringae. Thus, factors which contribute to increasing host resilience to freezing are paramount in combating bacterial canker:
- Maintain balanced nutrition and sound health of the trees
- Pay particular attention to avoiding excessive nitrogen fertilization
- While many elements are involved, sound calcium and boron nutrition are especially critical
- Do not induce premature dormancy in trees if the opportunity exists for continued working of leaves.
- Keep populations of plant-parasitic nematodes in check
- Wherever possible, especially in 1st to 7th leaf trees, do not prune until the dangers of frost are minimal
- Get copper sprays on early, before extensive freezing. Apply a second application if the winter cold persists into freezing temperatures.