The Question of Bacteria

Bacterial Introduction
From a production grower’s standpoint bacterial diseases of plants are very difficult to control, and although bacterial diseases when compared to fungal diseases are relatively few, they can cause grim monetary fatalities. For the backyard gardener they can be just as devastating, for bacterial diseases in addition to visible symptoms manifested by the plant, can become systemic in the plant’s vascular tissue making it unrealistic to try and eradicate the pathogen by pruning out the symptomatic tissues or by applying a pesticide to the plant surface. Moreover, bacteria undergo exponential growth, which means that their populations can double in several hours, depending on the bacterial species and favorable environmental conditions. To coin a catchy term, bacterial diseases are explosive; by the time symptoms are acknowledged, the pathogen is often entrenched and well on its way to not only destroying the plant but the crop as well.

Generally, managing bacterial diseases depends mostly on host resistance, which may not necessarily be available for the crop or plant you desire. Thank goodness there are cultural and sanitation practices and in some cases biological control agents and bactericides that can be effectively integrated into a disease management program.

Lets target Pseudomonas syringae
P. syringae is accountable for a number of significant plant diseases in the Pacific Northwest. This causal agent is responsible for diseases commonly referred to as bacterial canker, bacterial blast, or bacterial gummosis (diseases are named from the symptoms they cause, the host or the pathogen). So maybe it would be easier to start with the wide array of symptoms that P. syringae can cause to an extensive assortment of ornamental, vegetable and fruit plants.

Wide-ranging Plant Symptoms Caused by P. syringae
To narrow down diagnosis of plant disease we often rely heavily on the symptoms that a pathogen causes the plant. Sounds simple right? Unfortunately, its not simple at all, that’s why we keep plant pathologists employed, but don’t despair for you can narrow things down and make what we call ‘educated guesses’ and then confirm your diagnosis with a trained professional. So what happens to complicate matters, more than one symptom can appear simultaneously on a single plant. Symptoms will vary depending on the different strain of the pathogen, the species of plant infected as well as the part of the plant infected. What do you want to look for if you suspect P. syringae is attacking your plants? Common symptoms include stem cankers which can exude goo, officially called ‘gummosis’, the cankers can enlarge to girdle the stem or branch, tip or shoot dieback, leaf and fruit spots, dead dormant buds, flower blast and necrotic leaf veins which is a result of a systemic infection. The most common of these symptoms in our area is tip or shoot dieback which has been found on approximately 40 woody deciduous plants in the Pacific Northwest. Some of the plants that were infected include aspen, blueberry, dogwood, filbert, magnolia, lilac and oriental pear.

Why is My Plant Susceptible to P. syringae?
The jury is still out on this one, in other words the experts do not necessarily agree on the severity of the diseases, most feel that P. syringae is an opportunistic bacterium, which means that it waits until the host is weakened and then invades. But either way there are conditions that you can watch for, the first being if your plant happens to be attacked by something else and is under stress then the disease severity is greater. Wounding of any kind such as mechanical (pruning or weedwacking damage) as well as frost injury will allow the bacterium entrance. Certain factors such as soil pH and nutrient deficiencies or excesses may also predispose trees to infection. The moral of this story is “keep your plants happy and healthy” and they will better withstand attack and be able to fight off infection of a pathogen.

How is P. syringae spread?
Bacteria are ubiquitous, they are found everywhere and P. syringae can easily be moved around by wind, rain, insects, people (pruning or mechanical equipment), and infested plant material.

Where does P. syringae live?
There are several potential survival sources for P. syringae. The pathogen can overwinter in plant buds which appear to be perfectly healthy. These latent infections, establish P. syringae inside symptomless tissues signify a very important source of primary inoculum. Systemic incursion of the pathogen isolated from interior tissues assumes significant environmental implications and clearly poses a major challenge in control of the disease. Cankers left over from the previous years growth can also harbour inoculum. P. syringae can also exist on surfaces of many plants and when environmental conditions favour disease development, it can spring into action and infect. Some weeds and grasses have been known to be hosts to P. syringae.

What Can I do to Control P. syringae?
Keep your yard a happy and healthy place for your plants, in other words the right plant in the right place. Stress predisposes your plants to pest invasion, whether that pest be a pathogen, arthropod, weed or vertebrate. Cultural strategies such as scheduling overhead irrigation in the morning will allow plants to dry off; better yet use drip or bottom irrigation. Altering soil pH by liming can promote some stonefruit vigour. Pruning in late winter (February) can limit P. syringae infections. Cauterization of small cankers by a hand held propane burner limit the canker’s spread so it did not girdle the branch or trunk. A certain skill and of course care must be taken when doing this, don’t burn the tree down! Sanitation methods such as weeding can also help reduce inoculum levels. Plant resistant cultivars, ask for these at your local garden centre. There is also the option of biological control, but this is aimed at frost control using bacterial antagonists, not yet developed for the home gardener. Finally, as a last resort there are chemical controls, to be used in a judicious, vigilant manner.