Bacterial Canker
Bacterial canker is caused by the bacteria Pseudomonas syringae and Pseudomonas morsprunorum. The bacterium enters the tree through wounds left on the tree. These wounds may be as the result of previous pruning exercises, and in some cases, frost damage. Infections are common during autumn, winter and in early spring seasons. The bacteria thrives under damp conditions. The bacterium is spread by rainwater or poorly cleaned pruning tools.
Should the canker manage to form a ring around a branch or limb of the tree, all the leaves in that area will curl in and turn yellow. Also, the area stops growing and eventually dies. Similarly, the same result can be expected if a ring forms in a region around the trunk of the entire tree.
Both P. syringae and P. morsprunorum overwinter in the plant itself - in lesions, buds and towards the edges of the canker itself. During wetter or damper periods, the bacteria rapidly multiply in the ooze from the canker. They are then spread by rains and high winds. Outbreaks are strongly associated with cooler conditions, with freezing temperatures causing the plant to become predisposed to the disease.
The elevated temperatures of late spring and summer put a halt to the spread of the disease.
Symptoms
There are various symptoms that indicate the presence of bacterial canker. The first of these symptoms is the formation of gummy wounds on the tree trunk or on the twigs. These wounds produce a foul-smelling sap that oozes out of the tree. The affected areas also begin to appear darker than the unaffected healthy regions. The moment you notice such signs, it is time to start thinking of a line of action to treat bacterial canker.
Prevention and Treatment
When it comes to preventing the infection, the most important thing is to keep the tree healthy. Healthy trees are less susceptible to the causative bacteria. Trees most likely to get infected are those in locations where ring nematodes flourishes. Likewise, damp and frost environment are excellent breeding grounds for the bacteria. As mentioned, freezing predisposes the plant to infection, therefore consideration should be taken to suitable planting locations - including the potential to overwinter undercover.
Using any culture method like good irrigation methods and providing nitrogen to plants are a few ways to keep a tree healthy. Also, fumigation, before planting takes place, is encouraged as it helps to keep ring nematodes in check.
Taking caution to prevent ring nematodes is the best form of treatment for bacterial canker in stone fruits. It is often said that that prevention is better than cure. However, in the event of an infection, below are some simple treatments to follow to get rid of the bacterial canker infection.
Pruning in the warmer weather conditions of midsummer is additionally useful at preventing the fungal spores associated with silver leaf disease from infecting a potentially already weakened plant.
Treatment Steps
- Flowering trees should be pruned when they are blooming; it will make any wound heal faster.
- Cut off and get rid of dead tree limbs and any other infected parts.
- Bacteria are more functional in early spring and autumn. Avoid pruning during these periods.
- Using copper-based chemicals like Copper Oxychloride (also known as dicopper chloride trihydroxide or Demildex) to spray infected areas. It will prevent the re-infection of present lesions. The chemical is easy to prepare. Simply adding water and stirring it up in a plastic container is enough.
Bacterial Spot
Bacterial spots are caused by the bacterium Xanthomonas arboricola pv. pruni. It is known to affect leaves, fruits and branches. Chronic infections have been known to cause severe drops in fruit quality and harvest. Under favourable conditions, the disease can spread to more than half the produce.
In cases of chronic infections, the leaves fall earlier than expected – early defoliation. Early defoliation can cause the tree to become weak and less vigorous. As a result, it is prone to more damage in the winter. Also, when the infection is severe, the tree becomes susceptible to fruit diseases such as brown rot and rhizopus. There is also a chance that the tree will become a host for many insects and pests. In addition, the fruit size reduces, and the fruit is likely to become sunburnt. In some cases, the fruits start to crack while still on the tree.
The bacteria overwinters near the buds, in areas that are relatively sheltered and in cracks in the bark. When temperatures reach around 18 °C, buds begin to swell and the bacteria begins to multiply when new tissue begins to emerge. The disease is spread by falling dew and rain and is carried by the wind. It is most active in regions of warm temperatures and light rains. However, it is dormant in hot and dry weather. The bacteria can also cause infection via natural openings or wounds. As the primary method of transmission is by rain, regions with frequent heavy rainfall and lengthy heavy dews - the latter is particularly a concern for the infection of fruit. Leaves can be infected for as long as leaves are emerging. Soil conditions associated with bacterial spot disease include lighter soils such as those with a high sand content. Already stressed trees are more prone to infection - emphasising the need for constant vigilance and adherence to growing best practices.
Symptoms
Leaf: Irregular and angular spots appear on the leaves. The spots are usually brown or black. They can also appear as dark purple spot. When the spots converge into a larger form they make the tree leaves appear charred.
Fruits: On the fruits, the spots can appear to be brown or black. They are usually surrounded by a soaked margin. For peaches, the spots appear on the sides facing the sun. Meanwhile, plums show slight differences like the black spots may be larger or appear in small pit-like lesions forms.
Twigs: There are two types of infection that twigs exhibit. Spring cankers are known to form on young twigs that grew during the summer. At first, they look like dark blisters that have a lot of pus in them. With time, the blisters will break, releasing the bacteria onto the tree surface. Summer cankers form on twigs when the tree is already showing signs of leaf spot.
Treatment of canker and spot using copper oxychloride Demildex
Copper oxychloride Demildex is effective at the prevention of proliferation of the bacteria that cause stone fruit canker and black spot diseases.
Treatment for canker
Use copper-based chemicals like Copper Oxychloride (also known as dicopper chloride trihydroxide or Demildex) to spray infected areas. It will prevent the re-infection of present lesions. The chemical is easy to prepare. Simply adding water and stirring it up in a plastic container is enough.
Like the mechanism of action of Demildex in other bacteria, the use of Demildex prevents the reproduction of the bacteria itself.
Treatment for black spot
There is no sure-kill chemical for treating this infection. Nevertheless, using copper based treatments like copper oxychloride Demildex have shown significant results in regards to fighting off the infection. It’s best that these chemicals are used in preventing rather than treating the disease.
The initial application focuses on reducing the chances of contamination on the leaf. Later applications may be done to protect the fruit when they start to blossom out from the husk. The chemical’s effectiveness will vary with the length of the infection.
The copper oxychloride Demildex works by cutting off the operation of the enzyme system of spores. In so doing, there is irreversible damage to the plant. Nevertheless, it forms a protective chemical barrier against infection.
General considerations for the use of copper oxychloride Demildex
Standard applications of copper oxychloride Demildex are in the region 0.25 volume % in water solution, this represents an equivalence of approximately 50% metallic copper. Copper oxychloride is the sole active ingredient in Demilidex and is supplied as a fine light green powder that contains copper oxychloride at a concentration of 850 g kg-1. Demildex’s particle size is 95% smaller than 5 μm meaning that solubility is excellent and absorbability by the plant is good. Surface area is around 30,000 cm2 g-1. Demilidex is applied at the early stages of the growing season for both canker and spot, with additional applications as required, as part standard husbandry. Spraying is best done in the morning of a dry and sunny day. After application, the plants should be given time to dry off. As a precaution, the chemical mixture should be prevented from penetrating the tissue of the leaves.
Once applied to the plant, the copper oxychloride is slowly dissolved by moisture on the plant surface when there is a weak acid present.
As with all copper based treatments, great care should be taken during application to avoid spreading onto humans or animals - to which such high concentrations of copper are toxic. A time not less than two weeks should be left between the final spraying with Demildex and harvesting.
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