Botryosphaeria obtusa

Botryosphaeria obtusa is an important plant pathogen. Frogeye leaf spot/Black rot/ canker is a disease that is capable of infecting many plants, fruits, trees and shrubs, worldwide.^[4] On the leaf it is referred to as Frogeye leaf spot, this phase typically affects tree and shrubs, in fruit, such as the apple and cranberry, it is referred to as Black rot and in twigs and trunks it is known as Canker. This disease is caused by the Fungus known as Botryosphaeria obtusa.^[4]

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Symptoms

Black spot (in fruit)

The fungus Botryosphaeria obtusa enters the fruit through wounds in the fruit. These can be made by insects, birds or growth cracks. At first a brown spot, near the calyx, appears on the fruit.^[3] The spot on the fruit then enlarges and black/brown rings appear on the fruit. The fruit holds its shape, however, unlike other fruit diseases. The fruit will then wither up and can remain on the tree for another year before falling off. During this time pycnidia appear on the surface of the rotted fruit.^[4]

Frogeye Leaf spot (in trees and shrubs)

In leaves the fungus Botryosphaeria obtusa begins by causing purple specks on infected leaves.^[2] These then enlarge to cause large spots on the leaf, developing a brown colour. The spots appear to have rings of brown with a purple margin, thus giving it its frogeye appearance. The spots can then produce pycnidia which can separate this species of fungus from others possible leaf fungi’s.^[4]

Canker (on twigs, branches and trunks)

On twigs, branches and trunks the fungus Botryosphaeria obtusa can infect where there has been a winter injury or fire blight cankers. Slightly sunken reddish/brown spots appear on the infected areas in bark. These then enlarge to form cankers, which can then enlarge slightly more each year. The bark usually dies and can, after time, be pulled away from the tree. In older cankers the pycnidia appear on the bark.^[4]

Defence mechanisms

Passive protection

These defenses include the production of chemicals or the hardening of the epidermal structures. These allow protection against entrance to the plant by Botryospaeria obtusa. The hardening of the epidermal surface is known by the lignin production of the epithelial cell to form bark or a woody structure. The chemicals are known as phytoalexins, these strict the colonization of microorganisms. Both of these defenses are extremely important and effective in the protection of the plant against fungal pathogens.^[5]

Active protection

There are known as the systemic and localized defenses and are initiated when the pathogen gains entrance into the plant through wounds. The localized response is when the tissue around the infected area begins necrosis and cell death occurs, this is the plant attempting to rid of the pathogen by removing the infected tissue. Also accumulation of phytoalexins occurs; the phytoalexins are released in response to the pathogen and restrict colonization of the organism. The localized response is efficient but is damaging to the plant itself and is therefore not as productively effective as passive protection. With the systemic response it is not yet known if it occurs with Botryospaeria obtusa, but with most plants, chemicals are released elsewhere in the plant away from the infected site, to protect the rest of the plant against the disease.^[5]

Treatment and Control

The most effective treatment is to prune out the infected areas on trees, to ensure transfer between trees does not occur. Fruit that is infected can stay on the tree for over one years period of time, and therefore remaining fruit should be removed to avoid another source of inoculation for other trees in the orchard. The trimmed branches or dead fruit should then be burned or disposed of immediately as the organism can survive on the dead tissue for a long period of time. Infection of leaves and fruit can be avoided by spraying them with a fungicide. The treatment for the fungicide should be also kept up to date via the manufacturer’s instructions.^[3]

For further information refer to this link^[2] http://www.caf.wvu.edu/kearneysville/disease_descriptions/omblackr.html

References

1. Leuty, T. (2006). Summer pest activity in heartnut walnuts. Hort Matters, 6(21), 5.

2. Travis, J.W., Rytter, J.L., Biggs, A.R. (n.d.). Black rot, Botryosphaeria obtusa. Retrieved October 30, 2007, from http://www.caf.wvu.edu/kearneysville/disease_descriptions/omblackr.html

3. Ellis, M. A. (n.d.). Plant pathology: Black rot and frogeye leaf spot on apple. Retrieved October 30, 2007, from http://ohioline.osu.edu/hyg-fact/3000/3030.html

4. Hartman, J. (n.d.). Plant pathology fact sheet: Frogeye leaf spot, black rot and canker on apple. Retrieved October 30, 2007, from http://www.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/PPFS-FR-T-3.pdf

5. Biggs, A.R. (n.d.). Responses of Angiosperm Bark Tissues to Fungi Causing Cankers and Canker Rots. Retrieved November 11, 2007, from http://www.caf.wvu.edu/Bark/cankers.htm#Expression%20of%20Resistance