Sirococcus clavigignenti-juglandacearum

Sirococcus clavigignenti-juglandacearum (butternut canker) is a fungus that causes a lethal canker disease of Juglans cinerea (Butternut) trees. It is also known to parasitize other members of the genus Juglans (walnuts) on occasion, and very rarely other related trees including Carya (hickories).^[1][2]

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Distribution

Butternut canker was first discovered in 1967 in North America, where it is an invasive species, in Wisconsin. It has since spread to other states. Its native origin is unknown, but possibly in Asia given the resistance of Asian walnuts to the disease.^[2] It was reported that 84% of all Butternuts in Michigan as well as 58% of all trees from Wisconsin have been affected; later surveys revealed that 91% of all living trees were diseased or cankered. In Virginia and North Carolina, the butternut population has been reduced from 7.5 million to 2.5 million. It has since been reported in Canada.

Symptoms

Lenial and broad cankers on the main stem, branches, young twigs, and exposed roots. Most cankers are covered with bark cracks and shredded bark above cankers in late stages.

Life Cycle

  Pycnidiospores are released during rainy periods. When the spores make contact with wounds or broken branches, they germinate and penetrate deep into the tree to produce cankers. Infection hyphae typically penetrates through the parenchyma phloem intracellularly but they can also penetrate intercellularly through uni and multiceliate xylem ray cells and paranchyma cells. Later, the fungus will produce mycelial mats of stroma and mycelial pegs.

Stroma mats will produce uni or multilocular pycnidia. Inside the pycnidia are branched and unbranched conidiophores with two-celled pycniospores, which later are ejected from the pycnidial ostiole.

Additionally, the stroma will produce a peg of interwoven mycelium. These pegs put pressure on the outer peridium of the host bark, which exposes the pycnidia below. These pegs also produce pycnidia that are smaller than the pycnidia in the stroma. While different in size, the spores produced are identical.

Resistance

Many species of tree show varying degrees of resistance, such as the heartnut, Butternut, and the Japanese, Black, and Persian Walnuts. The heartnut and Japanese Walnut trees show the highest degree of resistance, as both have a peridium layers between 35-45 cells think which act as a barrier for pathogen penetration. Additionally, both trees produce phenolics immediately upon attack, later producing gums and tyloses to surround the pathogen.

Control

Breeding for resistance is important for fighting butternut canker. While standard practice has been that infected trees should be removed to prevent further spread, there is a growing opinion that the time for this is past. The disease has now been found in virtually all parts of the butternut range. Additionally, it is suggested that "removing diseased trees" is a guarantee that infected, but not dying trees, ie. those that are specifically "partially resistant" to the fungus, will be killed; eliminating any chance of increased resistance in progeny. Instances are known of long-term survival of pure butternuts infected by the canker.

Recent reports have shown that the fungus can be internally seed-borne, so seeds should be subjected to intense quarantine protocols; most especially if destined for plantings where the disease is not already established.

References

• "Survivor Butternuts"; discussion, documentation. http://www.badgersett.com/info/publications/Bulletin8v1_0.pdf