Fusarium circinatum is a fungal

Fungus

A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...

 plant pathogen that causes the serious disease pitch canker

Pine pitch canker

thumb|240px|Pitch canker affecting western white pinePine pitch canker is a virulent and incurable fungal disease of pine trees caused by the fungus Fusarium circinatum....

on pine trees. Like other Fusarium

Fusarium

Fusarium is a large genus of filamentous fungi widely distributed in soil and in association with plants. Most species are harmless saprobes, and are relatively abundant members of the soil microbial community. Some species produce mycotoxins in cereal crops that can affect human and animal health...

species in the phylum Ascomycota

Ascomycota

The Ascomycota are a Division/Phylum of the kingdom Fungi, and subkingdom Dikarya. Its members are commonly known as the Sac fungi. They are the largest phylum of Fungi, with over 64,000 species...

, it is the asexual reproductive state of the fungus and has a teleomorph

Teleomorph, anamorph and holomorph

The terms teleomorph, anamorph, and holomorph apply to portions of the life cycles of fungi in the phyla Ascomycota and Basidiomycota.*Teleomorph: the sexual reproductive stage , typically a fruiting body....

, Gibberella circinata.

Distribution

This fungus is believed to have originated in Mexico. It spread to the eastern United States in 1946 and by 1986 had reached the western United States. It was first recorded in Japan in the 1980s, in South Africa in 1990, in Chile and Spain in the mid 1990s and in Italy in 2007.

Host species

In California this canker has been recorded on nine different species of pine (Pinus) and on Douglas fir (Pseudotsuga menziesii). In Europe and Asia it has been recorded on over 30 other Pinus species. Monterrey pine (Pinus radiata) seems to be the most susceptible species and in California 85% of the native Monterey pine forests may be killed within a decade.

Biology

F. circinatum infects the twigs and branches of pine trees, causing a bark canker. Most infection is by macroconidia

Conidium

Conidia, sometimes termed conidiospores, are asexual, non-motile spores of a fungus and are named after the greek word for dust, konia. They are also called mitospores due to the way they are generated through the cellular process of mitosis...

 or microconidia

Conidium

Conidia, sometimes termed conidiospores, are asexual, non-motile spores of a fungus and are named after the greek word for dust, konia. They are also called mitospores due to the way they are generated through the cellular process of mitosis...

. The macroconidia are 3-septate, with slightly curved walls and the microconidia are single-celled, ovoid, and borne in false heads on aerial polyphialides. The aerial mycelium

Mycelium

thumb|right|Fungal myceliaMycelium is the vegetative part of a fungus, consisting of a mass of branching, thread-like hyphae. The mass of hyphae is sometimes called shiro, especially within the fairy ring fungi. Fungal colonies composed of mycelia are found in soil and on or within many other...

 is white or pale violet colour and slightly twisted below the proliferation of microconidiophores. In culture, perithecia

Ascocarp

An ascocarp, or ascoma , is the fruiting body of an ascomycete fungus. It consists of very tightly interwoven hyphae and may contain millions of asci, each of which typically contains eight ascospores...

 are readily produced. They are dark purple or black and ovoid. Cylindrical asci

Ascus

An ascus is the sexual spore-bearing cell produced in ascomycete fungi. On average, asci normally contain eight ascospores, produced by a meiotic cell division followed, in most species, by a mitotic cell division. However, asci in some genera or species can number one , two, four, or multiples...

 are released by oozing. There are eight ascospores which are 1-septate and ellipsoidal to fusiform. Because peritheca have not been observed in the field, it is not thought that ascospores are an important route for infection.

The infection is usually carried from tree to tree by the rain, the wind or by bark-feeding insects. These including weevil

Weevil

A weevil is any beetle from the Curculionoidea superfamily. They are usually small, less than , and herbivorous. There are over 60,000 species in several families, mostly in the family Curculionidae...

s in the genus Pityophthorus and bark beetle

Bark beetle

A bark beetle is one of approximately 220 genera with 6,000 species of beetles in the subfamily Scolytinae. Traditionally, this was considered a distinct family Scolytidae, but now it is understood that bark beetles are in fact very specialized members of the "true weevil" family...

s in the genera Ips and Conophthorus. These insects commonly infeat pine trees and the adults may disperse the pathogen. Additionally, these insects cause a wound when feeding and this may facilitate entry of the infection. Warmth and moisture encourage the development of the disease whereas cooler drier conditions restrict it. In California it is more severe in coastal areas.

Research was undertaken to see whether spores from the telemorph, Giberella circinata, might be responsible for spread of the fungus. It was found that very few vegetative compatibility groups existed among the California strains of the pathogen. This implied that asexual reproduction

Asexual reproduction

Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...

predominated and laboratory tests confirmed this.

Spread

In Chile, the infection was first reported on Pinus radiata in nurseries and was thought to be due to the import of contaminated seed. The symptoms were similar to other damping off diseases with seedlings wilting and dying and exuding resin from the root collar areas. Seedlings could also be infected by soil-borne contamination. A few years later, the disease had not spread to mature stands of trees. The same is true in South Africa, where it was reported to infect nursery stock but not forest trees.

F. circinatum is spread locally by wind and insects, but it is slow to spread into new areas. Over large distances it can be transported in contaminated pine seeds or by young plants. Although it could be carried as infected timber, this is considered unlikely especially if the bark has been removed. If timber had been a significant means of infection, the fungus would have spread more rapidly to other parts of the world as there is a considerable trade in pine.

Testing

A test has been developed for detecting contamination of seeds of Pinus species by F. circinatum. It is based on the biological enrichment of a sample followed by a real-time polymerase chain reaction assay. Many countries are imposing quarantine controls on movement of propagating materials and this test can help prevent the introduction of the pathogen through contaminated seed.