Fusarium circinatum

Fusarium circinatum is a fungal plant pathogen that causes the serious disease pitch canker on pine trees and Douglas fir (Pseudotsuga menziesii). The most common hosts of the pathogen include slash pine (Pinus elliottii), loblolly pine (Pinus taeda), Monterey pine (Pinus radiata), Mexican weeping pine (Pinus patula), and Douglas fir. Like other Fusarium species in the phylum Ascomycota, it is the asexual reproductive state of the fungus and has a teleomorph, Gibberella circinata. 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. 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. Monterey pine (Pinus radiata) seems to be the most susceptible species, and in California 85% of the native Monterey pine forests were initially thought to be threatened by the disease. Because of the activation of systemic acquired resistance in native Monterey pine trees, however, the impacts of the disease in California have been mitigated. F. circinatum infects the twigs and branches of pine trees, causing a bark canker. Most infection is by macroconidia or microconidia. 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 is white or pale violet colour and slightly twisted below the proliferation of microconidiophores. In culture, perithecia are readily produced. They are dark purple or black and ovoid. Cylindrical asci 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 weevils in the genus Pityophthorus and bark beetles in the genera Ips and Conophthorus. These insects commonly infect 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, Gibberella 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 predominated and laboratory tests confirmed this. The various symptoms of F. circinatum can help identify and distinguish it from other pathogens or common Pinus diseases. The symptoms are similar to other damping off diseases with seedlings wilting and dying and exuding resin from the root collar areas. Drooping from the resin production and the plant’s resistance mechanism can be observed along with die-back near the apical meristem. A discoloration of the stem and needles is usually present, with plants exhibiting purple or blue shades. Other symptoms include chlorosis of the needles turning a reddish brown color and lesions on the stems, root collars, and tap roots. Host factors that can trigger infection include plant stress with excessive nitrogen in the soil, unbalanced watering cycles, warmer temperatures, and wounds from pruning or insect damage. Numerous plant pathologists have noted F. circinatum as a serious threat to the pine tree species. Due to the high tree mortality rate, reduced growth, and degradation of wood quality, the economic and ecological importance are greatly affected by this disease. Not only can the spread of infection go from branch to branch, but also infect pine seeds, leading to damping-off of younger seedlings and resulting in death from the fungal infection. The environmental interactions that take place to favor the spread and development of this disease play a large role in transmission. Factors such as soil nutrient ratio, abiotic stressors, air pollution, temperature, and humidity can all contribute to the spread of this disease.