Fusarium (exotic strains), the fungi causing a flop
A number of emerging biosecurity risks should be on growers' radar – these are exotic pests or pathogens that are proving highly invasive around the world.
What are Fusariums?
Fusarium is a genus of soil-borne fungi containing more than 50 species, a number of which are important crop pathogens. Most species of the filamentous fungus have the ability to reproduce both sexually and asexually. Fusarium can live in the soil for up to 16 years, and survives from one season to the next on decaying organic material. Some species in the genus gain their nutrition from dead and decaying plant matter, whist others are parasitic and can cause rot in their host plants. The processes by which Fusarium infects its hosts are not well understood, but host plants infected with the fungus often show symptoms of disease known as Fusarium wilt.
Why are they a problem?
Fusarium species can be highly pathogenic or non-pathogenic, with symptom expression dependent on the species, the race, and the relationship with the host. The fungi causes a range of different conditions in host plants including seed, root, stem or crown rot, head or top blight, scabs, damping-off, wilts, stem and branch cankers and dieback. Internal symptoms can include discolouration of stem tissues. Fusarium infection can also cause storage rot in harvested produce.
Both fruit and vegetable species are known to succumb to the pathogen, as well as forestry trees and native plants. One of the most devastating examples of the pathogen is Panama disease in bananas, which is caused by Fusarium oxysporum f. sp. cubense (Foc). Severe impacts have been seen in banana growing regions worldwide, especially following the discovery of a particularly pathogenic strain, tropical race 4, which affects the widely grown cavendish variety.
Another species of fusarium, Fusarium circinatum or pine pitch canker, is such a severe threat to forestry that it is a Notifiable Organism in New Zealand under the Biosecurity Act 1993. This status means there is a legal obligation to report the organism.
Whilst most species of Fusarium are pathogenic to plants, some can cause infection in animals and even humans. In addition, some species of Fusarium produce mycotoxins that can be harmful to humans and animals if they ingest food contaminated by a toxinproducing Fusarium species.
What crops do they attack? Horticutural crops that have an association with a Fusarium species overseas include potatoes, peas, beans, corn, bananas, cereals, melons, peppers, tomatoes, rice, lettuces, strawberries, avocados and maize, among others.
Distribution and spread
Fusarium species are present all over the world. The widespread distribution may be due to their ability to grow on a range of substrates and their
efficient dispersal mechanisms. Spores can be spread over long distances via movement of infested soil or water. The pathogen can also be transferred in infected plant material, and some species take advantage of insect vectors to spread from plant to plant. Risk to New Zealand
A number of Fusarium species are already present in NZ, but many of the most damaging species and races are thankfully absent. In the early stages, symptoms of Fusarium infection can be mistaken for water stress or nutrient deficiencies. Once samples are taken, identification to species level can be time consuming - it can take up to four weeks for some of the fungal structures that are crucial for accurate identification to form in culture.
As is often the case with soil-borne pathogens, if an exotic Fusarium species were to establish there are few options for eradication. Resistance issues have been apparent overseas, limiting the efficacy of fungicides as a control tool. Add to this the ease with which the fungus can spread across fields, farms and regions, and even containment would be a challenge. There have been instances overseas where failure to control build-up of the disease has resulted in fields becoming no longer suitable for production of susceptible crops.
What to do if you see it
If you think you’ve seen unusual disease symptoms that could be associated with Fusarium species, call the MPI pest and disease hotline on 0800 80 99 66.
▴ fungus Fusarium verticillioides