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.

The Orchardist - - Our Most - By Leanne Stew­art

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 toxin-pro­duc­ing 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.

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