'Be aware of what you can't see in store'


04 January 2022
Bacteria and virus specialists at FERA offer some timely advice

STORING potatoes is an expensive exercise, costing between £7 and £21 per tonne depending on variety, store efficiency, length of storage and sprout suppressants. It therefore makes sense to only store sound, good quality crops. 

While potatoes affected by external defects such as poor skin finish and skin disease like scab can obviously be identified and marketed accordingly, internal defects are by their nature, harder to detect. Destructive sampling has obvious limitations, so for many crops and defects, such as spraing or soft rots, it is useful to adopt a risk-based approach.

Spraing is a symptom of tuber infection by either tobacco rattle virus (TRV) or potato mop top virus (PMTV). Both have soil-borne vectors, free-living nematodes in the case of TRV, and the powdery scab fungus, Spongospera subterranea, in the case of PMTV. Soil testing for both virus and vectors can be a useful part of risk assessment. 

Fera virologist Adrian Fox recommends a soil bait test which consists of taking 1 kg of soil from a 4-hectare area (ideally soon after the previous crop has been harvested). 

“If you are a ware grower looking to maximise potential and minimise risk, then you may want to consider
soil bait testing to ensure soil-borne viruses don’t affect your crop,” he said.

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In the lab, bait plants are grown in the soil and molecular diagnostics are used to check the roots for the presence of potato mop-top virus (PMTV) and tobacco rattle virus (TRV) to provide an assessment for the risk of virus transmission within the sampled field.

Fera nematode specialist Tom Prior says testing for free-living nematodes (FLN) can also provide an idea of the risk of virus transmission.

Although not strictly an internal defect, some crops may be more susceptible to soft rots in storage and bacterial soft rot testing is advisable, as it can give an idea of the actual risk to the crop. 

Bacteriology Diagnostic Team Manager, Brian Carter, said:  “If counts are below 1,000 then it’s considered a low-level risk, and between 1,000 and 100,000 represents an intermediate level and moderate risk. Anything over 100,000 you are looking at quite a high level and elevated risk of disease development.”

When combined with additional sampling throughout the storage period, these risk assessments can greatly improve the management of crops which have a higher risk of developing internal defects.

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