With visual appearance and processing quality so important to the potato industry, all stages of the supply chain must take responsibility for protecting the crop against factors which damage these characteristics.
Fluctuating prices, rising costs and market uncertainty present the industry with many challenges, making it vital that growers and store keepers do all they can to preserve tuber quality. With an industry under pressure to meet rapidly evolving opportunities for fresh and processed potatoes, it is essential that steps are taken to minimise losses through disease.
Minimising problems in tuber quality during production and storage demands knowledge of diseases, an understanding of risk; and an appreciation of how to reduce those risks. This article seeks to illustrate the risks faced, and to give pointers on how to manage the risks of economically damaging disease.
DISEASE IMPACTS
Potato disease experts at Central Science Laboratory (CSL) estimate that there are about 25 potato diseases of concern to the supply chain. Several of these exhibit internal symptoms that are not readily apparent until the tubers are washed or cut; others cause surface blemishes that become apparent only after tuber washing. A third group may cause slow or rapid rotting of tubers either externally, internally or both.
Most operations and management decisions during the production cycle have implications for disease risk, and good practice is critical at every stage to reduce the likelihood of disease.
MANAGING MAN-MADE DISEASE RISKS
There can be little or no excuse for disease problems caused by human error in well established fresh produce potato supply chains. Clearly one of the most obvious ‘disease’ opportunities in supply chains is that of tuber washing. High levels of bacterial or fungal inoculum can easily accumulate in poorly managed washing systems. Conversely over-managed washing systems will dispose or recycle washing water too often.
Problems associated with washing water could be minimised by daily monitoring of bacterial or fungal populations in washing systems. Simple diagnostic procedures for estimating the microbiological flora present in washing systems have been developed by CSL, allowing quantification of specific primary or ‘secondary’ potato pathogens. Use of these types of services will ensure fewer or no problems from tuber washing systems later in supply chain. They could also help reduce problems associated with frequent recycling and/or disposal.
A FRESH START?
Virgin potato land and disease-free, certified seed tubers of known provenance are clearly the best possible fresh start. Producers may wish to ensure that this is so by having such soils and seed tubers tested for the presence or absence of potato pathogens and act accordingly.
It may also be necessary to quantify the inoculum’s spatial incidence or density in the soil to inform decisions on the use of soil-applied fungicides. Similarly quantifying the inoculum loading on the seed tubers could inform decisions on its planting or fungicide use, particularly where the end-use of the subsequent crop is acutely sensitive to the presence of disease, even those that cause surface blemishes only.
GOING FULL CIRCLE
Rotations on good potato land are probably tighter now than ever. This being so, the potential for problems caused by soil-borne diseases such as powdery scab or spraing has never been greater, just when tuber quality has become so important. In this situation quantifying the spatial incidence and density of pathogen inoculum in field soils before planting can be very helpful in identifying the extent of disease risk and informing decisions on variety choice, whether or not to plant at all, the target market for the subsequent crop, the time to burn off, the time to lift and other agronomic variables.
Spraing provides a good example of the advantages of this new technology. For growers, it is extremely important to be able to detect the presence and differentiate between TRV-spraing and PMTV-spraing in soils because of the very different implications of these visually-similar conditions. A new, rapid soil has been developed giving results which could mean the difference between a marketable and an unmarketable crop, major savings in nematicide costs, powdery scab and PMTV in irrigated situations.
DISEASE PROFILING
The most obvious means of avoiding potential problems caused by long lag times in the supply chain is to reduce its length and increase its speed of operation. Putting potatoes into retail outlets within 24 hours of lifting demands consistent, assured tuber quality, local supply agreements, slick operational management and logistics, and above all absolute confidence based on knowledge, experience and/or accurate assessment of disease risk. In such a short supply chain no one can afford the delays caused by having to cope with the problems due to disease, particularly those affecting skin appearance.
One of the best risk assessment methods for such potential disease problems is seed tuber and soil-borne disease diagnostics. CSL’s TaqMan PCR tests can provide quantitative assessments of both tuber-borne and soil-borne inoculum of up to 25 diseases within a few hours of receiving the tubers. The interpretation of such quantitative assessments is the subject of joint projects between CSL, SAC, SCRI and Adas, currently co-funded by BPC and SEERAD. Availability of this level of information about the disease profile of tubers will vastly improve decision making on how crops are handled and marketed.
THE GROWING CROP
Once the potato crop is planted the only problems about which producers can take sensible action are late blight and aphid-borne virus. All other problems are seed or soil-borne so action against these should have been taken already.
Aphid population monitoring is used in the UK to estimate the risk of virus spread in seed and ware crops. Marked increases in the size of aphid populations are used as a guide to burning-off seed crops and for aphicide application. Aphid populations could be used to estimate the frequency of aphid-borne virus transmission because CSL is able to identify and quantify viruses and virus strains in single aphids. This could be particularly important for varieties sensitive to PVYNTN, which can cause potato tuber necrotic ringspot disease.
THE STORED CROP
The advent of sophisticated potato storage facilities should ensure that problems associated with short- to long-term storage are easily managed and no longer present any significant threat in terms of tuber breakdown due to blight and/or bacterial soft-rots, reductions in tuber physiological quality, or excessive development of skin blemish diseases.
Problems are likely to arise only when the store manager is ill-informed about the immediate history of one or more loads arriving at the potato store. Bruising assessments can be managed in real-time before store loading; bruised loads can be handled differentially from others and adjustments to lifting machinery advised to field operators.
Unfortunately disease assessments cannot be accomplished with the same rapidity, so it is as yet impossible to differentiate loads on the basis of disease incidence and severity. Therefore, disease ‘control’ in store must rely on the best possible store management and some other form of tuber quality monitoring such as ‘electronic nose’ technology.
NEW CONFIDENCE
Methods of detecting and quantifying disease levels are developing rapidly. The new insights into crop health that these tests allow will bring greater confidence into the supply chain, exactly what is required in this age of global trade. Decisions on seed sources, sites of production and end markets will all be aided by greater knowledge of diseases and associated risk, giving supply chain managers new tools to ensure consumer demands are fulfilled.
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