Prediction and manipulation of black dot development in potato crops

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The project examines factors influencing black dot development to help identifying crops at risk to reduce symptoms development through early harvesting.

One option for the management of black dot is early harvesting, which reduces symptom development. In order to select crops for early harvesting it is essential to know which crops are likely to be at risk by examining the factors influencing BD development. The research has shown that the ability to predict the risk of black dot occurring varies with potato variety. The methods of haulm destruction and the different storage conditions studied during the project did not effectively reduce the development of black dot.

Observations were made over 3-year study to try predicting black dot (BD) development on harvested tubers by assessing stems and stolons for the presence of the disease in 124 potato crops and related to the disease on the tubers at harvest. In some years significant associations were observed in some cultivars. In 2000 and 2002 in the cv. Estima if stems or stolons showed black dot symptoms as the crops were starting to senesce then the disease was likely to be present on tubers at harvest. In cv. Maris Piper a significant association between black dot on stems in September 1999 with disease at harvest was observed. No relationship between black dot on stems and stolons in the growing crop and disease on tubers at harvest were observed in the cvs. Saxon and King Edward. To explain these results a glasshouse and field experiment were performed to identify when symptoms of black dot develop on underground parts of a potato plant. These showed that infection by C. coccodes could occur soon after these plant parts had formed. Visual symptoms did not appear till just prior to senescence on stems and stolons and during senescence on tubers. Methods of haulm destruction and different storage conditions did not effectively reduce development of black dot on tubers. Reasons for this and its commercial implications are discussed.

Strategies for minimising virus infection of potato crops

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This project report combines published information with the results from this work to provide a Decision Support System for minimizing PVY infection.

The project combines close collaboration with participating seed growers. It implements a system of within field aphid trapping and virus testing with complementary experiments designed to provide information to evaluate the risk of virus (principally PVY) infection and examine the merit of various testing strategies for crop management. The experiments were particularly concerned with evaluating the importance of mature plant resistance to PVY infection. The aphid species which have been shown to transmit PVY were given an efficiency index based on their ability to transmit the virus.

Aphid trapping can be a useful management tool for controlling PVY. In the two years prior to the date of this interim report, seed crops were burnt down promptly once a substantial rise in the ‘PVY index’ was recorded and in most crops either no PVY or a low incidence (1- 5 %) was recorded in post harvest tuber tests. The data obtained from these crops allows estimation of conditions which preclude substantial PVY spread but the absence of high incidence of infection limits the potential for modelling PVY ingress. Results from the virus movement experiment carried out over two years have been largely consistent. ELISA tests carried out on leaves and stems gave substantially lower incidence of PVY than the results from tuber tests. The incidence of infection in tubers was negligible for tubers harvested immediately after one week exposure to infection but one week later, infected tubers were detected and the incidence increased after a further week although to a lesser extent. The proportion of tubers infected tended to increase with increase in aphid index up to c. 50 but there was considerable variation in incidence of PVY in relation to aphid indices between exposure dates. Older plants had a lower incidence of infection than young plants, but even for old plants (exposed 6 weeks after emergence) up to 30 % were infected during weeks when the vector index was high. The differences in tuber infection between young and old plants confirms the importance of mature plant resistance for modelling infection in crops

Project report: Independent variety trials

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Varietal performance for susceptibility to blight, black dot, black scurf, silver scurf, skin spot, common scab, powdery scab, blackleg, dry rot, potato cyst nematodes and other damages

To comply with national and European Community legislation for marketing seed potatoes, varieties must be placed on the official National List (NL) of a Member State, which is then automatically entered on to the Common Catalogue (EC National List). Part of the NL testing involves assessing a new variety for Value for Cultivation or Use. In the UK, this testing is largely concentrated on assessing varietal performance for susceptibility to diseases, pests and important tuber quality characteristics.

This 28-page experimental report describes tests conducted on 4 varieties undergoing their 2nd year of UK NL testing, 7 varieties which had completed UK NL tests and 5 Common Catalogue varieties. The above varieties were tested against foliage late blight, black dot, black scurf, silver scurf and skin spot. The Common Catalogue varieties were also tested for susceptibility to common scab, powdery scab, blackleg, dry rot, potato cyst nematodes, external (splitting) and internal (bruising) damages. The report presents a comprehensive summary that covers Project Aims, Work Undertaken and Findings (results on Susceptibility/resistance for 2005 are summarised in a table and rated on 1 (Low) – 9 (High) for resistance to disease, pest and defects) and conclusions.

Fungicide Resistance Action Group UK (FRAG-UK) Potato late blight: Guidelines for managing fungicide resistance

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This guide sets out actions to minimise the risk of resistance development to existing fungicides and new active substances for control of late blight.

The introduction in the late 1970s of the phenylamides brought a new dimension to blight control but there was a rapid development of resistance in the blight pathogen. This quickly brought about a change in the number of recommended applications. Of the current approved fungicides, resistance in Phytophthora infestans has developed only to the phenylamides, which was first identified in the UK in 1981.  The report provides information on the use of fungicide, the types and occurrence of fungicide resistance and how to reduce the risk of resistance developing in a pathogen population. A list of 12 fungicide groups, their mode of action and mobility, common name of active ingredient, product name, resistance risk and use is being presented. The Impact of new strains on fungicide use is also being addressed.

Managing fungicide resistance can be summarised as follows: where possible use a blight resistant cultivar, avoid growing large areas of highly susceptible cultivars, eliminate sources of blight (infected seed, ground-keepers and dumps), monitor local weather conditions, apply fungicides as protectants, select fungicides to suit local conditions, use mixed formulations of active ingredients with different modes of action or from a different fungicide families, or target specific products in blocks to appropriate growth stages, do not chase the epidemic with fungicides, burn off early if blight levels are significant and delay lifting until the haulm has been dead for at least 14 days.

Fight against blight: Potato outgrade hygiene

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This report provides the best practice advice for potato outgrade control, which can act as a reservoir for blight infection.

The report focused on minimizing the impact of potato outgrades. This included easy access to outgrade piles away from potato crops and farm buildings, site piles on land not intended for any crop or any potato crop in the future, not to risk polluting watercourses, locate piles away from ditches and rivers or groundwater etc.

Control measures emphasised on adopting a zero tolerance approach to green foliage on outgrade piles by tackling it early; relying on a late application of glyphosate or diquat + paraquat is a high risk strategy as blight may already have spread by this point. The report also provides a strategy for the prevention of green foliage and highlight regulations for agricultural waste management in the UK.

Fight against Blight Issue 4: Volunteer Control (Groundkeepers)

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Growers’ advice to reduce the impact of blight through cultural methods and the use of herbicides to control volunteer potatoes.

Volunteer potatoes act as a primary infection source by over-wintering infected tubers, and provide unprotected foliage that can act as an entry point throughout the season for blight. They can also act as a reservoir or a host for other problems such as spraing, potato cyst nematodes (PCN), black scurf, black dot, powdery scab etc. Volunteers are also a host for aphids and aphid transmitted potato viruses. The report provides facts backed by evidence from trials about the problem and describes cultural and chemical methods of control as no single herbicide treatment is entirely effective.

An integrated control strategy was suggested emphasising on optimising tuber size distribution, applying maleic hydrazide, setting the harvester to lift smaller tubers where possible, keeping returned tubers near the soil surface, following with a competitive crop, hitting volunteers whenever possible, using selective herbicides in the following crops, using pre-harvest glyphosate, cleaning up stubbles with glyphosate and watching out for volunteers in shaded crops like winter oilseed rape and maize.