Identifying and controlling potato storage diseases Louise Cooke & Gillian Young Sustainable Agri-Food Sciences Division AFBI Newforge What I’ll be covering today: • Dry rot • Gangrene • Common scab • Powdery scab • Black scurf • Silver scurf • Skin spot • Pink rot & rubbery rot • Tuber blight 1 Dry Rot 2 Dry Rot • Caused by soil-borne Fusarium spp. fungi • Infection occurs through wounds during harvest and handling • Encouraged by damage at/after harvest + humidity + warm storage temperatures • Can spread in store • Tubers become more susceptible after December • Favoured by grading in spring-time and de-budding Dry Rot Control • Prevent damage • Apply approved fungicides (next slide) ASAP after harvest (not pre-planting) • Quick dry and cool storage • Wash and disinfect stores, boxes and equipment every year • Aim to grade seed before December • Handle tubers gently at planting to avoid damage: rots starting then increase dry rot in progeny tubers 3 Dry Rot Control Approved fungicides for seed post-harvest • imazalil e.g. Fungazil • thiabendazole e.g. Storite Excel • imazalil + thiabendazole e.g. Storite Super Approved fungicides for ware post-harvest • as above, but excluding imazalil + thiabendazole formulations Pre-planting fungicides don’t reduce dry rot in progeny tubers Gangrene 4 Gangrene • Caused by seed-borne Phoma spp. fungi • Spores develop on stems as they senesce • Infection occurs through wounds during harvest and handling • Encouraged by damage at/after harvest, handling or moving in cold conditions, chilling, draughts, frost • Favoured by early grading (unlike dry rot) 5 Gangrene Control • Prevent damage • Avoid early grading • Keep tubers warm for 1 week after handling to cure wounds • Don’t let tubers get chilled or handle them in cold conditions • Don’t transport tubers in cold weather • Apply approved fungicides ASAP after harvest as for dry rot Common Scab 6 Common Scab • Caused by ubiquitous soil-inhabiting Streptomyces bacteria • Also seed-borne, but planting infected seed only increases risk slightly • Favoured by light soils and high pH, dry weather at tuber initiation, so a problem in 2013 • Symptoms variable: lesions tend to be straight-edged and rather flat (powdery scab lesions rounder, more raised) • Varieties differ in susceptibility e.g. Maris Piper susceptible 7 Common Scab Control • Avoid liming before planting potatoes, lower soil pH • Can be reduced by irrigation at tuber initiation • Select more resistant varieties (none is completely resistant) • Fludioxonil (Maxim) applied at planting may reduce common scab Powdery Scab 8 Powdery Scab On Premier Powdery Scab Cases on Carlingford, Désirée, Maris Piper in winter 2012-13 9 Canker phase on Cara Powdery Scab • Caused by Spongospora subterranea • Seed and soil-borne; forms spore balls that survive in soil for at least 6 years • Powdery scab can cause round, raised lesions or (more rarely) malformed, cankered tubers or galls on roots • Infection occurs as the tubers develop • Favoured by wet weather, high soil water and cool temperatures (9-17°C) 10 Powdery Scab Identification • Lesions tend to be rounder and more raised than those of common scab, but don’t diagnose on lesion appearance alone • Spore balls visible under the microscope • Sss AgriStrip test from Bioreba Powdery Scab Control • Use healthy seed • Improve drainage • Avoid fields with a history of powdery scab • An Extension of Authorisation for Minor Use (EAMU) allows application of fluazinam (Shirlan) as a ground spray or drench to seed crops only to control powdery scab • Growers who wish to use this must have a copy of the relevant EAMU and follow its conditions: it can be downloaded from the Chemicals Regulation Directorate website www.pesticides.gov.uk https://secure.pesticides.gov.uk/offlabels/search.asp • Fluazinam must not be used for blight control if it is applied to reduce powdery scab 11 Black Scurf 12 Severe Sever e stem canker on Kerr’s Pink (2012 advisory case) Black Scurf • Caused by fungus Rhizoctonia solani • Both seed and soil-borne • Present on tuber surface as black scurf (sclerotia) • Encouraged by infected seed, cold dry light soils, drought and late harvest • Delays emergence • Stem cankers on growing plant, sometimes curling of leaves, yield reductions, black scurf on progeny, elephant-hide appearance and malformed tubers 13 Black Scurf Control • Plant scurf-free seed • Use pre-planting fungicides (see handout for products) • Avoid late lifting • Aim for long rotations • Dry years increase risk Silver Scurf 14 Silver Scurf • Caused by fungus Helminthosporium solani • Mainly seed-borne, but can also spread in soil • Infection usually occurs before lifting, silver patches develop in store • Under high humidity, the patches turn black • Spreads in store, particularly if humid • Detracts from appearance of washed tubers • Causes weight loss from dehydration 15 Silver Scurf Control • Avoid planting infected seed • Pre-planting fungicides containing imazalil can reduce disease in progeny, but post-harvest and storage management more effective • Harvest early • Apply appropriate fungicides ASAP after harvest • Rapid drying after harvest using forced ventilation • Good store hygiene essential Skin Spot 16 Skin Spot • Caused by the fungus Polyscytalum pustulans • Seed and soil-borne • Favoured by wet harvest, dirty tubers + cool wet storage • Symptoms take several months to develop: not visible until January-February • Sprouts become infected under moist storage • Kills tuber eyes →blanking, weak plants, gaps, crop failure • Particularly damaging if combined with severe silver scurf 17 Skin Spot Control • Similar to those for silver scurf • Use disease-free seed • Harvest early, dry cure for 10-14 days • Remove excess soil from boxes and tubers after harvest • Apply appropriate fungicides ASAP after harvest and before planting if disease is present • Store dry, prevent condensation • Maintain good store hygiene Pink Rot and Rubbery Rot 18 19 Pink rot and Rubbery rot • Soil-borne, sporadic problem • Pink rot and rubbery rot look similar but are caused by different pathogens • Flesh feels rubbery rather than soft • Typical vinegary smell • Pink colour develops after cutting within 30-60 minutes for pink rot, longer and dirty pink for rubbery rot, then turns black • Encouraged by warm weather and heavy, waterlogged soils • Enter tubers via stolons, eyes, damage points, lenticels • Symptoms may not develop until some time after harvest Pink rot and Rubbery rot Control • Avoid fields with disease history • Improve drainage • Use a long rotation • Check seed for symptoms – do a hotbox test • Store cool with good ventilation 20 Tuber Blight 21 Tuber blight • Spores washed into soil from infected foliage • Encouraged by rainfall and high soil moisture • Enters tubers via eyes and lenticels • Symptoms take 14-21 days to develop, sometimes longer • Shouldn’t spread from tuber to tuber during handling unless tubers are wet • Encourages secondary soft rotting • Current blight fungicides good at preventing tuber blight • In 2013, 1st field foliar blight report was not until 17th July, apart from 2010 this was the latest in over 30 years Tuber blight Control • Earth up tubers well • Prevent foliage blight • Maintain protection of foliage until all haulm is dead • Wait 14-21 days after haulm death before lifting • Store cool with good ventilation 22 What you can do now: • Source healthy seed, wash a sample, check for disease symptoms • Select suitable land, check pH, nutrient, drainage • Dry rot, Gangrene Handle seed carefully to avoid damage, chit, check sprouting • Common scab, Powdery scab Check market requirements, consider at-planting treatments for specific markets, but be aware these only reduce scab at best • Black scurf, Silver scurf, Skin spot If symptoms present or your market requires it, consider using preplanting or at-planting fungicide treatment • Pink rot & rubbery rot Avoid planting on heavy land and hope for a year which doesn’t favour them • Tuber blight Plan a robust blight fungicide programme to prevent foliar infection http://www.pesticides.gov.uk/guidance/industries/pesticides/advisory-groups/Resistance-Action-Groups/frag 23 Soft Rot Soft Rot: Blackleg Symptoms • Heel end rot • Macerated brown/black rot • Fishy smell Control • Plant healthy seed free from blackleg • Minimise disease spread by harvesting early in dry conditions, use correct harvester setting and lift tubers with a good skin set 24 General soft rot Symptoms • A macerated often black/brown rot typically associated with a foul smell that can affect any part of the potato • The rot can become arrested due to environmental conditions – pit rot • Soft-rotting bacteria are almost always present naturally on the surface of potatoes Control • Dry crops immediately after harvest and reduce temperature steadily afterwards • Ventilate to avoid carbon dioxide build-up and condensation • Avoid moisture and high temperatures • Pick off rotted tubers early in the grading process • Avoid damage to the potato to avoid entry points • Use good hygiene – disinfect machinery and stores Store Hygiene Routine cleaning and disinfection is essential to control the spread of disease in store Post-harvest cleaning (empty store) • • • • • • Store cleaning should occur annually, preferably after the last crop has been removed Remove dirt and debris by vacuuming and wiping down surfaces Clean surfaces by power washing, and apply a disinfectant mist, fog or spray. Always read and follow the manufacturers instructions when using disinfectant Take care with electrical equipment and apply disinfectant by wiping around them with a cloth Clean potato boxes by removing debris, power washing and applying a disinfectant Daily cleaning • Vacuum floors and clean/tidy stores daily, especially around forklift routes, to avoid dust contamination • Clean and disinfect grading equipment particularly after grading a diseased crop • Avoid grading in store if possible to avoid dust and spore release into the air • Position grading lines away from storage areas and use dust extraction equipment 25 Thank you for your attention! 26
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