Know Your Enemy - Blight

• The same disease which resulted in the Potato Famine in Ireland and Western Scotland in 1840s and 50s.
• The most severe and spectacular disease of potato and tomato; foliage can be totally killed within ten days.
• Late blight usually becomes epidemic later in the growing season - July and August.
• Caused by Phytophthora infestans, a "fungal" pathogen that is more closely related to brown seaweeds than to other fungi.
• The disease was introduced from the Americas - probably from Mexico but its country of origin is controversial.

Trial at Glyn Farm, Anglesey, 2007: foreground - susceptible; background - Sárpo clone

A blight lesion (lower right) on a leaflet of a wild Solanum demissum in Mexico

• Blackish spots (lesions) appear on the leaves and stems.  These grow larger each day and are often surrounded by a halo of light green leaf tissue.
• If weather is humid, a frosting of white spores form on the lesion on the underside of the leaf.
• Usually yellowing of the infected leaf does not take place.
• Late-blight infection can be confused with wind damage, natural ripening of early varieties, nutrient deficiency and Early Blight (Alternaria) or Grey Mould (Botrytis).
• Early infections on the lower stems, often arising from the mother tuber, are often overlooked but once spores from the stem lesion infect the leaves, the disease becomes obvious.
• If weather continues warm and wet, blight will spread to all stems leaves and flowers until no green tissue is left.
• If weather turns dry, the lesions will dry up but stem lesions remain dormant and will recommence spore formation when wet weather returns.
• Tubers may become infected as spores are washed down into the soil by heavy rain.  The tubers of some varieties are particularly susceptible to blight.
• Infected tubers show patches of firm brown rot on the surface.  The rot develops first just under the skin then spreads inwards.  Blighted tissue is firm and has a chestnut brown in colour.
• Soft rotting bacteria frequently infect blighted tubers converting the flesh to a putrid semi-liquid mess. 

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Stem blight on cv. Dunbar Standard

Lesions on underside of leaf of variety Alpha: Anglesey, 2008

Symptoms of late blight on tubers of a very susceptible variety,
La Ratte 

Host Range

• P. infestans has a narrow host range compared with other species of Phytophthora.
• Hosts are confined to plants belonging to the Solanum family (Solanaceae).
• Tomato (now named Solanum lycopersicum) is susceptible to infection and will become infected by the same strains of the pathogen that attack potato.
• Symptoms on tomato are just like those on potato: leaves show dark brown or black lesions with spores on the undersides.  Fruits become infected at all stages of maturity.  They should be destoyed immediately as they will not ripen before rotting.
• It helps to grow tomato crops in a different area from susceptible potato crops.
• Aubergines (Eggplants) Solanum melongena, can become infected if foliage gets wet.
• Petunia grown as an ornamental can be destroyed by blight
• Solanum nigrum or black nightshade, a common weed in potato fields, sometimes becomes infected but is mostly resistant.
• Solanum dulcamara (bittersweet) is susceptible and shows greenish (biotrophic) lesions on leaves.
• Infection of Datura spp. and Solanum laciniatum (Kangaroo Apple)become infected particularly if grown with potato or tomato.

Blight also attacks all varieties of tomato, causing lesions on leaves and a firm rot on the fruit

Potato blight infecting the tender Australian Kangaroo Apple (Solanum laciniatum) growing in the National Botanic Garden of Wales

• The "fungus" cannot grow in the soil - it must feed on plant tissue like potato foliage or tubers.
• When tubers become infected in the autumn they can sometimes survive the winter and sprout the following spring to produce a blighted plant.
• Potato dumps on farms - where outgrade potatoes are discarded in winter - are dangerous sources of new infection in the spring. 
• Dumps must be destroyed - see the Potato Council website(www.potato.org.uk) for the best ways to do this.
• If the seed crop is infected, the new seed can carry blight into the new crop.  Most infected seed either dies after planting or produces a healthy plant. But the odd tuber can survive to initiate a new epidemic. 
• Unharvested tubers will survive over winter, particularly if weather is mild.  Volunteer (goundkeeper) plants will develop in the following crop and some of these could carry infection.  Destroy small tubers at harvest and remove volunteers as soon as possible in the spring.

A dump of diseased tubers.  If not destoyed, blight will grow into the new plants and generate spores

Row of Sarpo varieties and behind, a crop of Charlotte showing a primary focus of infection, starting from a single infected plant

• Usually no, because hyphae and spores die quickly in the soil being dependent on living tissue, like tubers, for survival
• But here is a possibility that sexual spores, oospores, form inside the leaves when both mating types are present.
• Oospores can lie dormant in the soil for many years then germinate to infect a new crop planted in the same field.  Long rotations of several years between potato crops can minimize this risk.
• In the garden, there is a small risk that oospores might be present in foliage at harvest and that they might survive composting, particularly if rotting is at low temperatures.
• On-going experiments at Henfaes, funded by Potato Council, have shown that new crops planted on the same site as crops infected with both mating types of blight become infected from the soil and that new strains of blight are generated.

Two oospores (35 micrometers in diameter), each produced from a mating

• The "fungus" within an infected plant can grow through the stomata on the stems and leaves and produce millions of spores (sporangia).
• Sporangia become air-borne in drying air and will travel on the wind for many miles.
• If a sporangium lands on wet foliage, it can hatch to release up to 10 zoospores.  These are biflagellate swarmers (just like the ones produced by seaweeds on the beach).
• Zoospores find a good place on the leaf surface to settle down and germinate.  A complicated physical and chemical mechanism results in penetration of the leaf.
• A branched mycelium develops inside the leaf as the "fungus" sucks nutrients from the leaf cells.  A visible lesion of fungus is formed, black dead leaf cells at the centre and living, parasitized cells at the periphery.
• Growth of the "fungus" through the stomata gives a new generation of spores (sporangia).
• The generation from spore to spore can be as short as 72 hours if the weather is warm (15 - 20C) and humid (>90%RH).
• The lesion will continue to enlarge and sporulate during spells of humid weather until the whole leaf is rotted.
• Many cycles of infection and spore formation take place during an epidemic.

Spore stalk (sporangiophore) emerging from the breathing pore (stoma) on undersurface of a leaf (Scanning Electron Microscope, SEM)

Sporangiophore bearing three spores (sporangia) SEM

Severely infected leaves drying up and showing white spores on underside cv. Charlotte

• During periods of heavy rainfall, sporangia are washed down through the soil to the surface of the new tubers.  Zoospores emerging from the spores will infect the tuber and colonise the layer just under the skin.
• Some tubers may develop an extensive rot.  These tubers are frequently infected with secondary soft rotting bacteria to produce an evil smelling, slimy mess.
• Tubers forming near the soil surface are more vulnerable to tuber blight.  Careful ridging helps shed the rain and protect the tubers.

Spores from leaves wash into soil during rainfall to infect tubers; click image to see tubers

Blighted tubers from Morocco on sale in S.W. France

• With great difficulty using either chemicals or resistant varieties or a combination of the two.
• Hygiene can help to reduce damage.
• A wide range of fungicides are available with differing modes of action
• Metalaxyl can cure an already established infection because of its systemic activity.
• The common new strain of blight, Blue-13, is resistant to metalaxyl
• Dithane, a dithiocarbamate fungicide, is prophylactic and has been used effectively for many decades without any signs of resistance
• Dithane is used in mixtures with other, newer fungicides to try to prevent new, resistant strains of blight arising.
• In wet seasons, foliage needs to be sprayed every 4 - 6 days to provide efficient control.
• This means that up to twenty sprays can be applied to a crop, just to control one disease.
• Efforts are being made to reduce the dose of fungicide applied when risk of weather conditions favouring sporulation and infection are low.

• Formulations of copper (Bordeaux Mixture; copper oxychloride etc.) have been used as a protectant fungicide.
• Because copper is a potent environmental poison, its use cannot be justified and will soon be phased out by EU legislation.
• Extracts from certain plants are efficient fungicides against late blight. None of these have been commercialised.

Chemical Control in Gardens And Allotments

• Bordeaux Mixture based on copper is an approved fungicide against blight in U.K.
• This potent environmental poison will soon be banned by European legislation.
• Dithane based on mancozeb is also approved for blight control.
• Dithane may also be banned by EU due to its action as an Endocrine Disruptor.
• Less hazardous chemicals, such as Shirlan (fluazinam), are not available to amateur growers.

• Good rotation and destruction of dumps and volunteers effectively reduce the incidence of primary infections.
• Use healthy seed free of blight infection.  Blighted seed can cause stem infections in the new crop.
• Early planting can allow even maincrop varieties to bulk before blight becomes widespread.
• Choosing an open site with good air flow can hasten drying of foliage after rain.  Accordingly, progression of the disease will be slower.
• Foliage should be removed from unprotected crops if blight becomes established.  This will prevent spores spreading to neighboring crops and will reduce tuber infection.
• Harvest should be delayed by three weeks or more so that tuber skins are well set and blight spores on foliage and in the soil have a chance to die off.

What a lovely harvest of Pink Fir Apple - but 4 weeks later :(

• Plots can be surrounded by polythene tents with open ends.
• Tents keep the foliage dry and prevent spores germinating
• Similar polythene protection can be used to keep tomatoes disease free.
• The first lesions to be seen on foliage can be destroyed to limit spread of the disease.  However, by the time the first lesions are seen, spores have probably spread to other plants and plots.
• Early removal of foliage is particularly essential to prevent tuber infection of certain varieties with high tuber susceptibility e.g. Pink Fir Apple; Nicola.
• Delaying harvest for three weeks after removal of foliage prevents tuber infection during harvesting.
• Harvest in dry weather and dry off the tubers before storing in dry, dark conditions.
• Inspect tubers in store periodically and destroy any which are diseased.

Demonstration on use of plastic tents to keep foliage dry and blight-free (Henfaes Research Centre)

Polythene hats keep tomatoes blight-free at Henfaes

• Varieties with a range of resistance to blight are available.
• Beware that some so-called blight resistant varieties can be destoyed by new strains of the fungus.
• Some varieties have a low level of resistance which can give some protection in drier seasons but offer little advantage
• Blight can be controlled in partially resistant varieties by using a reduced dose of fungicide.
• Other varieties, including Sárpo cultivars, have high levels of resistance, allowing them to be grown without chemical protection even in the wettest growing seasons like 2007 and 2008 when blight pressure across the UK was very high.
• Most resistant varieties have maincrop maturity.  Early-maturing varieties are usually susceptible but Sárpo Una has useful blight resistance.
• Some varieties have useful foliage resistance but poor tuber-blight resistance e.g. Toluca. 
• Yet others have good tuber-blight resistance but poor foliage-blight resistance e.g. Charlotte.
• Ideally, a variety should have good resistance to both foliage and tuber blight.

        Foreground - a susceptable variety killed by blight.
Background - a resistant Sárpo variety

• Up until 1976, the fungus existed throughout the world as a giant clone, a single type showing little variation.
• Exceptionally in Mexico, the most likely home of blight, many strains existed and new ones frequently arose from sexual interaction of A1 and A2 mating types.
• All blight outside Mexico was of a single mating type, A1 and the fungus remained celibate (asexual). Strains did develop which had the ability to overcome some of the genes determining blight resistance
• In 1976, potatoes were imported into Europe from Mexico to satisfy shortages; the Mexican blight hitched a ride.
• Many new strains of both A1 and A2 were detected in European populations of blight fungus; the fungus was now able to form oospores and generate new strains.
• In U.K. several A1 strains became common along with a larger number of rare strains. Strains of A2 mating type remained rare.

• In 2005, a survey of blight strains was undertaken by SRT, funded by the Potato Council.
• New strains were identified and many were of A2 mating type.
• olecular fingerprinting (using probe RG57) identified a new A2 strain, Blue, which was resistant to the systemic fungicide metalaxyl.
• Snce then, SCRI identified the same strain (referred to as type 13) using SSR fingerprinting.
• Strain Blue-13 is now the predominant strain in U.K.
• Strain Blue-13 is highly aggressive and can overcome the resistance of many formerly resistant varieties.
• Fortunately, most of our Sárpo varieties are resistant to Blue-13.
• There is always a risk that new strains of blight will arise without warning and render the blight resistance of promising new varieties useless.

 
The first seven samples belong to a new genotype coloured light blue
(now known as Blue-13)