Source of spores
- Phytophthora infestans overwinters in tubers and sporulation can start when stems begin to grow. More …
- Seed certification standards allow 1 in 200 tubers to be visibly infected with blight. Not all infected tubers will produce stems but within a crop there is always the potential for seed to be a source of blight. In humid conditions sporulation can occur on stems at the soil surface without development of lesions. These spores can be dispersed by rain splash (including irrigation) and wind. More …
- Spores may be carried long distances in light winds. More …
- Other Solanum species may be infected by P. infestans but at present there is no evidence that these act as over-wintering hosts for P. infestans in the UK.
- Oospores in soil could potentially become another source of spores if A1 and A2 blight strains occur together at similar frequencies. More …
- Sources of blight spores thus include:
- potato dumps
- infected seed tubers
- volunteer potatoes
- potatoes grown in gardens and allotments
- potato crops grown under polythene, when the cover is first removed
- organic potato crops, particularly mid season
Cultivar resistance to blight
- Cultivars differ in susceptibility to Phytophthora infestans but none are truly resistant.
- Durable resistance of a potato cultivar requires multiple resistance genes. More …
- P. infestans possesses many genes which have the potential to overcome cultivar resistance; appearance of the current very aggressive isolates caused the resistance ratings of most cultivars to be downgraded. More …
- The risk of blight infection and development increases as cultivar resistance decreases, particularly if fungicide application intervals are increased.
- Application of phosphite can help to stimulate plant resistance responses but there is some evidence that the response is greater the higher the cultivar’s natural resistance. More …
- Late blight resistance ratings for many cultivars can be found in the British Potato Variety Database but note that cultivars may have different resistance ratings for foliar and for tuber blight.
- There is an increasing number of cultivars with reported resistance to late blight. More …
Blight resistance to chemicals
- Within any population of Phytophthora infestans there will be isolates with differing susceptibility to each fungicide. Most of these will be controlled by the recommended rate of the fungicide. More …
- Resistance occurs when isolates which cannot be controlled by the recommended rate of a fungicide become prevalent.
- Resistance to phenylamide fungicides (e.g. metalaxyl-M) is widespread (more …) and resistance (more …) or reduced sensitivity (more …) to fluazinam has been detected.
- The Fungicides Resistance Action Committee (FRAC) considers P. infestans to have a high risk of developing resistance to fungicides but also indicates that there is a greater risk with some fungicide groups than others. FRAC lists the mode of action groups to which different products belong.
- The majority of blight fungicides have only single site activity, i.e. they affect only a single biochemical pathway, and there is therefore a risk of resistance when these active ingredients are used alone. More …
- An anti-resistance strategy should therefore mixtures or alternation of active ingredients from different mode of action groups.
- Mancozeb has multi-site activity (more …), while the mode of action of cymoxanil is poorly understood but is considered to include some stimulation of the plant’s resistance response (more …). These active ingredients are therefore useful additions to a blight control programme but the period for which use of mancozeb will still be permitted in the UK is uncertain (March 2021).
- Use only high quality seed potatoes.
- If growing seed, use the best possible blight control programme and ensure that any farm produced seed is tested for blight.
- Control growth of volunteer potatoes in other crops.
- Control sprout growth on any piles of cull potatoes.
Principles of control
- Control of Phytophthora infestans by fungicides should be prophylactic; it is much more difficult to control the disease once infection has occurred. More … and more …
- Start the programme early, as soon as emergence occurs (more information on an early start …). If emergence is uneven make two applications rather than waiting for all plants to emerge. Remember that there is a risk of infection whenever leaves are wet. More …
- Make fungicide applications frequently: it is better to be a day early than a day late. The maximum application interval should now be 5 – 7 days.
- Use the shortest application intervals when the canopy is expanding most rapidly: new leaves will need to be protected. Infection of leaves occurs more easily at this stage than when the canopy is complete. An application interval of 5 days may be required during rapid canopy expansion. Fungicides will not move out of older leaves to protect the growing point but some will spread in a new leaf as the leaf expands. More …
- Continue the programme until the canopy has senesced completely, including stems, but check harvest interval for final fungicide applications. More …
- As an anti-resistance strategy use products from different mode of action groups, alternating or mixing products as appropriate. More …
- Addition of phosphite can stimulate natural plant resistance, improve plant health and thus aid control of tuber blight in particular. Applications during tuber bulking may be particularly beneficial if tubers are to be stored. More …
- If an application interval has been stretched due to bad weather, make subsequent applications at narrower intervals, with additional mixer products (e.g. cymoxanil) as appropriate.
- Use of a drift retardant, appropriate spray technology (more …) or appropriate adjuvant may enhance fungicide efficacy (more …).
- Always follow the manufacturer’s recommendations.
Example control programme
- Products have been chosen with knowledge of their modes of action and mode of action group, efficacy in the field and risk of resistance developing.
- This programme provides 16 fungicide applications on 7 day intervals.
- For other possible programmes with / without mancozeb and propamocarb use this link.
- The period for which use of mancozeb will still be permitted in the UK is uncertain (March 2021).
See the manufacturers’ websites for details of agrochemical use.
Notes on late blight
Late blight 2021 (Genotype changes; blight resistant cultivars; control programmes with / without mancozeb / propamocarb)
Late blight 2020 (Genotype changes; pathogenicity of different genotypes; possible control programmes)
Late blight 2019 (Use of drift retardants and of low drift nozzles; importance of fungicide timing particularly on susceptible cultivars; fungicide / desiccant tank mixes supported by manufacturers)
Late blight 2018 (Insensitivity of some blight strains to fluazinam; virulence [ability to infect] of blight isolates; selection for aggressive isolates on more resistant cultivars; selection for aggressiveness with reduced fungicide dose rates; limitations to decision support systems; interactions between phosphite and cultivar resistance; use of oxathiapiproline; mode of action groups; post-infection application of fungicides)
Biology and control of Phytophthora infestans 2017 (The Hutton Criteria for blight risk conditions; introduction to oxathiapiproline; in-furrow treatment and the importance of early control; tests of blight sensitivity to fungicides; risk of resistance developing to different mode of action groups)
Surfactants for potatoes 2017 (Enhancement of fungicide efficacy)
Late blight 2016 (Trials with elicitors to stimulate plant defences; effects of mineral oils on fungicide efficacy)
Control of late blight 2015 (Trials with co-formulations and with adjuvants)
Late blight 2014 (Importance of early control; is there a potential for seed treatment to reduce blight risk? spore dispersal; product comparisons)
Late blight biology and control 2013 (Blight resistance of cv “Sarpo Mira”; induction of resistance by phosphite; application of fungicides in tank mixes with mineral oils [oils used for suppression of virus transmission]; product comparisons)
Late blight biology and control 2012 (Development of blight from infected seed tubers; trials on product comparisons)
Phosphite and disease resistance 2012 (Suppression of P. infestans with phosphite)
Late blight 2011 (Cultivar resistance; control of tuber blight; protection of new growth; product comparisons)
Elicitors, effectors and disease control 2011 (Plant resistance and its potential induction; mechanisms by which blight overcomes resistance)
Late blight 2010 (Introduction to initium; further information on generics; fungicide mode of action groups)
Late blight 2009 (Cultivar resistance to aggressive isolates; risks of oospore formation and survival; comparison of products for different aspects of blight control; risks of using generic products)
Late blight fungicides efficacy 2008 (Introduction to amisulbrom; mobility of phosphite; efficacy of fungicides for protection of new growth)
Late blight fungicides mobility 2008 (Mobility in the plant of: dimethomorph, propamocarb hydrochloride, metalaxyl, cymoxanil)
Late blight – protection of new growth 2007 (Application in relation to leaf production; mobility of fungicides)
Late blight fungicide trials 2007 (Results from product comparison trials)
Late blight fungicide resistance 2007 (Fungicide mode of action groups; risk of fungicide resistance developing)
Late blight control with mandipropamid 2007 (Introduction to mandipropamid)
Late blight and phosphorus acid 2007 (Reduction of tuber blight risk by foliar application of phosphite)
Late blight 2006 (Blight resistant cultivars; introduction to benthiavaldicarb-isopropyl and to flupicolide; results from product trials)
Late blight 2005 (Results from product comparison trials)
Late blight chemical control 2004 (Performance of products in trials; comparison of fluazinam and cyazofamid for control of tuber blight)
Late blight forecasting 2004 (Blight forecasting systems fail to predict blight conducive conditions before they occur)
Late blight resistant cultivars 2004 (Blight development in resistant cultivars; introduction to the Sarpo breeding programme)
Late blight 2003 (Fungicide persistence; introduction to fenamidone; blight control programme in relation to canopy development)
Late blight research 2002 (Contribution of cultivar resistance to blight control)
Blight life cycle 2001 (Sources of infection)
Late blight control 2001 (Modes of action of different active ingredient and activity in the blight life cycle; introductions to famoxate, zoxium and cyazofamid)
Blight control principles 2001 (Time of starting; application interval; duration of programme; protection of tubers)
Blight epidemiology 2001 (Influence of leaf wetness and environment on disease development)
Blight control during the life cycle 2001 (How easy is it to control blight at different stages in the life cycle? Example of control with dimethomorph)
Blight chemical comparison 2001 (Product comparisons; fungicide mobility; persistence of fluazinam in soil)
Blight fungicide concentrations 2001 (How does the concentration of fungicide applied compare with the concentration required for control?)
Blight phenylamide resistance 2001 (Development of phenylamide resistance; build up in resistance through the season)
Late blight control 2000 (Control of tuber blight; persistence of fluazinam in soil)
Late blight 1999 (Phenylamide resistant and aggressive strains of blight; A1 and A2 breeding types)
Late blight control 1998 (Use of drop leg sprayers; principles of a control programme)
Late blight 1996 (Mode of action of dimethomorph)
Late blight 1995 (Importance of leaf wetness and canopy microclimate in blight development; aggressive blight strains reported overseas)
Late blight 1994 (Influence of cultivar and disease intensity on control; mode of action of fluazinam; syngergism between active ingredients in co-formulations; phenylamide resistance)
Seminar 2 Notes 1992 (Chapter 5: Blight control – a surfactant may improve efficacy of protectant fungicides.)