Background and life cycle
- Phytophthora infestans is very aggressive and can result in rapid loss of foliage if infection is not controlled.
- Stems can also become infected but this may be less evident.
- Infection of tubers can cause rotting and increase susceptibility to soft rots.
- The life cycle of P. infestans is generally asexual and completed within a week or less, with potentially multiple generations within a season.
- If two breeding types of P. infestans occur together, sexual reproduction can occur, resulting in production of oospores, which give a potential for blight to become soil borne.
- Asexual P. infestans overwinters in tubers in cull piles or as seed or volunteers; sporulation on these produces the first generation of spores to infect other plants. More …
- Since the 1970s the old genotypes of Phytophthora infestans have been largely replaced by more aggressive genotypes. (First discussed in 1999 with further developments by 2009 and continuing annually)
- The “new” genotypes have a faster rate of development, faster rate of growth and greater spore production than have the “old” genotypes and can cause infections at lower temperature and humidity. More …
- Late blight is constantly evolving and a mixture of different strains can be expected each season; these may include different breeding types and strains with resistance to particular fungicides but the relative frequency of strains in the population cannot be predicted.
- There are two breeding types: A1 and A2; if these occur together there is a risk of sexual reproduction and formation of oospores. More …
- Sexual reproduction increases the risk of new strains forming (recombinant strains).
- Oospores are the resting spores of Phytophthora and have the potential to become soil borne.
- Naturally formed oospores have not yet been found in the UK but possible recombinant strains of blight have been found in Scotland. More …
- Recent information on blight strains in the UK is shown below. More …
N.B. “Pink-6” is “6_A1”; “Blue-13” is “13_A2”.
- Phytophthora infestans infection of foliage can be very aggressive and loss of canopy can result in loss of yield.
- Typical conditions for infection are mild temperatures and high humidity but the more aggressive strains of blight can cause infections in a wider range of conditions. More …
- Leaf wetness is important: a film of water is necessary for development of spores which are deposited on the leaf surface. The longer the period of leaf wetness, the less important is the temperature. More …
- In suitable conditions infection may occur within two hours or less of a spore landing on a leaf.
- Once infection has occurred hyphae develop within leaves and lesions appear as cells die.
- Spore bearing bodies are produced from lesions, particularly on the lower surfaces of leaves, and a lesion may produce spores for 7 days.
- The time from a spore landing on a leaf to production of new spores may be 5 – 7 days in conducive conditions and there may be many generations in a season.
- Fungicides for control of late blight provide prophylactic control and are most effective against spore stages; once blight has started to develop inside a leaf control is much more difficult.
- Stem blight may be more easily overlooked than foliar blight.
- A lesion on a leaf sporulates for approximately 7 days; a lesion on a stem may sporulate for 21 days.
- Water running down stems can wash spores into soil.
- Tuber blight is suggested to be more closely correlated with stem blight than with foliar blight.
- Stem blight is better able to withstand hot, dry conditions than is foliar blight and may sporulation in cooler, more humid conditions.
- It is suggested that the “new” genotypes of blight are more likely to cause stem infections than were the “old” genotypes of blight.
- Infection of growing tubers occurs mainly through lenticels and eyes but can also occur through growth cracks, holes made by wireworms, slugs, etc.
- Growing tubers become less susceptible to infection as they age and lenticels become suberised.
- Lenticels become more susceptible to infection if proliferation occurs under conditions of high soil moisture (“burst” lenticels). More …
- Infection of tubers requires rain to wash spores from foliage, a high soil moisture content to move spores through soil and a film of water on the tuber surface to allow disease development. Motile zoospores do not “swim” downwards in search of tubers. More …
- Infection of tubers may occur at any time when conditions are suitable.
- Control of tuber blight should be considered throughout the season. None of the blight fungicides is translocated to tubers to provide direct control; control of tuber blight is achieved by preventing spore formations. However, phosphite can be moved to tubers and can help tuber blight control by improving plant health and disease resistance.
- The higher the number of spores deposited on soil the greater the risk of tuber blight: spores in soil may remain viable for up to three weeks. More …
- Tuber blight infection can occur in crops where there has been little foliar blight apparent.
- The greatest risk of tuber blight infection occurs if there are spores in soil and tubers are damaged at harvest.
- Infected tubers are susceptible to infection by bacteria and other secondary rots and these may be a greater cause of yield loss than the actual blight infection.
Notes on late blight
Late blight 2021 (Genotype changes; blight resistant cultivars)
Late blight 2020 (Genotype changes; pathogenicity of different genotypes; possible control programmes)
Late blight 2019 (Genetic changes – spread and aggressiveness of the new isolates)
Late blight 2018 (Genetic changes including aggressive 37_A2 and 36_A2; cultivar resistance; virulence and aggressiveness of isolates)
Biology and control of Phytophthora infestans 2017 (Genetic changes; sources of inoculum; the Hutton Criteria as a replacement for the Smith Period)
Late blight 2016 (Genetic changes; elicitors to stimulate resistance; tests of the Smith Period to predict aggressive blight)
Control of late blight 2015 (Genetic changes)
Late blight 2014 (Genetic changes; model to simulate spore dispersal; spread of infection from seed to stems)
Late blight biology and control 2013 (Genetic changes; inadequacy of Smith Period; mechanisms of blight resistance; resistance induced by phosphite)
Late blight biology and control 2012 (Genetic changes including fluazinam-resistent “Green-33”; development of blight from infected seed tubers; oospore risk)
Phosphite and disease resistance 2012 (Suppression of blight infection by phosphite, due to increased plant resistance)
Overview of diseases 2012 (Includes: late blight life cycle; effect of leaf wetness)
Late blight 2011 (Aggressive blight isolates including “Pink 6”; changed ratings for blight resistance)
Elicitors, effectors and disease control 2011 (How is disease overcome or disease resistance stimulated?)
Late blight 2009 (“Blue-13” aggressive blight overcoming cultivar resistance; risk of oospore formation)
Late blight biology 2008 (Aggressive forms of blight; survival of spores in soil; cultivar resistance)
Late blight biology 2004 (Plant and leaf resistance)
Late blight 2003 (Aggressiveness of different isolates; sporangia dispersion and survival)
Late blight research 2002 (Virulence of blight; susceptibility of tubers to blight)
Blight life cycle 2001 (Asexual and sexual life cycles; influences on development and infection from sporangia; zoospores and oospores)
Blight epidemiology 2001 (Crop losses due to blight; influence of environment on disease development)
Blight genetic changes 2001 (Development of aggressive blight)
Tuber rotting diseases 2001 (Includes infection of tubers with blight)
Late blight control 2000 (Importance of soil moisture in development of tuber blight)
Late blight 1999 (Aggressive strains of blight; oospore formation)
Late blight 1995 (A1 and A2 blight; the importance of leaf wetness)
Late blight 1994 (Mechanism of cultivar blight resistance)