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Late Blight of Tomato

Photo gallery.

Mt-Fresh-Plus_Plot-46_Rep-4_9-14_Mt-Magic-behind_8246x400This very destructive disease of potato and tomato occurred routinely on Long Island, NY, for several years starting in 2009, often beginning to develop early in the growing season (May – June) and having major impact on crops and gardens.  Before that, most years it did not occur or was not observed until late (Oct) when impact was negligible; and afterwards late blight returned to occurring more sporadically; see table below.  Beginning in 2012 research was conducted at LIHREC under field conditions to evaluate resistant tomato varieties and the Late Blight Decision Support System for timing fungicide applications.  The pathogen spread naturally to these experiments.  Whether tomato fruit with late blight could be a source of inoculum for seedlings growing from the fruit was examined in greenhouse experiments.

First date late blight observed on Long Island, NY

29 June 1996
3 Oct 2002
5 July 2006
26 Aug 2007
3 Oct 2008
23 June 2009
18 June 2010
24 June 2011
29 May 2012
25 July 2013
20 June 2014
None in 2015
None in 2016
Late Aug – Early Sep 2017
7 Oct 2018
None in 2019

Resistant tomato varieties have been evaluated since 2012.  Varieties bred to have the Ph2 and Ph3 resistance genes have exhibited excellent resistance to US-23 genotype of Phytophthora infestans, which has been the dominant one in the USA.  Few symptoms developed on leaves and fruit.  Varieties evaluated with this resistance are Defiant PhR, Iron Lady, Mountain Merit, and Mountain Magic.  Several experimental varieties also performed well.  Plum Regal, which has the Ph3 gene, exhibited good resistance.  Legend (Ph2), New Yorker (Ph1), and Juliet (reported resistant) were as severely affected by late blight as susceptible Mountain Fresh Plus included for comparison.  This reflects the fact resistance genes are specific in their activity.  The genes in these varieties are not effective against the current genotype of the pathogen.  Varieties with unknown resistance that exhibited resistance in these experiments are Jasper, Matt’s Wild Cherry, Lemon Drop, Mr. Stripey, Pruden’s Purple, and Wapsipinicon Peach. Since 2018, evaluations have been conducted of experimental varieties developed by Cornell plant breeder Martha Mutschler-Chu. These are red-slicer types with resistance to several diseases including late blight.

Technical reports: 2012  2013a    2013b   2014

Varieties resistant to late blight and other foliar diseases developed at Cornell.

Web articles with photographs: 2012   2013  eOrganic (March 2018)

Late Blight Decision Support System (DSS) was evaluated using copper applied to organically-produced tomato.  This computer-based program determines when the first fungicide application of the season for late blight is warranted as well as when to make subsequent applications through out a growing season.  The DSS uses weather data from local stations and forecast weather.  Compared to the weekly schedule, the DSS recommended two more applications (16 total) to the susceptible variety and four fewer (10 total) to the moderately resistant variety (Legend) in 2014.  Late bight developed to a limited degree throughout this experiment, thus the DSS could not be adequately evaluated.  Other diseases occurring (powdery mildew and Septoria leaf spot) were effectively managed with the DSS application schedule.

Technical report: 2014

Role of affected tomato fruit as a source of the pathogen for seedlings growing from these fruit was investigated through greenhouse experiments.  If tomato seedlings growing from dropped fruit with late blight are infected with Phytophthora infestans, they could serve as a source of this pathogen for tomato and potato in nearby production fields or gardens.  Naturally-infected fruit (US-23 genotype) were obtained from a garden and research plantings.  They were put on potting mix in seeding trays then covered with more mix on 7 Sep 2011 and 17 Oct 2013.  Within 7 days seedlings began emerging.  Symptoms (dark brown tissue) appeared on lower stems starting 13 days after experiments were set up and continuing through 56 and 79 days after set up in 2011 and 2013, respectively.  Stems were incubated in a moist environment to obtain sporulation to confirm symptoms were late blight.  Cultivars with affected seedlings were Black Cherry (58% of seedlings in 2013), Juliet, Sakura Honey (100%), SunGold (13%), Sweet Treats, and Wapsipinicon Peach (32%).  Symptoms did not develop on any seedlings of Orange Banana (45 seedlings) or Black Zebra (13), or on any of the 40 seedlings that emerged from asymptomatic mature fruit of SunGold, thus these fruit were not latently infected.   In 2011 a set of trays was kept cool and dark for 75 days to delay germination. Symptoms did not develop on any seedlings that grew in these trays after they were put in the greenhouse.  Symptoms developed on seedlings growing from commercial seed put inside affected fruit in place of the seeds there, but not from seed removed from affected fruit.  In conclusion, P. infestans can survive for several weeks in affected tomato fruit tissue and can infect seedlings emerging from this fruit.  These seedlings could be an inoculum source where time between crops is less than about three months.

Meeting abstracts:

  • McGrath, M. T. 2012. Phytophthora infestans transmitted to seedlings growing from tomato fruit rottedby late blight but not their seed.  Phytopathology 102:S4.78.
  • McGrath, M. T. 2014. Suppression of late blight by resistant tomato cultivars.  Phytopathology 104:S3.78.

Tomato LB Res-APS 2014 Abstract-McGrath


Tomato LB Seedlings-APS 2012 Abstract-McGrath

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