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Late Blight and Tomato Transplant Production

Photos of Late Blight on Tomatoes, Many good close-up photos from the Long Island Horticultural Research and Education Center

Late Blight caused by the fungus-like organism, Phytophthora infestans, occurs sporadically in most of New England in most growing seasons. Late Blight outbreaks from 2005 to 2008 developed during late summer/early fall and were the result of inoculum (sporangia) carried on storm systems that originated in southern areas where the disease was active.  In 2009, the disease was widespread and developed early. Typically, potato is the main crop affected; in 2009, a strain aggressive on tomato was present on tomato transplants. This new genotype of late blight, US22, caused a disease epidemic throughout the Northeast. First reports of the pathogen were recorded in late June, the earliest incidence of widespread disease ever observed in our region. US22 was detected in tomato plants for sale in garden centers in numerous states by extension specialists. In 2012 and 2013, the continually evolving pathogen was again primarily a problem for tomato crops US23 (2012) and US24 (2013).

Background: Late Blight has occurred in New England in recent years and different strains of the pathogen have been responsible: US11 (1994-1998), US8 (1992-2010), US17 (1996-1997), and US 22 (2009-2010). These genotypes differ in mating type, host aggressiveness, and fungicide resistance. Sources of Phytophthora infestans in New England include potatoes saved year to year for seed, tubers that survive the winter unfrozen in the soil, and volunteer potato and tomato plants in compost, cull piles, or fields. Other more recent sources of the pathogen have been infected tomato transplants, infected petunia bedding plants, and infected crops in frost-free areas producing wind dispersed spores.

Pathogen Biology: Phytophthora infestans needs cool (below 77 F) wet weather. Hot, dry summers, like 2010 are not conducive to late blight development. An exception would be where plantings are located where fog occurs periodically. The late blight pathogen has recently undergone changes in Florida that affect disease occurrence there and in other eastern states. Diseased tomato plants in south Florida have survived cold periods in winter allowing the pathogen to persist. Late blight has also been active into the spring as late as May indicating an increased tolerance for warmer temperatures. This means a potential source of inoculum persists until crops are being produced north of Florida and a 'green bridge' exists for the pathogen to progress on until it reaches the northeast. Currently, the late blight pathogen is only known to survive on living plant tissue unlike Alternaria solani, the cause of early blight. If both mating types of the organism occur in the same field, sexual reproduction and therefore persistence outside a living host as oospores (long lived spores in the soil) is possible. Both mating types have been observed in the US and mating is known to occur in the Pacific Northwest. Oospore production is common in central and South America as well as Europe and the disease is a perennial problem in these regions.

Steps to prevent Late Blight from occurring in your greenhouse or garden center:

1). Grow tomato plants from seed or purchase starter plants produced locally. Phytophthora infestans does not survive in or on tomato seed.

2). Tomato plugs imported from the south may be a potential source of the disease organism.  If you must bring transplants from the south, carefully inspect all tomato plugs before accepting shipment and reject any with symptoms of disease (or insect infestations). If possible, isolate new plants and inspect them regularly until their disease-free status is confirmed.

3). Start transplant production in a clean greenhouse free from previous crops, pet plants, and weeds. Grow vegetable transplants separately from bedding plants. This prevents cross-infection from ornamental plants which can harbor Late blight, TSWV (tomato spotted wilt virus), INSV (impatiens necrotic spot virus) and other potential vegetable pathogens.  This also makes pesticide treatment easier as many materials registered on ornamentals are not registered for vegetable transplants.

4). Select resistant tomato varieties where available. 'Mountain Magic',  'Mountain Merit', 'Legend',  'Defiant PHR' and 'Plum Regal' have excellent resistance to Late Blight.  'Jasper', 'Red Pearl' and 'Matt's Wild Cherry' are small fruited tomatoes with good resistance. Some heirloom tomato varieties have good tolerance to late blight. A few cultivars also have resistance to Early Blight (Alternaria solani, A. tomatophila) and should be considered.  Triple resistant (LB, EB, Septoria leaf spot) 'Iron Lady' is now available. The development of triple (LB, EB, Septoria leaf spot) resistant varieties is continues at Cornell and other Universities.

5). Remove solanaceous weeds in and around the greenhouse. Examples include hairy nightshade, bittersweet nightshade, jimson weed, golden henbane, and others. The ornamental plants Petunia and Calibrachoa are also hosts for late blight.

6). Check the Late Blight forecast model at or  weekly. These websites provide forecasts of where late blight is active and when environmental conditions are favorable for epidemic development.

7). Scout greenhouse regularly for disease and insect infestations or hire an IPM scout.

8). Confirm possible late blight occurrences with the UMass Extension Plant Diagnostic Lab.

9) If Late Blight is confirmed by a diagnostic lab in your operation, immediately remove and destroy affected plant tissue and promptly inform neighbors that grow susceptible crops. Late Blight is a community disease due to its potential to spread rapidly.

10). Protect remaining healthy plants with preventive fungicide applications. See for a list of registered products. Continue scouting on a regular basis as late blight is difficult to manage once it has been established.

More Resources

Late Blight, UMass Vegetable Program

Recognizing Tomato Blights, UMass Vegetable Program

Photos of Late Blight on Tomatoes, Long Island Horticultural Research and Extension Center

 M. Bess Dicklow,  Updated 12/2013
UMass Extension Plant Diagnostic Laboratory