Update and Alert: Late Blight

Michael Celetti, Plant Pathologist Program Lead – Horticulture Crops, OMAFRA; Janice LeBoeuf, Vegetable Crop Specialist, OMAFRA

Late blight found in US North East: Recently late blight caused by Phytophthora infestans was confirmed on tomatoes in a greenhouse in Maine, where problems with the disease occurred in previous years, and in a Connecticut greenhouse on both potatoes and tomatoes that was likely introduced on the cut seed potatoes infected with the pathogen.  Infected plants at these sites were removed and no additional infections have been reported.  The disease was also found at one potato site in Michigan.

The wet, cool weather experienced this spring is ideal for late blight infection, development and spread. Given the early detections in the US northeast, this disease is a concern in the eastern region including Ontario tomato field and greenhouse crops, as well as potato production, this year. In fact, the New York State Agriculture Commissioner issued an alert on May 18, 2011, to home gardeners and commercial growers, of the potential introduction of late blight this growing season.

The disease caused significant problems for Ontario tomato and potato growers particularly in organic production systems in 2009 and again at the end of 2010. In previous years the source of the disease was traced to infected tomato transplants sold and distributed through home garden retail centers across the south and eastern US states. Fortunately there are no reports yet of the disease spreading on tomato transplants this year, however, that can change.

Late Blight Biology and Identification: Late blight can develop and spread rapidly when conditions are wet and temperatures are moderate. Infected potato tubers left in fields, that produce volunteer plants the following year, are the most common source of the disease. Infected tubers that develop shoots can also produce spore sacks called sporangia of the pathogen which are the main means of dispersal. These spore sacks are easily dislodged and can be rain splashed or wind blown to susceptible crops up to several kilometres away. Although ultraviolet light can kill these spore sacks, studies have shown that they can survive for up to 24 hours in storm clouds during cool wet conditions, much like what the eastern region has been experiencing over the past month. Once the disease is found in a region, all susceptible crops nearby are threatened.

Symptoms first appear as water soaked grey lesions on leaves and stems of tomato and potato plants. During warm wet or humid conditions the lesions expand rapidly, often surrounded by a light green or yellow halo (Figure 1). On stems and leaf petioles, late blight lesions appear chocolate brown. Depending upon the pathogen strain or tomato cultivar, the brown stem and petiole lesions may appear to have a silvery or grey sheen to them (Figure 2). Eventually the pathogen infects tomato fruit. Water soaked greasy patches develop on the top or along the sides of infected tomato fruit eventually becoming sunken and developing into a dry brown rot (Figure 3). Under favourable conditions, entire crops can be destroyed within 5 – 7 days.

Figure 1. Late blight symptoms on tomato leaves appear as water soaked grey-brown lesions often surrounded by a light green or yellow halo. Figure 1. Late blight symptoms on tomato leaves appear as water soaked grey-brown lesions often surrounded by a light green or yellow halo.
Figure 2. On tomato stems and leaf petioles, late blight lesions appear chocolate brown sometimes with a silvery or grey sheen.
Figure 2. On tomato stems and leaf petioles, late blight lesions appear chocolate brown sometimes with a silvery or grey sheen.
Figure 3.  Water soaked greasy patches develop on the top or along the sides of infected tomato fruit eventually becoming sunken and developing into a dry brown rot.
Figure 3. Water soaked greasy patches develop on the top or along the sides of infected tomato fruit eventually becoming sunken and developing into a dry brown rot.

Management Strategies: This spring, growers and scouts should be vigilant in monitoring for signs or symptoms of this disease as well as for volunteer potatoes that may sprout in last year’s potato fields. Cull piles should be either buried or completely covered with a black plastic tarp. All volunteer potato plants in last year’s fields should be removed and disposed of immediately.

Many growers follow an early-season bacterial disease management program that includes fixed copper products and fungicide.  This program provides some initial late blight protection to the crop.  Other growers who want to minimize their risk by initiating their fungicide program early could apply a protectant fungicide such as chlorothalonil tank mixed with mancozeb before the disease arrives and before the plants produce fruit or the rows close. If late blight is found in the region include the fungicides Tanos 50 DF, Revus or Presidio (new for this year), tank mixed with chlorothalonil, in a 5- 7 day fungicide schedule, particularly if the weather remains rainy, cloudy and temperatures are moderate to warm. Always rotate fungicides to avoid or delay resistance developing. If the disease is found in a small area of the crop, the area should be destroyed immediately and the rest of the crop, if salvageable, should receive an application of Tanos, Revus or Presidio, tank mixed with chlorothalonil, immediately.

2011 Survey: In recent years, new strains of late blight have been detected in both the US and Canada. Some of these new strains are very aggressive on tomatoes while others have the potential to be very aggressive on potatoes or both crops. OMAFRA is participating in a late blight survey organized by Agriculture and Agri-Food Canada and the University of Manitoba that is being conducted across Canada this year.  If late blight is detected or suspected in a tomato or potato crop, please contact the Michael Celetti, OMAFRA Plant Pathologist at (519) 824-4120 extension 58910 or michael.celetti@ontario.ca or Janice LeBoeuf OMAFRA Vegetable Specialist at janice.leboeuf@ontario.ca  as soon as possible so that we can arrange to collect samples of the disease for strain type identification.

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