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.