Michael Celetti, OMAFRA, Plant Pathologist – Horticulture Crops Program Lead
Late blight has been confirmed in a tomato field in Essex County, Ontario. The disease has been spreading around several US states this spring and summer. It was only a matter of time before it showed up in Ontario. Some tomato growers, particularly organic tomato growers, lost significant yields in 2009 and 2010 due to late blight. In fact, the late blight epidemic during 2009 has been referred to by some researchers as “the worst in modern history due to a “perfect storm” of widespread inoculum distribution and conducive weather.” Warm dry conditions that occurred in August of 2010 held the disease for most of the summer, however, the cool wet weather that occurred last September revived the disease and resulted in significant yield losses to several organic tomato growers at the end of the season. This disease can destroy unprotected tomato and potato crops within one week under ideal conditions and is the same disease that contributed to the Irish potato famine during the 1840’s. Foggy weather with intermittent thunder storms together with warm temperatures not to mention dew formation during the morning are ideal conditions for infection, disease development and potential spread.
With the arrival of late blight into Ontario it is important that all tomato and potato growers apply an effective fungicide to protect their crops on a regular basis. The disease is best kept under control when fungicides are applied prior to the arrival of the pathogen and infection. Scouting fields for diseased plants is also recommended at this time. If a few infected plants in a patch are found, it is best to pull out the infected plants, place them in garbage bag and dispose of them away from the field prior to spraying an effective fungicide. Do not transport infected plants through the field without placing them in a garbage bag first.
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 and often appear as a brown necrotic spot surrounded by a light green or yellow halo (Figure 1). On stems and leaf petioles, late blight lesions appear chocolate brown (Figure 2). 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.
There are several strains of the fungus, some that infect potatoes only, some that infect tomatoes primarily but many that infect both crops. Over the past decade, scientists in North America have been studying and monitoring the different strains that have developed through sexual recombination and spreading through out North America. Of the new strains, strain US-22 was the most widespread on tomato transplants and the one that spread around eastern North America in 2009. Two other new strains, US-23 and US-24 were also found infecting tomatoes and potatoes. Based on what we observed in 2009, strain US-22 appears to be particularly aggressive on tomatoes. Strain US-8 was the most prevalent strain on potatoes.
The pathogen, Phytophthora infestans that causes late blight is a “water mold” and can reproduce through asexual sporangia (spore sacks) and sexual persistent oospores produced during wet conditions. Sporangia are the main means of dispersal. They are easily dislodged and can be wind blown to susceptible crops several kilometers away. In a study several years ago, sporangia were found to survive for up to 24 hours during cool wet conditions. The researchers estimated that with a wind speed of 20 to 40 km/hour during a storm, sporangia could travel 80 to 160 km in four hours. That is why late blight is considered a community disease. Once it is found in a region, all susceptible crops near by are threatened.