By Ann Hazelrigg
Editor’s note: Dr. Ann Hazelrigg is the University of Vermont Extension plant pathologist and director of the UVM Plant Diagnostic Clinic.
With our recent rainy weather and storms, I am not surprised that we diagnosed late blight (Phytophthora infestans) in the University of Vermont Plant Diagnostic Clinic in late August. So far, it has been found in a garden and a high tunnel only on tomatoes although this aggressive pathogen can also attack potatoes, depending on the strain of the organism.
This destructive fungal-like organism was the cause of the Irish potato famine or the “Great Hunger” in the 1840s. If you have an Irish last name, you may be here as a result of your ancestors being among the million souls who left Ireland during the famine to search for a better life in America.
The pathogen cannot survive Vermont winters but typically blows in on storm fronts from the south or gets introduced on infected transplants, as occurred in Vermont in 2009. Late blight symptoms can appear on foliage, stems and fruit.
Leaf symptoms first appear as small, water-soaked areas that expand rapidly to form purplish blotches. The spots usually show up on the upper foliage first since the spores “rain down” during weather events. The disease can be hit or miss in an area, depending on where rain events have occurred.
When humidity is high, rings of whitish, spore-forming structures appear on the edges or undersides of the blotches. You can place suspect tomato tissue in a plastic bag with moist paper towels overnight and check for these rings of spores. Infected fruit remains firm and may exhibit whitish spores during high humidity. It should be destroyed and not be eaten or used for canning.
The pathogen likes cool, moist weather, spreads rapidly by airborne spores and can quickly kill an entire field of tomatoes within a week. If the weather becomes hot and dry (80 degrees Fahrenheit with humidity less than 90%), the spread of the disease slows only to pick up again when the weather turns cool and wet.
If your tomatoes do not have late blight yet and you know it is in the area, you can protect them with an organic (copper-based) or conventional fungicide, being sure to spray weekly or according to the label to keep new tissue protected. If your plants already are showing symptoms of the disease, it is best to destroy the plants since rescuing them with fungicides is difficult, especially if the weather is conducive.
The pathogen does not live on dead tomato plants, so cutting or pulling the plants will cause the spores to die. Leaving live infected plants in the garden increases the chances that the tomatoes at your neighbors or the farm down the road also will get the disease, so be a good neighbor!
Dead plants can be composted since the pathogen will not survive the winter. However, if you don’t have an active compost pile that heats up, other tomato diseases on the plant may not break down.
Rotation for this particular disease is not that important since the pathogen does not remain in our gardens although rotation should be followed for the other tomato leaf spot diseases that show up every year.
The best way to avoid the disease is to plant resistant varieties. There are several tomato varieties that are designated as resistant to late blight: ‘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.
