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| Ken MacDonald, garden club member, checks out information about late blight provided by Rick Peters, plant pathologist. |
“Over the past two years, I have received over 200
calls from gardeners across the Maritimes wondering what to do about the
effects of late blight on their gardens,” says Rick. "Many people have not been able to harvest a healthy tomato crop for years."
Late blight is a fungus disease which affects potatoes, tomatoes, eggplant, peppers, and even petunias. The fungus kills the top of the plants and also destroys potato tubers, making them inedible. The disease can destroy a plant within a few days. The spores survive over the winter in potato tubers.
The disease is responsible for the Irish potato famine in the 1840s. Most of Ireland’s poor were living on potatoes, when the crop failed, over 1 million people starved to death. About 2 million more emigrated, many of them to Canada. We can credit the Irish heritage of PEI to this disease.
Late blight still causes problems today – it causes about $20
to$30 million in damage to crops every year.
Farmers spray a variety of fungicides on their crops to prevent the
disease, but home gardeners do not have those options.
Late blight is closely related to algae. It produces a white fuzz on the underside of
leaves, which produces millions of spores which float through the air and
infect other plants. The spores land on
a susceptible leaf, germinate, and cause brown oily lesions. The spores splash on the ground and affect
potato tubers, which become brown and rusty looking, with a granular texture. Tomato fruit develop rotten spots that grow quickly. For potatoes, the tops of the plants can look
perfectly healthy, while the tubers are badly affected.
In 1993, the strain of fungus responsible for the
Irish potato famine was still active in North America and Europe. Three years later, a new strain appeared,
which was much more aggressive and resistant to pesticides. In 2010, newer strains appeared, and a year
later they were present in PEI and NB.
By 2013, the strain US 23 was present across the country. It is spread by diseased potato tubers and by
tomato transplants that are moved across the continent. Last year, US 23 was found in tomato
transplants sold in Charlottetown.
Late blight spreads most rapidly in humid conditions
between 15 and 24 C when the leaves do not get a chance to dry.
Tomato
varieties susceptible to late blight
- Scotia
- Brandywine
- Oxheart
- Monster
Blight
resistant varieties – available at Vesey’s Seeds
- Mountain Magic – a large cherry tomato – prolific - very resistant
- Mountain Merit – beefsteak – determinate - very resistant
- Defiant – medium size – determinate –high yield - very resistant
- Plum Regal – 50 % resistant
Prevention
- Pull out field bindweed – it is part of the same family (Solanaceae) and can be a carrier
- Don’t crowd plants – plant 3 feet apart
- Plant only certified disease free potato seed
- Purchase blight resistant tomato seeds and start them yourself
- Don’t grow potatoes, tomatoes, peppers, eggplant close together
- Water the soil, not the leaves
- Destroy volunteer plants – they are most likely not disease resistant
- Watch for diseased plants and remove immediately
- Don’t compost diseased plant material – put plastic bag over plant, pull it out of the ground, seal in bag, let it break down in the sun, put in garbage
- Encourage good air flow and plant in full sun
- Don’t overfertilize – too many leaves will form
- Plant early varieties that will ripen before late blight hits
- Organic sprays like copper-based Bordo can burn leaves and need to be applied after every rain. They are not foolproof
Prevention means you are being a good neighbour and not encouraging the spread of disease that could destroy someone's livelyhood.
