Late Blight

Ken MacDonald, garden club member, checks out information about late blight provided by Rick Peters, plant pathologist.

The January 2015 meeting of the Garden Club of PEI featured guest speaker Rick Peters, plant pathologist at Agriculture and Agri-Food Canada. Rick earned his degree at the university of Guelph, and has worked on PEI since 1994.  He spoke about his research into late blight, which has become a major problem for home gardeners as well as large commercial producers across Canada.  

“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

Even when a susceptible tomato variety is planted directly beside a resistant variety, the resistant variety is not affected by late blight.

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

     The potato industry is a $1 billion crop on PEI.  
     Prevention means you are being a good neighbour and not encouraging the spread of disease that could destroy someone's livelyhood.