Early spring prompts disease management warning

Workshops held to help identify late blight, zebra chip, fusarium and blackleg

Mild southern Alberta weather prompted scientists at Agriculture Canada’s Lethbridge Research Centre to erect a spore trap last week, much earlier than usual.

The trap, one of about a dozen erected each year to monitor pathogen activity, has already captured airborne sporangia.

It’s an early reminder for potato growers to watch for problematic fungal pathogens this season.

“We did detect a phytophthora sporangium, but there’s 232 different phytophthora species out there,” said research scientist Larry Kawchuk, who specializes in potato disease prevention.

“It’s going to be an early spring. We’re already starting to see spores of various fungal microorganisms in the air. So be aware.”

The exact type of sporangia captured is irrelevant now, when planting has not begun and there’s no potato crop to be infected.

However, a series of workshops held in Alberta last week were designed to inform growers of fresh market and table potatoes about identification and management of diseases, including late blight, zebra chip, fusarium dry rot and blackleg.

Late blight is caused by phytophthora infestans, a fungus that affects all parts of the potato plant, spreads quickly and can devastate an entire crop.

“There’s no such thing as a little bit of late blight,” Rob Spencer, a commercial horticulture specialist with Alberta Agriculture, said at the March 1 workshop in Lethbridge.

A one percent infection level in potato seed is equivalent to 100 to 150 tubers per acre, and a five percent infection is 550 to 725 tubers per acre.

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Spencer said one potato plant with late blight can infect plants within a 10-foot diameter.

“That’s why we worry about late blight,” he said.

Spore monitoring last year allowed quick control of the disease in a five metre patch of potatoes in Alberta, said Kawchuk, who considered it a major success story on the value of vigilance.

Spores can travel on the wind and by rain and water splash. They can even travel on equipment, though the survival window is short because they require living tissue.

Late blight has been found in Alberta in the past, but it is not endemic. To date, no A2 types of spores have been found in the province that can mate with A1 spores and create a permanent problem.

“That would be a real game changer for us, and we would have late blight issues every year, chronic late blight issues,” said Alberta Agriculture crop pathologist Mike Harding.

Spencer said late blight can be difficult to identify in its early stages. It is distinguished by the appearance of dark, water soaked lesions on leaves that are not contained by the leaf veins.

The lesions may develop yellow edges as they expand and move in from the leaf tip or margin. They later produce spores, sometimes visible under moist conditions as white fuzz.

Late blight in tubers appears as a reddish-brown rot.

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“I’ve always heard it described as foxy brown,” said Spencer.

The blight can spread in storage to healthy tubers and can overwinter in cull piles.

Late blight also affects tomatoes and can spread from backyard and urban gardens into commercial potato plots. It has also been known to infect petunias.

The disease thrives in moderate temperatures and wet, humid field conditions. Spencer said Alberta’s semi-arid climate has been a protection, but moisture in irrigated fields can create ideal conditions.

No treatment is available for late blight once plants are infected, but nearby growers can apply protective fungicide if it is identified in a region.

“The big thing is detection. Eternal vigilance, really,” said Spencer.

Growers should monitor fields and send suspicious plant material for testing. Controlling volunteer potatoes and adjusting plant density to reduce humidity are good strategies.

“Late blight is a community disease. It’s up to all of us and everybody else who’s not here to manage it,” said Spencer.

“It’s everybody’s problem.”

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