An ounce of prevention is worth a pound of cure

TABER, Alta. — Don’t treat late blight. Instead, prevent it from ever occurring.

That is the key message from plant pathologists and potato industry officials in this region, where the fungal disease caused major losses last year.

Late blight kills potato plants and causes tuber rot in storage. It also affects tomatoes, which can be a source of spread, so the potato industry has made a concentrated effort this year to inform growers and gardeners about late blight prevention.

Philip Hamm, professor emeritus and plant pathologist at Oregon State University, spoke to growers June 10 during the industry’s most recent meeting about late blight prevention and control.

Hamm, who has worked for 25 years in research and extension, primarily with potatoes, recommended protectant as opposed to systemic fungicides in the fight against blight.

He recommends the fungicides chlorothalonil, the active ingredient in Bravo, and dithane, based on their effectiveness and cost in preventive programs.

Aerial and ground spraying are the only two methods available to Canadian growers because few products are registered for application to potatoes through irrigation, as is commonly done in parts of the United States.

“There are issues with no matter what application method you use, but there are ways to take away from those issues,” said Hamm.

Key considerations include initial coverage, initial concentration of fungicide and the time needed for coverage of the entire plant canopy.

“We not only have to remember getting the fungicide to the canopy, however you do it, but you’ve got to get it throughout the canopy because you’ve got to protect the whole canopy, and often times the top of the canopy isn’t nearly as important as below because of the humidity issues that late blight likes.”

Aerial application on potatoes is relatively common in southern Alberta, and its effectiveness depends on many factors: wind, nozzle type, pressure, droplet size, water volume, obstructions, speed, density of canopy, effective swath width, humidity, drift, topography and cost.

However, Hamm said the primary considerations should always be initial coverage, initial concentration and redistribution of the product once it has been sprayed.

Similar considerations apply to ground spraying.

“The bottom line is, no matter what application method you use, it will control late blight. You just have to be thinking of a few things,” said Hamm.

“A ground rig will get more material to the canopy than an air application will and it will give a higher concentration within the canopy than an air application will.”

Agriculture Canada research scientist Larry Kawchuk agreed.

“If you have the ability to do ground application, you’re going to get better coverage, but if you don’t have that option, air coverage is fast. Phil brought up an interesting point how you can still get fairly good coverage with additional irrigation or rain washing some of the product into the under canopy.”

Hamm said fungicide application frequency of a week to 10 days will likely provide good protection.