Even the Farmer’s Almanac, in publication since 1818, has a section on the scourge of powdery mildew, a many-specied fungal disease that attacks a wide range of plants, particularly grapes.

At Oregon State University, Sarah Lowder wants to know more about grape powdery mildew and the fact that growers are having disease control issues due to fungicide resistance.

Speaking on the subject at the most recent Oregon Wine Symposium, she told of last year's season and why things were so challenging because of mildew. 

“Although growers have to deal with this issue every year, some seasons are worse than others — mysterious in some cases as to why the increase, although warm weather early in the season that prompts a lot of fast shoot growth and unprotected tissue could be a factor,” she said.

“If you have two different cultivars sitting next to each other that may be sprayed at the same time, they may have different growth patterns and bloom times resulting in one block blowing up faster than the other.”

Powdery mildew usually starts off as circular powdery-white spots on the upper part of leaves until the plant ends up looking like it has been dusted with flour.

“We had a lot of early season vine growth last year which left a lot of unprotected susceptible tissue before sprays could be applied,” Lowder said. “This season, it’s hard to tell as we’re still fairly early but once we hit late spring any rains could make it harder to get sprays on.  We’ll see how the weather turns out and how the vines grow as we move forward.”

Fungicide plays a key role

How bad a mildew presence is depends on the fungicide product being used.

“There’s a single mutation in the mitochondria, the powerhouse of the cell, that can cause resistance and when you build up enough of this in a population, your control benefits diminish because of the resistance.  As the season goes on, there’s an increase in the mutation,” she said. “If you do field monitoring, you can collect powdery mildew, extract the DNA from the spores, and determine if you have that mutation present, and if so, what kind of resistance you have in your field.”

One of Lowder’s projects has involved rapid-sampling techniques to monitor fungicide resistance.  “Previously, monitoring vineyard resistance was both cost- and time-prohibitive on any scale useful to growers,” she said. “We found it faster and more economical to use a cotton swab to collect fungal spores right off the gloves of workers already moving through the vines.

“Collecting powdery mildew samples in the field, particularly early season when you have very low disease levels, can be time-consuming and not necessarily logistically possible for most growers with wide swaths,” she said. “But as workers move throughout the vineyard doing various maintenance tasks, we met them at the end of the row and swabbed their gloves for any spores that many have fallen onto them from any of the tissues they touched.

“We found out the swab technique can do more than monitor for fungicide resistance.  It can help monitor diseases in general because it’s more sensitive and much faster than visual scouting or spore traps.”