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Reduced Risk of White Mold (07/01/21)

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Prolonged dry and hot conditions reduce the likelihood of white mold occurring, yet alone causing significant yield loss. Additionally, thinner stands and poorer crop development reduces the favorable microclimate that white mold thrives in.  While there are a range of crop conditions and recent localized rainfall in the state, we are generally looking at a reduced risk of white mold in 2021. 

However, weather patterns can change and I have learned to never count white mold out, so a quick review of white mold is warranted. The short story is: All broadleaf crops are susceptible, but only at (or after) bloom, and the disease is most severe when it’s cool and wet. Fungicides can help manage white mold, but are most valuable when there is disease risk.

But that needs a little more explanation…

All broadleaf crops are susceptible,

When the environmental conditions are favorable, white mold can occur on all broadleaf crops. We have the most consistent problems with white mold on sunflower, dry edible bean and canola, but yield loss certainly can occur in soybeans, pulse crops and many others.

but only at (or after) bloom,

White mold most frequently infects a crop by first establishing itself on floral tissue. Spores land on the florets, produce fuzzy white mycelium (mold) and invade the healthy green tissue from there. It’s why nearly all fungicide recommendations include application in the early bloom growth stages of our crops. It’s also why we are laser-beam focused on environmental conditions during bloom.

Infection radiating from infected floret on sunflower leaf.

and the disease is most severe when it’s cool and wet.

The spores that infect the crops are dispersed from small mushrooms (apothecia) that emerge on the overwinter structure of the pathogen (sclerotia). However, these small mushrooms do not germinate unless there is adequate soil moisture. A rule of thumb is at least 1-2 inches of rain 1-2 weeks before bloom. For those spores to cause infection, they need prolonged wet conditions, preferably at cool temperatures (60s-70s are optimal), during bloom. Anything that makes the canopy stay wet longer, such as a lush canopy or close proximity to a shelterbelt, makes the microclimate more favorable for infection and disease development. What does white mold dislike most (?) – prolonged dry soil conditions, infrequent rain, high temperatures, thin canopies.

 

Fungicides can help manage white mold,

Fungicides in most broadleaf crops (sunflower is an exception) can help reduce the incidence, severity and yield loss caused by white mold. For more information on timing, droplets and links to efficacy information, I refer you back to a crop and pest report article from last year.

 

but are most valuable when there is disease risk.

Every field is different. Some fields always seem to have white mold problems while it’s a rare event in others. As far as the environment is concerned, a couple erratic thunderstorms can move the field from low to high risk quickly. And since nobody want to add an input into a field if there is a low chance of return, it’s important to evaluate the disease risk in each field. 

A pair of helpful tools to determine risk are the Canola Risk Map and Risk Calculator, which was developed by the Canola Pathology Program with support from the Northern Canola Growers Association.  However, these tools take into account conditions conducive for white mold that broadly apply to all crops in bloom.  The tools are available at NDSU (https://www.ag.ndsu.edu/sclerotinia/riskmap.html) or through the Northern Canola Growers Association (https://www.northerncanola.com/growers/Forecast-Maps/), the Minnesota Canola Council (http://www.mncanola.org/maps.php), and the NDSU canola pathology program websites (https://www.ag.ndsu.edu/sclerotinia/).

 

Sam Markell

Extension Plant Pathologist, Broad-leaf Crops