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White Mold Update (07/23/15)

Frequent rains in much of the state have continued to provide a favorable environment for Sclerotinia ascospore production, the pathogen that causes white mold.

White Mold Update

Frequent rains in much of the state have continued to provide a favorable environment for Sclerotinia ascospore production, the pathogen that causes white mold.  When soils remain wet, the pathogen has the ability to generate new apothecia (small mushrooms), which will produce spores that can infect the plant through flowers.  A great tool to assess the potential spore production is the canola risk map, which was developed by NDSU and the Northern Canola Growers Association (Figure 1 and 2).  The map is useful for all broadleaf crops in bloom and shows high risk for much of the state, essentially, lots of Sclerotinia spores are in the air.

However, two other factors are needed for infection; a wet canopy and moderate temperatures.  While the canopies are wet in much of the state, forecasted temperatures are pretty high and may not be particularly favorable for infection.  In addition to drying out the canopy faster, temperatures above 85 F will damage spores and reduce the infection potential. 

Many growers have already made decisions about spraying, and some may be considering a second application on sensitive crops like dry beans.  However, white mold is difficult to predict when you have a ‘mixed bag’ for risk (high spore potential, moderately wet canopies but potentially unfavorable temperatures).  As a consequence, it is very important for growers considering a second application to ask themselves some questions (below) to determine their individual risk. 

1)      Does your field have a history of severe white mold? 

2)      Do you have a tight crop rotation of susceptible crops (dry beans, canola, sunflower, some soybeans, etc)?

3)      Is your canopy very dense (lush plants, rows closed, etc.)?

4)      Does the canopy stay wet for a long time (into late morning – early afternoon)?

 

The more questions you answer ‘Yes’ to, the higher your relatively risk. 

 

The more questions you answer ‘No’ to, the lower your relatively risk. 

 

One last thing to consider is the length of the season left.  Essentially, it is a race to harvest between the plant and the disease; will the plant mature before the new lesions develop and take yield?  At some point in the season fungicide applications are not helpful or economic, but it’s hard to know exactly when that is.  For some sensitive crops like dry beans, there is likely enough time for new infections to cause problems.  As an important note, fungicide applications help prevent NEW infections.  They do not rescue the plant from infections (and lesions) that have already occurred. 

Whether you have applied fungicides or not, cross your fingers for a dry spell and highs in the high 80’s or 90’s F. 

ppth.markell.fig 1

ppth.markell.fig 2

Sam Markell

Extension Plant Pathologist, Broad-leaf Crops

 

Michael Wunsch

Research and Extension Pathologist

NDSU Carrington Research Extension Center


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