White Mold Can't Take the Heat (7/14/11)

Although we have had plenty of rain for sclerotia germination, the heat is going to impair the ability of the pathogen to cause infections.

Crops are susceptible to white mold once they enter bloom.   Ascospores will colonize flower petals first, and from there the fungus will penetrate into the rest of the plant, eventually causing the characteristic cream colored and shredded lesion.  For this to occur, several environmental factors need to come together.  First, enough soil moisture must be present to allow sclerotia to germinate and produce ascospores.  Generally the minimum rainfall needed is 1 to 2 inches within 1 to 2 weeks of bloom.  Secondly, you need a favorable environment during bloom; namely, wet canopies (heavy dew, rain, etc), and cool to moderate temperatures.  According to the weather forecast, we are going to be HOT as soybeans, dry beans, and canola are blooming.  According to the National Weather Service, temperatures in North Dakota will reach the high 80's and low to mid 90's by weeks end, and stay there for the remainder of the 10- day forecast.  The white mold pathogen is much less of a threat when temperatures are above 85F.  If the weather forecast is accurate and your crops are blooming when temperatures are in the 90's I would expect few infections until the temperature drops.  This is great, because the infections that occur closest to bloom are the most yield limiting.  Those early infections are often lower on the plant and have the most time to develop and subsequently cause yield loss. When the temperature drops infections can occur, but these late infections are often on lateral branches, and have less time to develop lesions. 

Below are some thoughts on the effect of heat and management of white mold.

For soybean growers, particularly those in the southern part of the state, a fungicide application this year may be far less likely to provide a return if your plants are soon entering bloom.  Further, data suggests that once soybean reaches R3, fungicides for white mold are unlikely to impact yield under any temperature.  This is simply because the infections occurring after that are usually not yield- limiting.

For dry bean growers, it is important to remember that dry beans are very susceptible to white mold, and we have lots of inoculum potential.  Even under non-ideal conditions you can get some disease.  A fungicide application may still be warranted, particularly in the Northern part of the state where white mold has been terrible in the last two years.  However, In beans entering bloom in this heat, i would expect disease severity to be less than in either 2009 or 2010.

For canola, which is already blooming in many areas, white mold may be much less of an issue than in years past.  Some infection could have already occurred, but I would expect far fewer new infections once we get hot.

For sunflowers, this heat will reduce the incidence of mid-stalk rot.  However, bloom is still weeks away and this week’s temperatures will have no impact on head rot.

For other crops that are naturally less susceptible, namely peas, flax, lentils, chickpeas etc, white mold will likely be a non- issue if your plants are blooming during the heat. 

Sam Markell - Plant Pathologist

Samuel.markell@ndsu.edu