Tuesday, August 18, 2009 10:01 PM CDT
Fungi to look for as fall approaches
Suzanne Bissonnette, University of Illinois Extension.
Our August days have been warm, nights have been cooling, backpacks are full of new pencils, and happy smiling parents are prevalent — all signals that fall is fast approaching. As the growing season begins its wind down, a different set of pests need to be monitored in the field.
Area soybeans have a few fungal pests that have been causing some concern. The first is a fungal stem disease called “white mold.”
The name says it all. This is an easy disease to diagnose in the field because there will be a lot of fluffy white mold growing on the stem of the infected soybean plants.
Typically, white mold tends to be a northern Illinois disease, or, more confined to fields that have had green bean or dry bean production in the past. Its presence in area fields this year points to two particularly necessary elements for white mold to develop.
First, the weather needs to tend toward cool and be wet when the soybean plant is flowering. Well, that’s exactly the weather we had this year during flowering.
And second, the seeming widespread development of this disease this year points to the tenacity of the resting survival structures of white mold, called sclerotia, waiting for just the right environmental conditions to produce spores and infect senescing soybean flowers.
White mold is caused by the fungus Sclerotinia sclerotiorum and it is has a bit of an unusual life style. White mold is a necrotroph; in other words, it only grows on dead tissue.
So what, you may ask, is it doing on living soybean plants? Well, the fungus solves this life style problem by producing a toxin that kills soybean tissue in advance of its growth — pretty clever.
The presence of white mold in your soybean field may cause you some alarm, but unfortunately, once you see the development of the disease on plants in the field it is too late to control the disease. For those fields in which white molds develop significantly in this season, a management plan should be developed for future soybean rotations.
White mold is not an easy disease to manage; several strategies need to be integrated. First, variety selection; while there is no specific resistance to white mold, there are varieties that are more tolerant.
Next, row spacing and seeding population need to be considered. Wider rows and lower seeding rates can increase airflow to the soybean plant, thus decreasing white mold infections. Changing row spacing and seeding rate will only have an impact in very severely infested fields.
Next, there are a couple of fungicides that are registered for white mold management, but application timing is critical, as mentioned and expected weather during flowering needs to be a substantial deciding factor for application.
Another management strategy is to not use bin run seed. Bin run may have sclerotia of the fungus hanging out with the seed just waiting to infest a field where you plant.
And finally, there is a biological control that is registered for managing the disease. The biological control product is a fungus called Coniothyrium minitans, and this fungus is a parasite of the survival sclerotia of white mold. The biological control is not applied in season but either in the fall or spring.
Sudden death syndrome also has made its presence visible in some area soybean fields. Luckily, the SDS symptom development is very late this year and the disease is very unlikely to cause significant yield losses.
Suzanne Bissonnette is an integrated pest management educator for the University of Illinois Extension.
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