By Ashok Chanda (Assistant Professor & Extension
Sugarbeet Pathologist), Department of Plant Pathology and Northwest Research
and Outreach Center, University of Minnesota – Twin Cities
Ashok Chanda can be reached at 218-281-8625 or achanda@umn.edu or Twitter @BeetPath
Hopefully the official sugarbeet planting season will start
in a couple of weeks in MN and ND and as usual there may be a lots of questions
on your mind about how to get a hold on Rhizoctonia diseases, this is also
confirmed by the highest number of Rhizoctonia samples diagnosed in our laboratory
during the past 3 years. If this year’s planting is delayed due to weather,
fields will be at high risk for Rhizoctonia, as warm soil favors development of
Rhizoctonia. Here are some things that can help you to develop your customized
Rhizoctonia Management Plan for 2018.
Why Rhizoctonia is
becoming a problem?
Rhizoctonia can kill sugarbeet seedlings before or after
emergence and adult plants during the growing season, depending on the soil
moisture levels. Rhizoctonia can survive as dormant sclerotia in the soil for at least 2 years, when you have
previously infected-roots that are buried in the soil. For some of you who are
only doing 2-year rotation for beets, the risk of Rhizoctonia development is
much higher compared to 3- or 4-year rotations. If you have shorter rotations
and/or susceptible host crops in the rotation, the risk will be much higher.
The strains of Rhizoctonia (AG 2-2) that infect sugarbeet
are also capable of infecting soybean, corn, edible beans, and sunflower. Based on what we know about these strains,
the susceptibility (from highest to lowest) of common rotation crops is as
follows:
Seedlings: High Ã
sugarbeet > soybean > pinto bean > corn > sunflower Ã
Low
Adult plants: High Ã
sugarbeet > soybean > pinto bean > sunflower and corn Ã
Low
What can I do about
seedling Rhizoctonia?
Rhizoctonia damage at seedling
stage can be seen as uneven stands in parts of the fields and dark lesions can
be observed on the wilting seedlings at or below the soil line (Figure 1). Since
2017 all the sugarbeet seed sold in MN and ND comes with some sort of fungicide
as a seed treatment. These include Kabina, Systiva, Vibrance , and Metlock
Suite, individually or in combinations. Any of these seed treatments are
effective in protecting the seedlings for up to 4-5 weeks and any activity afterwards
will depend on factors such as soil conditions and microbial activity.
 |
Figure 1. Rhizoctonia damage on the seedlings at the soil
line.
|
How can I manage Rhizoctonia
during the season?
Typical collapse of crowns (Figure 2, Top) or
ladder like pattern of lesions (Figure 2, bottom) can be observed on the roots
of wilting plants, which are indications of Rhizoctonia infection. Most of the
growers are applying a POST fungicide when the 4 in. soil temperatures are at
62 - 65 F, but it can be challenging if you cannot get in to the fields,
especially if they are wet. As seed treatments are offering early protection,
you have some window of time to apply these POST fungicides at 4- to 8-leaf
stage (ideal at 6-leaf) depending on your convenience and other factors. Fungicides
such as Quadris, Azteroid or other generic formulations of Quadris, Priaxor or
Proline can be used for POST application.
 |
| Figure 2. Rhizoctonia damage leading to collapse of crowns (top) or ladder like pattern of lesions (bottom) on the roots. |
How can I apply POST
fungicides?
Fungicides applied as a 7 inch band over the plants, to
cover the crown area and some soil surrounding the crown area, should work very
well. We also looked at broadcasting Quadris in 2016 and 2017, where efficacy
was comparable to band application. However, broadcasting should only be
considered as a second choice compared to band application. It may be beneficial
to time the POST application prior to a rainfall (about ½ inch) event so that
the fungicide can be re-distributed in the root zone.
What about the root rot
occurring at the tip of the roots?
We are also seeing some fields where root rot
is only occurring at the tip of the root (Figure 3) at or below 4 inches from
the soil line. This can be a challenge for some growers as we don’t know if the
POST fungicides can go that deep to offer protection. One way that my work very
well for these fields is by applying fungicides in the furrow at the time of
planting sugarbeets. Based on previous research done at the University of
Minnesota- Northwest Research and Outreach Center in Crookston, Quadris applied in-furrow offered protection up to 8 weeks after planting. The key here
is to keep the beets as healthy as possible during the season so that they are
able to withstand some late season infections, if there are any. Fungicides
such as Quadris, Azteroid or other generic formulations of Quadris, Headline,
and Xanthion can be used for in-furrow application.
 |
| Figure 3. Rhizoctonia Rot observed on the tip of the roots (top and bottom). |
What about my management plan if I am using a Rhizoctonia
specialty (tolerant) variety?
The specialty
varieties that are available in the market still need to reach 4 to 6 leaf
stage before the natural plant resistance will be active, so you have to
protect these plants until that stage.
Rhizoctonia being a
soilborne pathogen can move with your farm machinery into clean fields. As a
best management practice, you should pressure wash your farm machinery after
using in the Rhizoctonia fields.
If you are not sure about how to proceed, below is a comprehensive management plan for
Rhizoctonia that should work for most of you. As always, there will be
exceptions, and I will be happy to assist you in finding the most appropriate
way for you. You can answer the questions in columns 1 and 2 and see which options work best in columns 3 and 4. Seed treatment is not listed as an option here because every sugarbeet seed has a Rhizoctonia treatment in 2018.
Rhizoctonia risk (no of beets/100 ft. row based on the
field history)
|
Use of Specialty (tolerant) variety
|
In-furrow fungicides
|
Postemergence (POST) fungicides
|
Low (170-200)
|
No
No
|
No
No
|
No
Yes (if following Beans)
|
Moderate (130 – 170)
|
Yes
Yes
No
|
No
No
No
|
No
Yes (if following Beans)
Yes
|
Severe (less than 130)
|
Yes
|
Yes
|
Yes
|
Our Sugarbeet
Pathology Lab in Crookston will be offering diagnostic services for sugarbeet root
diseases again in 2018. If you are not sure about the problem, please send in your
plant samples (without soil especially for samples out of MN), we can help you to
find out what is causing the problem. Alternatively, you can also request your agriculturists to pull samples for diagnosis; they are very familiar with our service.
For a detailed comparison
of fungicides that are commonly used for managing Rhizoctonia and other management
options including diseases caused by Aphanomyces, I highly encourage you to see this presentation and publications from 2018.
Mention of trade names or commercial
products in this article is solely for the purpose of providing information and
does not imply endorsement.
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