The Utah State University onion research team
has been interested in looking at a systems or landscape scale
approach to managing onion thrips
and iris yellow spot virus.
So we've been focused on evaluating some of these farm
scape factors.
In particular, we've been looking
at things like the nitrogen rate that's
applied to the field, how much nitrogen is
available in the soil and the plant tissue.
Previous crop rotation scheme, soil quality,
insecticide sprays, their frequency
and the different modes of action.
But we've also expanded to look at more of a landscape scale.
What are the crops growing nearby?
We found that alfalfa is a particularly good host
for overwintering onion thrips, not nearly
as good as winter wheat or even summer
corn that could provide a respite during the summer.
We've looked at coal piles of onions and volunteer onions.
And then more recently we've been
focused looking at weedy ditch banks, the types of weeds that
grow along the edges of fields.
These in particular can be green bridge plants that
help the thrips and virus overwinter
and move into the onion crop the next year.
I'm Claudia Nischwitz.
I'm the extension plant pathologist
at Utah State University.
We're here in one of the onion fields in Syracuse,
and I'm going to talk about iris yellow spot
virus who is the most important disease Utah onion growers see.
The virus is transmitted by onion thrips.
Both the virus and the thrips can survive on weeds.
The virus has to survive on weeds that overwinter
during the field season.
And then so next spring the plants grow out again,
thrips reproduce on those weeds and the larvae
have to acquire the virus from those plants
and then they can transmit the virus throughout their lives.
Onion thrips themselves, if they're adults cannot acquire
the virus and then transmit it.
I'm Shawn Olsen with Utah State University Extension.
I'd like to talk to you about four weeds that
have been found to be important in thrips and iris
yellow spot getting into the field.
These four weeds, three are winter annuals,
the other is field vine weed.
But they all green up early in the spring
and so they serve as kind of a overwintering reservoir
for the virus and the thrips.
Now as we look out at this field,
it's very well taken care of, not a lot of weeds in it.
But if we look here on the edge, a lot of these weeds
are found along the edge of the field.
And so I want to show you some of the key characteristics
of these weeds.
And as I mentioned, these have been
found to be a reservoir for thrips overwintering
and also iris yellow spot virus.
So the first weed that I want to mention is flixweed.
Now it's dried up already, this time of year.
It has yellow flowers.
Normally it'll have some finely dissected leaves
here along the base.
And you can see by these scars that they're alternately
spaced along the stem and it also has this unique seed pod.
So this is flixweed.
The next one that I want to mention is common mallow
and it has these round or kind of heart shaped leaves.
It has pink or white flowers.
And one characteristic that a lot of people
notice about this plant, this is the fruit
and it's a small round button and a lot of people
will tell stories about eating these buttons when
they were young growing up.
So common mallow.
The next wheat is prickly lettuce
and it deserves its name.
It has these little spines along the back of the leaf,
also along the stem.
These leaves kind of come around the stem plus the stem.
And again, they are ultimately spaced along the plant.
These will grow quite tall up to five feet tall.
So prickly lettuce.
The last one I wanted to mention is field bindweed.
This is not a super healthy sample but this is a perennial
and the leaves are kind of shaped like an arrowhead.
This plant can grow 20 feet deep.
The flowers are white or pinkish.
Very hard to control weed.
But again, it's a source, reservoir source
of iris yellow spot virus and the thrips.
And so they can move from the edge of the field
out into the onion field and cause problems later
in the season.
So field bindweed.
So a possible strategy is if you have
a lot of these weeds along the edge of the field, target
and see if you can control them because these four weeds have
found to be in particular host for these insects and diseases.
You may not be able to eliminate all the weeds along the ditch
bank and edge but if you could target these four weeds,
it may reduce the amount of thrip and disease
going out into the field.
So you can see here in this field
where they've burned the edges, the ditch at banks of the field
and that's a good way to help reduce those weed populations.
Also, targeted herbicide applications, cultivation
in your field to remove weeds.
Those are all good techniques to keep down
the populations of weeds.
We have learned by sampling thrips and fields
that we tend to see more onion thrips near the field edges
as compared to the more center of the field.
We feel this is due to the presence of the weeds
and other alternate crops around onion fields
which the ones that have we found the most
strips on our alfalfa and to a lesser extent wheat and corn.
And so we set up our sampling plots
so that we have a plot near the edge of the field
and then one more interior so that we can compare
those different numbers of thrips,
but we do tend to find more near the field edges.
I wanted to mention a resource.
This is a book called Weeds of the West.
It's 600 plus pages, nice color pictures of weeds.
Good description as the scientific name, common name,
often several common names.
So this is a great resource if you're
looking to identify weeds in your field.
Weeds of the West.
It's available from the county Extension office or state
publications office.