Fusarium Wilt of Blackberry Confirmed in Arkansas
Fusarium wilt is a new and devastating disease of blackberry that is present in California,
Mexico, North Carolina, and now in Arkansas. See below for information on identifying
and mitigating this disease.
Fusarium wilt is a disease that many Arkansas specialty crop growers are familiar
with in watermelons, tomatoes, and other fruit and vegetable crops. This disease is
caused by a soil dwelling fungus, Fusarium oxysporum, and can be very difficult to
manage. Fusarium is known to persist in soils for several years and limit the ability
of farmers to grow susceptible crops in those areas.
Historically Fusarium wilt had not been observed in commercially grown blackberry
cultivars, but it was found in the last 10 years in California, Mexico, and North
Carolina, and more recently in commercial blackberries grown in Arkansas
(Picture 1). Genetic studies have indicated that isolates of the Fusarium fungus infecting blackberry
across the continent are all from the same form ( Fusarium oxysporum formae specialis mori) but there is some genetic diversity among isolates found in different areas. Research
suggests that this disease can be found in wild blackberries, which may indicate that
a Fusarium species that causes this disease in blackberry has existed for some time. It has
only recently been observed on commercially grown cultivars.
It is unclear whether these new cases of Fusarium wilt are due to new, more susceptible
cultivars being infected by pathogen that was already present, or introduction of
the pathogen somewhere in the plant supply chain. What we do know is that there is
likely a large amount of variation in susceptibility to this disease by cultivar,
with ‘Twilight’ being very susceptible and where the disease was first observed in
many areas including Arkansas. Fusarium wilt has also been observed on other commonly
grown commercial blackberry cultivars such as ‘Ouachita’, while cultivars such as
‘Navajo’ appear to be resistant to some degree. We also know that there appears to
be patterns in the relatedness of the specific pathogen across several areas. Samples
from Arkansas have been sent to North Carolina to be compared to other isolates from
across the continent to hopefully help piece together the current puzzle.
Picture 1. Blackberry plants of the cultivar ‘Twilight’ with Fusarium wilt.
Should blackberry growers in Arkansas be worried?
It’s hard to know exactly what we are dealing with right now, but the short answer
is that all growers should be on the lookout for Fusarium on their farms. When plants
get this disease they will die, just like in other crops where this disease is prevalent.
There is an inherent risk to not finding this disease early, as farmers in North Carolina
have documented the capacity of the disease to move across a farm. What we know for
sure is that we found it in the state and all growers need to be looking for it.
How can Fusarium wilt be identified on blackberry?
Fusarium wilt on blackberry isn’t very well understood yet but it does have key diagnostics
which makes it relatively easy to ID in the field. Blackberry plants that have this
disease will exhibit wilting accompanied by the appearance of longitudinal black streaks
on canes, that will begin only on one side of the cane
(Picture 1 and 2). Black streaks will generally originate from the base of canes or near the ground.
These canes will wilt rapidly, and leaves will die and remain attached to the plant.
Sometimes canes will die while others will remain healthy, although plants do collapse
eventually. Spores can often be seen as masses present on the surface of the cane
in whitish-pink or tan-colored streaks on necrotic portions of the canes (Pictures 1 and 2). Ultimately if you think you have found Fusarium wilt on your farm you need to contact
your county agent and send in a sample to the Plant Health Clinic. Make sure to send a full plant with crown and roots along with infected canes to
Picture 2. Fusarium wilt pictured on blackberry plants in Arkansas. Necrotic lesions and discoloration
can be seen predominantly on one side of canes. Spore masses can also be seen as tan/pink
areas on necrotic areas in each plant pictured here.
How does Fusarium move?
The disease cycle of Fusarium wilt in blackberry is not very well understood. What
we do know is that fusarium wilt is normally considered soil-borne with infection
occurring through roots, but large masses of spores present on blackberry canes suggest
that there could be some level of above-ground spread. Infection of blackberry plant
material is currently thought to occur via the movement of above-ground spores. These
aerial spores may be moving via rain-splashing, through air movement, or may be physically
transferred plant-to-plant during harvest or trellising. Soil-borne spores can also
be spread via farm equipment such as bed layers, through transplanting plants in infected
soil, or even by infected soil moved by workers shoes.
What we know for sure is that this disease showed up in North Carolina and easily
spread across some farms either by the movement of soil-borne or aerial spores. Although
some growers may think of soil-borne diseases being relatively slow spreading, this
has not been the experience of blackberry growers dealing with Fusarium wilt.
What should you do if you find it?
What we know right now is that plants that become infected are likely to die. This
means that you shouldn’t expect to get any more yield off those plants and should
remove them ASAP and burn them to prevent the spread further. We also know that there
isn’t any remediation for the soil near infected plants. Growers in North Carolina
tried fumigating and replanting, just to see plants become reinfected in those same
areas. Do not plan on going back with blackberry in those areas any time soon.
As for the fields where Fusarium shows up, it is likely that you need to do more than
just remove the infected plants. I think the current assumption is that plants of
a similar cultivar nearby are likely to be infected, but not all growers are going
to be willing to get rid of these plants. If you are a small grower with not much
acreage, it is likely worth your time to remove plants as soon as they are found to
be infected and try to be as sterile as possible when working from plant to plant.
Larger growers may want to think about cutting their losses in those specific fields
to prevent spread to others. We are still waiting to see what the spread is like here
in Arkansas, but growers in North Carolina have really preached isolation and cutting
your losses on infected fields.
How can you avoid Fusarium wilt?
Although this disease is not well understood in blackberry yet, there are some best
management practices you can follow. First, growers should be sure to avoid introduction
of the pathogen by only buying disease free, tissue-cultured plants from reputable
sources. It’s a good idea to always rely on these reputable sources of plants to avoid
the many diseases that could possibly be moved with blackberry plant material.
Next, practice good sanitation when working in your blackberry fields. This starts
with removing wild blackberries that are nearby your plantings, as some work in California
has confirmed that Fusarium can move between wild and cultivated blackberry. Growers
should be sure to avoid handling plants when wet and also be sure to handle, prune,
or harvest fields that are not known to have issues before going to any infected or
suspected Fusarium infected plantings. There is some thought that this disease was
moved around during bed making or planting, so be sure to clean off equipment (even
shoes!) when moving field-to-field to limit the spread if it is present in the soil.
Avoid replanting in previously infected areas as this could promote movement of aerial
spores to other fields.
Lastly, we currently don’t think fumigation will be an effective tool and don’t know
of any beneficial fungicides to limit spread through aerial spores. Ultimately this
disease can spread through root infection, which presents a large problem as blackberries
are known to have expansive root zones that will outstrip a lot of your hopeful fumigation
or foliar fungicide efforts.
If you suspect you may have a Fusarium issue, contact your agent, and give me a call
at 479-249-7352. I also recommend taking a look at the extension article from
NC State for more info. Bill Cline and Sara Villani are on the ground working with this disease
and have been an excellent source of information for us here in Arkansas for this
emerging issue and others in the past.