Disease Update: Tomato
corky root and vine decline
In recent seasons, poor growth or premature decline of the foliage
has been seen in a number of processing tomato fields. The symptoms
are usually associated with brown root lesions, loss of fine feeder
roots, or root rot. The 2009 season seemed to be especially severe,
but symptoms were again widespread across the growing region in
2010. Impacts on yield and quality in severely affected fields can
be significant. The term vine decline is being used to denote
what we believe is a complex of factors contributing to poor health
in these fields. Corky root rot seems to be one of the key
factors involved. However, if substantial and universal yield reductions
are part of the definition of a plant disease epidemic, then vine
decline or corky root rot do not qualify as yet. Average tomato
yields in Ontario were high in 2009 and 2010 even though there were
severely affected corky root hot spots.
It is likely that the corky root pathogens have been present in
our soils for a long time and are part of a complex of root and
foliar pathogens interacting with other stresses. Similar symptoms
have been seen over many years, but the corky root rot pathogens
were not detected because these fungi are slow-growing and not easily
cultured using standard media. The root symptoms can be found across
the processing tomato growing area and the patterns of incidence
do not suggest a recent introduction.
Although foliar bacterial disease can cause loss of foliage and
stunted growth, it does not cause the root symptoms. The combination
of foliar bacterial disease and root rot infections, however, may
increase the impact on the crop.
Surveys and research projects in 2009 and 2010 have looked into
the causes of these problems.
Corky root rot
A key diagnostic symptom seen in problem fields is the presence
of brown, corky bands on the roots. A team led by Dr. Jim Traquair
at Agriculture and Agri-Food Canada in London has found that the
corky root symptoms are caused by Pyrenochaeta terrestris,
Pyrenochaeta lycopersici, and Rhizopycnis vagum (alone
or in combination).
Fusarium and some degree of associated brown rot of fine
roots were found at all sample sites in 2009 and 2010, both at sites
that appeared healthy and at those with vine decline symptoms. Black
dot root rot, caused by Colletotrichum was also present at
many healthy and vine decline sites. All the pathogens were frequently
isolated from severely rotted roots and in these cases, Colletotrichum
black dot root rot may have been there as a secondary infection,
after the roots were weakened by the corky root rot pathogens.
This is the most frequently found corky root rot fungus in tomato
root samples from Essex and Kent counties. This pathogen is normally
associated with pink root of onion in warm climates. However it
has a very wide host range, including many of the crops commonly
grown in rotation with processing tomatoes in Ontario. It is associated
with a pink discolouration of some roots. This pathogen was also
isolated from red roots of corn (volunteer corn or a neighbouring
field) and from nightshade (in the tomato field).
P. terrestris has been reported to survive on soybean, pea,
millet, oats, barley, wheat, corn, squash, cucumber, cantaloupe,
muskmelon, tomato, pepper, eggplant, cauliflower, carrot, spinach,
and onion. It is not reported to cause significant disease on all
of these crops, but it does cause red root rot of corn and pink
root of onion.
This pathogen occurs in many soil types and can survive in soil
for many years.
This corky root rot fungus was found in some Essex county samples,
but was not as common as P. terrestris. This pathogen is
more commonly a problem on greenhouse tomatoes and occurs worldwide,
but it has been reported to be sporadic on field tomatoes in European
countries, California and Florida.
Host crops include tomato, pepper, eggplant, cucumber, melon, and
squash. Beet is also reported to be a symptomless host. Some solanaceous
weeds such as Jimsonweed and nightshade may also be hosts.
This fungus was found in many tomato samples and also in corn from
Essex and Kent counties. It has been reported to cause vine decline
of melons in Spain, the south-western USA, and Central America,
and to cause root lesions on onion in Iran and tomato in Italy.
In some of these fields, additional stresses were also identified
such as other root pathogens, foliar pathogens, and/or high nematode
levels. In many fields, examination of the root systems showed that
they were also being physically restricted by soil compaction. We
are likely dealing with a complex of stresses that include pathogens
and the growing environment. Preliminary observations point to differences
in susceptibility among tomato varieties, as well.
What is corky root?
Symptoms (see photos below)
- stunting, slow growth
- premature defoliation
- brown bands on roots that may develop into dark, rotted roots;
loss of small feeder roots
- no discolouration of internal root tissue
- on severely affected roots, the outer layer (cortex) can be
easily pulled off the root core (stele)
All of the corky root rot fungi identified produce microsclerotia
which provide a means of survival in the soil from one growing season
to the next and probably over several years. One of more of the
corky root fungi may attack the roots under almost any soil temperature.
In addition, each of the pathogens and particularly, P. terrestris,
has a fairly wide host range.
P. terrestris has an optimum temperature for growth and
development of 25-28°C (77-82°F), but infection may occur
down to 16°C (61°F). P. lycopersici develops best
at 15-20°C (59-68°F). Rhizopycnis vagum is favoured
by temperatures of 25-27°C but will infect roots at 20°C.
All can survive deep in the soil.
These pathogens can produce pycnidia containing conidia (spores)
on diseased roots and root debris but in the field, the major means
of dispersal is thought to be through movement of soil and plant
debris containing the microsclerotia (survival structure of the
fungi). The soil could be moved within the field and from field
to field by wind, runoff and farm equipment. Preliminary tests in
the laboratory have shown that Pyrenochaeta and Rhizopycnis
conidia and microsclerotia can survive in water for 6 months. In
fact, the microsclerotia can survive in a wide range of conditions.
One of the ways of preserving strains of these fungi is to air dry
microsclerotia in sterile soil and to freeze them in dry soil at
Management of corky root rot and the vine decline complex in field
tomato will be difficult and will require an integrated approach
involving many mechanisms, especially those focussed on maintaining
general soil and plant health. There are limited products available
for the management of soil-borne pathogens such as the corky root
fungi. Fields with severe and yield-reducing infestations will warrant
intensive and probably expensive corky root rot management measures.
Chemical and biological control
Results with fumigation in other affected areas have been inconsistent,
even with three-tiered injection depths. Yield losses have been
reported from tomato vine decline in both non-fumigated and fumigated
fields in Ontario. Methyl bromide, chloropicrin and thiocarbamates
(metam sodium) are listed for corky root rot management in California
and other parts of the world, but in practice, results are inconsistent.
Chloropicrin and metam sodium are currently registered for some
soil-borne diseases and nematodes in Canada, but it is expected
that restrictions on their use will become more stringent over time.
Research is underway to test efficacious rates of metam sodium on
these pathogens in Ontario. The efficacy of biofumigants such as
mustard green manures that release inhibitory isothiocyanates is
being investigated in some field sites in Ontario. Management is
made more difficult because the fungi can be found deep in the soil
and will be able to infect the plant through roots that penetrate
below the fumigated zone, or simply outside the fumigated band.
Research is also underway to investigate the impact of biological
control through the establishment of protective microbial endophytes
(bacteria and fungi antagonistic to growth of corky root pathogens)
in the roots of tomato plug transplants and plant defense inducers
applied before and after transplanting. Research in other areas
has shown conventional fungicide treatments to be ineffective.
Crop rotation has been the primary cultural practice aimed at slowing
the build-up of many soil pathogens, but will probably do little
to reduce levels of the corky root fungi in infected fields. Reports
of these diseases in other parts of the world have shown that each
pathogen has a fairly wide host range including most if not all
the traditional rotation crops. Our surveys have shown that Pyrenochaeta
terrestris and Rhizopycnis vagum can be found on corn,
onion and solanaceous weeds such as nightshade. Brassica crops,
such as turnip and canola, or leguminous forage crops, such as clovers
and alfalfa, are not reported hosts. Incorporating these crops into
a rotation program may limit the buildup of corky root pathogens
in the soil.
Other cultural practices
Other cultural practices that promote healthy plants and production
of adventitious roots such as proper fertilization, irrigation,
management of other diseases and management of plant parasitic nematodes
may help the tomatoes to avoid or compensate for infection by corky
root fungi. Finally, the ultimate control measure would be the growing
of corky root resistant varieties. Limited host resistance to corky
root rot and other root rots has been reported by breeders for greenhouse
and some fresh market varieties of tomato. Screening for root vigour
and corky root resistance, and incorporation of these characteristics
into field varieties is underway but it will be a slow process.
Figure 1: Early decline of foliage.
Figure 2: Early decline of foliage.
Figure 3: Range of root symptoms.
Figure 4: Range of root symptoms.
Figure 5: Healthy roots for comparison.
Figure 6: Healthy roots for comparison.
Figure 7: Symptoms of Pyrenochaeta terrestris
and Rhizopycnis vagum infection on corn.
Figure 8: Symptoms of P. terrestris on nightshade
The vine decline surveys were conducted in cooperation with:
- Tomecek Agronomy Services
- Tomato scouts from numerous agribusinesses in Essex and Kent
- Agriculture and Agri-Food Canada - London
- Ridgetown Campus - University of Guelph
This infosheet was authored by Janice LeBoeuf, Ontario Ministry
of Agriculture, Food and Rural Affairs, Ridgetown, Ontario; Dr.
Jim Traquair, Agriculture and Agri-Food Canada, London, Ontario;
Cheryl Trueman, Ridgetown Campus - University of Guelph, Ridgetown,
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