Black root rot is caused by a complex interaction
of fungi, nematodes and environmental factors. Several fungi are implicated
in the disease including Rhizoctonia spp., Pythiumspp., and Fusarium
spp. When root lesion nematodes are present in the planting, the disease
is often more severe. This indicates that root feeding by the nematodes
may predispose the roots to infection by the fungi. The disease most
commonly occurs in low areas, on compacted soils or where organic matter
is low.
One or more of the fungi listed above is usually present in the field at
the time of planting. Thus, black root rot is not usually introduced into
a new planting through nursery stock or contaminated equipment. The exception
to this would be recontamination of fungi after a field is sterilized by
chemical fumigation. If an environmental factor or other factor (e.g.,
nematode feeding) is present, the disease complex may be triggered. Factors
that favor black root rot include poorly drained soils, winter injury,
nutrient imbalances and herbicide damage.
Symptoms
Aboveground: Symptoms of black root rot usually begin in the first fruiting
year. The injury will be most noticeable in low or soil compacted areas
of a field where drainage is poor. Strawberry plants with black root rot
show a general lack of vigor with poor runner growth and small berries.
Plants may collapse when water demand is high such as during spring growth,
during or after fruiting, or during drought stress. Aboveground symptoms
may resemble symptoms caused by other root disorders; therefore, roots
also need to be examined.
Belowground: To diagnose black root rot, strawberry plants should be sampled
in April or May. When taking a plant sample, dig, rather than pull the
plant from the ground. Examine the white roots of plants that are showing
signs of decline. In healthy plants, the interior of woody roots is yellowish-white.
Also, there are abundant fleshy white roots and lateral roots with white
cores. If black root rot is present, roots may be rotted at their tips
or may appear mottled with black lesions along the white roots. The root
core in early stages of black root rot is white, rather than red (which
indicates red stele disease) (FigureI). If plants are severely affected,
both the core and outer tissue will be black.
Disease Management
Black root rot is favored by wet soils and soils low in organic matter.
As a result, proper site selection and preparation are both important management
tools for this disease complex.
Site Selection: The future planting site should be selected at least one
year before planting. Soil drainage should be good. Avoid low-lying areas
that have a tendency to be poorly drained.
Site Preparation: If the site had been planted in strawberries previously,
practice crop rotation by planting the area into cover crops or annual
cash crops for at least two years, but preferably three to five years.
Do not rotate with crucifers or legumes because these are alternate hosts
for Rhizoctonia spp. Sweet corn or pumpkins might be a good choice.
Cover crops should be planted for at least one growing season, even if
strawberries were not grown previously, to build up organic matter in the
soil. Do not use a legume if black root rot is of concern. Cover crops
include annual ryegrass, sudan grass and sorghum/sudan (Sudex). More information
on choosing a preplant cover crop can be found in the Bramble Production
Guide (see References).
If the site selected does not have good soil drainage, the strawberry planting
should be established on raised beds. The raised beds will allow excess
soil water to drain from the strawberry root system, creating an environment
less favorable to the disease causing fungi. In addition, less soil compaction
will occur near the root system.
Soil tests for fertility and pH should be conducted during site preparation.
Fertilizer and/or lime should be added before or at planting based on the
results.
Preplant soil fumigation is often recommended for control of soil fungi
and nematodes. However, there have been mixed results after fumigation
with regard to black root rot. In some instances, black root rot has been
reported to build up more quickly in fumigated soils than in nonfumigated
soils. Researchers suspect that the reason for this may be that when the
soil is sterilized by fumigation, beneficial or competitive soil microorganisms
are eliminated. In their absence, Rhizoctonia and other fungi involved
in the black root rot complex may increase more rapidly.
Planting: Use only healthy, white rooted plants which have been purchased
from a reputable nursery. There are no strawberry varieties, at this time,
that are resistant to black root rot however, some varieties appear to
be more tolerant than others. In varietal trials conducted on infested
soils in New York, Raritan, Lester and Sparkle showed more tolerance than
Blomidon and Honeoye.
After Planting: Always use cultural practices which favor good plant growth
and development. A soil and/or tissue analysis should be performed each
year to determine optimum fertilizer applications. Try to minimize soil
compaction created by equipment and cultural practices. Avoid over or under
irrigation of the strawberry field. Strawberry plants should be mulched
in late fall to prevent winter injury, which predisposes plants to black
root rot. In particular, compacted, wet soils tend to heave more during
the winter if mulching is insufficient.
 Figure 1. Comparison of root structures of healthy plants and those infected
with red stele or black root rot. Adapted from Soil-borne Diseases of Strawberries
in New York State by Arneson, PA. and A.J. Braun. Cornell Coop Ext. Info
Bull. No. 73.
References:
Pritts, M. and D. Handley (eds.). 1991. Bramble Production Guide. Northeast
Regional Agricultural Engineering Service Pub. No. NRAES-35. 189 pp.
Available from UConn Department of Natural Resources Management and
Engineering
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