Agriculture Reference
In-Depth Information
may cause the replant trees signifi cant stress, particularly
in situations were the replant trees will be receiving the
same amount of water and nutritional supplements as
mature trees.
Roots of nursery trees may be infested with nematodes
from the rootstock stoolbeds or the nursery soils.
Nematodes are mobile and can move short distances (a few
metres per year) while migrating to feed. Spread over long
distances occurs mainly through transport of soil on
cultivation implements and farm machinery.
Remove as many tree roots from as deep in the soil as
possible when replanting a block where trees have been
previously affected by white root rot.
Importance
Replant problems associated with root-lesion nematodes
can be serious, particularly where apples are replanted in
areas where old apple trees have recently been removed, or
where crops that are hosts of lesion nematodes have been
grown recently.
Clean machinery after use in a block affected by white
root rot.
Never pile up roots removed from blocks affected by
white root rot on clean soil; instead, they should be
carefully disposed of in an area well away from apple
production blocks.
Consider using a long-term (several years) crop rotation
for severely affected blocks, after a short-term (one year)
rotation of green manure crops. Anecdotally successful
rotation crops for the Granite Belt include stone fruit,
grapes and vegetables.
Management
In replant sites, pull out old trees and then rip and plough
the soil thoroughly to remove tree roots. Delay replanting
for as long as possible, preferably 12 months.
During spring and summer, grow a cover crop of a
sorghum
To facilitate short-term crop rotation: plant new trees in
nursery blocks, in a separate location, for their fi rst year,
before planting them as two-year-old trees into
remediated blocks. Then crop these trees within one year
of planting instead of two.
×
sudan-grass hybrid.
Incorporate the cover crop in late summer. At the time
of incorporation, add urea to encourage the trash to
break down and to achieve ammonia concentrations
in soil that are toxic to nematodes.
Where apples have to be replanted soon after removing
old trees, maintain the area free from weeds and fumigate.
NEMATODES
After replanting, maintain a mulch of organic material
1-2 m wide along the tree row.
ROOT-LESION NEMATODE
( PRATYLENCHUS SPP.)
Do not plant cover crops such as cowpea, rye or lupins,
which are susceptible to root-lesion nematode.
Cause
Root-lesion nematode ( Pratylenchus spp.).
DISEASE COMPLEXES
Symptoms
Young replanted trees are stunted and unproductive, and
established trees show reduced growth and yield. Root
systems of affected trees are small and discoloured, often
grow in tufts, and lack well-developed feeder roots.
APPLE REPLANT DISEASE
Symptoms
Apple replant disease can cause a range of variable
symptoms. However, typical symptoms include severe
stunting, shortened internodes, rosetted leaves, and
reduced fruit production. Affected trees tend to have
small root systems with lots of fibrous roots, and often
fruit two to three years later than unaffected trees.
Source of infection and spread
The root-lesion nematode has a wide host range and is best
suited to sandy soils low in organic matter.
When orchard trees are removed, high numbers of
nematodes remain in the roots or in the soil. In weed-free
fallow soils, nematode numbers tend to decline with time,
whereas they will increase if old apple roots, weeds or host
crops are present.
Less distinctive symptoms include general unthriftiness
and reduced growth, and can appear similar to the initial
symptoms of other soil-borne pathogens, including
Armillaria , Phytophthora and white root rot.
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