Rhizomania

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KWS SAAT AG
P. O. Box 14 63
37555 Einbeck
Phone: 00 49 / 55 61/ 311 -349
Fax: 00 49 / 55 61/ 311 -907
E-Mail: [email protected]
www.kws.com
It begins with us.
Table of Contents
2
What is Rhizomania ?
4
Origin, History and Spread
of the Disease
5
Lifecycle of the Virus
5
Leaf and root symptoms
6
Damages caused by Rhizomania
· Reduced yield and quality
· Change in quality components
9
Disease Diagnosis
10
What can farmers do to
reduce the risk of Rhizomania ?
· Use of tolerant varieties
13
Rhizomania Breeding
· Breeding by conventional Methods
· Breeding by biotechnical Methods
15
Rhizomania at a glance
· Symptoms
· Additional characteristic
18
Prospects
18
3
What is Rhizomania?
Rhizomania is a plant disease of the Chenopodiaceae family caused by the development of a virus
called BNYVV (Beet Necrotic Yellow Vein Virus),
which is introduced and transmitted by the soil fungus Polymyxa betae.
Origin, History and Spread of the Disease
Rhizomania disease caused considerable yield
losses in sugar beet cultivation in Northern Italy
in the mid-fifties. Since then, it has been found in
many European locations over the last few decades (see picture opposite).
Today, Rhizomania is found in all major sugar beet
growing areas of the world including the United
States, the CIS, China and Japan.
Fig. 1:
Spread of
the Disease
Lifecycle of the Virus
0,1–10%
10–30%
30– 70%
+70%
Rhizomania is a disease which is carried by the
soil fungus Polymyxa betae. Under optimum conditions, i. e. at about 15 – 25°C soil temperature, a
pH value in the neutral to weakly alkaline range and
high soil moisture content (field capacity excessive), so-called swimming spores (zoospores)
develop from the fungal resting spores in the soil.
After infecting the host plants which, besides
sugar beet, also include other Beta and Chenopodium species (e.g. fodder beet, chard, red beet,
spinach), plasmodia of the fungus develop which
either release further zoospores to cause new
infections or form resting spores which can remain
viable and infectious for at least 20 years.
Rhizomania can be spread by soil movement
during cultivations, drilling and spraying, irrigation
equipment, harvesters and transport vehicles.
Soil particles can also be spread through water
(drainage water, waste water, irrigation) and wind,
even through shoes, game and wildlife. Vehicles
carrying soil over longer distances and products
with minimal dirt tare (e.g. seed potatoes) are commonly responsible for the contamination.
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Fig. 2:
Microscopic
picture of the soil
fungus Polymyxa
betae which
shows the survival
organs of the
fungus in the
tissue of a young
sugar beet root.
Wind erosion may also favour the spread of rhizomania. According to the latest scientific research
the virus cannot be transferred through beet seed
(by systemic infection) and the virus-carrying
vectors cannot be spread through processed,
pelleted seed. Neither insects nor nematodes are
vectors.
Fig. 3:
Yellow vein
effects on
the leafs
During the growing season numerous leaf and root
symptoms develop under high infection pressure
which suggest deficiencies in nutrients (nitrogen,
boron, manganese) and water.
Leaf and root symptoms
The symptoms of Rhizomania which are essential
for field diagnosis occur on both leaves and roots.
Only in extreme cases do infected plants wilt in the
early stage of development. The symptom of the
disease which is responsible for the name “yellow
vein virus”– striking yellow chlorosis along the
leaf veins – is seldom observed and results from
systemic infection by the virus.
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Fig. 4:
Yellow patches
in the field
Typical foliar symptoms show up as yellow patches
in the field and can be confused with poor nitrogen
supply or nematode attack. In dry conditions plants
may occasionally wilt in the field despite sufficient
moisture.
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Individual plants with brighter leaf colour, long petioles and unusually narrow leaf blades indicate
Rhizomania infection.
On cutting the lower part of the tap root a brownish
discoloration of the vascular bundle rings is visible, occasionally combined with symptoms of
decay caused by secondary organisms.
Typical symptoms in the root area are stunting of
the plants, mainly in the case of early infection, the
development of a root beard due to continuous
roots death and new development of lateral roots
as well as often a turnip-shaped beet.
Fig. 5:
Infected plant
showing long
petioles
Fig. 7:
Cross Selection
of root showing
brown vascular
tissue
Fig. 6: Bearded root symptoms
Damages caused by Rhizomania
Reduced yield and quality
The symptoms described above cause root yield
losses of up to 80 per cent depending on the
severity of the attack. This yield loss will be higher
the more severe the infection. However, before
large yield losses are caused, there are a number
of symptoms indicating an infection with the disease, without necessarily resulting in low root yields.
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9
Change in quality components
Possible indicators may be the change in quality
components which are evident in individual loads
delivered to the factory:
disease symptoms may strengthen the suspicion.
A particularly alarming sign is a highly reduced root
yield combined with low sugar content and simultaneous increase in potassium and especially
sodium values.
· a very low sugar content
(depending on the severity of the infection up
to 6 % below non-infected beet)
· a relatively high sodium content (an increase
above 10 mmol/100 g sucrose or 2 mmol/100 g
beet may be initial signs of infection)
· a relatively low amino-nitrogen content
(about 5 mmol/100 g sucrose or 1 mmol/100 g
beet lower than in non-infected beet)
· a relatively high invert sugar content
The low sugar and high sodium content lead to increased impurities in the beet.
If all the parameters referred to above are present
as described, it is most likely that Rhizomania is
involved. Ultimate reliability can only be achieved
by a laboratory test.
NB. Soil compaction does not cause internal browning of the vascular system.
All symptoms mentioned may also be due to other
biotic and abiotic factors and are therefore not
absolutely reliable features for Rhizomania detection. Confomation of disease presence can only
be established by applying the ELISA (EnzymeLinked Immuno Sorbent Assay) test.
In this test sap is extracted from suspicious root
samples and mixed with antibodies that are specific for the virus. After further analyses the presence of the virus in the sample is established by means of colour reaction.
Fig. 8:
Running an
Elisa Test
Disease Diagnosis
Diagnosis by the farmer of whether the damage
caused is due to Rhizomania infection is possible
by means of certain criteria. Observation of the
symptoms described (patchy leaf yellowing, wilting
in spite of adequate soil moisture, stunting in the
field) may indicate the presence of the disease.
However, confusion with symptoms of nitrogen deficiency or nematode attack cannot be excluded.
The presence of a bearded root as well as vascular browning within the tap roots of plants with
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The test can be carried out directly on a plant sample as described, or indirectly on soil samples, in
which a susceptible variety is grown for 6 to 8
weeks under optimum conditions. This so-called
bait plant test is applied where direct evidence in
sugar beet on suspect fields is lacking.
Fig. 9:
Bait Plant Test
and Direct Test
on the Roots
sowing of
a susceptible
variety
accumulation
of water
grow at 22 – 25°C
for 6 – 8 weeks
Spread of Rhizomania in soil normally involves a
carrier and a frequent source of contamination is
of human origin. Rhizomania can be spread by machinery and equipment as well as by soil on shoes
when moving from one field to another.
cutting of
the roots
Two reliable tests
for Rhizomania:
Use of a bait plant
test as well as a
direct test on the
roots of suspect
beet.
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Fig. 10:
Bait plants
growing in
soil sample
Further spread may occur by irrigation, if the virus
is present in irrigation water.
sap extraction
ELISA laboratory analysis
What can farmers do to
reduce the risk of Rhizomania?
field sample from
suspect beet
Therefore, the risk of spreading the virus is difficult
to minimize. Farmers and advisors who visit a
large number of fields should clean their shoes
between each field, especially in affected areas.
However, one cannot expect every farmer to clean
his equipment after every soil cultivation before
moving to the next field. This is more important
where machinery is shared or where contractors
are involved. For these reasons it seems unlikely
that the spread of the disease can be halted completely.
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Rhizomania Breeding
Use of tolerant varieties
Previously, the use of Rhizomania tolerant varieties
have come with a yield penalty. However the new
tolerant KWS varieties are considerably improved
from the beet quality aspect. Due to their tolerance to Rhizomania, the Amino-N content remains at
the same low level as the control varieties under
healthy conditions and invert sugars are not increased.
Breeding by conventional methods
Classical breeding of Rhizomania tolerant varieties
has always been the main field of KWS activities.
These tolerant varieties have been developed by
integrating tolerance of different sources of wild
species. This tolerance level has been combined
with the high yield potential of KWS breeding material in a backcrossing programme by means of
glasshouse tests.
In areas where Rhizomania is prevalent the use of
tolerant varieties can reduce the spread of the
disease. It has been demonstrated that the multiplication of the virus is considerably reduced when
using these varieties compared with normal
susceptible ones. It is important to minimize the build up of the virus, because even the best tolerant
varieties under very severe infection conditions will
suffer a yield penalty. In this way sugar beet can
be grown successfully until totally resistant varieties are available. It is envisaged that ultimately all
varieties will be totally resistant to Rhizomania.
KWS has thus obtained tolerant varieties which
show under conditions of infection or non-infection
almost similar performance to the susceptible varieties under healthy conditions.
Breeding by biotechnical methods
Besides classical breeding, other methods have
been tested by biotechnological techniques. Therefore, genetically modified beet could lead to a
longterm solution of the Rhizomania problem, as
the researchers have in a way vaccinated the beet
against this disease.
Using genetic engineering, KWS has been working
since 1988 on the development of sugar beet varieties which are resistant to the virus disease Rhizomania. The researchers have successfully isolated the gene producing the coat protein from the
virus. Using cell biology and molecular methods
this gene could be transmitted into the beet, thus
producing transgenic beet. Depending on the vaccine, the gene transferred into the beet has a resistance effect against the disease.
Constant progress is expected in the field of genetic engineering research to control Rhizomania.
The cultivation of tolerant varieties compensates
for yield loss, but does not yet prevent the complete spread of the Rhizomania virus. On the contrary, with genetically modified beet the development of the pathogenic agent in the plant host is
totally restricted.
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15
The first field tests have yielded encouraging
results. By means of genetic engineering specific
traits can be introduced. Nevertheless, gene technology will not completely replace conventional
breeding methods; they will be complimentary.
With the help of
gene technology
GENE
RESOURCES
Traditional
Fig. 12:
Breeding Method
Plants of the
same species
Plants, bacteria,
fungi, viruses
Evaluation, isolation,
synthesis of genes
Integration
integratie
ofgenen
genes into
van
plant cells
in plantencellen
Variation
2–4 years
Regeneration
regeneratie
tot
of planten
a whole plant
Fig. 11:
How to produce
virus resistant
Plants
Rhizomania
affected
sugar beet
1st step:
Isolation of the
virus from an
affected beet.
Crossing
2nd step: Isolation
of the gene that
produces the virus
coat protein and its
transfer into the
nucleus of single
sugar beet cells.
3rd step: Regeneration of these cells
into plants.The virus
coat protein in all
cells of these plants
constitutes a
defence mechanism against
invading Rhizomania
viruses.
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Evaluation
evaluatie
for important
van belangrijke
characteristics
karaktereigenschappen
TEST
jaren,Years,
locaties
sites,
herhalingen
replications
Selection
8–12 years
Yield,opbrengst
resistentie
resistance
Selection
In addition
a marker gene
is transferred
which serves to
identify the cells
that have become resistant.
Official test
4th step:
Rhizomania
resistant
sugar beet
Variety
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Rhizomania at a glance
Rhizomania is a virus disease which may cause
heavy losses in yield and quality.
Symptoms
patchy yellowing of beet leaves in the field
long petioles, narrow leaf blade
stunting and development of a root beard
browning of the vascular bundle rings
after cutting
·
·
·
·
Additional characteristics
· increased sodium content
· reduced sugar content
· reduced amino-nitrogen content
· yield loss
Evidence of the disease: ELISA test
Chemical control of Rhizomania is not possible.
The only useful method at present is the growing
of tolerant varieties. A permanent solution to the
problem of controlling Rhizomania can only be obtained by resistance breeding and may be achieved by genetic engineering.
Prospects
The further spread of the virus disease Rhizomania which could have ended sugar beet growing
for many farmers, has become controllable through
the advance of sugar beet breeding programmes.
The new tolerant KWS varieties which are currently available to the grower give high yields and
a quality similar to the standard varieties under
disease-free conditions.
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