Symptomology, agronomy, and economic considerations in aster yellows management

16 Pages·2008·0.6 MB·English

by  Philip Northover

Symptomology, Agronomy, and Economic Considerations in Aster Yellows
Management
Philip Northover, Manitoba Agriculture, Food and Rural Initiatives
Crops Knowledge Centre, Box 1149 Carman MB R0G 0J0
E-mail: [email protected]

Abstract
The aster yellows phytoplasma (formerly a mycoplasma like organism or MLO), is a single-
celled prokaryotic microorganism, lacking a cell wall, capable of inciting disease in over 300
plant species. Vectored by the aster leafhopper (Macrosteles quadrilineatus Forbes =M.
fascifrons), aster yellows (AY) symptoms, include proliferation, alteration of tissue pigments
(red, orange, yellow, and purple), phyllody, and reduced palatability in food crops. Once a plant
is infected, there is no curative treatment. Symptoms of AY often mimic other diseases or
chemical damage making identification difficult.

In 2007, aster yellows diseases were observed in a wide range of crops in Manitoba raising
concerns among many producers. A survey of canola fields in 2007 determined aster yellows
levels at 80% of fields compared to 3% in 2006, with mean incidence levels of 5.3% in 2007
compared to 1.0% incidence levels in 2006.

Aster leafhoppers were monitored in Manitoba carrot fields in 2007 at levels that were seven
times that of 2006, at the highest level recorded. The Aster Yellows Index (AYI) is a value that
indicates when chemical application is warranted. In 2007, this level was exceeded for carrots
on July 6th and reached levels as high as 35 times the economic threshold, in managed
commercial fields. Incidence levels in four fields in September ranged from 10-13%, in one field,
the incidence level exceeded 50%, a significant economic loss.

Market considerations must be made for pest management, and management strategies
employed in certain crops will not necessarily be feasible in others. The sporadic nature of AY in
many field crops such as canola makes management difficult and is not an economically
prudent option. In Manitoba, crops with significant economic returns (eg. carrots) in
combination with regular AY outbreaks enable an integrated strategy implementing use of
chemical inputs for AY management, to be employed.

General Phytoplasma Biology
Phytoplasmas (formerly referred to as mycoplasma like organisms or MLO’s), are highly
modified bacterial plant pathogens, lacking a cell wall. Belonging to the Class Mollicutes (which
includes another plant parasitic group-- the spiroplasmas), they are among the smallest of all
free-living organisms. Their small size does not permit them to be viewed using light
microscopy, instead the techniques of fluorescence and electron microscopy are required, to
determine the presence of a phytoplasma. Pleomorphic in nature, when viewed in cross section
using electron microscopy, phytoplasmas often appear as oval shaped bodies within the sieve
elements of the phloem tissue.

Unlike many bacterial and fungal plant pathogens phytoplasmas cannot be cultured, making
diagnosis of phytoplasma associated diseases difficult. When it was learned that phytoplasmas
respond to the use of antibiotics, this provided a means of determining an association between
the presence of a phytoplasma and development of disease. With the advent of recombinant
DNA technology, PCR based tests made identification of specific phytoplasmas and their
relatedness to each other possible. PCR amplication of the 16S r RNA genes and Restriction
Fragment Length Polymorphisms (RFLP) form the basis of phytoplasma diagnostics. This does
pose challenges to laboratories that are not equipped or have staff who are not familiar with
molecular techniques.

Aster Yellows Phytoplasma
Capable of inciting disease in over 300 plant species, the aster yellows (AY) phytoplasma is
present across Canada, and around the world (see Appendix 1 for a partial list). In Manitoba, a
wide range of crops and weed species can become infected by aster yellows. Vegetable crops
(carrot, parsnip, cabbage, lettuce, cucurbits, tomato, potato, broccoli, onions, garlic, cauliflower,
rutabaga, celery), typically suffer the greatest damage relative to other crops, due in part to the
reduction in quality, the unmarketable appearance, but also due to the unpalatable taste that
can be imparted to the plant by infection with the AY phytoplasma.

Field Crops including oilseeds such as canola, flax, sunflower; grains including corn, wheat,
barley, and oats; can all serve as host to the aster yellows phytoplasma. Leguminous crops,
such as alfalfa and soybean can also serve as hosts. Among fruit crops strawberry, can be
severely impacted, as well as special crops such as buckwheat, echinacea, caraway, and
coriander.

Grasses such as brome and ryegrasses, can also serve as hosts to the phytoplasmas. Many
weed species such as plantain, dandelion, and sow thistlecan serve as reservoirs for the
phytoplasma.

The aster yellows or six spotted leafhopper (Macrosteles
quadrilineatus Forbes =Macrosteles fascifrons) is
considered the chief vector of the aster yellows phytoplamsa
in Manitoba. The leafhoppers are phloem feeders, which
acquire the phytoplasma, through feeding on infected plants.
The phytoplasma is capable of reproducing within the
tissues of the leafhopper, and is able to transmit the
phytoplasma to healthy
plants, inciting disease. Figure 1. Aster leafhopper (Macrosteles
quadrilineatus) (B.Elliott)
The association is
beneficial to the leafhopper and phytoplasma. The
phytoplasma gains the benefit of a dispersal mechanism and
an additional location to reproduce, while the leafhopper has
been shown to have a longer life span and lay more eggs
(Beanland et al. 2000), relative to leafhoppers that do not
carry the phytoplasma.
As there is no practical way of controlling the phytoplasma
Figure 2. Size comparison of the aster directly, the leafhopper vector, must be managed in high
leaf hopper to a penny. value crops, such as carrots, lettuce, and many other
vegetables.
Symptom Expression
Residing in the phloem, the phytoplasmas inhabit the region where nutrients, sugars, and
hormones are transported. Disruption of the hormone balance in a plant can have significant
effects on plant development, essentially the “blue print” of the plant is rewritten. The cause of
the symptoms caused by aster yellows is not well understood, but a
number of explanations offer some insights, alteration of phloem sap
content, changes in the expression of the flower development gene,
clogging of sieve elements, and the action of long distance toxins.
One symptom that strongly suggests a phytoplasma is associated with
a plant disorder is phyllody, the replacement of floral parts with leaves
(or more accurately the prevention of the development of development
of floral parts). The flowers are replaced with green parts that may
appear as flattened leaf like or thickened “bladder-like structures
(Figure 3) depending on the time of infection and the host plant
involved.
Figure 3. Bladder-like Pigment expression is also altered by infection with the aster yellows
pods (phyllody) on canola.
phytoplasma. Purple, yellow, red, and bronze discolouration have been
(C.Olivier)
observed in a number of hosts. In carrots, leaves can appear in a
range of colours, brassica crops such as canola (Figure 4) and cabbage, and rutabaga may
show reddish purple pigments in leaf tissue.
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Discolouration by itself is not necessarily a . D
good diagnostic feature, as the degree of es
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altered pigment expression varies with the crop. rd
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Symptoms on canola can be confused with s
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sulfur deficiency, anthocyanin production, and A
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symptoms of BYDV virus can be very difficult to I Cro
distinguish from infections by Aster Yellows. p D
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Another symptom associated with aster yellow rt
infection is proliferation, also referred to as
“witches’-brooms”. This is the extensive (over)
production of shoot and in some cases root
tissue. Multiple branching may occur and the
Figure 4. Discolouration of canola leaves due to
result is a “bunchy” appearing plant (Figure 5). aster yellow infection.(M.Desjardins)
This may be the first symptom that is likely to
be observed, especially under dry conditions, when other plants may be reduced in size, these
plants will be much more distinctive and easier to notice. In potatoes, the symptoms are quite
unusual with tubers (which is are modified stem tissue) produced above ground (Figure 6).
Generally, not all floral parts are replaced, on a plant, and depending
on the host, any remaining “normal looking” seeds may or may not be
viable. Work with canola, determined that between 0.3% and 0.7% of
the seeds were misshapen and 50-90% of the “normal looking”
appearing seeds could germinate (Olivier et al 2007).
While work has been conducted with the objective of determining if
aster yellows could be spread through seed, the prevailing view is that
seed transmission of phytoplasma diseases does not occur, With no
vascular connection between the phloem and the developing embryo, it
appears improbable that seed transmission is possible. The presence
of aster yellows phytoplasma has been reported on canola seed
Figure 5. Proliferation-
“bunchy growth” on carrot. however (Olivier et al 2006).

Agronomic Considerations for Management
The percentage of canola fields in Manitoba and Saskatchewan,
surveyed for the presence of aster yellows, from 2001 through to
2007 is shown in Figure 7. The number of fields surveyed for each of
the provinces in each of the years are shown in Table 1.
In Manitoba, the number of fields that have plants with symptoms of
aster yellows, varies considerably, ranging from 2% of the fields
(2003) to as high as 94% (2004). Levels in 2007 were 80% in MB,
83% in SK. The changes in the number of fields with symptoms of
the disease in Manitoba from year to year do not suggest any
predictive ability. It must be considered that fields with trace levels
(symptoms observed at levels too low to be detected by the sampling
procedures employed) to levels in excess of 20% were reported. This
information suggests there is no guarantee that a region that had
aster yellows in one year, will have the same or an increased level of
symptomatic plants the following year.
Figure 6. Stem proliferation on
potatoes (WCPD Potato Image
collection).
Figure 7. Plot of the number of the percentage of canola fields with symptoms of aster yellows in Manitoba and
Saskatchewan for 2001 to 2007. Due to drought conditions, there was no Saskatchewan survey in 2002.

Table 1. Number of fields examined in each of the years for canola disease surveys,
conducted in each of the three prairie provinces from 2001-2007.
Year Manitoba Saskatchewan Alberta
2001 277 95 N/A
2002 297 N/A (no survey-drought) N/A
2003 259 85 N/A
2004 68 87 182*
2005 81 107 N/A
2006 33 101 N/A
2007 40 99 N/A
*Alberta data is not shown in graphs, trace amounts (<0.1% to 0.7% were detected in 2004, no
other annual surveys in Alberta have been reported).
The unpredictability of the appearance of aster yellows in canola crops is contrasted to the
situation with carrots, grown in Manitoba (Figure 8). In carrot production, aster yellows is an
annual problem and of considerable concern. It is expected that aster yellows will be observed
in any given year to some degree, in any of the major production fields. Considering the high
value of the carrot crop relative to that of most field crops, and developing a management
strategy becomes more feasible. Conversely the sporadic nature of the disease in canola and
other field crops make chemical uses impractical.

Figure 8. Plot of the percentage of canola (Brassica napus) fields with symptoms of aster yellows, compared to
carrot (Daucus carota) fields in Manitoba over the past seven years.
The average incidence levels (the % of plants in a canola field with symptoms of aster yellows)
within production fields, both 2004 and 2007 in Manitoba were higher than the rest of the values
in MB and SK. In 2007, incidence levels in SK were higher than reported for the previous six
years in SK. Aster yellows levels on average in canola are generally low on average, rarely
exceeding 1-2% in both provinces (See Figure 9).
Figure 9. Plot of the incidence of canola (Brassica napus) with symptoms of aster yellows in Manitoba and
Saskatchewan over the past seven years.
The results of the weekly monitoring of carrots conducted as part of the Manitoba Weekly
Vegetable Report in 2007 are shown in Figure 10. Five carrot fields in Manitoba are
represented on the graph. Seeding of some carrot fields occurred in Mid-May, and harvest took
place in September. Sampling of leaf hoppers was conducted each week unless rainfall events
or spray operations were in progress. On June 7th, sampling began and the first leaf hoppers
were collected on July 5th, which exceeded the economic threshold level for carrots. Multiple
local maxima (peaks) and local minima (valleys) were observed on the graph and indication of
rising and falling population levels. The declines in the population could be attributed to
application of insecticides, corresponding increases could be due to both the increase in local
populations and the entrance into the field of migratory populations.
The 2007 “average” canola growing season is overlaid on the graph, seeding generally occurred
around mid to late May. Flowering took place at approximately June 21st to June 28th.
If an application of dimethoate (CygonTM or Lagon®), the only active ingredient registered for
aster leafhopper control in canola, was applied prior to June 28th,to canola, based on the levels
caught in carrot fields, there would be no leafhoppers to spray (in addition, flowering of the crop
needs to be considered to avoid negative effects on the bee population). It appears that in the
carrot field, a migratory population appeared quite rapidly. An early season spray would be of
little use, as the rapid influx of leafhoppers could not have been predicted. An insecticide
application would be based on a high level of uncertainty

Figure 10. Weekly changes in the aster leafhopper (Macrosteles quadrilineatus) populations in five carrot fields in
Manitoba in 2007, with key events in the canola growing season superimposed over the graph. The horizontal black
line represents the economic threshold for application of insecticides to for leafhopper management in carrots. The
preharvest interval of dimethoate for use in canola is 21 days prior to harvest.
A single application of a dimethoate product, would also be of limited use due to the multiple
generation of leaf hoppers in Manitoba (2-4 per year) and multiple migrations into fields.
Generations may overlap and migrations at various times may occur which would be unaffected
by a single application of dimethoate. In 2003 to 2006 aster leaf hoppers generally appeared in
late June to early July, applying an insecticide prior to this would be of little use (Figure 11).
It has not been demonstrated that the aster leafhopper can overwinter in Manitoba, as the
winter conditions are too harsh. Previous sweeping work in multiple years appears to confirm
this, as no leafhoppers have been caught (See Figure 11). It is possible that if they were
present and infective, they could be at such low levels that they cannot be detected by
sweeping. Any contribution of any “overwintering leafhoppers” to disease levels could be of
limited impact, as the high number of migratory leafhoppers would account for the vast majority
of the leaf hopper population.
Figure 11. Weekly changes in the populations of aster leafhoppers (Macrosteles quadrilineatus) in Manitoba carrot
fields from 2003 to 2006, showing the multiple generation and migration events. The orange line represents the
economic threshold for application of insecticides to manages the aster leaf hopper.

Economic Considerations for Aster Yellows Management
Figure 12, depicts the relationship between bushel price, gross revenue, net revenue before and
after one, two or three applications of dimethoate for control of aster leaf hoppers at a yield of
30 bushels/acre. At yields below 25 bushels/acre, net revenue was below $0 for all of the price
levels depicted. Only the cost of the insecticide is considered. Costs associated with the
application of insecticide such as labour, fuel, machinery maintenance, etc is not considered.

At $7.75 per bushel, the net revenue is already in a loss position prior to any application of
insecticide. Any additional costs would only put a producer further in a loss position. At a price
of $9.21 a bushel, net revenue would be negative after a third application. At $10.00 an acre, all
applications still leave the producer with a positive revenue position.
Figure 12. The relationship between gross revenue at three bushel prices, and effect on the net revenue of 0, 1, 2,
or 3 applications of dimethoate (CDN$9.50) for a yield of 30 bushels/acre of canola. Cost of dimethoate application is
only the cost of the chemical per acre, other costs associated with application have not been considered.
Considering the sporadic nature of aster yellows, the use of an insecticide is largely
uneconomical. The typically low levels of crop loss observed in most fields, do not warrant
management through chemical applications. Seeding of canola as early as possible would be
worth considering, though this does not guarantee that some degree of loss will not occur. In
short aster yellows will be more of an annoyance for most producers of canola, and remains a
significant concern for vegetables, notably carrot grower.

Summary
Aster yellows is a disease caused by a phytoplasma, which is vectored by the aster or six
spotted leaf hopper. Crop rotation is ineffective for aster yellows management due to the wide
host range of the phytoplasma, and the annual migratory populations that come into Manitoba
from the south. Diagnosis is difficult and requires specialized equipment and techniques that
are not available in more basic laboratories. Visual assessment of symptoms may be sufficient
in limited cases, but in some hosts symptoms can be easily confused with stress responses,
nutrient deficiencies, or herbicide damage.
The sporadic nature of this disease in canola and other field crops make chemical use
impractical. Planting crops as early as possible may have some benefit as the arrival of