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Anthracnose Problems in 2003: Current data, Research
and Recommendations
Dr. Frank Louws, Dr. Leonor Leandro, Rosemary Schwegel, and Jim Driver
Department of Plant Pathology, North Carolina State University
Anthracnose problems have been widespread this year in plug operations.
We have isolated the pathogen from many different sites and have consistently
identified Colletotrichum acutatum. This pathogen is particularly aggressive
in the spring when it causes ripe fruit rot. Clearly, it also is aggressive
on young green and succulent tissue, causing lesions on leaf petioles
and plant collapse. In the last 2 years, we have been able to isolate
it from infected roots also.
Can you treat plug trays and reduce disease levels? To determine the
effect of C. acutatum on young plug plants and to determine if fungicides
could have a curative effect, we conducted a trial last fall. Four
week old plug plants (in 50 cell trays) were inoculated with a spore
suspension of C. acutatum and covered with plastic for 48 hrs (to allow
for infection). Several trays were not inoculated (non-inoculated controls).
Seven days later, plants were sprayed until runoff and trays were drenched
from above until soil saturation with a fungicide solution. Plants
were field set the following morning. No additional fungicides were
applied until 25 March 2003. Data on plant mortality, vigor, and yield
were collected. Young and old leaves were collected in February and
assayed for the presence of C. acutatum.
RESULTS: Lesions began to form on leaf petioles 7 days after inoculation.
The inoculation level was moderate. Some plant mortality occurred soon
after planting, and by the spring differences were noted (Figure 1).
Captan treatments increased plant mortality. C. acutatum alone (with
no treatments) did not significantly reduce plant stand compared to
non-inoculated controls.
The pathogen caused stunting of inoculated plants (Figure 2). All plants
inoculated with C. acutatum were distinctly stunted (6 Mar 2003) compared
to plants not inoculated. The plant growth in each plot of inoculated
plants was variable i.e. some plants were larger than others. Therefore,
we used a weighted scale using values from 1 to 6, rating each plant
and then weighting this rating based on the number of plants in each
category. No fungicide “cured” the plants from this stunting
symptom and Captan tended to increase stunting problems.
Ten to 12 newly emerged and old leaves (that emerged after inoculation)
were collected 6 Feb. Leaves were surface sterilized and then treated
with paraquat (GramoxoneTM) to kill plant tissue, and sprayed with
RovralTM to inhibit saprophyte growth. The leaves were incubated in
moist chambers to allow C. acutatum to grow out, if present in the
leaf. On average 27% and 45% of the new (range = 4% to 50%) and old
leaves (range = 10% to 64%), respectively, were infected by the pathogen.
Non-inoculated controls tended to have the lowest number of infected
leaves. These data suggest that the pathogen had successfully spread
from plot to plot and colonized new leaves during the winter months.
RECOMMENDATIONS: Based on this trial, recommendations
to treat infected plug plants cannot be provided. The fungicides in
this trial offered
no advantage. (Fall fungicide applications in the field were not evaluated
in this trial). We have good data that demonstrate the benefits of
spring fungicide programs and therefore suggest it may be best to save
all fungicide uses for spring applications.
Can you cull out infected plug plants and plant the rest? Last year,
many growers culled out infected and surrounding trays, planting the
remainder of plants. This appears to be a successful strategy for growers
who cannot secure an alternative source of plants. The main factor
that determines the success of culling is the level of disease in the
plug trays. Early problems diagnosed in August of 2003 were severe.
We mapped “hot spots” in an area of 60,000 trays (Figure
3). This map represents hotspots where plant symptoms such as stunting
and desiccation were clearly visible.
 The site also had 21 sprinklers evenly distributed throughout the area.
We collected 5 petioles from four sites around each sprinkler (N, S,
E, and W). These petioles were placed in sterile tubes, then taken
to the lab. The tissue was frozen (-20 C) for 2-3 hrs and then surface
sterilized in a bleach solution. The petioles were incubated for 7
days and scored for presence of C. acutatum using microscopes . Two
sites were sampled in a similar fashion. Up to 15% and 7% of the samples
at Sites 1 and 2, respectively, had the pathogen present. The majority
of this tissue had no symptoms.
RECOMMENDATIONS: Infestation levels as high as those observed at these
sampling sites were too high for culling to be an effective control
strategy. These growers secured an alternative source of plants. A
short while later, at two other grower sites in the mountains, the
level of infestation based on the presence of hot spots was lower.
However, leaf petioles and (especially) runners with diagnostic lesions
were scattered throughout the trays.
There has been some indication that later shipments of tips have not
been as heavily infested. In such cases, and given the shortage of
plants, the only option a grower may have is to cull out hot spots
and surrounding trays. Remember that although surrounding plants may
show no symptoms, the pathogen may still be present. In such cases,
limit overhead watering once plants are rooted and do not handle plants
when wet. Plant sooner rather than later.
Are bare-root plants from the same source a viable alternative? Dr.
Poling has provided extensive details on his trip to Canada and the
sampling of strawberry tissue. Dr. Poling walked 2 fields of strawberry
plants (Field A & B) that were destined for supplying North Carolina.
A systematic sampling scheme was developed and 400 leaf petioles from
the mother plants were collected from each field (roughly estimated
to represent 1.5% of the mother plants). Forty petioles, sampled from
each of 10 rows, were placed in ziplock plastic bags and delivered
to our lab. One bag was previewed (Field A, Bag 8), based on Dr. Poling’s
suggestion, since lesions were present on 2 of the petioles. The lesions
were diagnostic for anthracnose and characteristic spores were confirmed
in one of the lesions.
All of the bags were placed in the -20 C freezer for 6 hrs in an attempt
to kill the green tissue (as we did above). The petioles were then
surface sterilized and placed on wire racks in incubation chambers
that contained wet paper towels. Such high relative humidity chambers
allow the pathogen to grow out and presence of the pathogen can be
scored using microscopes. The petioles remained green, even after 6
days. The petiole tissue from the field nursery, unlike those from
plug houses, was not susceptible to the freeze-thaw method of desiccation.
The samples were then treated using a paraquat technique and scored
again after 4 days (13 Sep) and again after 7 days (16 Sep). Most of
the petiole tissue remained green. The tissue needs to be killed for
the pathogen to grow out. On the tissue that did die, no evidence of
C. acutatum was observed.
RECOMMENDATIONS: Field A had symptoms and spores consistent with C.
acutatum. There is a chance that the bare-root plants will have anthracnose
associated with them. The pathogen will multiply during the winter
and early spring – producing more inoculum on the green tissue.
Then, with the warm temperatures and favorable wet weather in the spring,
flower blight and fruit rot can occur. The vigilant use of fungicides
in the spring, according to the schedules we have developed, should
help minimize these crop losses.
Should the bare-root plants be dipped in Quadris? Growers do have an
option to dip their bare-root plants in Quadris. It is now a labeled
use. Research in California and Florida has shown this to be of benefit
if the bare-root plants are heavily infected. In one study, bare-root
transplants were inoculated with C. acutatum and incubated for 4 days.
The transplants were then dipped in fungicide solution for 5 minutes
directly before hand-planting into the field and strawberry plants
were assessed for mortality, vigor and yield. The use rate for Quadris
was 4 oz/100 gal. Treatments were also complemented with a spring fungicide
program or not treated further.
Inoculated plants treated with no fungicide had a mortality rate of
83% - 88%. Quadris dip treatments reduced this level to 5% - 15% mortality.
Likewise, Quadris dip treatments boosted plant vigor. Quadris dip treatments,
combined with a spring fungicide program that included Quadris, Switch
and Captan, were among the best in the trial.
RECOMMENDATIONS: If there is strong evidence of a heavy infestation
of anthracnose present in the bare-root plants, a Quadris drip treatment
could be beneficial for suppression of root and crown rot caused by
Colletotrichum species. Use 5-8 fluid oz per 100 gallons. Where practical,
wash and remove excess soil prior to dipping; completely immerse planting
stock in solution; DO NOT reuse water solution (ideally, each set of
plants should be dipped in fresh solution); dip to expose plants for
a minimum of 2 to 5 minutes; plant immediately.
There are some concerns and issues. First, the tips with the anthracnose
problem have also had a moderate level of angular (bacterial) leaf
spot. The question arises: “Will the bare-root plants have the
same disease?”. Dip treatments will cause this pathogen to spread
from plant to plant. Thus, it would be best, if practical, that fungicide
solutions not be reused. Alternatively, growers may wish to explore
how they can treat only the roots in the dip process. Removal of free
soil has in itself reduced anthracnose problems in Florida and helps
to keep the Quadris fungicide active. Workers must be properly protected
when dipping and planting. The solution must be properly disposed of.
IN SUMMARY: Anthracnose concerns have been high for 2003, primarily
from one nursery source. It appears that tips, especially in early
shipments, have had a very high incidence of infection. This high level
of disease precludes culling and there is a serious concern about plant
establishment and plant vigor if infected plants are used. Fungicide
applications to plug trays have not proven useful and no data is available
concerning the utility of fall applications in the field. There is
considerable data that indicates spring fungicide programs work and
should be the focus of anthracnose management. Bare-root plants from
the same nursery may have anthracnose but the likelihood of disease
development is unknown. Assays were not effective for determining a
rough estimate of incidence. If the data suggest a high infestation
level in the nursery plants, Quadris root dips could be implemented.
Otherwise, growers may choose to focus on good plant establishment
and management with a focus on spring fungicide use.
DISCLAIMER:
From: The University Office of Legal Affairs (Aug. 29, 2003)
1. The strawberry specialist inspected fields that are of concern to
inspect for proper disease management practices and visual signs of
the disease. A representative sample of plants was obtained upon which
laboratory examinations were conducted to determine if the disease
was possible.
2. By the nature of the disease,
it may be hidden at the time of visual and/or laboratory examination,
and thus there is some
risk that the
disease may be present but not detected. Therefore, the risk of a
false negative is relatively high.
3. We have done the in-field and
laboratory screening to help reduce the risk to growers, but because
the risk of false negatives is
relatively high, the grower assumes risk for any disease that
develops subsequent
to the screening.
4. Therefore, our screening process in no way states or implies
that plants are free from disease or the risk of developing disease.
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